[3 Mark Question Answer]

Question 13 Marks

Decomposition reactions can occur by heat.
Give two balanced reactions for each.

Answer

Heat:
In these types of reactions, a single reactant breaks down to give simpler products. For example, when ferrous sulfate crystals are heated, they lose water and the colour of the crystals changes. This is because ferrous sulfate decomposes to ferric oxide, sulfur dioxide and sulfur trioxide.
$\underset{\text { (Ferrous sulphate) }}{2 \mathrm{FeSO}_4(\mathrm{~s})} \xrightarrow{\Delta} \underset{\text { Ferric oxide }}{\mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s})}+\mathrm{SO}_2(\mathrm{~g})+\mathrm{SO}_3(\mathrm{~g})$
Thermal decomposition of calcium carbonate to produce calcium oxide and Carbon dioxide is very important in many industries. The product calcium oxide, which is also called lime or quicklime, is used to manufacture cement.
$\underset{\text { (Lime stone) }}{\left.\mathrm{CaCO}_3(\mathrm{~s})\right)} \xrightarrow{\Delta} \underset{\text { (Quick lime) }}{\mathrm{CaO}(\mathrm{s})}+\mathrm{CO}_2(\mathrm{~g})$

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Question 23 Marks

What do you understand by precipitation reaction? Explain with examples.

Answer

Precipitation reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate. A precipitate is a solid that forms out of solution.
A common example is that of the mixing of two clear solutions: Silver nitrate $\left(\mathrm{AgNO}_3\right)$ and sodium chloride $(\mathrm{NaCl})$
$
\operatorname{AgNO}_3(\mathrm{aq})+\mathrm{NaCI}(\mathrm{aq}) \longrightarrow \mathrm{AgCI}(\mathrm{s}) \downarrow+\mathrm{NaNO}_3(\mathrm{aq})
$
The precipitate forms because the solid $(\mathrm{AgCl})$ is insoluble in water.

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Question 33 Marks

(a) Define the neutralization reaction with an example.
(b) Give a balanced equation for this reaction.
(c) Give three applications of neutralization reactions.

Answer

(a) The reaction between an acid and a base to form salt and water only is referred to as a neutralisation reaction.
(b) $NaOH + HCl → NaCl + H_2O$
(c) Applications of neutralisation reactions:
When someone is stung by a bee, formic acid enters the skin and causes pain, which can be relieved by rubbing the spot with slaked lime or baking soda, both of which are bases.
Acid which is accidentally spilled on to our clothes can be neutralised with ammonia solution.
If soil is somewhat acidic and thus unfavourable for growing of certain crops, slaked lime is added to neutralise the excess acid.

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Question 43 Marks

Explain, giving one example of the following chemical change:
Thermal dissociation

Answer

A reversible decomposition reaction brought about only by heat is called thermal dissociation reaction. Heat some solid ammonium chloride in a test tube. Two colourless gases, ammonia and hydrogen chloride, are produced. As these gases move up to the upper part of the cooler test tube, they combine to form ammonium chloride, which appears as a white sublimate on the upper cooler side o the test tube.
$ \mathrm{NH}_4 \mathrm{Cl} \rightleftharpoons \mathrm{NH}_3 \uparrow+\mathrm{HCl} \uparrow $

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Question 53 Marks

Match the following

A . $\mathrm{Zn}_{(\mathrm{s})}+\mathrm{H}_2 \mathrm{SO}_{4(\mathrm{aq})} \rightarrow \mathrm{ZnSO}_{4(\mathrm{aq})}+\mathrm{H}_{2(\mathrm{~g})}$	i. Photochemical decomposition
B . $ 2 \mathrm{AgCI}(\mathrm{s}) \xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{CI}_2(\mathrm{~g}) $	ii. Thermal decomposition
C . $2 \mathrm{KCI} \xrightarrow{\text { Electricity }} 2 \mathrm{~K}+\mathrm{CI}_2$	iii. Displacement reaction
D . $2 \mathrm{HgO}(\mathrm{s}) \xrightarrow{\Delta} 2 \mathrm{Hg}(\mathrm{s})+\mathrm{O}(\mathrm{g})$	iv. Electrolytic decomposition

Answer

Match the following

A . $\mathrm{Zn}_{(\mathrm{s})}+\mathrm{H}_2 \mathrm{SO}_{4(\mathrm{aq})} \rightarrow \mathrm{ZnSO}_{4(\mathrm{aq})}+\mathrm{H}_{2(\mathrm{~g})}$	i. Displacement reaction
B . $ 2 \mathrm{AgCI}(\mathrm{s}) \xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{CI}_2(\mathrm{~g}) $	ii. Photochemical decomposition
C . $2 \mathrm{KCI} \xrightarrow{\text { Electricity }} 2 \mathrm{~K}+\mathrm{CI}_2$	iii. Electrolytic decomposition
D . $2 \mathrm{HgO}(\mathrm{s}) \xrightarrow{\Delta} 2 \mathrm{Hg}(\mathrm{s})+\mathrm{O}(\mathrm{g})$	iv. Thermal decomposition

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