AS & A Level Chemistry 2.2 The mole and the Avogadro constant: Exam Style Questions Paper 2

Question

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

(a) (i) Explain the meaning of the term bond polarity [1]
(ii) Suggest why the boiling point of HF is much higher than the boiling points of the other hydrogen halides. [2]
(iii) Describe and explain the relative thermal stabilities of the hydrogen halides.

(b) 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}
$
(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]
(ii) Explain why the reaction of concentrated sulfuric acid and sodium iodide is not suitable for the preparation of hydrogen iodide.[2]

(c) 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.
(i) Use the data in the table to calculate the overall enthalpy change of reaction.

enthalpy change of reaction $=$
$\mathrm{kJmol}^{-1}[2]$
(ii) State the type of catalyst used in this reaction. Explain how a catalyst is able to increase the rate of a chemical reaction.[2]
(iii) 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]

(iv) When $1.60 \mathrm{~mol}$ of $\mathrm{HCl}$ are mixed in a sealed container with $0.500 \mathrm{~mol}$ of $\mathrm{O}_2$ at $400^{\circ} \mathrm{C}$, $0.600 \mathrm{~mol}$ of $\mathrm{Cl}_2$ and $0.600 \mathrm{~mol}$ 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 $\mathrm{HCl}$ and $\mathrm{O}_2$ in the equilibrium mixture.

$
\begin{aligned}
& \mathrm{HCl}= \ldots \ldots . \ldots \mathrm{mol} \\
& \mathrm{O}_2=\ldots \ldots \mathrm{mol}
\end{aligned}
$
$\mathrm{mol}$

• Calculate the mole fraction of $\mathrm{Cl}_2$ and hence the partial pressure of $\mathrm{Cl}_2$ in the equilibrium mixture.

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

$
K_{\mathrm{p}}=\frac{\left(p_{\mathrm{Cl}}\right)^2 \times\left(p_{\mathrm{H}_2 \mathrm{O}}\right)^2}{\left(p_{\mathrm{HCl}}\right)^4 \times \mathrm{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 $K_{\mathrm{p}}$.
$
K_{\mathrm{p}}=
$
units $=$[2]
(vi) The reaction is repeated without a catalyst.
State the effect of this on $K_p$.[1] [Total: 22]