Chemistry Equilibrium MDCAT 2015
PMDC Verified Question 92 of 102
During the manufacture of nitric acid, nitric oxide is oxidized to nitrogen dioxide. This reaction is given as:

$$ 2\text{NO}_{(g)} + \text{O}_{2(g)} \rightleftharpoons 2\text{NO}_{2(g)} \quad \Delta H = -114 \text{ kJ/mol} $$

According to Le Chatelier's Principle:
A
Reaction must not be temperature dependent
B
Reaction must be carried out at room temperature
C
Reaction must be carried out at low temperature
D
Reaction must be carried out at high temperature
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: Reaction must be carried out at low temperature
Concept:

Le Chatelier's Principle predicts how an equilibrium shifts in response to temperature changes, depending on whether the reaction is endothermic or exothermic.

Formula:

$$ \Delta H < 0 \implies \text{Exothermic Reaction} $$

Solution:

  • The negative sign in \( \Delta H = -114 \text{ kJ/mol} \) indicates the forward reaction releases heat (exothermic).


  • To maximize the yield of \( \text{NO}_2 \), the equilibrium must be shifted forward.


  • Decreasing the temperature (removing heat) shifts an exothermic reaction forward to produce more heat.


  • Therefore, it must be carried out at a logically low temperature.


Why other options are incorrect:

High temperature would favor the reverse endothermic reaction, breaking down \( \text{NO}_2 \). It is fundamentally dependent on temperature.

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