Book cover for Chemistry The Science in Context

Chemistry The Science in Context

Thomas R. Gilbert

ISBN #9780393615142

5th Edition

2,675 Questions

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191,124 Students Helped

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter emphasizes that chemical equilibrium is not a static state but a dynamic one where the forward and reverse reactions occur at equal rates. Key topics include the equilibrium constant (K), how to calculate and interpret it using the law of mass action, and the use of the reaction quotient (Q) to understand system shifts. Tools like RICE tables help in systematically determining equilibrium concentrations, while Le Châtelier’s principle provides insights into how changes in system conditions affect the equilibrium position. Understanding these fundamental concepts is crucial for applications in industrial and natural systems alike.

Learning Objectives

1

Explain the concept of chemical equilibrium as a dynamic balance between forward and reverse reactions.

2

Calculate and interpret the equilibrium constant (K) and understand its application using the law of mass action.

3

Utilize tools such as the reaction quotient (Q) and RICE tables to predict system behavior under various perturbations.

4

Apply Le Châtelier’s principle to determine how changes in concentration, pressure, volume, and temperature affect the equilibrium position.

5

Differentiate between equilibrium constants for homogeneous and heterogeneous systems, including the relationships between Kc and Kp.

Key Concepts

CONCEPT

DEFINITION

Chemical Equilibrium

A dynamic state in which the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant over time, even though reactions continue to occur at the molecular level.

Equilibrium Constant (K)

A quantitative measure of the ratio of the concentrations (or partial pressures) of products to reactants at equilibrium, derived from the law of mass action.

Reaction Quotient (Q)

A ratio similar to the equilibrium constant but calculated using the current concentrations of reactants and products; it helps predict the direction in which a system not at equilibrium will shift.

RICE Tables

A systematic method of organizing the Initial concentrations, the Change that occurs as the reaction approaches equilibrium, and the Equilibrium concentrations to solve equilibrium problems.

Le Châtelier’s Principle

A qualitative guideline that predicts how an equilibrium system responds to disturbances (changes in concentration, pressure, volume, or temperature) by shifting to counteract the changes.

Heterogeneous Equilibria

Equilibria involving more than one phase (e.g., gas-solid, liquid-solid) where the equilibrium constant expression is written only in terms of the concentrations or pressures of species in a single phase.

Kc vs. Kp

Kc refers to equilibrium constants expressed in terms of molar concentrations, while Kp refers to those expressed in terms of partial pressures; their relationship depends on the reaction stoichiometry and temperature.

Example Problems

Example 1

Consider the graph of concentration versus time in Figure $P 14.1$ a. What is the mass action expression for the reaction? b. What is the value of $K_{c} ?$ (FIGURE CANNOT COPY)

Example 2

In Figure $\mathrm{P} 14.2,$ the red spheres represent reactant A and the blue spheres represent product B in equilibrium with A. a. Write a chemical equation that describes the equilibrium. b. What is the value of the equilibrium constant $K_{\mathrm{c}} ?$ (FIGURE CANNOT COPY)

Example 3

The equilibrium constant $K_{c}$ for the reaction $$\text { A (red spheres) }+\mathrm{B} \text { (blue spheres) } \rightleftharpoons \mathrm{AB}$$ is 3.0 at $300.0 \mathrm{K} .$ Does the situation depicted in Figure P14.3 correspond to equilibrium? If not, in what direction (to the left or to the right) will the system shift to attain equilibrium? (FIGURE CANNOT COPY)

Example 4

The diagrams in Figure P14.4 represent equilibrium states of the reaction $$ \text { A (red spheres) }+\text { B (blue spheres) } \rightleftharpoons \mathrm{AB} $$ at $300 \mathrm{K}$ and $400 \mathrm{K},$ respectively. Is this reaction endothermic or exothermic? Explain. (FIGURE CANNOT COPY)

Example 5

Does the reaction $A \rightarrow 2$ B represented in Figure $P 14.5$ reach equilibrium in $20 \mu$ s? Explain your answer. (FIGURE CANNOT COPY)
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Step-by-Step Explanations

QUESTION

Given a reaction aA + bB ⇌ cC + dD, how do you calculate the equilibrium constant K?

STEP-BY-STEP ANSWER:

Step 1: Write the balanced chemical equation.
Step 2: Write the equilibrium constant expression based on the law of mass action: K = ([C]^c [D]^d) / ([A]^a [B]^b).
Step 3: Insert the equilibrium concentrations of the reactants and products into the expression.
Step 4: Calculate the value of K from the expression.
Final Answer: K is determined as the ratio of product concentrations raised to their stoichiometric coefficients to the reactant concentrations raised to their stoichiometric coefficients.

Equilibrium Constant (K)

QUESTION

How do you determine the direction in which a reaction will shift by comparing Q to K?

STEP-BY-STEP ANSWER:

Step 1: Write the expression for the reaction quotient, Q, similar to the equilibrium constant expression.
Step 2: Substitute the current concentrations (or pressures) into the Q expression.
Step 3: Compare Q to the equilibrium constant K.
Step 4: If Q < K, the reaction will shift towards the products to reach equilibrium; if Q > K, it will shift towards the reactants.
Final Answer: The comparison of Q and K predicts the direction of the shift required to attain equilibrium.

Reaction Quotient (Q)

QUESTION

What will be the effect on the equilibrium if additional reactant is added to the system?

STEP-BY-STEP ANSWER:

Step 1: Recognize that adding more reactant increases its concentration.
Step 2: According to Le Châtelier’s principle, the system will respond by shifting the equilibrium to decrease the disturbance, favoring the forward reaction to produce more products.
Step 3: Conclude that the equilibrium position shifts to the right.
Final Answer: Adding a reactant causes the reaction to shift toward the products.

Le Châtelier’s Principle

QUESTION

How can you use a RICE table to determine equilibrium concentrations?

STEP-BY-STEP ANSWER:

Step 1: List the initial concentrations of all reactants and products.
Step 2: Write down the changes that occur as the reaction proceeds towards equilibrium, using variables to represent unknown changes.
Step 3: Write the equilibrium concentrations by combining the initial values and the changes.
Step 4: Substitute these equilibrium expressions into the equilibrium constant expression and solve for the unknown variables.
Final Answer: RICE tables allow systematic calculation of how quantities change until equilibrium is reached.

RICE Tables

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Common Mistakes

  • Believing that equilibrium means the reaction has stopped rather than being dynamic.
  • Confusing the equilibrium constant (K) with the rate constant.
  • Incorrectly setting up the equilibrium expression, especially forgetting to raise concentrations to their stoichiometric coefficients.
  • Mixing up the reaction quotient (Q) with the equilibrium constant (K) and misinterpreting their relationship.
  • Overlooking the role of different phases in heterogeneous equilibria when forming the equilibrium constant expression.