Test: Chemical Equilibrium - MCAT MCQ

Analyze each statement and eliminate any answer choices as we move along.

Catalysts more effectively lowers the activation energy in the forward reaction is an incorrect statement. If we look at any energy profile, it will become apparent that by lowering the activation energy of the forward reaction that of the reverse reaction is also lowered.

Catalysts generally react with one or more reactants to form intermediates that subsequently given the final reaction product is a true statement. What needs further qualification is the process of giving the final reaction product. In that process, what is regenerated is the catalyst. A catalyst may react with the reactants as long as it is regenerated by the reaction.

If a catalyst affects the equilibrium of the reaction, it must be consumed as the reaction proceeds is an incorrect statement. Catalysts can only affect the kinetics or the rate at which the reaction approaches equilibrium, but not the equilibrium constant of the reaction. Concentration, pressure, temperature can affect the equilibrium, but not a catalyst.

Catalysts may increase the reaction rate or selectivity or enable the reaction at a lower temperature. Lowering the activation energy would allow for the reaction to occur at a lower temperature since less heat is needed to overcome the barrier. Statement IV is correct.

Statements II and IV are correct.

Increasing the K_eq will increase the amount of product formed at equilibrium. When we increase the pressure, the equilibrium will shift to the right according to Le Chatelier’s Principle. The concentrations will change, but will readjust such that K_eq remains the same.

Increasing or decreasing the concentration will cause the equilibrium to shift to the right or left, respectively, but the value of K_eq remains constant since it is concentration-independent.

Only by increasing or decreasing the temperature will change the value of K_eq since Keq = e(-ΔG/RT).

By decreasing the temperature for an exothermic reaction, the equilibrium will shift to the right such that the concentration of the products increase and the concentration of the reactants decrease. That results in an increased value of K_eq

Since the equilibrium concentrations of the reactants and products are given, the value of K_eq can be calculated:

= 0.1
Let’s evaluate when all the concentrations are 0.3 M using the reaction quotient Q, which has the same formula as K_eq:

Let’s evaluate when all the concentrations are 3.0 M using the reaction quotient Q, which has the same formula as K_eq:

Since Q > K_eq for both, the reaction will shift to the left.

Standard state conditions should not be confused with standard temperature and pressure for gases or standard conditions for procedures carried out in laboratory settings. Standard state conditions is commonly used in the thermodynamic evaluations of a reaction.

Most tables of thermodynamic quantities are collected at one of two temperatures, 273 K or 298 K more commonly. Standard state does not technically specify a temperature.

For a substance in solution, the standard state molality is 1 mol kg⁻¹ while standard state amount concentration is 1 mol dm⁻³. It is assumed that water does not change its concentration of 55.6 M appreciably.

The pH of a solution is not mentioned in standard state conditions.

Some of us may be more familiar with seeing 1 atmosphere, but IUPAC does recommend a standard pressure of 10⁵ Pascals for standard state conditions. Since 10⁵ Pascals is 0.99 atmospheres, they are essentially the same. Therefore, an ambient pressure of 100,000 Pascals is necessary as part of the standard state conditions.

Let’s evaluate each statement individually. A large rate constant k means that the reaction will go to equilibrium quickly, while a large equilibrium constant K_eq means that the forward reaction will go to completion, and the concentration of products will exceed that of the reactants.

K_eq is formed from the concentrations of the reactants each raised to their stoichiometric coefficients, while k is formed from the concentrations of the reactants raised to the stoichiometric coefficients only if dealing with elementary reactions. Otherwise, the order of the reaction must be determined by experimental data.

The rate constant indicates how quickly it will take to reach equilibrium, which is similar to how long the reaction will take to reach completion. The K_eq does give an indication of the relative concentrations of the reactants versus the products at equilibrium. If we are talking about ratio, then its the concentrations set to their stoichiometric coefficients.

The correct answer is that both constants are dependent on temperature. K_eq is independent of concentration. Once at equilibrium, if the concentrations of the reactants and products change, they will readjust such that the ratio equals K_eq again. Rate is not independent of concentration; the greater the concentration, the greater the initial speed of reaction.

An exothermic reaction releases heat, and increasing the temperature is a stress on the equilibrium. According to Le Chatelier's principle, the system will respond by shifting in a direction that counteracts the stress, which in this case is towards the reactants.

If the coefficients of a balanced chemical equation are multiplied by a factor, the value of the equilibrium constant (K) is raised to the power of that factor. This means that if the coefficients are multiplied by n, the value of K will be raised to the power of n. Therefore, the value of K increases.

In a system at equilibrium, the rates of the forward and reverse reactions are equal. While the concentrations of reactants and products may not be equal, their relative concentrations remain constant over time.

Increasing the pressure on a system at equilibrium involving only gases is a stress on the equilibrium. According to Le Chatelier's principle, the system will respond by shifting in a direction that reduces the pressure. This means that the equilibrium will shift towards the side with more moles of gas, as this will result in a decrease in the total pressure.

An endothermic reaction absorbs heat, and decreasing the temperature is a stress on the equilibrium. According to Le Chatelier's principle, the system will respond by shifting in a direction that counteracts the stress, which in this case is towards the reactants.