Rate of Reaction of Magnesium With Hydrochloric Acid

Rate of reaction of magnesium with hydrochloric acid Magnesium reacts with dilute hydrochloric acid in a conical flask which is connected to an inverted measuring cylinder in a trough of water. The volume of hydrogen gas produced is measured over a few minutes, and the results are used to plot a graph. Read our standard health & safety guidance Lesson organisation This is intended as a class practical. It is best if the students work in pairs because setting up and starting the experiment requires more than one pair of hands. One student can add the magnesium ribbon to the acid and stopper the flask, while the other starts the stopclock. During the experiment, one student can take the readings while the other records them. The experiment itself takes only a few minutes. But allow at least 30 minutes to give students time to set up, take readings and draw graph. Hydrogen gas (Extremely flammable) is generated in the experiment. Students should not have access to any source of ignition. Apparatus and Chemicals Eye protection Each group of students will need: Conical flask (100 cm3) Single-holed rubber bung and delivery tube to fit conical flask (see note 3) Trough (or plastic washing-up bowl) Measuring cylinders (100 cm3), 2 Clamp stand, boss and clamp Stopclock Graph paper Each group of students will need access to: Magnesium ribbon (Low hazard) cut into 3 cm lengths (see note 1) Dilute hydrochloric acid, 1 mol dm-3 (Low hazard at this concentration) (see note 2) Below is a diagram of the set-up Technical notes Magnesium ribbon (Low hazard) Refer to CLEAPSS Hazcard 59A Hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS Hazcard 47A and CLEAPSS Recipe Card 31 Hydrogen gas (Extremely flammable) Refer to CLEAPPS Hazcard 48

1 The magnesium ribbon should be clean and free from obvious corrosion or oxidation. Clean if necessary by rubbing lengths of the ribbon with fine sandpaper to remove the layer of oxidation. 2 The hydrochloric acid should be about 1 mol dm-3 for a reasonable rate of reaction. Each experiment run will need 50 cm3. Though low hazard, eye protection is necessary as you may get a spray as tiny bubbles burst. 3 The bungs in the flasks need to be rubber. Corks are too porous and will leak. The tube through the bung should be a short section of glass, and then a flexible rubber tube can be connected. 4 Gas syringes can be used instead of troughs of water and measuring cylinders. But these are very expensive and are probably best used by the teacher in a demonstration. Syringes should not be allowed to become wet, or the plungers will stick inside the barrels. 5 Ensure that all naked flames are extinguished, and that there are no other sources of ignition available to students. Procedure SAFETY: Wear eye protection throughout. Ensure that there are no naked flames. a Measure 50 cm3 of 1 mol dm-3 hydrochloric acid using one of the measuring cylinders. Pour the acid into the 100 cm3 conical flask. b Set up the apparatus as shown in the diagram. Half fill the trough or bowl with water. c Fill the other measuring cylinder with water, and make sure that it stays filled with water when you turn it upside down. d When you are ready, add a 3 cm strip of magnesium ribbon to the flask, put the bung back into the flask as quickly as you can, and start the stopclock. e Record the volume of hydrogen gas given off at suitable intervals (eg 10 seconds). Continue timing until no more gas appears to be given off. Teaching notes The equation for the reaction is: magnesium + hydrochloric acid → magnesium chloride + hydrogen Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) Students follow the rate of reaction between magnesium and the acid, by measuring the amount of gas produced at 10 second intervals.

3 cm of magnesium ribbon typically has a mass of 0.04 g and yields 40 cm3 of hydrogen when reacted with excess acid. 50 cm3 of 1 mol dm-3 hydrochloric acid is a six-fold excess of acid. In this reaction, the magnesium and acid are gradually used up. However the acid is in excess, so it is mainly the loss of magnesium (surface area becomes smaller) that causes the change in the rate. If a graph of volume (y-axis) against time (x-axis) is drawn, the slope of the graph is steepest at the beginning. This shows that the reaction is fastest at the start. As the magnesium is used up, the rate falls. This can be seen on the graph, as the slope becomes less steep and then levels out when the reaction has stopped (when no more gas is produced). The reaction is exothermic, but the dilute acid is in excess and the rise in temperature is only of the order of 3.5˚C. There is some acceleration of the reaction rate due to the rise in temperature. Some students might notice the flask becoming slightly warm and they could be asked how this would affect the rate of reaction, and how they might adapt the experiment to make it a ‘fair test’.

Reaction of Magnesium with Hydrochloric Acid Purpose To develop a method for measuring the volume of gas generated in a reaction between Mg and HCl. To develop a relationship between the mass of magnesium reacted and the volume of hydrogen generated. To acquire an understanding of limiting reactants. Materials plastic bin rubber hose glass bend 1.0 M HCl 100 mL graduated cylinder test tube (25 x 200 mm) 5.00 mL pipet 0.5 g Mg ribbon ringstand and clamp 1-hole rubber stopper pipet filler tap water Safety Avoid skin contact with hydrochloric acid. Wear goggles at all times. Rinse skin thoroughly if contact is made. Hydrogen gas is flammable. Groups You will work in groups of 3-4, assigned at random or by the professor. One member of the group will be assigned as a project manager; one as a computer operator; one or two as a laboratory technician. See the group work page for more information. Procedure Design an apparatus that will enable you to collect and measure the volume of hydrogen gas generated using only materials found in the list above. To facilitate speedy collection of data, your group may choose to set up two separate sets of equipment. Check your system(s) for leaks or cracks. React eight different masses of magnesium in the range of 0.01 g to 0.10 g with exactly 5.00 mL of 1.0 M HCl in eight separate trials. To the extent possible, try to spread the masses of magnesium evenly over the entire range 0.01 g to 0.1 g. Use an analytical balance to record the masses. For each trial, collect the hydrogen gas until there is no more than one bubble produced per minute, then record the mass of Mg you used and the volume of hydrogen generated, and make a note if any magnesium metal remains. Repeat any suspicious trials Questions Using Excel, prepare a graph of the volume of hydrogen gas generated vs. the mass of the magnesium used, plotting only the points (do not connect the dots).

Which reactant, if either, is the limiting reactant when smaller masses of magnesium are used? when larger masses of magnesium are used? Use the concept of limiting reactants to explain the shape of your graph. Using the necessary data from your results, use Excel to develop a mathematical relationship between the grams of Mg reacted and volume of H2 produced. Note: Use only the points that make an approximately straight line. If there is more than one linear portion on your graph, plot a trendline for the portion of the graph that is not "level". Based on your experiment, how many milliliters of H2 would be produced had 0.0400 g of Mg been reacted with 5.00 mL of 1.0 M HCl? What if 0.0800 g of Mg had been used? 0.120 g? Write a balanced chemical equation, including phase labels, of the reaction that took place in this experiment. Consult your text, if necessary. Hint: This is a single replacement reaction. Using your balanced equation, calculate the theoretical volume of hydrogen gas, in mL, that should be produced at at temperature of 0°C and and a pressure of 760 mm Hg when 0.0400 g of Mg reacts. Note: 1 mol of any gas at a temperature of 0°C and a pressure of 760 mm Hg has a volume of 22.4 L. How does this result compare to the volume calculated for 0.0400 g of Mg in Question #4 above? How would your results and your graph change, if at all, if the concentration of HCl was twice as large as was actually used in the experiment? There were, as in all experiments, some "uncertainties" in this lab. Explain some actual causes of error and/or inaccurate measurements. How could you design a future experiment to eliminate these? Laboratory Report (Review the Guidelines for Writing Laboratory Reports) The laboratory report for this experiment will be a group report. All members of the group should contribute in some way to the writing of the report. You will need to include a title, a detailed procedure, results, discussion and conclusion, and references. After the references, also include the answers to questions 4 and 6 above. The answers to the other questions should be incorporated into your results or discussion, as appropriate. As this is a group report, please include a table at the end showing the percent contribution of each group member on the entire lab, including the report. Follow your instructor's directions for submitting the lab report. You may be allowed to submit this report by paper, by the D2L dropbox, or by email (using a filename convention of Lastname1 Lastname2 Etc Limiting and a subject line in your email of "Chem 1061: Limiting Reactant Lab").

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