(a)(i) Define hard water (ii) Name two substances responsible for hardness in water (iii) Give two methods for the removal of hardness in water. (b)(i) What...

(a)(i) Definition of hard water

Hard water is water that does not readily form a lather with soap.

(a)(ii) Substances responsible for hardness

• Calcium hydrogencarbonate, \( \mathrm{Ca(HCO_3)_2} \)
• Magnesium sulphate, \( \mathrm{MgSO_4} \)

Other acceptable substances include calcium sulphate, magnesium hydrogencarbonate and magnesium chloride. Calcium and magnesium ions cause hardness.

(a)(iii) Removal of hardness

• Boiling the water, which removes temporary hardness caused by hydrogencarbonates.
• Adding washing soda, sodium trioxocarbonate(IV), \( \mathrm{Na_2CO_3} \), which removes calcium and magnesium ions as insoluble carbonates.

Ion-exchange resins (permutit) and distillation are also acceptable methods.

(b)(i) Raw materials for the Contact process

Sulphur and air (or oxygen) are the main raw materials. Sulphur is burned in air to form sulphur dioxide.

(b)(ii) Catalysed reaction in the Contact process

\[ 2\mathrm{SO_2(g)} + \mathrm{O_2(g)} \rightleftharpoons 2\mathrm{SO_3(g)} \]

(b)(iii) Catalyst

Vanadium(V) oxide, \( \mathrm{V_2O_5} \).

(c)(i) Uses of sodium tetraoxosulphate(VI), \( \mathrm{Na_2SO_4} \)

• It is used in the manufacture of glass or paper.
• It is used in the manufacture of detergents.

(c)(ii) Type of reaction

\[ \mathrm{Na_2SO_4 \cdot 10H_2O(s) \rightarrow Na_2SO_4 \cdot H_2O(s) + 9H_2O(g)} \]

This reaction is called efflorescence. The hydrated salt loses some of its water of crystallisation.

(c)(iii) Solubility of \( \mathrm{Na_2CO_3} \) at \(25^\circ\mathrm{C}\)

Mass of anhydrous \( \mathrm{Na_2CO_3} \) in \(30.0\ \mathrm{cm^3}\) of saturated solution \(=1.80\ \mathrm{g}\).

\[ \text{Mass in }1000\ \mathrm{cm^3} = \frac{1000 \times 1.80}{30.0} = 60.0\ \mathrm{g} \]

\[ M_r(\mathrm{Na_2CO_3}) = 2(23)+12+3(16)=106 \]

\[ \text{Solubility} = \frac{60.0}{106}=0.566\ \mathrm{mol\,dm^{-3}} \]

Therefore, the solubility is \(0.566\ \mathrm{mol\,dm^{-3}}\) (or \(60.0\ \mathrm{g\,dm^{-3}}\)).

(d)(i) Activated complex

An activated complex is a temporary, unstable, high-energy arrangement of atoms formed when reacting particles collide effectively, before products are formed.

(d)(ii) Reason a collision may not cause a reaction

The colliding particles may have energy less than the activation energy. Therefore, they cannot form the activated complex. A collision may also fail if the particles have an unsuitable orientation.

(d)(iii) Energy profile diagram for the formation of water gas

The reaction has \( \Delta H=+131\ \mathrm{kJ\,mol^{-1}} \), so it is endothermic: the products are at a higher enthalpy level than the reactants.

The key feature is that the products are higher than the reactants, because a positive \( \Delta H \) means energy is absorbed overall.