CHEMISTRY SPM FORM 4 Short Notes Chapter 7 ACIDS AND BASES

STUDYSMART CHEMISTRY FORM 4 CHAPTER 7 : ACIDS AND BASES

7.1 Analysing characteristics and properties of acids and bases 7.2 Synthesising the concepts of strong acids, weak acids, strong alkalis and weak alkalis 7.3 Analysing concentration of acids and alkalis 7.4 Analysing Neutralization

7.1 ANALYSING CHARACTERISTICS AND PROPERTIES OF ACIDS AND BASES Acids An acid is a chemical substance which ionizes in water to produce hydrogen ions, H+ (hydroxonium ion, H3O+) Example : 1. HCl H2O HH+ + ClHCl + H2O  H3O+ + Cl2. HNO3 3. H2SO4

H2O

H+ + NO3-

H2O

4. CH3COOH Acid can be classified based on its basicity a) Monoprotic acid – (1mol – 1mol) Example : HCl H2O H+ + ClH O HNO3 2 H+ + NO3b) Diprotic acid – (1mol – 2mol) HO Example H2SO4 2 Bases and alkalis A base is a chemical substance which ionizes in water to produce hydroxide ion, OH-. Example : 1. Hydroxide Metal NOTE!! NaOH H2O Na+ + OHAll alkalis are bases, but all bases 2. Oxide Metals are not alkalis Li2O + H2O  2LiOH B Li2O + H2O  2Li+ + 2OHA All soluble bases are known as alkalis NH3 + H2O NH4+ + OH

Chemical Properties of Acids___________ Acid + Base  Salt + water H2SO4 + ZnO  ZnSO4 + H2O

Acid + metal  salt + hydrogen gas 2HCl + Mg  MgCl + H2

CHEMICAL PROPERTIES OF ACIDS Acid + carbonate  Salt + Water + Carbon Dioxide 2HNO3 + CuCO3  Cu(NO3)2 + H2O + CO2

Chemical Properties of Alkalis___________ Alkali + Acid  Salt + Water NaOH + HCl  NaCl + H2OS

Alkali + Metal Ion  Insoluble Metal Hydroxide 2OH- + Cu2+  Cu(OH)2 [Blue Precipitate] CHEMICAL PROPERTIES OF ALKALIS

Alkali + Ammonium salt  Salt + water + ammonia gas NaOH + NH4Cl  NaCl + H2O + NH3

7.2 SYNTHESISING THE CONCEPTS OF STRONG ACIDS, WEAK ACIDS, STRONG ALKALIS AND WEAK ALKALIS The pH scale The pH is used to indicate the degree of acidity or alkalinity of a solution. It consist of pH value that range from 0 – 14. pH value less than 7 indicate an acidic solution pH value equals 7 indicate a neutral solution pH value greater than 7 indicate an alkaline solution INCREASING ACIDITY NEUTRAL INCREASE ALKALINE pH

0

1

2

3

4

5 6 7 8 9 10 11 12 13 14 + Concentration of H ions increases Concentration of OH- ions increases The pH is actually a measure of the concentration if hydrogen ions. The higher the pH value, the higher the concentration of hydroxide ions.

Strong and weak acids The strength of an acid depends on the degree of ionization or dissociation of the acid in water An acid which ionizes completely in water to produce high concentration of hydrogen ion is called a strong acid. HCl H2O H+ + OHA weak acid is only partially ionizes in water to produce a low concentration of hydrogen ion CH3COOH H+ + CH3COO-

Strong and weak acids The strength of an alkali depends on the degree of ionization of the alkali in water A strong alkali is completely ionized in water to produce a higher concentration of hydroxide ion, OHNaOH  Na+ + OHA weak alkali ionizes partially in water to produce a low concentration of hydroxide ion, OHNH3 + H2O NH4+ + OH7.3 ANALYSING CONCENTRATION OF ACIDS AND ALKALIS Concentration is the quantity of solute in a given volume of solution, which is usually 1dm3 Concentration can be defined in two ways

The two units of concentration can be inter-converted Molarity x molar mass -3 g dm-3 mol dm ÷ molar mass Preparation of standard solution Standard solution is a solution in which its concentration is accurately known. For preparation of solution by dilution method, add water to a concentrated solution changes the concentration of the solution but does not change the amount of solute present in the solution. Thus, moles of solute before dilution = moles of solute after dilution M 1 x V1 = M 2 x V 2 Where, M1 = Molarity of solution before dilution V1 = Volume of solution before dilution M2 = Molarity of solution after dilution V2 = Volume of solution after dilution 7.4 ANALYSING NEUTRALIZATION Neutralization is the reaction between acid and bases to form salt and water. ACID + BASE  SALT + WATER Examples: a. HCl + NaOH  NaCl + H2O b. HNO3 + Ca(OH)2  c. H2SO4 + Ba(OH)2  d. CH3COOH + KOH  e. H3PO4 + NH4OH  When acid neutralizes an alkali/base, the pH value is 7. The point of which this happens is called the end point At the end point, H+ ion (from acid) combine with OH- ions (from base/alkali) to produce water H+ + OH-  H2O Acid-base titration. Acid of known concentration is delivered from a burette to completely neutralizes a known volume of an alkali in a conical flask

Numerical problems involving neutralization. a, acid + b, base / alkali  salt + water MaVa = a MbVb b Ma = molarity of acid Va = volume of acid Mb = molarity of alkali/base Vb = volume of alkali/base a = coefficient of acid in a balance equation b = coefficient of base/alkali in a balance equation TRY THIS 1 50 cm3 of 1 mol dm-3, sodium hydroxide solution is neutralized by 25 cm3 of sulphuric acid, Calculate the concentration of acid in mol dm-3 and g dm-3 [RAM = H,1 ; S,32 ; O,16 ; Na, 23]

TRY THIS 2 Calculate the volume of 2 mol dm-3 potassium hydroxide that is needed to neutralize 100 cm3 of 1 mol dm-3 hydrochloric acid.