Nitrogen dioxide gas is dark brown in color and remains in equilibrium with dinitrogen tetroxide gas, which is colorless.
2NO2(g) ⇌ N2O4(g)
When a light brown colored mixture of the two gases at equilibrium was moved from room temperature to a higher temperature, the mixture turned dark brown in color.
Which of the following conclusions about this equilibrium mixture is true?
This reaction is exothermic because the system shifted to the left on heating.
This reaction is exothermic because the system shifted to the right on heating.
This reaction is endothermic because the system shifted to the left on heating.
This reaction is endothermic because the system shifted to the right on heating.
The Correct Answer and Explanation is :
The correct conclusion is: “This reaction is endothermic because the system shifted to the left on heating.”
Here’s why:
The given reaction is an equilibrium between nitrogen dioxide (NO₂), which is dark brown, and dinitrogen tetroxide (N₂O₄), which is colorless:
[ 2NO₂(g) ⇌ N₂O₄(g) ]
When the system is at room temperature, a mixture of NO₂ and N₂O₄ gases gives a light brown color, indicating that some NO₂ is present along with the colorless N₂O₄.
Effect of temperature change on equilibrium:
According to Le Chatelier’s Principle, when a system at equilibrium is subjected to a change in temperature, the system adjusts to counteract that change. If the temperature is increased, the equilibrium will shift in the direction that absorbs heat. In other words, if the reaction is endothermic (heat-absorbing), the equilibrium will shift to favor the forward reaction. Conversely, if the reaction is exothermic (heat-releasing), the equilibrium will shift to favor the reverse reaction.
Color change upon heating:
When the mixture is heated, the system turns dark brown, indicating an increase in NO₂ concentration. Since NO₂ is on the left side of the equilibrium, the shift towards the left (formation of more NO₂) suggests that heat is absorbed when NO₂ forms. This implies that the reverse reaction (N₂O₄ breaking down into NO₂) is endothermic. Thus, increasing the temperature favors the endothermic process, shifting the equilibrium to the left, producing more NO₂.
Conclusion:
The reaction is endothermic because heating causes the equilibrium to shift left, producing more brown-colored NO₂ gas. Hence, the statement “This reaction is endothermic because the system shifted to the left on heating” is correct.