Thursday, September 22, 2011
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Question 1
Zinc nitrate reacts with aqueous ammonia (NH4OH) to form a salt and a base.
(i) Name the salt and the base.
The salt is Ammonium Nitrate while the base is Zinc Hydroxide .
(ii) Describe the observations for the reaction.
Zinc Nitrate and Ammonia Hydroxide are colorless solutions . When added together,
a colorless solution, Ammonium Nitrate, and a white insoluble precipitate, Zinc Hydoxide is formed .
(iii) Write a chemical equation for the reaction (include state symbols).
Zn(NO3)2 (aq) + 2NH4OH (aq) --> 2NH4NO3 (aq) + Zn(OH)2 (s)
(iv) Write an ionic equation for the reaction.
Zn2+ (aq) + 2OH- (aq) -->Zn(OH)2 (s)
Question 2
Ammonium sulfate is heated with sodium hydroxide.(i) Write a chemical equation (include state symbols) for the reaction.
(NH4)2SO4 (aq) + 2NaOH (aq) --> Na2SO4 (aq) + 2H2O (l) + 2NH3 (g)
(ii) Describe a test for the gas.
Hold a piece of damp red litmus paper over the mouth of the test tube containing ammonia gas. It will turn damp red litmus paper to blue .
Question 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.The gas evolved is Ammonia gas.
(ii) This is a confirmatory test for an anion. Name this anion.It is Nitrate.
(iii) Give a possible cation which gives the green solution.Iron(II) ion.
Question 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.
Titration method is to prepare Group (I) salt or Ammonium salt. Potassium sulfate is Group (I) salt and it is soluble. Sulfuric acid and Potassium Hydroxide are both colourless solution. Hence by using titration, we add the indicator to ensure that the neutralization change to a permanent colour to show that neutralization has completed and record the volume. Then we can use the correct volume of 2 solutions for preparation of this salt.
(ii) Write an ionic equation for the reaction.H2SO4 (aq) + 2KOH (aq) --> K2SO4 (aq) + 2H2O (l)
Question 5
Excess zinc carbonate is added to hydrochloric acid in the preparation of zinc chloride salt.
(i) Write an ionic equation for the reaction
ZnCO3 (aq) + 2HCl (aq) --> ZnCl2 (aq) + H2O (l) + CO2 (g)
(ii) Why excess zinc carbonate is used?
Excess zinc carbonate to ensure that all the hydrochloric acid has fully and completely reacted with the zinc carbonate before filtering out the excess unreacted solid from the salt solution.
(iii) Briefly explain how the zinc chloride crystals can be obtained.
Add excess zinc carbonate to a beaker the hydrochloric acid first until excess solid is left unreacted behind. Filter to remove the excess unreacted zinc carbonate. Evaporate the zinc chloride solution to remove water to make a saturated salt solution. The hot saturated solution form zinc chloride crystals when it is cooled. This is how the zinc chloride crystals can be obtained.
Lee Hwee Leng 
