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Thursday, 22 September 2011
Q1. Zinc nitrate reacts with aqueous ammonia (
NH
4
OH) 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.
Answers:
(i) The salt formed is ammonium nitrate and the base formed is zinc hydroxide.
(ii) When zinc nitrate reacts with aqueous ammonia, sodium nitrate and zinc hydroxide (white precipitate) is formed.
(iii)
Zn(NO)
3(aq)
+ 2NH
4
OH
(aq)
Zn(OH)
2(s)
+ 2NH
4
NO
3 (aq)
(iv) Zn
2+
(NO
3
-
)
2
(aq) 2
NH
4
+
OH- (aq)
Zn(OH)
2 (s)
+
2NH
4
+
NO
-
(aq)
Zn
2+
(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. (ii) Describe a test for the gas.
Answer:
(i) 2NaOH
(aq)
+ (NH
4
)
2
SO
4 (aq)
Na
2
SO
4(aq)
+ 2H
2
O
(L)
+ 2NH
3(g)
(ii)
Hold a piece of damp red litmus paper over the mouth of a test tube. It will turn damp 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. (ii) This is a confirmatory test for an anion. Name this anion. (iii) Give a possible cation which gives the green solution.
Answer:
(i)The gas evolved is ammonia gas.
(ii) The anion is nitrate (NO
3
-
)
(iii) Iron(II) , Fe
2+
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. (ii) Write an ionic equation for the reaction.
Answers:
(i)Titration method is use to prepare Group(I) or ammonium salt. Potassium sulfate salt belongs to Group(I) salt. Thus, the titration method is recommended for the preparation of this salt.
(ii)
H
+(aq)
+ 2OH-
(aq)
2H
2
O
(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. (ii) Why excess zinc carbonate is used? (iii) Briefly explain how the zinc chloride crystals can be obtained
.
Answers:
(i) Zn(CO
3
)
2 (aq)
+ 2HCL
(aq)
ZnCl
2 (aq)
+ H
2
0
(L)
+ CO
2
(g)
Zn
2+
(CO
3
)
2 - (aq)
+ 2H
+
CL-
(aq)
Zn
2+
Cl
2 -
(aq)
+ H
2
0
(L)
+ CO
2
(g)
(CO
3
)
2
- (aq) + 2H
+
(aq)
H
2
O
(L)
+ CO
2 (g)
(ii) Excess zinc carbonate will ensure that all the hydrochloric acid is completely reacted.
(iii) Zinc chloride crystals can be obtained by the excess solid + acid method. This method is used to prepare soluble but not Group(I) salt, usually Group (II), (III) or transition metals salt.
(1) Add excess zinc carbonate solid to a beaker of hydrochloric acid until there is unreacted excess solid .
(2) Filter to remove the excess unreacted solid.
(3) Evaporate the zinc chloride solution to remove the water and make a saturated salt solution.
(4) The hot saturated solution form zinc chloride salt when it is cooled.
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9/22/2011 11:01:00 pm
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