Scenes A, B, and C below depict this reaction at three temperatures:

${\mathbf{NH}}_{\mathbf{4}}\mathbf{CI}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{\rightleftharpoons }{\mathbf{NH}}_{\mathbf{3}}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\mathbf{HCI}\mathbf{\left(}\mathbf{g}\mathbf{\right)}{\mathbf{\Delta H}}_{\mathbf{rxn}}^{\mathbf{0}}\mathbf{}\mathbf{=}\mathbf{176}\mathbf{kj}$

(a) Which best represents the reaction mixture at the highest temperature? Explain. (b) Which best represents the reaction mixture at the lowest temperature? Explain.

The chemical equilibrium of a reaction can be described as the point at which the forward and backward reactions occur at an equal rate.

Once a system reaches equilibrium, there is no further change in the concentration of reactants and products. The ratio of the concentration of products to reactants in equilibrium is called the equilibrium constant.

The equilibrium constant for a reaction is constant for a particular temperature. Therefore, equilibrium can be defined as the point at which the reaction quotient of a reaction becomes equal to its equilibrium constant.

Le-Chatelier’s principle is used to describe the equilibrium of a reaction. It states that if the determining factors of equilibrium are changed, causing a disturbance in equilibrium, the system adjusts itself to counteract the effect of the change.

The determining factors of equilibrium include pressure, temperature, volume, and concentration. The equilibrium constant of the reaction remains constant during any change except for the temperature change. Le-Chatelier’s principle determines the optimum conditions to carry out a reaction.

In the given reaction, solid ammonium chloride is converted to ammonia gas and gaseous hydrogen chloride. Therefore, the solid deposited at the bottom represents the reactant ammonium chloride and the gas molecules represent the product in the figure given.

The enthalpy of the reaction is given to be 176 kJ. It is a positive value; therefore, ammonium chloride decomposition is endothermic. For an endothermic reaction, the reactant absorbs heat to carry out the reaction.

Figure A contains four molecules of gases: 2 molecules of ammonia and two hydrogen chloride molecules. There are eight gas molecules in figure B representing four molecules each of ammonia and hydrogen chloride. Figure C has six gas molecules representing three molecules each of ammonia and hydrogen chloride.

(a) At the highest temperature, the heat content in the system will be maximum. Therefore, more heat will be available for the reactant. Higher temperature favors endothermic reaction. Therefore, the forward reaction rate increases, and more ammonium chloride will be decomposed.

At higher temperatures, the concentration of products will be highest; therefore, the figure containing the maximum number of gas molecules will be at the highest temperature.

Figure B represents the reaction at the highest temperature.

(b) At the lowest temperature, the heat content of the system will be minimum. Therefore, exothermic reactions are favored at a lower temperature. The backward reaction will be exothermic, and therefore, more products will be converted into reactants.

The forward reaction rate will be lowest at a lower temperature; therefore, the figure representing the reaction at the lowest temperature will have minimum gas molecules.

Figure A represents the reaction at the lowest temperature.