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CAIE A-Level Chemistry AS 7.1 Chemical Equilibria and Dynamic Equilibrium Questions

Practise defining dynamic equilibrium and predicting how temperature, concentration, pressure or a catalyst affects a system.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • state that forward and reverse rates are equal in a closed system while concentrations remain constant
  • use Le Chatelier’s principle to predict the direction of an equilibrium shift after a condition changes
  • interpret yield, composition or colour evidence without confusing equilibrium position with reaction rate

Covered syllabus points

7.1.1—What is meant by a reversible reaction7.1.2—Le Chatelier’s principle as: if a change7.1.3—Le Chatelier’s principle to deduce (from7.1.4—Expressions for equilibrium constants7.1.5—The terms mole fraction and partial pressure7.1.6—Expressions for equilibrium constants7.1.7—The Kc and Kp expressions to carry out7.1.8—The quantities present at equilibrium7.1.9—Whether changes in temperature, concentration7.1.10—Conditions used in the Haber process

Question 1

[Maximum number: 1]

When a sample of ammonium chloride is warmed it decomposes into ammonia and hydrogen chloride gas.

Figure for Question 1 — CAIE A-Level Chemistry AS
Figure for Question 1 — CAIE A-Level Chemistry AS

When the mixture of hot ammonia and hydrogen chloride gases hit a cold surface, a white solid of ammonium chloride reforms.

Which statements are correct?
1 Reaction 1 is in dynamic equilibrium.
2 Reaction 1 is reversible.
3 Reaction 1 is an endothermic reaction.

A

1, 2 and 3 are correct

B

1 and 2 only are correct

C

2 and 3 only are correct

D

1 only is correct

Question 2

[Maximum number: 7]

Question (a)

(a)

Define Le Chatelier's principle.

[ 2 ]

Question (b)

(b)

Reaction 1 describes the reversible reaction between yellow Fe3+(aq)\mathrm{Fe}^{3+}(\mathrm{aq}) and colourless SCN−(aq)\mathrm{SCN}^{-}(\mathrm{aq}) to produce red FeSCN2+(aq)\mathrm{FeSCN}^{2+}(\mathrm{aq}).
reaction 1

Fe3+(aq) yellow +SCN−(aq) colourless ⇌FeSCN2+(aq) red ΔH=−x kJ mol−1\underset{\text { yellow }}{\mathrm{Fe}^{3+}(\mathrm{aq})}+\underset{\text { colourless }}{\mathrm{SCN}^{-}(\mathrm{aq})} \rightleftharpoons \underset{\text { red }}{\mathrm{FeSCN}^{2+}(\mathrm{aq})} \quad \Delta H=-\mathrm{x} \mathrm{~kJ} \mathrm{~mol}^{-1}

A mixture of Fe3+(aq),SCN−(aq)\mathrm{Fe}^{3+}(\mathrm{aq}), \mathrm{SCN}^{-}(\mathrm{aq}) and FeSCN2+(aq)\mathrm{FeSCN}^{2+}(\mathrm{aq}) is at equilibrium at 20∘C20^{\circ} \mathrm{C}.
The temperature of this mixture is then increased to 50∘C50^{\circ} \mathrm{C} and allowed to reach equilibrium.
Deduce the changes that occur, if any, in the equilibrium mixture at 50∘C50^{\circ} \mathrm{C} compared to the equilibrium mixture at 20∘C20^{\circ} \mathrm{C}.
- change in appearance
- change in relative concentration of FeSCN2+(aq)\mathrm{FeSCN}^{2+}(\mathrm{aq})
- change in value of the equilibrium constant, KcK_{\mathrm{c}}

[ 3 ]

Question (c)

(c)

In another experiment, equimolar amounts of Fe3+(aq)\mathrm{Fe}^{3+}(\mathrm{aq}) and SCN−(aq)\mathrm{SCN}^{-}(\mathrm{aq}) are mixed together and allowed to reach equilibrium. The total volume of the mixture is 25.0 cm325.0 \mathrm{~cm}^{3}.
reaction 1Fe3+(aq)+SCN−(aq)⇌FeSCN2+(aq)1 \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{SCN}^{-}(\mathrm{aq}) \rightleftharpoons \mathrm{FeSCN}^{2+}(\mathrm{aq})
At equilibrium the mixture contains:
- [SCN−]=1.30×10−3moldm−3\left[\mathrm{SCN}^{-}\right]=1.30 \times 10^{-3} \mathrm{moldm}^{-3}
- [FeSCN2+]=0.300×10−3moldm−3\left[\mathrm{FeSCN}^{2+}\right]=0.300 \times 10^{-3} \mathrm{moldm}^{-3}.

[ 2 ]

Question (i)

(i)

Calculate Kc\mathrm{K}_{\mathrm{c}} for reaction 1 and state its units.

Show your working.

Kc=K_{\mathrm{c}}=

units

[ 2 ]

Question 3

[Maximum number: 1]

Graphs can be drawn to show the percentage of ammonia at equilibrium when nitrogen and hydrogen are mixed at different temperatures and pressures.

Which diagram correctly represents these two graphs?

Figure for Question 3 — CAIE A-Level Chemistry AS
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