Ionic Equations, Colours of Cations & Anions, Acids, Bases, Alkalis, Salts

1. Zinc nitrate reacts with aqueous ammonia (NH4OH) to form a salt and a base. (i) Name the salt and the base. (ii) Describe the observations for the reaction. (iii) Write a chemical equation for the reaction (include state symbols). (iv) Write an ionic equation for the reaction.

Answer:
1. (i) The salt formed is ammonium nitrate and the base is zinc hydroxide.

(ii) When colourless zinc nitrate solution reacts with colourless ammonium hydroxide, white precipitate of zinc hydroxide is produced and is soluble, giving a colourless solution. A colourless solution of ammonium nitrate solution is also formed.

(iii) Zn(NO3)2 (aq) + 2NH4OH (aq) --> 2NH4NO3 (aq) + Zn(OH)2 (s)

(iv) Zn 2+(aq) + 2OH - (aq) --> Zn(OH)2 (s)

2. Ammonium sulfate is heated with sodium hydroxide. (i) Write a chemical equation (include state symbols) for the reaction. (ii) Describe a test for the gas.

Answer:
2. (i) (NH4)2 SO4 (aq) + 2NaOH (aq) --> Na2 SO4 (aq) + 2H2O (l) + 2NH3 (g)

(ii) Turns damp red litmus paper blue.

3. 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. (ii) This is a confirmatory test for an anion. Name this anion. (iii) Give a possible cation which gives the green solution.

Answer:
3. (i) The gas evolved is ammonia.

(ii) It is nitrate ions.

(iii) It is iron(II) ions.

4. 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. (ii) Write an ionic equation for the reaction.

Answer:
4. (i) Titration is used to prepare the salt reacted from Sulfuric acid and Potassium hydroxide because the salt is a Group (I) salt and it is soluble. The acid, alkali and salt are also colourless solutions. Hence, titration is used to prepare the salt.

(ii) 2H+ (aq) + 2OH- (aq) --> 2H2O (l)

5. Excess zinc carbonate is added to hydrochloric acid in the preparation of zinc chloride salt. (i) Write an ionic equation for the reaction. (ii) Why excess zinc carbonate is used? (iii) Briefly explain how the zinc chloride crystals can be obtained.

Answer:
5. (i) ZnCO3 (s) + 2H+ (aq) --> Zn 2+ (aq) + H2O (l) + CO2 (g)

(ii) Excess zinc carbonate is used to make sure that the zinc carbonate has react completely with the hydrochloric acid before filtering out the excess unreacted zinc carbonate solid from the zinc chloride salt solution.

(iii) Excess zinc carbonate solid is added to the hydrochloric acid until there is excess zinc carbonate solid left behind. Filter the excess unreacted zinc carbonate solid and evaporate the zinc chloride solution to remove the water and make a saturated salt solution. The hot saturated solution from zinc chloride crystals when it is cooled.