Question

If the pressure in a reaction vessel for the following reaction is increased by decreasing the volume, what will happen to the concentrations of \(\mathrm{CO}\) and \(\mathrm{CO}_{2}\) ? $$ \mathrm{H}_{2} \mathrm{O}(g)+\mathrm{CO}(g) \rightleftharpoons \mathrm{H}_{2}(g)+\mathrm{CO}_{2}(g)+\text { Heat } $$ (a) both the [CO] and [CO \(\left._{2}\right]\) will decrease (b) neither the [CO] nor the \(\left[\mathrm{CO}_{2}\right]\) will change (c) the [CO] will decrease and the [CO \(_{2}\) ] will increase (d) both the [CO] and [CO \(\left._{2}\right]\) will increase

Short answer

The concentration of CO will increase and the concentration of CO2 will decrease, corresponding to choice (c).

Step-by-step solution

Understand Le Chatelier's Principle

Le Chatelier's principle states that if an external condition is applied to a system at equilibrium, the system will adjust in such a way as to partially counteract the change. In this case, we must consider the response of the system to an increase in pressure by decreasing the volume.

Apply the Principle to the Reaction

Increasing the pressure by decreasing the volume will shift the equilibrium to favor the side with fewer moles of gas. In the given reaction, 2 moles of reactant gases produce 2 moles of product gases. Therefore, changing the volume and pressure does not favor either side based on mole numbers.

Consider the Effect of Heat

Since the reaction is exothermic (releases heat), increasing the pressure is analogous to adding heat to the system. The system will shift the equilibrium away from the production of additional heat. This means the reaction will shift to the left, favoring the reactants.

Determine Changes in Concentrations

Shifting the reaction to the left will result in an increase of the concentrations of reactants (H2O and CO) and a decrease in the concentrations of the products (H2 and CO2).

Key concepts

Chemical Equilibrium

Chemical equilibrium represents a state in which a chemical reaction and its reverse are occurring at the same rate, resulting in no net change in the concentration of the reactants and products. It's a dynamic balance between the forward and reverse reactions, meaning that the substances are constantly reacting, but the overall concentrations remain constant.

Central to understanding equilibrium is that it can be disturbed by changing conditions such as concentration, pressure, and temperature. When such a change occurs, the system will adjust according to Le Chatelier's Principle, to re-establish equilibrium. The system's response aims to minimize the effect of the change imposed, which is key to predicting how the concentrations of substances will vary when conditions change.

Reaction Quotient

The reaction quotient (Q) helps us determine the direction in which the reaction will proceed to achieve equilibrium. It is calculated in the same way as the equilibrium constant (K), but unlike K, which pertains to the state of equilibrium, Q can be calculated for any concentration of reactants and products, not just at equilibrium.

The value of Q is compared with K to predict how the system will shift to reach equilibrium. If Q < K, the reaction will move forward to produce more products. If Q > K, the reaction will move in the reverse direction to produce more reactants. If Q = K, the system is already at equilibrium, and no shift will occur.

Pressure Effects on Equilibrium

When dealing with gases, pressure changes can significantly impact the state of chemical equilibrium. According to Le Chatelier's Principle, if the pressure of a system in equilibrium increases due to a decrease in volume, the system will shift towards the side with fewer moles of gas to counteract the pressure change.

In cases where there is the same number of moles of gas on both sides of a balanced equation, pressure changes will have no direct effect on the concentrations of reactants and products based on the quantity of gas alone. Other factors, such as whether the reaction is endothermic or exothermic, must then be considered to determine the equilibrium shift.

Exothermic Reactions

Exothermic reactions are those that release energy in the form of heat to the surroundings. In the context of Le Chatelier's Principle, for an exothermic reaction in equilibrium, an increase in temperature will result in the system favoring the reverse, or endothermic, reaction to absorb some of the added heat and reduce the temperature increase.

Conversely, a decrease in temperature would cause the system to shift towards the exothermic reaction to produce more heat, raising the temperature. Therefore, when a reaction vessel for an exothermic reaction at equilibrium is subjected to a pressure increase, the system will counteract this by favoring the reactants, thus producing less heat.