Question

For the allotropic change represented by the equation $C$ (graphite) $\to$ C (diamond), the enthalpy change, $\Delta H=$ $1.9kJ$. If $6g$ of diamond and $6g$ of graphite are separately burnt to yield $C{O}_2$, the heat liberated in first case is

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

Answer 1

Answer: (C)

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

$C$ (graphite) $+O_2\to C{O}_2,\dots \dots -\Delta H_1$

$C$ (diamond) $+O_2\to C{O}_2,\dots \dots -\Delta H_2$

$\left(-\Delta H_1\right)-\left(-\Delta H_2\right)=1.9kJ$

or $\Delta H_2=\Delta H_1+1.9$

For combustion of $6g,\Delta H_2>\Delta H_1$ by $1.9/2=0.95kJ$