Textbook Question
The diagram shown here represents the equilibrium state for the reaction A2(𝑔) + 2B(𝑔) ⇌ 2AB(𝑔). (a) Assuming the volume is 2 L, calculate the equilibrium constant 𝐾𝑐 for the reaction.
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Ch.15 - Chemical Equilibrium
Chapter 15, Problem 14b
The equilibrium constant for the dissociation of molecular iodine, I2(g) ⇌ 2 I(g), at 800 K is Kc = 3.1×10−5. (b) Assuming both forward and reverse reactions are elementary reactions, which reaction has the larger rate constant, the forward or the reverse reaction?
Verified step by step guidance1
Identify the reaction and its reverse: The given reaction is I2(g) ⇌ 2 I(g). The forward reaction is the dissociation of I2 into 2 I atoms, and the reverse reaction is the recombination of 2 I atoms to form I2.
Understand the equilibrium constant (Kc): Kc is defined as the ratio of the rate constant of the forward reaction (kf) to the rate constant of the reverse reaction (kr). The formula is Kc = kf / kr.
Analyze the value of Kc: Given that Kc = 3.1×10−5, which is a small number, it indicates that the equilibrium heavily favors the reactants over the products at 800 K.
Determine the larger rate constant: Since Kc is small and represents the ratio of kf to kr, this implies that kr (rate constant for the reverse reaction) is much larger than kf (rate constant for the forward reaction).
Conclude which reaction has the larger rate constant: The reverse reaction, which is the recombination of I atoms to form I2, has the larger rate constant compared to the forward reaction of dissociation of I2 into I atoms.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Equilibrium Constant (Kc)
The equilibrium constant, Kc, quantifies the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. For the dissociation of I2, Kc = [I]^2 / [I2]. A small Kc value, like 3.1×10^−5, indicates that at equilibrium, the concentration of reactants (I2) is much greater than that of products (I), suggesting the reaction favors the reactants.
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Equilibrium Constant Expressions
Rate Constants and Reaction Rates
Rate constants (k) are specific to each reaction and temperature, determining the speed of the reaction. The forward reaction rate constant (k_forward) relates to the formation of products, while the reverse reaction rate constant (k_reverse) relates to the formation of reactants. The relationship between Kc and the rate constants is given by Kc = k_forward / k_reverse, which helps in comparing their magnitudes.
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Elementary Reactions
Elementary reactions are single-step processes where reactants convert directly to products without intermediates. Each elementary reaction has a specific rate law that reflects its molecularity. In this case, both the forward and reverse reactions are assumed to be elementary, allowing for a straightforward comparison of their rate constants based on the equilibrium constant.
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Related Practice
Textbook Question
Suppose that the gas-phase reactions A → B and B → A are both elementary reactions with rate constants of 4.7×10−3 s−1 and 5.8×10−1 s−1, respectively. (a) What is the value of the equilibrium constant for the equilibrium A(g) ⇌ B(g)? (b) Which is greater at equilibrium, the partial pressure of A or the partial pressure of B?
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Textbook Question
The equilibrium constant for the dissociation of molecular iodine, I2(g) ⇌ 2 I(g), at 800 K is Kc = 3.1 × 10–5. (a) Which species predominates at equilibrium I2 or I?
Textbook Question
Write the expression for Kc for the following reactions. In each case indicate whether the reaction is homogeneous or heterogeneous.
(a) 3 NO(g) ⇌ N2O(g) + NO2(g)
(b) CH4(g) + 2 H2S(g) ⇌ CS2(g) + 4 H2(g)
(c) Ni(CO)4(g) ⇌ Ni(s) + 4 CO(g)
(d) HF(aq) ⇌ H+(aq) + F-(aq)
(e) 2Ag(s) + Zn2+(aq) ⇌ 2 Ag+(aq) + Zn(s)
(f) H2O(l) ⇌ H+(aq) + OH-(aq)
(g) 2 H2O(l) ⇌ 2 H+(aq) + 2 OH-(aq)
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Textbook Question
Write the expressions for Kc for the following reactions. In each case indicate whether the reaction is homogeneous or heterogeneous.
(a) 2 O3(g) ⇌ 3 O2(g)
(c) 2 C2H4(g) + 2 H2O(g) ⇌ 2 C2H6(g) + O2(g)
(d) C(s) + 2 H2(g) ⇌ CH4(g)
(e) 4 HCl(aq) + O2(g) ⇌ 2 H2O(l) + 2 Cl2(g)
(f) 2 C8H18(l) + 25 O2(g) ⇌ 16 CO2(g) + 18 H2O(g)
(g) 2 C8H18(l) + 25 O2(g) ⇌ 16 CO2(g) + 18 H2O(l)
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Textbook Question
Write the expressions for 𝐾𝑐 for the following reactions. In each case indicate whether the reaction is homogeneous or heterogeneous.
(b) Ti(𝑠) + 2Cl2(𝑔) ⇌ TiCl4(𝑙)
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