IN120085064 Chemistry TeacherLabManual 2013 Inv14

I n vest i gati on Acid-Base Titration

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How Do the Structure and the Initial Concentration of an Acid and a Base Influence the pH of the Resultant Solution During a Titration? Timing and Length of Investigation ■

40 minutes: Teacher Preparation Time Making solutions and gathering materials

■

185 minutes: Total Student Time



10 minutes: The prelab assessment



30 minutes: Animation viewing



20 minutes: Designing a procedure



60 minutes: Data collection and making of graphs



30 minutes: Class sharing and pooling of data



20 minutes: Final calculations and analysis



15 minutes: Whole-class wrap-up discussion

■■Central Challenge While there are times when the students only need to know if a solution is acidic, basic, or neutral, often the exact concentration is important, such as when making biodiesel fuel from vegetable oil. When vegetable oil degrades it becomes acidic. A base such as lye is added to neutralize the acid. The exact concentration of the acid must be known because if too much base is added instead of biodiesel fuel the result will be soap! Students need to know that a titration of the acid with a base will determine the exact concentration. Besides doing the titration, the students must be able to analyze the resultant titration curve.

■■Context for This Investigation Many foods taste as they do due to the presence of acidic or basic content. All foods, beverages, pharmaceuticals, biofuels, water in aquariums, drain cleaners, surface cleaners, and vitamins contain acids or bases, or a mixture of acids and bases. The amount of acid, base, and the pH of solutions and solids must be 273

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maintained at an optimal level. If a solution is too acidic, some base can be added to react with some of the acid. For example, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) By carefully controlling the amount of base added while doing an acid-base titration, and knowing when to stop adding base by using an indicator or a pH meter, one can determine the amount of acid present in the substance. The food industry uses titrations to determine the amount of sugar, free fatty acid content, and the concentration of vitamin C or E present in products.

■■Alignment to the AP Chemistry Curriculum

Primary Learning Objective ■■

Learning Objective 6.13: The student can interpret titration data for monoprotic or polyprotic acids involving titration of a weak or strong acid by a strong base (or a weak or strong base by a strong acid) to determine the concentration of the titrant and the pKa for a weak acid, or the pKb for a weak base. [See SP 5.1, 6.4]

Secondary Learning Objectives ■■

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Learning Objective 1.18: The student is able to apply conservation of atoms to the rearrangement of atoms in various processes. [See SP 1.4] Learning Objective 1.20: The student can design, and/or interpret data from, an experiment that uses titration to determine the concentration of an analyte in a solution. [See SP 4.2, 5.1, 6.4] Learning Objective 6.11: The student can generate or use a particulate representation of an acid (strong or weak or polyprotic) and a strong base to explain the species that will have large versus small concentrations at equilibrium. [See SP 1.1, 1.4, 2.3] Learning Objective 6.12: The student can reason about the distinction between strong and weak acid solutions with similar values of pH, including the percent ionization of the acids, the concentrations needed to achieve the same pH, and the amount of base needed to reach the equivalence point in a titration. [See SP 1.4, 6.4]

■■Skills

Prior Skills Students should be able to: ■■

Identify and use basic laboratory instruments, including graduated cylinders and burets, to accurately measure volume, and pH meters (probes) to measure pH;

Acid-Base Titration

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Determine the limits of precision and accuracy afforded by each piece of equipment;

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Compute values using equations with one unknown, including logarithms;

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Use stoichiometry to perform calculations involving: the mole concept, limiting reagents, and excess reagents in chemical reactions; Use the Lewis structure of acids and bases, definitions of acids and bases, along with experimental data of pH and titration curves, to help identify strong and weak acids and bases;

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Be able to make graphs and critically analyze data; and

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Calculate percent error of a calculated Ka or Kb to a known value.

This laboratory experiment fits best when the students are studying acid-base equilibria. This experiment is not designed to be the first exposure students have to acids, bases, and acid-base titrations. Prior to doing this activity, students should have laboratory experience with limiting reagents, volumetric measurement, molarity, preparation of aqueous solutions, classification of substances including acids and bases, primary acid standards, quantitative acid-base titrations, the pH scale, acidity and alkalinity, pH meters, and equilibrium systems.

Developing Science Practices, Instrumentation, and Procedural Skills Lab Activities

Associated Science Practice, Instrumentation, Procedure

The students will write appropriate acid-base equilibrium equations for the resultant solutions for each equilibrium system. The students will draw models showing how the atoms in the reaction are rearranged during different parts of the titration.

SP 1.1: The student can create representations and models of natural or man-made phenomena and systems in the domain.

Looking at a titration curve, the students will explain how the system changes throughout the titration.

SP1.2: The student can describe representations and models of natural or man-made phenomena and systems in the domain.

The students will draw Lewis structures of the acids to analyze acid strength.

SP 1.4: The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.

The students will explain the choice of equations to use to solve for an unknown molarity during a titration, the pH, and Ka of the acid.

SP 2.1: The student can justify the selection of a mathematical routine to solve problems. (Appropriateness of selected mathematical routine)

The students will perform the calculations for an unknown molarity, pH, and Ka.

SP 2.2: The student can apply mathematical routines to quantities that describe natural phenomena. (Correctness of application of mathematical routine)

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Lab Activities

Associated Science Practice, Instrumentation, Procedure

The students will choose a question to test.

SP 3.1: The student can pose scientific questions.

The students will determine if their question is a valid question.

SP 3.3: The student can evaluate scientific questions.

The students will explain what data needs to be collected to determine the unknown molarity.

SP 4.1: The student can justify the selection of the kind of data needed to answer a particular scientific question.

The students will choose the appropriate equipment SP 4.2: The student can design a plan for collecting data to answer a and design a procedure applying the principles of particular scientific question. acid-base chemistry, including understanding the difference between weak and strong acids and bases, equivalence, titration curves, pKb, and pKa, to solve the lab question. While performing the lab, the students will collect the data indicated in the procedure.

SP 4.3: The student can collect data to answer a particular scientific question.

The students can decide if the data they collected and the data collected by the class can answer the question they posed.

SP 4.4: The student can evaluate sources of data to answer a particular scientific question.

Looking at the data collected and curves drawn, SP 5.1 The student can analyze data to identify patterns or relationships. the students will identify a monoprotic acid and a diprotic acid and a weak and strong acid. Using the titration curves produced, the students will label the titration curve with the initial, halfway, and equivalence points. After looking at their own analysis, the students will compare their results with the class analysis and adjust their analysis if needed.

SP 5.2: The student can refine observations and measurements based on data analysis.

Using the class data, the students will evaluate the evidence to support their analysis of how structure and concentration affect the pH of the resultant solution. They will also perform a search of the literature to find appropriate information about acid-base titration curves.

SP 5.3: The student can evaluate the evidence provided by data sets in relation to a particular scientific question.

The students will use titration cures and acid-base neutralization equations to represent the reaction occurring at each part of the titration.

SP 6.1: The student can justify claims with evidence.

The students can explain how structure and concentration affect the shape of a titration curve from the lab performed.

SP 6.2: The student can construct explanations of phenomena based on evidence produced through scientific practices.

The students can explain two different examples of acid-base reactions in everyday life. Students can explain how acid base concepts affect taste.

SP 7.2: The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/ or big ideas.

Acid-Base Titration

■■Preparation

Materials Below is a list of materials for 30 students in eight groups of 3–4 students. Hydrochloric acid (HCl) 2.0 L of a 0.20 M solution

Calcium hydroxide Acetic acid (CH3COOH) – (Ca(OH)2) – 2.0 L of a 2.0 L of a 0.10 M solution 0.10 M solution

8 utility stands

Sulfuric acid (H2SO4) 2.0 Ammonia (NH3) – 2.0 Sodium hydroxide (NaOH) 8 stirring L of a 0.10 M solution L of a 0.20 M solution –2.0 L of a 0.10 M solution rods Nitric acid (HNO3) – 2.0 16–100 mL graduated L of a 0.050 M solution cylinders

16–250 mL Erlenmeyer flasks or beakers

8 pH meters or pH probes

16–50 mL burets

Teacher Tip If available and proper safety procedures are followed, maleic acid, a diprotic weak acid, and glutaric acid can also be used.

Safety and Disposal Acidic and basic solutions can be dangerous. Working with acids and bases requires adhering to all safety guidelines, including wearing gloves. Students need to look up the specific MSDS of their possible acids and bases prior to doing this activity. The following URL has freely accessible MSDS for the acids and base used in this activity: http://www.ehso.com/msds.php

Acids and bases can cause skin damage and eye damage. Some of the acids you will be working with are extremely corrosive and hygroscopic. Acid-base reactions are exothermic. When preparing dilute acid or base solutions from concentrated acids and bases, take care because the solution process is exothermic. Do not add water to any acid. This may cause the water to sputter and some acid may splash out of the container. If you are uncertain of any process or procedure, check with your teacher. Splash-proof goggles and rubber gloves must be worn at all times when working with acids and bases. If solutions are spilled, students should inform you immediately. If solutions get on skin, it should be rinsed with running water for 15 minutes; other lab safety procedures should also be followed. At the end of the lab, the solutions should be neutralized and the pH tested so that the waste can be safely disposed of following the procedures you outline to your students.

Prelab Preparation The materials listed here assume the student groups will be composed of three to four students and a total of eight lab groups. The students should follow a general procedure for doing acid-base titrations previously learned in prior lab

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work, but the students should have some input into the design of the experiment. Some students may not end up using all the equipment listed. Either have all the equipment at each lab station, or have them check out what they need from a common stock area. The solutions selected by each group should include two acids and two bases. Be sure the students record all the information about their starting materials and lab equipment in their notebooks/records. You will also want to designate a discard container or waste container for students to use. Students should not pour solutions down the drain. Teacher Tip After you review all the possible choices the students have made, if there is a particular titration not done such as using the base as the titrate after the student labs are completed, do a demonstration of it. This will ensure all the various types of weak/strong/polyprotic titrations are done and the students will be exposed to graphs going in both directions (base to acid and acid to base).

The amount of species required by each lab group will vary as not all groups will use each solution. If you have 20–24 students in a class, making 2.0 L of each solution should be more than adequate and will allow for multiple trials. Teacher Tip Another way to minimize preparation time is to give the students the possible known molarities students can use. This ensures you do not have to make additional solutions.

To prepare: For all acids measure out 800 mL of water, slowly add the needed amount of acid to the water, stir, and then slowly add water to 2.0 liters. Remember, NEVER add water to acid (or base). Acids: 2.0 L of 0.20 M hydrochloric acid (HCl) using 11.65 M concentrate — add 34.3 mL of acid 2.0 L of 0.10 M sulfuric acid (H2SO4) using 18.4 M concentrate — add 10.9 mL of acid 2.0 L of 0.050 M nitric acid (HNO3) using 15.8 M concentrate — add 6.33 mL of acid 2.0 L of 0.10 M acetic acid (CH3COOH) using 17.4 M concentrate — add 11.5 mL of acid Bases: 2.0 L of 0.10 M sodium hydroxide (NaOH) — dissolve 8.0 g of NaOH(s) in 800mL of water, then add water to 2.0 liters 2.0 L of 0.10 M calcium hydroxide (Ca(OH)2) — dissolve 14.8 g of Ca(OH)2(s) in 800 mL of water, then add water to 2.0 L

Acid-Base Titration

2.0 L of 0.20 M ammonia (NH3) using 18.1 M concentrate — In the fume hood to 800 mL of water add 22.1 mL of ammonia, stir, and then add water to 2.0 L Teacher DIRECTED

Start each group with 50 mL of each of their chosen solutions. To save time these can be premeasured out in 50 mL beakers with parafilm cover on top.

■■Prelab Guiding Questions/Simulations

Part I: Questions Teacher Tip At this point the students need to generate or receive questions to investigate. You can give them questions such as the following to consider OR you can give them a question of the day to answer.

Give students are given the following questions to choose from, or the students can be asked to generate their own questions: 1. Given 50 mL of 0.10 M HCl and 50 mL of 0.10 M acetic acid, will the amount of 0.10 M

NaOH required to neutralize each solution be the same, more, or less?

Expect a majority of students to say it will take more base to neutralize the stong acid compared to the weak acid. Allow this response at this time in the prelab and challenge students to design an experiment to test this. 2. Will the pH at the equivalence point of 50 mL 0.10 M HCl be the same, more, or less as

the pH at the equivalence points for 50 mL of 0.10 M acetic acid?

Expect a majority of students to say that the equivalence point is the same, pH = 7 for both acids. Allow this response at this time in the prelab and challenge students to design an experiment to test this. 3. What are some structural features that might help us classify an acid as a strong acid or

weak acid?

Have students draw Lewis diagrams of the acids. Students have trouble drawing Lewis diagrams of oxy-acids. Expect students to say there are only six or seven strong acids, especially if they have been taught this. Have the students focus on structural features. Carboxylic acids have a carboxylic acid function group. Other weak acids have less oxygen, compared to a similar system. Examples include HNO2 versus HNO3 and HF is a weak acid. 4. Draw a molecular and particulate view of what is happening in the steep part of a

general acid-base titration curve (such as Figure 1).

For a strong acid, neutralization pH = 7, 100% ionization. For a weak acid, only about 8% ionization, interaction of anion with water to create a basic solution, pH > 7.

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Or use the following as the possible Questions of the Day questions: (Answers will be determined by performing the investigation. Students’ initial answers will vary.) 5. How does the structure of an acid affect the shape of the titration curve? 6. How can a pH titration curve be used to help classify the resultant solution at the

endpoint, as acidic, basic, or neutral?

7. How do the structure and the initial concentration of an acid and a base influence the

pH of the resultant solution during a titration?

Part II: Simulation An animation simulation of different types of titrations has been provided for students to complete, but it is not required to do the experiment. The website Chemistry Experiment Simulations and Conceptual Computer Animations includes an animation entitled “Determination of the Molarity of an Acid or Base Solution,” found here: http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfiles/ stoichiometry/a_b_phtitr.html

This animation shows macro and micro levels of what is happening in a titration, and the students are allowed to choose amounts of acids and bases and see different curves and calculations pertaining to titrations. Showing an animation and having the students determine the relative concentrations of each species as the titration progresses will help facilitate student understanding.

■■Explanation to Strengthen Student Understanding Everyone has tastes and textures of food they prefer. Taste and texture are often linked to the acidity or alkalinity of a food or beverage detected by the tongue, which has sensors for different tastes. A sour, acidic lemon has a different taste compared to a drink of green tea or herbal tea, which are alkaline. Acids and bases are unique compounds that play an important role in influencing the pH of a solution. Chemists use several definitions to help classify compounds as an acid or a base. Svante Arrhenius defined acids as compounds containing the hydrogen ion, H+, and bases as compounds containing the hydroxide ion OH-. Brønsted-Lowry acids are defined as proton donors in a reaction and bases are proton acceptors in a reaction. The proton referred to is an H+ ion. A hydrogen atom has one proton and one electron and when the electron is removed to form an H+ ion, only a proton remains. It is not possible though for a single H+ ion to exist in water. The H+ combines with a water molecule to form the hydronium ion, H3O+. The pH of an aqueous solution is a measure of the amount of hydronium ion [H3O+] species which is also simply represented as the hydrogen ion H+ in the solution; pH = –log[H+].

Acid-Base Titration

Acids and bases can be considered weak or strong by the amount of ionization occurring in solution. Strong acids will ionize nearly 100 percent into ions while weak acids will ionize only a small percentage. For example the strong acid HBr will ionize almost completely into H+ and Br–, while the weak acid CH3COOH will remain primarily CH3COOH even though some CH3COO– and H+ form. The common strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4. The strong bases also ionize completely and are the Group 1 and some of the Group 2 hydroxides: LiOH, NaOH, KOH, RbOH, CsOH, Ba(OH)2, Sr(OH)2, and Ca(OH)2. Even though some Group 2 hydroxides are only slightly soluble, the amount dissolving ionizes completely. In acid-base titrations, the titrant in the buret is the chemical solution added to the chemical solution in the flask or beaker called the titrate. The objective of this experiment is to determine the concentration or molarity of a solution by doing a titration. Often in titrations, the base is added to the acid. When the moles of acid (really the moles of H+ released) present are equal to the moles of base added (or H+ consumed), the reaction has reached the equivalent point. The calculation of the unknown molarity involves finding the moles of acid and the moles of base. If the known molarity is that of the base then the base molarity times its volume in liters times the number of hydroxides in its formula will equal the total moles of hydroxide present: Mbase × Vbase × #OH- ions in the formula = moles OH-. This will be equal to the total moles of acid present at equivalence. The total moles of acid is then set equal to the molarity of the acid times the volume of the acid used in liters times the number of hydrogen ions in the acid: moles acid = Macid × Vacid× #H+ ions in the formula. The equivalence point can be determined by graphical means or by using an indicator. An indicator is a solution containing an organic compound, either a weak acid or weak base, which exhibits a different color in certain pH ranges. A common indicator used in reactions of strong acids and strong bases is phenolphthalein which is clear in acidic solutions and pink in basic solutions. Choosing the right indicator is important since the indicator color change is supposed to indicate the equivalence point. If the equivalence should be at a pH of 8 the indicator needs to change color around pH = 8. Done this way, the titration stops at the indicator change point which is called the end point. This method does not monitor the pH throughout the titration. A titration curve cannot be made using a single acid-base indicator (universal indicator, a mixture of multiple indicators would work). If a graph is made of how the pH changes as the titrant is added, this is called a titration curve. The graph below shows how this might look.

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A 1

B

pH versus Volume of NaOH Added

Vol OH- pH added (mL)

2 3 4 5 6 7 8 9 10 11 12 13 14

0

2.5

3.0 mL

2.5

6.0 mL

2.5

9.0 mL 12.0 mL

2.5 2.5

15.0 mL

2.6

18.0 mL 21.0 mL

3.4 8.2

24.0 mL

10.5

27.0 mL

11.0

30.0 mL

11.1

33.0 mL

11.3

36.0 mL

11.4

12.0 10.0 8.0

pH

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6.0 4.0 2.0 0.0

0

2

4

6

8

10

12

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Volume of NaOH Added (mL) Figure 1. Acid-base titration curve

Teacher Tip Depending on the type of acid and base used, the graphs will look slightly different. For weak acids and bases, important points on the graph are pKa and pKb. If you are titrating a weak acid, halfway to the equivalence point is pKa. To find the Ka of the acid use the equation 10–pKa = Ka. For a weak base the equation changes to 10–pKb = Kb. This information should not be given to students during the prelab.

■■Practice with Instrumentation and Procedure Teacher DIRECTED

You will need to teach students the procedure to use in an acid-base titration before they can complete this lab because the investigation that follows should not be the first time students have performed an acid-base titration. Students should follow a general procedure for doing an acid-base titration. The following procedures are to develop an understanding of not only a titration, but why it must be done when calculations are required.

Procedure Acids and bases can be tested in several ways. One way is just to test to see if it is an acid or base using litmus paper or pH Hydrion Paper. Students should follow the steps below. Step 1: Test an acid and a base with litmus paper. Does the litmus test provide you any quantitative data about the substance? What does the pH Hydrion Paper test indicate that the litmus paper test did not?

Acid-Base Titration

Step 2: Measure 5.0 mL of acid and 5.0 mL of base. Pour them together. What can you tell is happening with the acid and base just by observing the reaction at this point? Step 3: Do Step 2 again, but this time add 1 drop of the indicator phenolphthalein to the acid and slowly pour the base into the acid. When you see a color change, test the pH. Pour the rest of the base into the acid and test the pH again. Compare the two trials. How was the data different between the trials? Many experimental procedures require exact concentrations, and, to get this type of quantitative data, you need to take more exact measurements. A titration produces this type of data. In titration, there will be two solutions, an acid and a base. A solution whose molarity is known is called a titrant, and this titrant is added to another solution until the chemical reaction is complete. Pour a measured volume (such as 25 mL) of the unknown solution to be titrated into an Erlenmeyer flask. Rinse a buret with the titrant, and then pour the titrant into a buret held up by a ring stand. The buret is set up over the Erlenmeyer flask so the titrant can be slowly added to the unknown solution to be titrated. Monitor the pH throughout the reaction with either a pH meter or a probe. Continue the titration until the pH remains constant after a steep change in pH. Make a graph of the data (pH versus titrant added in mL). From the equivalence point on the graph, determine the amount of titrant added to reach equivalence.

■■Investigation Student DIRECTED

Each group will write a procedure to use different combinations of two acids, one with a known molarity and the other with an unknown molarity, and two bases, one with a known molarity and one with an unknown molarity, to perform acid-base titrations to collect data to draw titration curves, calculate unknown molarities, and answer the question their group has chosen to investigate from the prelab Guiding Questions section. Teacher Tip The students should titrate their unknown acid with a known base and then titrate their unknown base with the known acid. Each titration should be done twice but if time is limited all the other student data can be used for the multiple trials. You will want to ensure each titration combination is done by at least two groups for data comparison.

Procedure After having each group choose a question from the prelab section, have the students write a procedure to test their question using an acid-base titration method. After the students finish writing their procedures, you should check and initial them before allowing them to proceed.

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Two of the four samples given to each group will have the molarity identified and two will not. After titrations are complete, use the data to make a graph. All major points should be identified on the curves. If a sample includes a weak acid and/or base, percent ionization needs to be calculated, Ka and/or Kb needs to be calculated, and the percent error of the calculated Ka or Kb to the accepted value should be calculated. Teacher Tip The students who chose to rinse the buret with water instead of with titrant will have error in their final analysis. The presence of water in the buret will dilute the titrant, resulting in more titrant being needed to reach equivalence and thus a higher unknown molarity.

In-lab Discussion Questions To stimulate thinking, ask the students to think about the following questions: a. What is happening at the particulate level during a titration of a weak acid with a

strong base?

Initially the weak acid establishes an equilibrium system HX(aq) + H2O(l)

 X-(aq) + H3O+

For a titration of a weak acid HX, with a strong base, the hydroxide ion reacts with the hydronium ion OH–(aq) + H3O+ → 2H2O at the first level area [X–] is very close to the [HX], the pH is changes slightly as it is acting as a buffer region. b. What is happening at the particulate level when there is a steep part of the titration

curve?

The pH is changing very rapidly near the reaction completion point. c. How can the steep part of the curve be used in calculations?

At the midpoint of the steep part of the curve the acid and base amounts are equal so that is the equivalence point. d. Does the steep part tell you anything about the endpoint or the equivalence point?

Explain

If the curve is steep, and the indicator or meter changes near the middle of the steep range, then the measurement should be quite accurate. It will be less so if the curve is not very steep (as for weak acids and bases).

Acid-Base Titration

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e. Using one of your pH curves, predict and explain what the shape of the pH curve will

look like if the experiment was repeated with a lower concentration of analyte.

If the initial concentration of the analyte were 0.00100 M instead of 0.100 M, the initial pH is higher, the buffer region is shorter, the length of the equivalence point is shorter. A 1

B

C

Titration Comparison of 0.100 M Acid and 0.00100 M Acid versus Addition NaOH

Vol OH- 0.100 M 0.001 added (mL) acid

14

0

1.0

2.5

12

3

10

1.8

4.3

10

4

20

1.9

5.2

8

5

30

2.0

5.7

6

35

2.1

5.9

4

7

39

2.6

6.2

2

8

40

10.2

10.2

0

9

45

11.8

11.8

10

50

11.9

11.9

11

60

12.0

12.0

12

70

12.1

12.1

pH

2

6

0

20

40

60

Volume of NaOH Added (mL) 0.100 M Acid

0.00100 M Acid

Figure 2. Sample student titration curve for in-lab question (e)

Data Collection and Computation Have the students think about and discuss how to process the data. They need to come up with some or all of the following: 1. Completion of data table(s) they have prepared 2. Drawn titration curves for each titration performed, labeled appropriately 3. Determination of the percent error of Ka and/or Kb if appropriate

After completing the data collection and drawing their titration curves, groups will then pool data and compare the titration curves. Using this data, they should then try to answer the question of the lab: How do the structure and the initial concentration of an acid and a base influence the pH of the resultant solution during a titration?

Argumentation and Documentation Make sure students incorporate answers to their initial investigation questions (from the prelab) in the conclusion of their lab reports. Make sure students justify their claims and conclusions with evidence from their investigations. Below are sample student answers to questions that students may have chosen or that you may have given them as the Question of the Day to guide their investigation:

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1. How does the structure of an acid affect the shape of the titration curve?

The structure of an acid will determine if it is a strong or weak acid. See below for the shapes of the curves. 2. How can a pH titration curve be used to help classify the resultant solution at the

endpoint, as acidic, basic, or neutral?

The midpoint of the steep curve is the equivalence point. Determination of the pH at the equivalence point will indicate if the resultant solution is acidic pH < 7, neutral pH = 7, or basic pH > 7.

Teacher Tip A strong monoprotic acid and a weak monoprotic acid will have curves that look like those in Figure 3, below.

A 1

B

C

Vol OH- strong weak added (mL) acid acid

Weak and Strong Acid Titration Curves versus mL 0.1 M Titrant Added 14.0

2

0

1.5

3.6

3

5

1.6

4.0

4

10

1.7

4.5

5

15

1.8

4.7

10.0

6

20

1.9

4.9

7

25

2.5

5.6

8.0

8

30

11.0

11.0

9

35

12.1

12.1

10

40

12.2

12.2

pH

12.0

6.0 4.0 2.0 0.0

0

5

10

15

20

25

30

35

40

mL Titrant Added Strong Acid

Weak Acid

Figure 3. Strong and weak acid titration curve 3. How do the structure and the initial concentration of an acid and a base influence pH of

the resultant solution during a titration?

The type of acid, weak or strong, and the initial concentration will influence the shape of the pH titration curve.

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Acid-Base Titration

4. How will the shape of the pH curve change if the experiment is repeated with a lower

concentration of analyte (i.e., compare 0.10 M to 0.0010 M)?

Using a more concentrated acid solution will cause the pH curve to start at a lower pH; the pH curve would be lower, the buffer region would be extended, the equivalence point line would be extended, the alkaline region would remain the same. Using a less concentrated acid solution will cause the pH curve to start at a higher pH; the pH curve would be higher, the buffer region would be shorter, the equivalence point line would be shorter, the alkaline region would remain the same.

Make sure all students answer the following questions in the conclusions of their lab reports. 1. How do the process and the titration curves drawn vary if the acids or bases are weak or

strong? Justify your answer.

A weak acid will have a higher initial pH compared to a strong acid, at the same initial concentration. The initial portion of a strong acid pH curve rises slowly, while the weak acid rises a bit quicker. Both have a steep middle part. After the equivalence point, both curves are the same. 2. What would a titration curve look like if an indicator were used to know when to stop

the titration?

The titration curve would stop at the equivalence point.

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288

3. Commercials about antacids are on television all the time. How would you go about

investigating bases like antacids, which are solid?

Do a titration with an acid. The curve would look like the one below: A B C 1 mL 0.1 strong weak

Titration Curves Strong Base and Weak Base versus mL of 0.1 M HCl Added

HCl added 0 2

acid

acid

12.2

10.2

3

5

12.1

10.1

4

10

11.0

9.8

5

15

2.4

6.5

10.0

6

20

1.9

4.4

7

25

1.8

2.0

8.0

8

30

1.7

1.6

9

35

1.6

1.6

10

40

1.5

1.5

14.0

pH

12.0

6.0 4.0 2.0 0.0

0

5

10

15

20

25

30

35

40

45

Volume 0.1 M HCl Added (mL) Strong Base

Weak Base

Figure 4. Possible titration curve of acid into base 4. How would you investigate which antacid neutralizes the most acid or is the most

cost-effective?

Set up an experiment to see which antacid can neutralize the most base and then perform a cost analysis calculation based on moles of acid present. 5. Does it matter whether you start with pure acid or pure base as the titrate? Does it matter

if you add water during the course of the titration? Why or why not?

Yes, it matters. It depends what you are trying to determine. If you are determining the pKa and equivalence point of an acid, the base should be the solution in the buret and the acid the solution in the Erlenmeyer flask.

Acid-Base Titration

■■Postlab Assessment Ask students to answer the following questions. 1. Explain how rinsing the buret with water instead of the titrant before starting the

investigation will affect the calculated unknown molarity of the titrate.

Rinsing the buret with water instead of the titrant would dilute the titrant. It would take more volume of titrant to neutralize the acid than it should, thus the calculation will show more acid present than what actually is present. 2. Explain why there is a steep slope in a section of the titration curve and explain how it

can be used in calculations.

There is a steep slope in the titration curve near the neutralization point because the pH rapidly changes from below 7 to near 7 with just a few drops of titrant added. 3. What types of data needs to be collected to perform molarity calculations of the

unknown?

Volume of base, molarity of base, number of moles of acid initially present. 4. Does the presence of weak or strong acids and weak or strong bases make a difference to

when the equivalence point occurs? Justify your answer.

A lower concentration of acid means the initial pH is higher. The curve would start higher.

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investigation 14

5. Using one of your titration curves, explain how the ratio of the acid species to the

conjugate base concentration changes as the titration proceeds and draw particulate representations to show these changes at the beginning, half equivalence point, equivalence point, and beyond the equivalence point.

Yes, it makes a difference. The type of weak acid or weak base determines the type of salt formed in the resultant solution. When the salt forms in water, either the cation or anion reacts with water to establish an equilibrium system, which determines the pH at the equivalence point. For example, if acetic acid is being titrated with sodium hydroxide, sodium acetate is salt produced. The acetate ion reacts with water to form an equilibrium system with a pH around 8.2

Initial

Neutralization

Halfway

Beyond the equivalence point

Figure 5. Particulate views

Acid-Base Titration

291

6. Explain how to determine the Ka of an acid and the Kb of a base from a titration curve.

At the halfway point to equivalence the pH = pKa for a weak acid and for a weak base pOH = pKb at the halfway point. A 1

B

Titration Curve for a Diprotic Acid

pH of Vol of NaOH (mL) diprotic acid

14.0 12.0

2

0

1.0

3

1

1.4

4

2

1.6

5

3

1.8

6

4

2.0

7

5

2.2

8

6

2.3

4.0

9

7

2.4

2.0

10

8

2.7

11

9

3.8

12

10

5.8

13

11

6.4

14

12

6.5

15

14

6.6

16

15

6.8

17

16

7.0

18

17

8.0

19

18

11.0

20

19

11.4

21

20

11.6

22

21

11.8

23

22

11.9

24

23

12.0

25

24

12.1

26 27

25

12.2

26

12.3

28

27

12.3

29

28

12.4

30

29

12.4

10.0

pH

8.0

Equivalence Point 2

pH = pka2

6.0

0.0

Equivalence Point 1

0

pH = pka1 10 5

15

20

25

30

Volume of Base Added (mL of NaOH)

Figure 6. Titration curve for a diprotic acid

35

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investigation 14

For a diprotic acid, at the halfway point, the concentration of H2X(aq) remaining in the solution is equal to half the initial concentration of H2X. The concentration of NaHX(aq) produced is also numerically equal to half the initial concentration of H2X. H2X(aq) + H2O(l) → HX–(aq) + H3O+ Ka = [H3O+][HX–]/[H2X] or [H3O+] = Ka[H2X]/[HX-] at the midpoint of a titration [H3O+] = Ka[1/2H2X]initial/[1/2H2X]initial [H3O+] = Ka From the graph we can determine the pH at this point pH=-log10[H3O+], determine [H3O+] at this point. Calculate Ka for this equilibrium system. For a diprotic acid this is Ka1.] 7. Your car’s battery blows up, spraying sulfuric acid all over the engine’s hoses and yourself.

Explain how you might neutralize the acid using available household chemicals.

Car battery acid is sulfuric acid. Sulfuric acid can be neutralized by reacting it with a baking soda, sodium hydrogen carbonate.

2NaHCO3(aq) +H2SO4 (aq) →2CO2(g) + H2O(l)+ Na2SO4 (aq) 8. Include possible equations to help explain why taking an antacid is recommended when

a person has heartburn from consuming too many acidic foods or has acid reflux.

Antacid contains calcium carbonate or magnesium carbonate as the active ingredient. The carbonate anion reacts with acid. The calcium cation is a spectator ion.

CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) 9. Challenge question: Amino acids are essential to carbon-based sentient life forms.

Isoleucine is an α-amino acid with the chemical formula HO2CCH(NH2)CH(CH3) CH2CH3. Since carbon-based life forms cannot synthesize isoleucine, this amino acid must be obtained through eating various foods. The IUPAC name for isoleucine is 2-amino-3-methylpentanoic acid. Draw the structure for isoleucine. Given the following acid-base titration curve, for the titration of isoleucine, determine the pKa values for isoleucine.

Acid-Base Titration

A 1

B

Titration Curve for an Amino Acid

pH of Vol of NaOH (mL) amino acid

16.0 14.0

2

0

1.0

3

5

1.8

4

10

2.2

5

15

2.6

6

20

2.8

7

25

3.0

8

30

3.2

6.0

9

35

3.6

4.0

10

39

5.0

11

40

8.0

12

42

8.4

13

45

8.6

14

50

8.7

15

55

8.8

16

60

8.9

17

65

9.2

18

70

9.8

19

75

14.0

12.0

pH

10.0 8.0

2.0 0.0

0

10

20

30

40

50

60

70

Volume of Base Added (mL of NaOH)

Figure 7. Amino acid titration curves and pKa values pKa1 = 2.4; pKa2 = 9.7 CH3

O

H3C

OH NH2

Figure 8. Amino acid structure of isoleucine

■■Connecting the Lab to the Classroom and Beyond Acid-base titrations can be done at several points in the curriculum. This lab is directly related to investigating different types of acid-base reactions, indicators to use in acid-base titrations, acid-base equilibria, hydrolysis of salts in acidbase reactions, and buffering solutions. The optimal time would be with acidbase equilibria to lead directly into hydrolysis of salts in acid-base reactions and buffering solutions. Understanding the titration process will also lead into oxidation–reduction titrations.

80

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investigation 14

This lab can facilitate mastery of acid-base reaction equation writing. Have the students practice writing fully balanced molecular, ionic, and net equations for each reaction they did in the lab. Students can also master identifying acid-base pairs in the reactions they performed.

Extension Activity One extension activity is to have the students redo the experiment in microscale. Equipment needed would be pipettes and well plates. In 5.0 mL well plates using 1.0 mL of the titrate, the titrant would be added by drops. The total volume used should be less than 5.0 mL. Volume added is determined by number of drops. The drops can be calibrated (let the students work out how), by measuring the volume of 10, 50, or 100 drops. The students should compare their resulting data and graphs and determine the advantages and disadvantages of each method.

Follow-up Experiment Experiments using household chemicals bring relevancy to the classroom. Easy inquiry experiments to do include: a. “Plop. Plop. Fizz. Fizz oh what a relief it is” is the beginning of a common commercial for Alka-Seltzer®, which can be watched at http://www.youtube.com/ watch?v=bxjb2UJZ-5I. Design an investigation to determine the amount of acid

actually neutralized by an Alka-Seltzer tablet.

b. Determine which type of antacid is the best at neutralizing acid per dollar amount

and is thus the best to buy.

c. Other investigations could look at different acids and bases in the home such as

tannic acid in tea, citric acid in orange juice, comparing their pH’s and Ka’s.

■■Supplemental Resources

Links “Acid-Base Interactions.” Oklahoma State University Chemistry Department. Accessed July 31, 2012. http://genchem1.chem.okstate.edu/ccli/CCLIDefault.html

“Acid-Base Solutions.” University of Colorado at Boulder, PhET Interactive Simulations. Accessed July 31, 2012. http://phet.colorado.edu/en/simulation/acid-base-solutions

“Acid-Base Titrations.” About Chemistry. Accessed July 31, 2012.

http://chemistry.about.com/od/chemistryquickreview/a/titrationcalc.htm

“Titration.” 101 Science. Accessed July 31, 2012.

http://www.101science.com/Chemistry.htm#TITRATION

Acid-Base Titration

References Barnum, Dennis. W. “Predicting Acid-Base Titration Curves without Calculations.” Journal of Chemical Education 76, no. 7 (1999): 938. Glaister, Paul. “A Unified Titration Formula.” Journal of Chemical Education 76, no. 1 (1999): 132. Wildman, Randall. J., and Coleman, William. F. “Acid-Base Equilibria in Aqueous Solutions.” Journal of Chemical Education 79, no. 12 (2002): 1486.

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