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.

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 reaction 7.1.2—Le Chatelier’s principle as: if a change 7.1.3—Le Chatelier’s principle to deduce (from 7.1.4—Expressions for equilibrium constants 7.1.5—The terms mole fraction and partial pressure 7.1.6—Expressions for equilibrium constants 7.1.7—The Kc and Kp expressions to carry out 7.1.8—The quantities present at equilibrium 7.1.9—Whether changes in temperature, concentration 7.1.10—Conditions used in the Haber process

[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

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.

1, 2 and 3 are correct

1 and 2 only are correct

2 and 3 only are correct

1 only is correct

[Maximum number: 7]

(a)

Define Le Chatelier's principle.

[ 2 ]

(b)

Reaction 1 describes the reversible reaction between yellow $\mathrm{Fe}^{3+}(\mathrm{aq})$ and colourless $\mathrm{SCN}^{-}(\mathrm{aq})$ to produce red $\mathrm{FeSCN}^{2+}(\mathrm{aq})$ .
reaction 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 $\mathrm{Fe}^{3+}(\mathrm{aq}), \mathrm{SCN}^{-}(\mathrm{aq})$ and $\mathrm{FeSCN}^{2+}(\mathrm{aq})$ is at equilibrium at $20^{\circ} \mathrm{C}$ .
The temperature of this mixture is then increased to $50^{\circ} \mathrm{C}$ and allowed to reach equilibrium.
Deduce the changes that occur, if any, in the equilibrium mixture at $50^{\circ} \mathrm{C}$ compared to the equilibrium mixture at $20^{\circ} \mathrm{C}$ .
- change in appearance
- change in relative concentration of $\mathrm{FeSCN}^{2+}(\mathrm{aq})$
- change in value of the equilibrium constant, $K_{\mathrm{c}}$

[ 3 ]

M1 change in appearance paler red / more yellow / less orange

M2 change in relative concentration of $\mathrm{FeSCN}^{2+}(\mathrm{aq})$

lower

M3 change in value of the equilibrium constant, $K_{\mathrm{c}}$

lower

(c)

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

[ 2 ]

(i)

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

Show your working.

K_{\mathrm{c}}=

units

[ 2 ]

M1 value for $K_{\mathrm{c}}=178$

M2 units $=\mathrm{mol}^{-1} \mathrm{dm}^{3}$

[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

[Maximum number: 2]

Define Le Chatelier's principle.

[Maximum number: 1]

The units of $K_{\mathrm{c}}$ for an equilibrium reaction are $\mathrm{mol}^{-1} \mathrm{dm}^{3}$ .
What could be the equation for the equilibrium?

1 $\mathrm{~A}(\mathrm{aq})+\mathrm{B}(\mathrm{aq}) \rightleftharpoons \mathrm{C}(\mathrm{s})+\mathrm{D}(\mathrm{aq})$
2 $\mathrm{P}(\mathrm{aq})+\mathrm{Q}(\mathrm{aq}) \rightleftharpoons \mathrm{R}(\mathrm{aq})$
3 $\mathrm{~W}(\mathrm{aq})+2 \mathrm{X}(\mathrm{aq}) \rightleftharpoons \mathrm{Y}(\mathrm{aq})+\mathrm{Z}(\mathrm{aq})$

1, 2 and 3 are correct

1 and 2 only are correct

2 and 3 only are correct

1 only is correct

[Maximum number: 1]

A mixture of the three gases, oxygen, nitrogen and argon, is at a total pressure of 500 kPa . There is a total of 1.2 moles of gas in the mixture.

If the oxygen gas alone occupied the entire volume of the mixture, it would exert a pressure of 150 kPa .

At room conditions the amount of nitrogen gas in the mixture would occupy a volume of $5.76 \mathrm{dm}^{3}$ . What is the partial pressure of the argon gas in the mixture?

150 kPa

200 kPa

250 kPa

300 kPa

[Maximum number: 1]

Which reaction has an equilibrium constant, $K_{p}$ , that has no units?

$\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{HI}(\mathrm{g})$

$3 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{N}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g})$

$2 \mathrm{NO}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{N}_{2} \mathrm{O}_{4}(\mathrm{~g})$

$2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g})$

[Maximum number: 1]

An alcohol, ROH, reacts reversibly with ethanoic acid to produce an ester.

\mathrm{ROH}(\mathrm{I})+\mathrm{CH}_{3} \mathrm{COOH}(\mathrm{I}) \rightleftharpoons \mathrm{CH}_{3} \mathrm{COOR}(\mathrm{I})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I})

3.0 mol of $\mathrm{ROH}, 2.0 \mathrm{~mol}$ of ethanoic acid and 1.0 mol of water are mixed together. At equilibrium, 1.5 mol of $\mathrm{CH}_{3} \mathrm{COOR}$ is present.

What is the value of the equilibrium constant, $K_{\mathrm{c}}$ , for this reaction?

0.20

0.25

2.00

5.00

[Maximum number: 1]

The reaction between sulfur dioxide and oxygen is reversible.

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad K_{\mathrm{c}}=280 \mathrm{~mol}^{-1} \mathrm{dm}^{3} \text { at } 1000 \mathrm{~K}

In an equilibrium mixture at 1000 K the sulfur trioxide concentration is $6.00 \mathrm{~mol} \mathrm{dm}^{-3}$ .
The sulfur dioxide concentration is twice the oxygen concentration.
What is the sulfur dioxide concentration?

$0.175 \mathrm{~mol} \mathrm{dm}^{-3}$

$0.252 \mathrm{~mol} \mathrm{dm}^{-3}$

$0.318 \mathrm{~mol} \mathrm{dm}^{-3}$

$0.636 \mathrm{~mol} \mathrm{dm}^{-3}$

[Maximum number: 1]

Sulfur dioxide reacts with oxygen as shown.

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta H^{\ominus}=-197 \mathrm{~kJ} \mathrm{~mol}^{-1}

Which change will move the equilibrium position to the right side and change the value of the equilibrium constant, $K_{\mathrm{c}}$ ?

the addition of a catalyst

increasing the pressure of the reaction

decreasing the temperature of the reaction

decreasing the concentration of product

[Maximum number: 1]

The diagram represents the Haber process for the manufacture of ammonia from nitrogen and hydrogen.

Figure for Question 11 — CAIE A-Level Chemistry AS

What is the purpose of the heat exchanger?

to cool the incoming gas mixture to avoid overheating the catalyst

to cool the reaction products and separate the $\mathrm{NH}_{3}$ from unused $\mathrm{N}_{2}$ and $\mathrm{H}_{2}$

to warm the incoming gas mixture and shift the equilibrium to give more $\mathrm{NH}_{3}$

to warm the incoming gas mixture and speed up the reaction

All question bank results loaded

Covered syllabus points

Question 1

Question 2

Question (a)

Question (b)

Question (c)

Question (i)

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11