CAIE A-Level Chemistry: Equilibria

[Maximum number: 11]

The elements sodium to chlorine, in the third period, all form oxides.

(a)

$\mathrm{SO}_{3}$ is produced by the reaction between $\mathrm{SO}_{2}$ and $\mathrm{O}_{2}$ in the Contact process. A dynamic equilibrium is established.

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta H=-196 \mathrm{~kJ} \mathrm{~mol}^{-1}

[ 5 ]

(i)

Explain why increasing the total pressure, at constant temperature, increases the rate of production of $\mathrm{SO}_{3}$ and increases the yield of $\mathrm{SO}_{3}$ .
rate
yield

The graph shows how the concentrations of all three species in the system change with time for a typical reaction mixture. The gradients of all three lines decrease with time and then level off in this dynamic equilibrium.

Figure for Question (i) — CAIE A-Level Chemistry AS

[ 4 ]

(ii)

Explain why all three lines become horizontal.

[ 1 ]

(b)

2.00 moles of $\mathrm{SO}_{2}(\mathrm{~g})$ and 2.00 moles of $\mathrm{O}_{2}(\mathrm{~g})$ are sealed in a container with a suitable catalyst, at constant temperature and pressure. The resulting equilibrium mixture contains 1.98 moles of $\mathrm{SO}_{3}(\mathrm{~g})$ .
The total volume of the equilibrium mixture is $40.0 \mathrm{dm}^{3}$ .

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

[ 6 ]

(i)

Write the expression for the equilibrium constant, $K_{\mathrm{c}}$ , for the reaction between $\mathrm{SO}_{2}(\mathrm{~g})$ and $\mathrm{O}_{2}(\mathrm{~g})$ to produce $\mathrm{SO}_{3}(\mathrm{~g})$ .

K_{c}=

[ 1 ]

$K_{c}=\frac{\left[\mathrm{SO}_{3}\right]^{2}}{\left[\mathrm{SO}_{2}\right]^{2} \times\left[\mathrm{O}_{2}\right]}$

(ii)

Calculate the amount, in moles, of $\mathrm{SO}_{2}(\mathrm{~g})$ and $\mathrm{O}_{2}(\mathrm{~g})$ in the equilibrium mixture.

\begin{array}{r} \mathrm{SO}_{2}(\mathrm{~g})=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . \mathrm{mol} \\ \mathrm{O}_{2}(\mathrm{~g})=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . \mathrm{mol} \end{array}

[ 2 ]

$\mathrm{SO}_{2}=0.02(\mathrm{~mol})$ $\mathrm{O}_{2}=1.01(\mathrm{~mol})$

(iii)

Use your answers to (d)(i) and (d)(ii) to calculate the value of $K_{\mathrm{c}}$ for this equilibrium mixture. Give the units of $K_{\mathrm{c}}$ .

\begin{array}{r} K_{\mathrm{c}}= \\ \text { units }= \end{array}

[ 3 ]

$\begin{aligned} K_{c} & =\frac{(1.98 / 40)^{2}}{(0.02 / 40)^{2}(1.01 / 40)} & =3.88 / 3.882 \times 10^{5} \end{aligned}$

Units $=\mathrm{dm}^{3} \mathrm{~mol}^{-1} / \mathrm{mol}^{-1} \mathrm{dm}^{3}$

M1

[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: 7]

NO and $\mathrm{NO}_{2}$ react at $25^{\circ} \mathrm{C}$ to give $\mathrm{N}_{2} \mathrm{O}_{3}$ as shown in the equation.

\mathrm{NO}(\mathrm{~g})+\mathrm{NO}_{2}(\mathrm{~g}) \rightleftharpoons \mathrm{N}_{2} \mathrm{O}_{3}(\mathrm{~g}) \quad \Delta H=-7.2 \mathrm{~kJ} \mathrm{~mol}^{-1}

The reaction is reversible and reaches equilibrium in a closed system.

(a)

Fig. 2.1 shows how the rate of the forward reaction changes with time.

Initially, the rate of the reverse reaction is zero.
Complete Fig. 2.1 to sketch how the rate of the reverse reaction changes with time.

Fig. 2.1

[ 1 ]

(b)

State how the position of equilibrium changes, if at all, when the reaction takes place at $100^{\circ} \mathrm{C}$ .

Explain your answer.
Assume the pressure remains constant.

[ 2 ]

(c)

Table 2.1 shows the composition of an equilibrium mixture of $\mathrm{NO}(\mathrm{g}), \mathrm{NO}_{2}(\mathrm{~g})$ and $\mathrm{N}_{2} \mathrm{O}_{3}(\mathrm{~g})$ at 101 kPa .

Table 2.1

Calculate $K_{p}$ , the equilibrium constant with respect to partial pressures.
Deduce the units of $K_{\mathrm{p}}$ .

K_{p}=

units

[ 3 ]

NO
$\frac{0.605}{1.60}(=0.378125)$

(partial pressure / kPa $38.19 \quad 38.19 \quad$ 24.62)
$K_{p}=24.62 \div 38.19^{2}=0.0169$ $\mathrm{kPa}^{-1}$

(d)

A student titrates nitric acid with a base to form a solution containing aqueous magnesium nitrate.

[ 1 ]

(i)

Identify a base that the student could use.

[ 1 ]

magnesium hydroxide / Mg(OH) ${ }_{2}$

[Maximum number: 10]

Hydrogen halides are compounds formed when halogens (Group 17 elements) react with hydrogen. The bond polarity of the hydrogen halides decreases from HF to HI.

Some relevant data are shown in the table.

Table for Question 7. Equilibria question 4 — CAIE A-Level Chemistry AS

(a)

The equation for the preparation of hydrogen chloride using concentrated sulfuric acid is shown.

\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{NaCl} \rightarrow \mathrm{NaHSO}_{4}+\mathrm{HCl}

[ 2 ]

(i)

Use the Brønsted-Lowry theory of acids and bases to identify the base and its conjugate acid in this reaction. Explain your answer.

Brønsted-Lowry base (base-I) =
conjugate acid (acid-II) =

[ 2 ]

M1

base is $\mathrm{Cl}^{-}$ AND conjugate acid is HCl

OR

base is $\mathrm{HSO}_{4}{ }^{-}$ AND conjugate acid is $\mathrm{H}_{2} \mathrm{SO}_{4}$

M2

$\mathrm{Cl}^{-} / \mathrm{HSO}_{4}^{-} /$ base is a proton acceptor

OR

$\mathrm{HCl} / \mathrm{H}_{2} \mathrm{SO}_{4} /$ (conjugate) acid has one more $\mathrm{H}^{+}$

(b)

Hydrogen chloride undergoes a reversible reaction with oxygen.

4 \mathrm{HCl}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{Cl}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{~g})

The reaction is carried out at $400^{\circ} \mathrm{C}$ in the presence of a copper(II) chloride catalyst.

[ 8 ]

(i)

The reaction exists in dynamic equilibrium.

The reaction was repeated at $1000^{\circ} \mathrm{C}$ and the same pressure.
State and explain the effect on the composition of the equilibrium mixture of the change in temperature.

[ 2 ]

Reaction is exothermic.

Increased temperature shifts the equilibrium to the left AND decreases the yield of products $\mathrm{Cl_2}$ and/or $\mathrm{H_2O}$ , so less product is formed.

(ii)

When 1.60 mol of HCl are mixed in a sealed container with 0.500 mol of $\mathrm{O}_{2}$ at $400^{\circ} \mathrm{C}$ , 0.600 mol of $\mathrm{Cl}_{2}$ and 0.600 mol of $\mathrm{H}_{2} \mathrm{O}$ are formed.

The total pressure inside the container is $1.50 \times 10^{5} \mathrm{~Pa}$ .
- Calculate the amounts, in mol , of HCl and $\mathrm{O}_{2}$ in the equilibrium mixture.

\begin{aligned} \mathrm{HCl} & =\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . \mathrm{mol} \\ \mathrm{O}_{2} & =\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . \mathrm{mol} \end{aligned}

- Calculate the mole fraction of $\mathrm{Cl}_{2}$ and hence the partial pressure of $\mathrm{Cl}_{2}$ in the equilibrium mixture.
mole fraction of $\mathrm{Cl}_{2}=$

p_{\mathrm{Cl}_{2}}=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . P a

[ 3 ]

(iii)

In a separate experiment, an equilibrium reaction mixture was found to contain the four gases at the partial pressures shown in the table.

Table for Question (iii) — CAIE A-Level Chemistry AS

K_{\mathrm{p}}=\frac{\left(p_{\mathrm{Cl}_{2}}\right)^{2} \times\left(p_{\mathrm{H}_{2} \mathrm{O}}\right)^{2}}{\left(p_{\mathrm{HCl}}\right)^{4} \times p_{\mathrm{O}_{2}}}

Use this information and the expression given for $K_{p}$ to calculate a value for $K_{p}$ . State the units of $K_{\mathrm{p}}$ .

\begin{array}{r} K_{\mathrm{p}}= \\ \text { units }= \end{array}

[ 2 ]

$K_p=\frac{(3.6\times10^4)^2\times(3.6\times10^4)^2}{(4.8\times10^4)^4\times3.0\times10^4}=1.05\times10^{-5}$
Units $=\mathrm{Pa^{-1}}$ .

(iv)

The reaction is repeated without a catalyst.

State the effect of this on $K_{\mathrm{p}}$ .

[ 1 ]

$K_p$ would not change.

CAIE A-Level Chemistry 7 Equilibria

Covered syllabus points

Covered syllabus points

7. Equilibria question 1

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)

Question (ii)

Question (iii)

7. Equilibria question 2

Question (a)

Question (b)

Question (c)

Question (i)

7. Equilibria question 3

Question (a)

Question (b)

Question (c)

Question (d)

Question (i)

7. Equilibria question 4

Question (a)

Question (i)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question (iv)