Chapter 6: Thermodynamics

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?

\[\begin{array}{*{35}{l}}   \left( \mathbf{a} \right)\mathbf{ C }\left( \mathbf{g} \right)\mathbf{ + 4 H }\left( \mathbf{g} \right)\mathbf{ }\to \mathbf{ C}{{\mathbf{H}}_{\mathbf{4}}}\mathbf{ }\left( \mathbf{g} \right)\mathbf{;}{{\mathbf{\Delta }}_{\mathbf{r }}}\mathbf{H = x kJ mo}{{\mathbf{l}}^{\mathbf{1}}}  \\   \left( \mathbf{b} \right)\mathbf{ C}{{\mathbf{ }}_{\left( \mathbf{graphite,s} \right)}}\mathbf{ + 2}{{\mathbf{H}}_{\mathbf{2}}}\mathbf{ }\left( \mathbf{g} \right)\to \mathbf{ C}{{\mathbf{H}}_{\mathbf{4}}}\mathbf{ }\left( \mathbf{g} \right)\mathbf{; }{{\mathbf{\Delta }}_{\mathbf{r }}}\mathbf{H = y kJ mo}{{\mathbf{l}}^{\mathbf{1}}}  \\\end{array}\] 

(i) x = y 

(ii) x = 2y

(iii) x > y

(iv) x < y

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

(i) is always negative

(ii) is always positive 

(iii) may be positive or negative 

(iv) is never negative

Enthalpy of sublimation of a substance is equal to

(i) enthalpy of fusion + enthalpy of vaporisation 

(ii) enthalpy of fusion

(iii) enthalpy of vaporisation 

(iv) twice the enthalpy of vaporisation

Which of the following is not correct?

(i) ∆G is zero for a reversible reaction

(ii) ∆G is positive for a spontaneous reaction

(iii) ∆G is negative for a spontaneous reaction

(iv) ∆G is positive for a non-spontaneous reaction