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Ch.15 - Chemical Equilibrium
Brown - Chemistry: The Central Science 14th Edition
Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232Not the one you use?Change textbook
All textbooksBrown 14th EditionCh.15 - Chemical EquilibriumProblem 19
Chapter 15, Problem 19

Which of the following statements are true and which are false? (a) The equilibrium constant can never be a negative number. (b) In reactions that we draw with a single-headed arrow, the equilibrium constant has a value that is very close to zero.

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Step 1: Understand the concept of the equilibrium constant (K). It is a ratio of the concentrations of products to reactants, each raised to the power of their stoichiometric coefficients, at equilibrium. The equilibrium constant is always a positive number because concentrations cannot be negative.
Step 2: Analyze statement (a): 'The equilibrium constant can never be a negative number.' Since concentrations are always positive, the equilibrium constant, which is a ratio of these concentrations, must also be positive. Therefore, statement (a) is true.
Step 3: Consider the meaning of a single-headed arrow in chemical reactions. A single-headed arrow indicates a reaction that goes to completion, meaning the reactants are fully converted to products, and the reverse reaction is negligible.
Step 4: Analyze statement (b): 'In reactions that we draw with a single-headed arrow, the equilibrium constant has a value that is very close to zero.' For reactions that go to completion, the equilibrium constant is actually very large, not close to zero, because the concentration of products is much higher than that of reactants at equilibrium.
Step 5: Conclude that statement (b) is false because a large equilibrium constant indicates a reaction that favors the formation of products, not a value close to zero.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Equilibrium Constant

The equilibrium constant (K) is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a reversible chemical reaction. It is always a positive value because it is derived from concentrations, which cannot be negative. A K value greater than 1 indicates that products are favored, while a value less than 1 indicates that reactants are favored.
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Equilibrium Constant K

Single-Headed Arrow in Reactions

A single-headed arrow in a chemical equation indicates a one-way reaction, meaning that the reactants convert to products without reverting back. In such cases, the reaction does not reach equilibrium, and thus the concept of an equilibrium constant does not apply. Therefore, the value of K for these reactions is not defined in the same way as for reversible reactions.

Equilibrium vs. Non-Equilibrium Reactions

Equilibrium reactions are characterized by the ability to reach a state where the rates of the forward and reverse reactions are equal, allowing for the calculation of an equilibrium constant. In contrast, non-equilibrium reactions, indicated by a single-headed arrow, do not reach this state, and thus the concept of an equilibrium constant is irrelevant. Understanding this distinction is crucial for interpreting the behavior of chemical reactions.
Related Practice
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) β‡Œ TiCl4(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.

(a) 2 O3(g) β‡Œ 3 O2(g)

(c) 2 C2H4(g) + 2 H2O(g) β‡Œ 2 C2H6(g) + O2(g)

(d) C(s) + 2 H2(g) β‡Œ CH4(g)

(e) 4 HCl(aq) + O2(g) β‡Œ 2 H2O(l) + 2 Cl2(g)

(f) 2 C8H18(l) + 25 O2(g) β‡Œ 16 CO2(g) + 18 H2O(g)

(g) 2 C8H18(l) + 25 O2(g) β‡Œ 16 CO2(g) + 18 H2O(l)

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Textbook Question

Consider the following equilibrium: 2 H2(g) + S2(g) β‡Œ 2 H2S(g) Kc = 1.08 Γ— 107 at 700Β°C (b) Does the equilibrium mixture contain mostly H2 and S2 or mostly H2S?