Chemical Thermodynamics Question 442

Question: The heat of combustion of $ C{H_{4(g)}},,{C_{(graphite)}} $ and $ {H_{2(g)}} $ are $ 20,kcal,,-40,kcal $ and $ -10,kcal $ respectively. The heat of formation of methane is

Options:

A) $ -4.0,kcal $

B) $ +40,kcal $

C) $ -80,kcal $

D) $ +80,kcal $

Show Answer

Answer:

Correct Answer: C

Solution:

  • Given, (i) $ CH_4+2O_2\to CO_2+2H_2O $ , $ \Delta H=+20 $ kcal (ii) $ C+O_2\to CO_2 $ , $ \Delta H=-40kcal. $

(iii) $ H_2+\frac{1}{2}O_2\to H_2O $ , $ \Delta H=-10kcal. $

Aim: $ C+2H_2\to CH_4 $

(ii) + 2 × (iii) - (i) gives. $ \Delta H=-40+2,(-10)-(+,20)=-80, & ,kcal. $



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