Equilibria

7.1 Chemical equilibria: reversible reactions, dynamic equilibrium
Learning outcomes
Candidates should be able to:
1 (a) understand what is meant by a reversible reaction
(b) understand what is meant by dynamic equilibrium in terms of the rate of forward and reverse reactions
being equal and the concentration of reactants and products remaining constant
(c) understand the need for a closed system in order to establish dynamic equilibrium
2 define Le Chatelier’s principle as: if a change is made to a system at dynamic equilibrium, the position of
equilibrium moves to minimise this change
3 use Le Chatelier’s principle to deduce qualitatively (from appropriate information) the effects of changes in
temperature, concentration, pressure or presence of a catalyst on a system at equilibrium
4 deduce expressions for equilibrium constants in terms of concentrations, Kc
5 use the terms mole fraction and partial pressure
6 deduce expressions for equilibrium constants in terms of partial pressures, Kp
(use of the relationship between Kp and Kc is not required)
7 use the Kc and Kp expressions to carry out calculations (such calculations will not require the solving of quadratic equations)
8 calculate the quantities present at equilibrium, given appropriate data
9 state whether changes in temperature, concentration or pressure or the presence of a catalyst affect the value of the equilibrium constant for a reaction
10 describe and explain the conditions used in the Haber process and the Contact process, as examples of the importance of an understanding of dynamic equilibrium in the chemical industry and the application of LeChatelier’s principle

7.2 Brønsted–Lowry theory of acids and bases
Learning outcomes
Candidates should be able to:
1 state the names and formulae of the common acids, limited to hydrochloric acid, HCl, sulfuric acid, H2SO4, nitric acid, HNO3, ethanoic acid, CH3COOH
2 state the names and formulae of the common alkalis, limited to sodium hydroxide, NaOH, potassium hydroxide, KOH, ammonia, NH3

3 describe the Brønsted–Lowry theory of acids and bases
4 describe strong acids and strong bases as fully dissociated in aqueous solution and weak acids and weak bases as partially dissociated in aqueous solution
5 appreciate that water has pH of 7, acid solutions pH of below 7 and alkaline solutions pH of above 7
6 explain qualitatively the differences in behaviour between strong and weak acids including the reaction with a reactive metal and difference in pH values by use of a pH meter, universal indicator or conductivity
7 understand that neutralisation reactions occur when H+(aq) and OH–(aq) form H2O(l)
8 understand that salts are formed in neutralisation reactions
9 sketch the pH titration curves of titrations using combinations of strong and weak acids with strong and Equilibria As Level Chemweak alkalis
10 select suitable indicators for acid-alkali titrations, given appropriate data (pKa
values will not be used)