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Question

In a flask colourless N2O4 is in equilibrium with brown coloured NO2. At equilibrium, when the flask is heated to 100oC the brown colour deepens and on cooling, the brown colour became less coloured. The change in enthalpy ΔH of the system is:

A
negative
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B
positive
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C
zero
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D
not defined
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Solution

The correct option is A positive
In a flask colourless N2O4 is in equilibrium with brown coloured NO2. At equilibrium, when the flask is heated to 100oC the brown colour deepens and on cooling, the brown colour became less coloured. The change in enthalpy ΔH of the system is positive.
Colourless N2O4 brown NO2
When temperature is increased (by heating the flask), brown colour deepens, so more and more NO2 is formed and forward reaction is favoured.
When temperature is decreased (by cooling the flask), brown colour became less coloured, so more and more NO2 is converted to N2O4 and reverse reaction is favoured.
This is characteristic of endothermic reaction with positive value of enthalpy change.

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