Question

For the allotropic change represented by the equation $C$ (graphite) $\rightarrow$ C (diamond), the enthalpy change, $\Delta H=$ $1.9 \,kJ$. If $6 g$ of diamond and $6\, g$ of graphite are separately burnt to yield $CO _{2}$, the heat liberated in first case is

• A. less than in the second case by $1.9\, kJ$
• B. more than in second case by $11.4 \,kJ$
• C. more than in the second case by $0.95\, kJ$
• D. less than in the second case by $11.4\, kJ$

Answer 1

Answer: (C)

$C$(graphite) $\rightarrow C$ (diamond), $\Delta H=1.9\, kJ$

$C$ (graphite) $+ O _{2} \rightarrow CO _{2}, \ldots \ldots-\Delta H_{1}$

$C$ (diamond) $+ O _{2} \rightarrow CO _{2}, \ldots \ldots-\Delta H_{2}$

$\left(-\Delta H_{1}\right)-\left(-\Delta H_{2}\right)=1.9 kJ$

or $\Delta H_{2}=\Delta H_{1}+1.9$

For combustion of $6 g , \Delta H_{2} >\Delta H_{1}$ by $1.9 / 2=0.95\, kJ$