Question

Consider this reaction at equilibrium. $$ \mathrm{C}(s)+\mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons \mathrm{CO}(g)+\mathrm{H}_{2}(g) $$ Predict the effect (shift right, shift left, or no effect) of these changes. (a) adding \(C\) to the reaction mixture (b) condensing \(\mathrm{H}_{2} \mathrm{O}\) and removing it from the reaction mixture (c) adding \(\mathrm{CO}\) to the reaction mixture (d) removing \(\mathrm{H}_{2}\) from the reaction mixture

Short answer

The equilibrium will (a) not shift upon adding C, (b) shift to the right when H2O is condensed and removed, (c) shift to the left with the addition of CO, and (d) shift to the right when H2 is removed.

Step-by-step solution

- Analyze the addition of C

According to Le Chatelier's Principle, adding a reactant to a system at equilibrium will cause the system to counteract this change by producing more product. Since carbon (C) is a solid reactant and its amount does not directly affect the pressure or concentration of the gaseous species in the reaction, adding more C will have no effect on the position of the equilibrium.

- Analyze the removal of H2O

Condensing and removing water (H2O) from the reaction mixture means removing one of the products. Le Chatelier's Principle states that the system will respond by shifting to the right to replace the removed product, thus producing more CO and H2.

- Analyze the addition of CO

Adding more of the product carbon monoxide (CO) to the system will result in a shift in the equilibrium to counteract this addition, according to Le Chatelier's Principle. The system will shift to the left, favoring the production of reactants, C and H2O.

- Analyze the removal of H2

Removing hydrogen gas (H2) from the reaction mixture is equivalent to reducing the concentration of a product. The equilibrium will shift to the right, favoring the formation of more products (CO and H2) to compensate for the lost H2.

Key concepts

Chemical Equilibrium

When a chemical reaction reaches a state where the rate of the forward reaction equals the rate of the reverse reaction, it has established a chemical equilibrium. At this point, the concentrations of reactants and products remain constant over time, but not necessarily equal. It’s important to understand that equilibrium does not mean that the reactants and products are in equal amounts, but instead, the processes of reaction and reformation are occurring at the same rate.

Imagine two teams of equally skilled players playing tug-of-war. Neither side wins, and the rope's position stays constant—this scenario is akin to chemical equilibrium. In our exercise, the reaction between carbon (C), water vapor (H₂O), carbon monoxide (CO), and hydrogen gas (H₂) has reached such a state. The system will naturally oppose any change to this 'tug-of-war' balance, an idea formalized by Le Chatelier’s Principle.

Reactant and Product Concentration

The concentrations of reactants and products in a reaction at equilibrium can be changed by adding or removing substances. These changes can disrupt the equilibrium, and the reaction responds to restore balance. This restoration doesn't revert the concentrations to their original values but adjusts the rates of the forward and reverse reactions to reach a new equilibrium state.

Consider a balanced scale, with reactants on one side and products on the other: adding more of one component is like putting extra weight on that side of the scale. The system will respond to level the scale again. In the textbook exercise, adding carbon (a reactant) does not affect the equilibrium because it's a solid and doesn’t contribute to the concentration of the gaseous substances in the mixture. On the other hand, modifying the concentration of gaseous components has a direct impact, which the system tries to counteract to maintain the 'weight balance' of the scale.

Equilibrium Shift

An equilibrium shift refers to the change in the position of equilibrium that occurs in response to a disturbance. This shift can be toward the production of more products (right shift) or reactants (left shift), depending on the nature of the change imposed on the system, as predicted by Le Chatelier's Principle.

When we manipulate the concentrations of either reactants or products, the equilibrium will 'shift' to oppose this change. If we remove a product, such as water or hydrogen gas, in our textbook reaction, we create an 'imbalance.' The system seeks to 'rebalance' by creating more of the removed substance, hence moving the equilibrium to the right. Conversely, adding extra product (like CO) will result in a leftward shift, as the system attempts to reduce the product concentration by making more reactants. These shifts are the system's way of maintaining a dynamic balance amidst the continuous push and pull of the chemical reaction.