Q1 Zinc nitrate reacts with aqueous ammonia (NH4OH) to form a salt and a base.
(i) Name the salt and the base.
Ans: The salt is Zinc hydroxide, Zn(OH)2. The base is Ammonium nitrate, NH4NO3.
(ii) Describe the observations for the reaction.
Ans: When Zinc hydroxide reacts with aqueous ammonia, a white precipitate, Zn(OH)2, is formed.
(iii) Write a chemical equation for the reaction (include state symbols).
Ans: Zn(NO3)2 (aq) + 2NH4OH (aq) -> Zn(OH)2 (s) + 2NH4NO3 (aq)
(iv) Write an ionic equation for the reaction.
Ans: Zn2+ (aq) + 2OH- (aq) -> Zn(OH)2 (s)
Q2 Ammonium sulfate is heated with sodium hydroxide.
(i) Write a chemical equation (include state symbols) for the reaction.
Ans: (NH4)2SO4 (aq) + 2NaOH (aq) -> Na2SO4 (s) + 2H2O (l) + 2NH3 (g)
(ii) Describe a test for the gas.
Ans: Turns damn red litmus paper blue.
Q3 An unknown green solution is heated with a piece of aluminium foil and sodium hydroxide solution.
(i) The gas produced turns damp red litmus paper blue. Name the gas evolved.
Ans: Ammonia, NH3
(ii) This is a confirmatory test for an anion. Name this anion.
Ans: Nitrate, NO3-
(iii) Give a possible cation which gives the green solution.
Ans: Iron (II), Fe2+
Q4 Sulfuric acid is titrated with potassium hydroxide in the preparation of potassium sulfate salt.
(i) Explain why this method is recommended for the preparation for this salt.
Ans: This method, titration, is recommended because potassium sulfate is soluble and a Group (I) salt, and sulfuric acid and potassium hydroxide are colourless solutions, therefore titration method is needed with an indicator to determine the end-point of the neutralisation between sulfuric acid and potassium hydroxide before we prepare the salt.
(ii) Write an ionic equation for the reaction.
Ans: OH- (aq) + H+ (aq) -> H2O (l)
Q5 Excess zinc carbonate is added to hydrochloric acid in the preparation of zinc chloride salt.
(i) Write an ionic equation for the reaction.
Ans: ZnCO3 (s) + 2H+ (aq) -> Zn2+ (aq) + H2O (l) + CO2 (g)
(ii) Why excess zinc carbonate is used?
Ans: To ensure that all the hydrochloric acid is completely reacted with the zinc carbonate.
(iii) Briefly explain how the zinc chloride crystals can be obtained.
Ans: After the hydrochloric acid is completely reacted with excess zinc carbonate until there is excess zinc carbonate solid left behind, filter to remove the excess unreacted zinc carbonate solid. Evaporate the zinc chloride solution to remove the water and make a saturated salt solution. The hot saturated solution form zinc chloride crystals when it is cooled.
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