Question

$C\left(diamond\right)\rightleftharpoons C\left(graphite\right)$ ${\Delta }_r=-1.9kJ/mole$
Densities of diamond and graphite are $3.5$ and $2.3g/mL$. Favourable conditions for formation of diamond are :

• A. high pressure and low temperature
• B. low pressure and high temperature
• C. high pressure and high temperature
• D. low pressure and low temperature

Answer 1

Answer: (C)

high pressure and high temperature

Conversion from diamond to graphite is an exothermic reaction as ${\Delta }_r$ is negative. Hence, it is an exothermic reaction. So, on increasing the temperature, we are increasing the heat of the reaction. This excess heat can be absorbed in an endothermic reaction. Endothermic reaction takes place during formation of diamond.
Also, during the conversion from diamond to graphite the density of carbon decreases. And density is directly proportional to the molecular mass or the number of moles formed. Hence, number of moles decreases on formation of graphite. If we increase the pressure, density of graphite will decrease. So, the reaction will move such as to increase the density, that is in backward direction.
Thus the reaction takes place at a higher temperatures and higher pressure.