(a) Crystals of zinc sulfate are made by warming excess solid zinc oxide with dilute sulfuric acid.
\[ \text{ZnO(s)} + \text{H}_2\text{SO}_4(\text{aq}) \rightarrow \text{ZnSO}_4(\text{aq}) + \text{H}_2\text{O(l)} \]
(i) State the meaning of the state symbol (aq).
(ii) State the method used to separate the excess solid zinc oxide from the reaction mixture.
(b) Crystals of sodium nitrate can be made by neutralising an acid with an alkali.
(i) Name the acid and the alkali used.
(ii) Complete the equation for all neutralisation reactions.
\[ \text{H}^+ + …… \rightarrow …… \]
(iii) Neutralisation reactions are exothermic.
Define the term exothermic.
(iv) Fig. 7.1 shows the reaction pathway diagram for an exothermic reaction.
Explain how Fig. 7.1 shows that the reaction is exothermic.
(c) Methyl orange is an acid-base indicator.
State the colour of methyl orange at pH2 and at pH12.
▶️ Answer/Explanation
(a)(i) aqueous / dissolved in water
The state symbol (aq) indicates that the substance is dissolved in water, forming an aqueous solution.
(a)(ii) filtration
Filtration is used to separate the undissolved excess zinc oxide (solid) from the zinc sulfate solution.
(b)(i) acid: nitric acid; alkali: sodium hydroxide
Sodium nitrate is formed by the neutralization reaction between nitric acid (HNO₃) and sodium hydroxide (NaOH).
(b)(ii) \[ \text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O} \]
This is the ionic equation for all neutralization reactions, showing the combination of hydrogen ions and hydroxide ions to form water.
(b)(iii) release of thermal energy
An exothermic reaction is one that releases energy, usually in the form of heat, to the surroundings.
(b)(iv) The energy of the products is lower than the energy of the reactants.
In an exothermic reaction, the products have less energy than the reactants, with the difference being released as heat. The diagram shows this by having the products at a lower energy level than the reactants.
(c) pH2: red/pink; pH12: yellow
Methyl orange is a pH indicator that changes color depending on the acidity. In strongly acidic conditions (pH2), it appears red/pink, while in strongly alkaline conditions (pH12), it turns yellow. The transition range is between pH 3.1 (red) and pH 4.4 (yellow).
This question is about acids, bases and salts.
(a) Sodium hydroxide is a base.
(i) Name the products formed when sodium hydroxide reacts with dilute nitric acid.[2]
(ii) Describe the effect of sodium hydroxide on a named indicator.[2]
(iii) Complete the word equation for the reaction of sodium hydroxide with ammonium chloride.
(b) Describe how to prepare pure, dry crystals of the salt zinc sulfate from an aqueous solution of zinc sulfate.[2]
(c) The rate of reaction of zinc powder with dilute sulfuric acid is found by measuring the increase in volume of hydrogen gas produced as time increases.
Describe the effect, if any, of each of the following on the rate of this reaction.
- The reaction is carried out with large pieces of zinc instead of zinc powder.
All other conditions stay the same.
- The reaction is carried out using a catalyst.
All other conditions stay the same.
- The reaction is carried out with dilute sulfuric acid of a lower concentration.
All other conditions stay the same. [3]
▶️ Answer/Explanation
(a)(i) Ans: Sodium nitrate and water
Neutralization reaction: \(NaOH + HNO_3 → NaNO_3 + H_2O\). Sodium hydroxide (base) reacts with nitric acid to form sodium nitrate (salt) and water.
(a)(ii) Ans: Turns litmus blue or methyl orange yellow
Sodium hydroxide is alkaline. Litmus turns blue in alkalis, while methyl orange changes from red to yellow in basic conditions.
(a)(iii) Ans: Sodium hydroxide + ammonium chloride → ammonia + sodium chloride + water
The reaction produces ammonia gas (\(NH_3\)), sodium chloride (\(NaCl\)), and water: \(NaOH + NH_4Cl → NH_3 + NaCl + H_2O\).
(b) Ans: Evaporate to crystallisation point, filter, and dry
1. Heat the solution to evaporate water until crystals form.
2. Filter to separate crystals.
3. Dry between filter paper or in a warm oven.
(c) Ans:
Large zinc pieces: Rate decreases (smaller surface area)
Catalyst: Rate increases (lowers activation energy)
Lower acid concentration: Rate decreases (fewer reacting particles)