Chemistry 18.2

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Chemistry 18.2

© Copyright Pearson Prentice Hall

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Reversible Reactions and Equilibrium

In the early 1900s, German chemists refined the process of making ammonia from elemental nitrogen and hydrogen.This process allows the manufacture of nitrogen fertilizers. You will learn how reaction conditions can influence the yield of a chemical reaction.

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Reversible Reactions

Reversible Reactions

How do the amounts of reactants and products change in a chemical system at equilibrium?

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> Reversible Reactions

At chemical equilibrium, no net change occurs in the actual amounts of the components of the system.

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If the rate of the shoppers going up the escalator is equal to the rate of the shoppers going down, then the number of shoppers on each floor remains constant, and there is an equilibrium.

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A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

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> Reversible Reactions18.2

At equilibrium, all three types of molecules are present.

SO2 and O2 react to give

SO3

SO3 decomposes to SO2 and O2

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When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance called chemical equilibrium.

The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction.

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> Reversible Reactions18.2


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Animation 23

Take a close look at a generalized reversible reaction.


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> Factors Affecting Equilibrium: Le Châtelier’s Principle

Factors Affecting Equilibrium: Le Châtelier’s Principle

What three stresses can cause a change in the equilibrium position of a chemical system?

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Stresses that upset the equilibrium of a chemical system include changes in the concentration of reactants or products, changes in temperature, and changes in pressure.

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The French chemist Le Châtelier proposed what has come to be called Le Châtelier’s principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.

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Simulation 24

Simulate Le Châtelier’s principle for the synthesis of ammonia.

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Concentration

Rapid breathing during and after vigorous exercise helps reestablish the body’s correct CO2:H2CO3 equilibrium, keeping the acid concentration in the blood within a safe range.

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Temperature

Dinitrogen tetroxide is a colorless gas; nitrogen dioxide is a brown gas. The flask on the left is in a dish of hot water; the flask on the right is in ice.

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Pressure

Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium

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Conceptual Problem 18.1


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Practice Problems for Conceptual Problem 18.1

Problem Solving 18.6 Solve a similar problem with the help of an interactive guided tutorial.



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Equilibrium Constants

Equilibrium Constants

What does the value of Keq indicate about the equilibrium position of a reaction?

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> Equilibrium Constants

The equilibrium constant (Keq) is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation.

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> Equilibrium Constants

A value of Keq greater than 1 means that products are favored over reactants; a value of Keq less than 1 means that reactants are favored over products.

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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Practice Problems for Sample Problem 18.1

Problem Solving 18.7 Solve Problem 7 with the help of an interactive guided tutorial.


SAMPLE PROBLEM

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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Practice Problems for Sample Problem 18.2

Problem Solving 18.9 Solve Problem 9 with the help of an interactive guided tutorial.


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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

18.2 Section Quiz.

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18.2 Section Quiz.

1. In a reaction at equilibrium, reactants and products

a. decrease in concentration.

b. form at equal rates.

c. have equal concentrations.

d. have stopped reacting.


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18.2 Section Quiz.

2. In the reaction 2NO2(g) 2NO(g) + O2(g), increasing the pressure on the reaction would cause

a. the amount of NO to increase.

b. the amount of NO2 to increase.

c. nothing to happen.

d. the amount of O2 to increase.


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18.2 Section Quiz.

3. Changing which of the following would NOT affect the equilibrium position of a chemical reaction?

a. concentration of a reactant only

b. concentration of a product only

c. temperature only

d. volume only


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4. For the following reaction, Keq = 1.

A(g) + B(g) C(g) + D(g) Therefore, at equilibrium

a. [C] = [A].

b. [A][B] = 0.

c. [AB] = [CD] = 1.

d. [A][B] = [C][D].

18.2 Section Quiz.

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Chemistry 18.2