Question

In a flask colourless $N_2{O}_4$ is in equilibrium with brown coloured ${NO}_2$. At equilibrium, when the flask is heated to ${100}^{o}C$ the brown colour deepens and on cooling, the brown colour became less coloured. The change in enthalpy $\Delta H$ of the system is:

• A. negative
• B. positive
• C. zero
• D. not defined

Answer 1

Answer: (B)

positive

In a flask colourless $N_2{O}_4$ is in equilibrium with brown coloured ${NO}_2$. At equilibrium, when the flask is heated to ${100}^{o}C$ the brown colour deepens and on cooling, the brown colour became less coloured. The change in enthalpy $\Delta H$ of the system is positive.
Colourless $N_2{O}_4\rightleftharpoons$ brown ${NO}_2$
When temperature is increased (by heating the flask), brown colour deepens, so more and more ${NO}_2$ is formed and forward reaction is favoured.
When temperature is decreased (by cooling the flask), brown colour became less coloured, so more and more ${NO}_2$ is converted to $N_2{O}_4$ and reverse reaction is favoured.
This is characteristic of endothermic reaction with positive value of enthalpy change.