Although both N2 and 02 are naturally present in the air we breathe, high levels of NO and NO2 in the atmosphere occur mainly in regions with large automobile or power plant emissions. The equilibrium constant for the reaction of N2 and 02 to give NO is very small. The reaction is, however, highly endothermic, with a heat of reaction equal to +180 kJ (Equation 7). N2(g) + O2(g) 180 kJ 2NO(g) Equation 7 +

(a) Use LeChâtelier's Principle to explain why the concentration of NO at equilibrium increases when the reaction takes place at high temperatures.

(b) Use LeChâtelier's Principle to predict whether the concentration of NO at equilibrium should increase when the reaction takes place at high pressures.

(a) Increasing the temperature adds heat which is a reactant shifting the equilibrium rightwards.

(b) Pressure has no effect since the change in the number of moles is zero.

Explanation:

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In this case, one could represent the given reaction as:

Since it is endothermic. Thus, solving the (a) statement, one identifies the heat as a reagent, that is why the reaction cools down as it progress, therefore, by increasing the temperature, heat is added, that is, a reagent is added, which shifts the equilibrium rightwards, in other words, more NO is produced so its concentration increases.

Furthermore, for the (b) statement, since the change in the number of moles is zero, based on the stoichiometric coefficients as shown below:

Such value implies that the pressure has no effect on the concentration, taking into account the following form of the law of mass action:

Thus, since , , so no effect in concentration is due to the pressure.

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