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Question

The heat of combustion of $$C_{2}H_{4}, C_{2}H_{6}$$ and $$H_{2}$$ gases are -1409.5 KJ, -1558.3 KJ and -285.6 KJ respectively. The heat of hydrogenation of ethene is :


A
-136.8 kJ
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B
-13.68 kJ
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C
273.6 kJ
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D
1.368 kJ
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Solution

The correct option is C -136.8 kJ
Hydrogenation reaction of ethene: $$C_2H_4+H_2\rightarrow C_2H_6$$.

Heat of hydrogenation of ethene = $$\Delta H_{C_2H_4}+\Delta H_{H_2}-\Delta H_{C_2H_6}$$.

Now, the combustion reactions are: 

(i) $$C_{2}H_{4}+3O_{2}\rightarrow 2CO_{2}+2H_{2}O;\: \Delta H_{1}=-1409.5\: KJ$$

(iii) $$H_{2}+\frac{1}{2}O_{2}\rightarrow H_{2}O;
\: \Delta H_{2}= -285.6\: KJ$$

(ii) $$C_{2}H_{6}+\frac{7}{2}O_{2}\rightarrow 2CO_{2}+3H_{2}O;\: \Delta H_{3}=-1558.3\: KJ$$
Now, (i) + (ii) - (iii) gives:

$$C_{2}H_{4}+H_{2}\rightarrow C_{2}H_{6}$$

$$\Delta H=\Delta H_{1}+\Delta H_{2}-\Delta H_{3}$$

$$=-1409.5-285.6 - (-1558.3)$$

$$=-136.8$$ KJ

Chemistry

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