Reaction Equilibrium — Practice Quiz

A Chemistry cheat sheet for Reaction Equilibrium — every key formula with its symbols defined — plus a medium-level practice quiz to test recall.

Formulas & key concepts

Equilibrium Constant (Concentration): Ratio of product concentrations to reactant concentrations at equilibrium, raised to stoichiometric coefficients.

$$K_c = \frac{[D]^d[E]^e}{[A]^a[B]^b}$$

Where: \(K_c\) = equilibrium constant

Haber Process Equilibrium: Specific equilibrium constant expression for \(N_2 + 3H_2 \rightleftharpoons 2NH_3\).

$$K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}$$

Equilibrium Constant (Pressure): Ratio of product partial pressures to reactant partial pressures at equilibrium.

$$K_p = \frac{(P_D)^d(P_E)^e}{(P_A)^a(P_B)^b}$$

Where: \(K_p\) = equilibrium constant (pressure)

Pressure-Concentration Relation: Relates partial pressure of a gas to its molar concentration.

$$P_A = [A]RT$$

Where: \(P\) = pressure, \([A]\) = concentration, \(R\) = gas constant, \(T\) = temperature

Kp-Kc Relationship: Relates equilibrium constants based on pressure and concentration.

$$K_p = K_c(RT)^{\Delta n}$$

Where: \(\Delta n\) = change in moles of gas

Change in Moles: Difference between sum of gaseous product coefficients and gaseous reactant coefficients.

$$\Delta n = (d+e) - (a+b)$$

Reaction Quotient: Same expression as \(K_c\) but calculated with current concentrations, not necessarily at equilibrium.

$$Q_c = \frac{[D]^d[E]^e}{[A]^a[B]^b}$$

Where: \(Q_c\) = reaction quotient

Endothermic Reaction: Heat acts as a reactant; increasing temperature shifts equilibrium to products (right).

$$\text{Reactants} + \text{Heat} \rightleftharpoons \text{Products}$$

Exothermic Reaction: Heat acts as a product; increasing temperature shifts equilibrium to reactants (left).

$$\text{Reactants} \rightleftharpoons \text{Products} + \text{Heat}$$

Practice quiz

  1. For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, which of the following is the correct expression for the equilibrium constant $K_c$?

    • $K_c = \frac{[SO_3]^2}{[SO_2]^2[O_2]}$
    • $K_c = \frac{[SO_2]^2[O_2]}{[SO_3]^2}$
    • $K_c = \frac{[SO_3]}{[SO_2][O_2]}$
    • $K_c = \frac{[SO_3]^2}{[SO_2]^2 + [O_2]}$

    Answer: $K_c = \frac{[SO_3]^2}{[SO_2]^2[O_2]}$

  2. Consider the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$. Which of the following is the correct expression for the equilibrium constant $K_p$?

    • $K_p = \frac{(P_{NH_3})^2}{(P_{N_2})(P_{H_2})^3}$
    • $K_p = \frac{(P_{N_2})(P_{H_2})^3}{(P_{NH_3})^2}$
    • $K_p = \frac{P_{NH_3}}{P_{N_2}P_{H_2}}$
    • $K_p = \frac{(P_{NH_3})^2}{(P_{N_2}) + (P_{H_2})^3}$

    Answer: $K_p = \frac{(P_{NH_3})^2}{(P_{N_2})(P_{H_2})^3}$

  3. For a reaction $A(g) \rightleftharpoons B(g)$, the equilibrium constant $K_c$ is $4.0$. If at a certain moment, the concentrations are $[A] = 2.0 \text{ M}$ and $[B] = 6.0 \text{ M}$, in which direction will the reaction proceed to reach equilibrium?

    • To the right (towards products)
    • To the left (towards reactants)
    • The reaction is already at equilibrium
    • Cannot be determined without temperature

    Answer: To the right (towards products)

  4. The decomposition of calcium carbonate is an endothermic process: $CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$. How will an increase in temperature affect the equilibrium concentration of $CO_2(g)$?

    • It will increase.
    • It will decrease.
    • It will remain unchanged.
    • It depends on the initial amount of $CaCO_3$.

    Answer: It will increase.

  5. Consider the exothermic reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$. If the temperature of the system is increased, what will happen to the equilibrium concentration of $NH_3(g)$?

    • It will increase.
    • It will decrease.
    • It will remain unchanged.
    • It depends on the pressure.

    Answer: It will decrease.

  6. For the reaction $2NO(g) + O_2(g) \rightleftharpoons 2NO_2(g)$, what is the value of $\Delta n$?

    • $1$
    • $-1$
    • $0$
    • $2$

    Answer: $-1$

  7. For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ at $298 \text{ K}$, if $K_c = 2.4 \times 10^3$, what is the value of $K_p$? (Use $R = 0.0821 \text{ L} \cdot \text{atm} / (\text{mol} \cdot \text{K})$)

    • $K_p = 2.4 \times 10^3$
    • $K_p = 5.8 \times 10^4$
    • $K_p = 9.8 \times 10^1$
    • $K_p = 9.8 \times 10^4$

    Answer: $K_p = 9.8 \times 10^1$

  8. According to the relationship $P_A = [A]RT$, if the temperature $T$ and the gas constant $R$ are held constant, how does the partial pressure of a gas $P_A$ relate to its molar concentration $[A]$?

    • $P_A$ is inversely proportional to $[A]$.
    • $P_A$ is directly proportional to $[A]$.
    • $P_A$ is proportional to the square of $[A]$.
    • $P_A$ is independent of $[A]$.

    Answer: $P_A$ is directly proportional to $[A]$.

  9. Which of the following expressions correctly represents the equilibrium constant $K_c$ for the Haber process, $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$?

    • $K_c = \frac{[N_2][H_2]^3}{[NH_3]^2}$
    • $K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}$
    • $K_c = \frac{[NH_3]}{[N_2][H_2]}$
    • $K_c = \frac{2[NH_3]}{[N_2] + 3[H_2]}$

    Answer: $K_c = \frac{[NH_3]^2}{[N_2][H_2]^3}$

  10. Consider the equilibrium $CO(g) + Cl_2(g) \rightleftharpoons COCl_2(g)$. If the concentration of $CO(g)$ is increased, what will be the immediate effect on the reaction quotient $Q_c$?

    • $Q_c$ will increase.
    • $Q_c$ will decrease.
    • $Q_c$ will remain unchanged.
    • $Q_c$ will become equal to $K_c$.

    Answer: $Q_c$ will decrease.

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