Question

Using Le Châtelier's principle, predict how the following reaction will respond to the specified changes. $$ \mathrm{HCHO}_{2}(g) \rightleftharpoons \mathrm{CO}(g)+\mathrm{H}_{2} \mathrm{O}(g) $$ (a) increase the concentration of \(\mathrm{HCHO}_{2}(g)\) (b) increase the concentration of \(\mathrm{CO}(g)\) (c) decrease the concentration of \(\mathrm{H}_{2} \mathrm{O}(g)\) (d) decrease the volume of the reaction vessel

Short answer

Le Châtelier's principle predicts that (a) the reaction will shift right, favoring product formation; (b) the reaction will shift left, favoring reactant formation; (c) the reaction will shift right, increasing product formation; (d) there will be no shift in equilibrium as the number of moles of gas on both sides of the reaction is the same.

Step-by-step solution

Analyze the effect of increasing \text{HCHO}_2 concentration

According to Le Châtelier's principle, increasing the concentration of a reactant will shift the equilibrium to the right to counteract this change, thus favoring the production of more products.

Analyze the effect of increasing CO concentration

Le Châtelier's principle states that increasing the concentration of a product will shift the equilibrium to the left to counteract this change, thus favoring the production of more reactants.

Analyze the effect of decreasing H2O concentration

Decreasing the concentration of a product will shift the equilibrium to the right according to Le Châtelier's principle, as the system will try to produce more product to balance the decrease.

Analyze the effect of decreasing the volume of a reaction vessel

Decreasing the volume of the reaction vessel will increase the pressure, and the equilibrium will shift towards the side with fewer moles of gas. In this reaction, the number of moles is the same on both sides, so there would be no shift due to a change in volume.

Key concepts

Chemical Equilibrium

Chemical equilibrium represents a state in a reversible chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. As a result, the concentrations of the reactants and products remain constant over time, although they are not necessarily equal.

The fascinating aspect of the equilibrium is that it's a dynamic process—molecules are continuously reacting, and no observable changes occur without external disturbances. This balance can be disrupted if the conditions of the reaction are altered, such as changes in concentration, pressure, or temperature. When equilibrium is disturbed, the system will adjust itself to counteract the effect of the disturbance, according to Le Châtelier's principle.

Reaction Quotient

Understanding the reaction quotient, denoted by Q, is crucial when dealing with equilibrium. It is a measure that compares the concentrations of the products raised to the power of their stoichiometric coefficients to the reactants, similarly raised, at any point during the reaction. The formula for Q is given by: \[Q = \frac{{[\text{{Products}}]^\text{{coefficients}}}}{{[\text{{Reactants}}]^\text{{coefficients}}}}\]An important aspect of Q is how it relates to the equilibrium constant, K. When Q = K, the system is at equilibrium. If Q < K, the forward reaction is favored, pushing the system towards the products. Conversely, if Q > K, the reverse reaction is favored, and the system shifts to form more reactants. During the process to attain equilibrium, Q changes until it matches K.

Shifts in Equilibrium

Le Châtelier's principle is vital in predicting how a system in equilibrium responds to disturbances. It explains that if a system at equilibrium experiences a change in concentration, pressure, or temperature, the equilibrium will move to counteract the imposed change, thereby restoring a new equilibrium state.

For instance, if a reactant's concentration is increased, the principle predicts that the system will shift towards the products, aiming to reduce the concentration of that reactant. Similarly, if the concentration of a product is increased, the system will shift toward the reactants to consume the excess product. This principle is a key tool for chemists to manipulate conditions and drive reactions towards a desired product.

Application in Exercises

When applying Le Châtelier's principle to exercises like predicting reaction shifts due to concentration changes or volume adjustments, it is essential to consider the mole balance and how the system will adjust to restore equilibrium. Observing how the reaction quotient, Q, reacts to these changes provides valuable insight into the direction of the shift.