Equilibrium Constant

X(g) + Y(g) ⇄ XY(g)

In an experiment, X(g) and Y(g) were combined in a rigid container at constant temperature and allowed to react as shown in the equation above. The table provides the data collected during the experiment. Based on the data, which of the following claims is most likely correct?

• The reaction was about to reach equilibrium 15 minutes after the reactants were combined because the concentrations of X and XY were almost the same.

• The reaction reached equilibrium between 75 minutes and 155 minutes after the reactants were combined because the concentrations of X and XY remained constant.

• The reaction did not reach equilibrium because only 86% of the initial concentration of X was consumed.

• The reaction did not reach equilibrium because initially there was no XY inside the container.

A sample of N₂O₄(g) is placed into an evacuated container at 373K and allowed to undergo the reversible reaction N₂O₄(g) ⇄ 2 NO₂(g). The concentration of each species is measured over time, and the data are used to make the graph shown above. Which of the following identifies when equilibrium is first reached and provides a correct explanation?

• At 14 seconds, because [N₂O₄] is twice [NO₂], which implies that the forward and reverse reaction rates are equal.

• At 23 seconds, because [NO₂] equals [N₂O₄], which shows that equal concentrations are present at equilibrium.

• At 40 seconds, because [NO₂] is twice [N₂O₄], which matches the stoichiometry of the balanced chemical equation.

• At 60 seconds, because [NO₂] and [N₂O₄] remain constant, indicating that the forward and reverse reaction rates are equal.

X(g) + Y(g) ⇄ XY(g)

The particle diagram above illustrates the changes that take place when X(g) and Y(g) are placed inside a rigid container at constant temperature. Which of the following is a characteristic of a system at equilibrium that is best represented by the particle diagram?

• The particle diagram shows that initially the reaction proceeds to the right to form products, which is a characteristic of a system at equilibrium.

• The particle diagram shows that after 200s the rate of the reverse reaction is slower than the rate of the forward reaction, which is a characteristic of a system at equilibrium.

• The particle diagram shows that after 200s there are no observable changes in the amounts of reactants and products, which is a characteristic of a system at equilibrium.

• The particle diagram shows that between 0s and 200s the rates of the forward and reverse reactions are the same, which is a characteristic of a system at equilibrium.

An equimolar mixture of X(g) and Y(g) is placed inside a rigid container at constant temperature. The particle diagram above represents the changes that occur over time. Based on the particle diagram, which of the following best predicts whether or not the system has reached equilibrium by 300s?

• It is not possible to determine that the system has reached equilibrium by 300s because the stoichiometry of the reaction is not known.

• It is not possible to determine that the system has reached equilibrium by 300s because the amounts of X, Y, and XY have continued to change.

• The system has reached equilibrium by 300s because the rate of formation of XY is constant.

• The system has reached equilibrium by 300s because the rates of consumption of X and Y are equal.

2 X(g) + Y₂(g) ⇄ 2 XY(g)

A reversible reaction is represented by the equation above. The amounts of reactants and products at time 1 are shown in the particle diagram on the left. The particle diagram on the right shows the amounts of reactants and products at time 2. Based on the diagrams, what can be inferred about the relative rates of the forward and reverse reactions between time 1 and time 2 ?

• Nothing can be inferred because the total number of X and Y atoms is the same in each diagram.

• Nothing can be inferred because the temperature of the system may have been changed.

• The rate of the reverse reaction is greater than the rate of the forward reaction.

• The rate of the forward reaction is greater than the rate of the reverse reaction.

The diagram above represents the equilibrium between the two isomers of C₂H₂Cl₂ , and the table provides the data collected in an experiment to determine its equilibrium constant, K_c , at 490K . In a second experiment done at the same temperature, [Z]_eq ≈ 1.0 M . Which of the following is the approximate equilibrium concentration of Y in the second experiment, and why?

• [Y]_eq ≈ 1.4 M because [Y]_eq−[Z]_eq should be the same for the same reaction.

• [Y]_eq ≈ 2.0 M because ([Y]_initial−[Y]_eq) = ([Z]_initial−[Z]_eq) should be the same for the same reaction.

The table above shows data for two reactions carried out in two separate evacuated 1.0-liter rigid containers at constant temperature of 298K. To each container 0.50mol of the appropriate reactants was added, and the reaction was allowed to reach equilibrium. Based on this information, which of the following correctly compares the relative concentrations of BrCl and NO present inside their respective containers at equilibrium?

• [BrCl]_eq=[NO]_eq because equimolecular mixtures of the reactants were allowed to reach equilibrium at the same constant temperature.

• [BrCl]_eq>[NO]_eq because Br₂ and Cl₂ are larger molecules that can collide more frequently to form products.

• [BrCl]_eq>[NO]_eq because the much larger K_eq for reaction 1 means that a much higher concentration of products will be present at equilibrium for reaction 1 compared with reaction 2.

• [BrCl]_eq<[NO]_eq because the much larger Keq for reaction 1 means that hardly any products will be present at equilibrium compared with reaction 2.

At night, an equilibrium reaction between two different nitrogen compounds generates N₂O₅ in the atmosphere, as represented below.

Reaction 1: NO₃(g) + NO₂(g) ⇄ N₂O₅(g)   K = 2.6×10⁻¹¹

A mixture of NO₃ and a ten-fold excess of NO₂ are placed inside a rigid container at constant temperature and allowed to reach equilibrium. Which of the following provides a correct comparison of the equilibrium concentrations of these chemical species, and why?

• [N₂O₅]<<[NO₃] , because a small K value indicates that the consumption of the reactants is favored at equilibrium.

• [NO₂]<<[N₂O₅]] , because a small K value indicates that the consumption of the reactants is favored at equilibrium.

• [NO₂]<<[NO₃] , because a small K value indicates that the formation of products is not favored at equilibrium.

• [N₂O₅]<<[NO₂] , because a small K value indicates that the formation of products is not favored at equilibrium.

HCl(aq) + H₂O(l) ⇄ H₃O⁺(aq) + Cl⁻(aq)

In 1.0 M HCl(aq), HCl is nearly 100 percent dissociated, as represented by the equation above. Which of the following best helps to explain why, in 0.01 M HCN(aq), less than 1 percent of HCN is dissociated?

• The CN⁻ ion is not very soluble in water, and a solid precipitate would form if more of the HCN dissociated.

• Compared to the HCl(aq) solution, the concentration of the HCN(aq) solution is much too dilute to achieve 100 percent dissociation.

• The equilibrium constant for the dissociation of HCN(aq) is much smaller than that for the dissociation of HCl(aq).

• HCN(aq) reacts with water to form a basic solution, and the high concentration of OH⁻(aq) interferes with the dissociation process.