Q12 of 62 Page 68

The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound

Example:



Now, from the above example it is clear that enthalpy of a formation of compound can be positive or negative.

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10

On the basis of thermochemical equations (a), (b) and (c), find out which of the algebric relationships given in options (I) to (iv) is correct.

(a) C (graphite) + O2 (g) → CO2 (g); ΔrH = x kJ mol–1


(b) C (graphite) + O2 (g) → CO (g);ΔrH = y kJ mol–1


c) CO (g) + O2 (g) → CO2 (g);ΔrH = z kJ mol–1


 11

Consider the reactions given below. On the basis of these reactions find out which of the algebric relations given in options (I) to (iv) is correct?

(a) C (g) + 4 H (g) → CH4 (g); ΔrH = x kJ mol–1


(b) C (graphite, s) + 2H2 (g) → CH4 (g); ΔrH = y kJ mol–1


 13

Enthalpy of sublimation of a substance is equal to

 14

Which of the following is not correct?