[5 Mark Question Answer]

Question 15 Marks

Is it essential that oxidation and reduction must occur side by side in a chemical reaction? Explain

Answer

In a chemical reaction, if one substance is oxidised, the other substance must necessarily be reduced. This is because the electrons lost during oxidation are simultaneously gained during reduction andvice versa.
For example, Zinc reacts with copper sulphate to form zinc sulphate and copper.
CuSO4+ Zn→ ZnSO4+ Cu
Cu + 2SO42-+ Zn →Zn + 2SO42-+ Cu
Writing the half reaction,
Zn→Zn2++ 2e-(Oxidation)
Cu2++ 2e-→ Cu (Reduction)
They occur simultaneously as
Cu2++ Zn→Zn2++Cu
Thus, oxidation and reduction always occur simultaneously.

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Question 25 Marks

Describe briefly the ionic concept of oxidation and reduction. Give an equation to illustrate.

Answer

In the electronic concept, oxidation is a process in which an atom or ion loses an electron(s).
$Zn→Zn^{2+}+ 2e^-$
Oxidation is also defined as a chemical process which involves
Addition of oxygen
Addition of electronegative ion
Removal of hydrogen
Removal of electropositive ion (element)
In the electronic concept, reduction is a process in which an atom or ion gains electrons.
$Cu^{2+}+ 2e^-→ Cu$
The reduction is also defined as a chemical process which involves
Removal of oxygen
Addition of electropositive ion
Addition of hydrogen
Removal of electronegative ion

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Question 35 Marks

Steam can react with metal and non-metal to liberate hydrogen. Give the necessary conditions and equations for the same.

Answer

With metals:
$3Fe + 4H_2O⇋Fe_3O_4+ 4H_2$
With non-metals:
Steam is passed over hot coke (1000°C) in furnaces of a special design called inverters giving water gas.
$C + H_2O(CO + H_2) - \triangle$
Water is mixed with excess steam and passed over heated ferric oxide which acts as a catalyst and chromic oxide which acts as a promoter.
$(CO + H_2) + H_2OCO_2+ 2H_2+ \triangle$
The above mixture $CO_2+ H_2$ is formed through cold water under pressure (30atm) or through caustic potash solution, which dissolves the more soluble carbon dioxide leaving hydrogen.
$2KOH + CO_2→K_2CO_3+ H_2O$
The mixture is passed through an ammoniacal cuprous chloride solution in order to dissolve any uncombined carbon monoxide.
$CuCl+ CO + 2H_2O →CuCl.CO.2H_2O$

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Question 45 Marks

When hydrogen is passed over a black solid compound A, the products are a 'colorless liquid' and a 'reddish-brown metal B'.
Substance B is divided into two parts each placed in separate test tubes.
Dilute HCl is added to one part of substance B and dilute $HNO_3$ to the other.

Name the substances A and B.
Give two tests for the colourless liquid formed in the experiment.
What happens to substance A when it reacts with hydrogen? Give reasons for your answer.
Write an equation for the reaction between hydrogen and substance A.
Is there any reaction between substance B and dilute hydrochloric acid? Give reasons for your answer.

Answer

a. $\mathrm{A}=\mathrm{CuO}, \mathrm{B}=\mathrm{Cu}$
b. Blue and red litmus paper, when dipped in the colourless liquid, do not change colour. This confirms the liquid formed is neutral and is water. It changes white anhydrous copper sulphate to blue salt.
c. Black copper oxide (A) on heating with hydrogen reduces copper oxide to reddish-brown copper and itself gets oxidised to water. Hydrogen is a strong reducing agent and removes oxygen from less active metals, i.e. it removes oxygen from heated metal oxides when passed over them and itself gets oxidised to water.
d. $\mathrm{CuO}+\mathrm{H}_2 \xrightarrow{\Delta} \mathrm{Cu}+\mathrm{H}_2 \mathrm{O}$
e.$\mathrm{Cu}+\mathrm{HCl} \rightarrow$ No reaction
Copper is less reactive than hydrogen and hence cannot displace it from $\mathrm{HCl}$.

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Question 55 Marks

How can these impurities be removed?

Answer

The impurities can be removed from hydrogen by passing it through

Silver nitrate solution to remove arsine and phosphine.

$AsH_3+ 6AgNO_3→ Ag_3As + 3AgNO_3+ 3HNO_3$
$PH_3+ 6AgNO_3→ Ag_3P + 3AgNO_3+ 3HNO_3$

Lead nitrate solution to remove hydrogensulphide.

$Pb(NO_3)_2+ H_2S → Pbs + 2HNO_3$

Caustic potash solution to remove sulphur dioxide, carbon dioxide and oxides of nitrogen.

$SO_2+ 2KOH → K_2SO_3+ H_2O$
$CO_2+ 2KOH→ K_2CO_3+ H_2O$
$2NO_2+ 2KOH →KNO_2+ KNO_3+ H_2O$

A drying agent used to dry the gas. Common drying agents such as fused calcium chloride, caustic potash stick and phosphorus pentoxide remove water vapor.

So, the gas is purified and dried and then collected over mercury because mercury does not react with it.

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