Chemistry 5070 · O Level · Solids, liquids and gases

Solids, liquids and gases — practice question

Bromine, Br$_2$, is placed in Group VII of the Periodic Table.
(a(i))[1]

The melting point of bromine is $-7^{\circ}\text{C}$ and the boiling point is $59^{\circ}\text{C}$.\n\nExplain why bromine remains a solid at $-50^{\circ}\text{C}$.

(a(ii))[3]

Describe the arrangement and movement of bromine molecules at $-50^{\circ}\text{C}$.

(b)[2]

A sample of liquid bromine has $2.408 \times 10^{25}$ molecules.\n\nOne mole of bromine liquid has $6.02 \times 10^{23}$ molecules.\n\nCalculate the mass of the sample of bromine liquid.

(c(i))[2]

The ionic equation represents bromine reacting with warm concentrated aqueous sodium hydroxide.\n\n$3\text{Br}_2 + 6\text{OH}^- \rightarrow \text{BrO}_3^- + 5\text{Br}^- + 3\text{H}_2\text{O}$\n\nState the oxidation number of bromine in Br$_2$ and in Br$^-$.

(c(ii))[1]

Bromine is reduced in the reaction.\n\nExplain why, in terms of electrons.

(d(i))[2]

Bromine reacts with hydrogen in a closed vessel to establish an equilibrium mixture. The forward reaction releases thermal energy to the surroundings.\n\n$\text{Br}_2(g) + \text{H}_2(g) \rightleftharpoons 2\text{HBr}(g)$\n\nThe temperature of the equilibrium mixture is increased while the pressure stays constant.\n\nState and explain what happens to the position of equilibrium.

(d(ii))[2]

The pressure of the equilibrium mixture is increased while the temperature stays constant.\n\nState and explain what happens to the position of equilibrium.

(e)[3]

A phosphorus bromide contains 7.2% phosphorus by mass.\n\nCalculate the empirical formula of this bromide. Show your working.

Worked solution & mark scheme

This 16-mark question has a full step-by-step worked solution and mark scheme. One marking point: -50 ^{\circ}C lies below the melting point

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