7 EquilibriaAS Level

7.1 Chemical equilibria: reversible reactions, dynamic equilibrium

10 of 10 objectives have lessons · 0 studied · 0 secure

Lessons

Reversible reactions and dynamic equilibriumSome reactions never finish. In a closed system the forward and reverse rates become equal and the amounts stop changing, even though both reactions continue. 9 min, not started
Le Chatelier's principle: concentration and pressurePush an equilibrium and it pushes back. Change a concentration or a pressure and the position of equilibrium moves to oppose the change — you can predict the direction without any numbers. 10 min, not started
Le Chatelier's principle: temperature and catalystsHeating an equilibrium always favours the endothermic direction, so the value of the equilibrium constant changes. A catalyst does neither — it only gets you there sooner. 9 min, not started
Writing $K_c$ expressionsThe equilibrium constant packages the whole equilibrium mixture into one number. Build it from the balanced equation — products on top, each raised to its coefficient. 10 min, not started
Mole fractions and partial pressuresIn a gas mixture each gas contributes part of the total pressure. Work out its share with a mole fraction, then multiply by the total pressure. 10 min, not started
Writing $K_p$ expressionsFor gas-phase equilibria the constant can be built from partial pressures instead of concentrations. The pattern is identical — but now the units need checking. 10 min, not started
Calculating with $K_c$ and $K_p$Two routines cover almost every equilibrium calculation — turn moles into concentrations, or mole fractions into partial pressures, then substitute. 10 min, not started
Finding the quantities at equilibriumWork backwards from the constant to the mixture. One unknown, one table, one substitution — and the arithmetic stays simple because the syllabus avoids quadratics. 10 min, not started
What changes the equilibrium constant?Concentration, pressure and catalysts move the mixture around, but only temperature changes the value of $K_c$ or $K_p$. This is the single most examined distinction in the topic. 10 min, not started
The Haber and Contact processes: equilibrium in industryBoth processes use the same logic — push the equilibrium with pressure and removal of product, then accept a compromise temperature because a high yield at no rate earns nothing. 10 min, not started

Syllabus objectives

10 objectives

What Cambridge says you should be able to do, in our words. Codes are the topic number plus the objective’s position.

7.1.1

(a) Understand what is meant by a reversible reaction; (b) understand what is meant by dynamic equilibrium, in terms of the rates of the forward and reverse reactions being equal and the concentrations of reactants and products remaining constant; (c) understand that a closed system is needed for dynamic equilibrium to be established.

Taught in: Reversible reactions and dynamic equilibrium · 6 questions

7.1.2

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.

Taught in: Le Chatelier's principle: concentration and pressure, Le Chatelier's principle: temperature and catalysts · 3 questions

7.1.3

Use Le Chatelier's principle to deduce qualitatively (from appropriate information) the effects of changes in temperature, concentration or pressure, or of the presence of a catalyst, on a system at equilibrium.

Taught in: Le Chatelier's principle: concentration and pressure, Le Chatelier's principle: temperature and catalysts · 13 questions

7.1.4

Deduce expressions for equilibrium constants in terms of concentrations, Kc.

Taught in: Writing $K_c$ expressions · 6 questions

7.1.5

Use the terms mole fraction and partial pressure.

Taught in: Mole fractions and partial pressures · 6 questions

7.1.6

Deduce expressions for equilibrium constants in terms of partial pressures, Kp.

Use of the relationship between Kp and Kc is not required.

Taught in: Writing $K_p$ expressions · 6 questions

7.1.7

Use the Kc and Kp expressions to carry out calculations.

Such calculations will not require the solving of quadratic equations.

Taught in: Calculating with $K_c$ and $K_p$ · 6 questions

7.1.8

Calculate the quantities present at equilibrium, given appropriate data.

Taught in: Finding the quantities at equilibrium · 6 questions

7.1.9

State whether changes in temperature, concentration or pressure, or the presence of a catalyst, affect the value of the equilibrium constant for a reaction.

Taught in: What changes the equilibrium constant? · 7 questions

7.1.10

Describe and explain the conditions used in the Haber process and the Contact process, as examples of the importance of understanding dynamic equilibrium in the chemical industry and of applying Le Chatelier's principle.

Taught in: The Haber and Contact processes: equilibrium in industry · 6 questions