5. How Do You Determine the Reaction Mechanism of a Reaction?

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5. How do you determine the reaction mechanism of a reaction?

Reaction Mechanisms

6. A(n) reaction is one in which reactants are converted to products
in a single step.

7. What is a reaction progress curve?

8. Is the following sentence true or false? A reaction mechanism includes all of the
elementary reactions of a complex reaction.

9. What is an intermediate of a reaction?

10. What determines the rate of a multi-step chemical reaction?

11. Look at Figure 18.11. What is one difference between this graph and the chemical
equation for this reaction?

18.3 Reversible Reactions
and Equilibrium

All reactions are reversible. Reactants go to products in the
forward direction. Products go to reactants in the reverse direction.

Reading Strategy

Cause and Effect A cause and effect chart is a useful tool when you want to describe
how, when, or why one event causes another. As you read, draw a cause and effect chart that
shows the relationship between stresses on a chemical system at equilibrium and the shift in
the equilibrium of the system.

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As you read Lesson 18.3, use the cause and effect chart below. List two changes in
concentration that result in a shift in equilibrium toward more product.

EXTENSION Add another cause to the cause and effect chart that shows how a change in
pressure results in more product in this reaction: N2(g) + 3H2(g) → 2NH3(g).

Lesson Summary

Reversible Reactions A reversible reaction is a chemical reaction that does not go to
completion and can occur in both the forward and reverse directions.

During a reversible reaction, reactants form from products at the same time products
form from reactants.

When forward and reverse reactions occur at the same rate, equilibrium exists.

Factors Affecting Equilibrium: Le Châtelier’s Principle Any stress on an
equilibrium causes the equilibrium to shift in the direction that releases the stress.

A change in concentration of any reactant or product will shift the equilibrium.

Changing the temperature of an equilibrium causes the equilibrium to shift.

Changes in pressure or volume change the equilibrium of reactions that involve gases.

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Equilibrium Constants Equilibrium constants can be used to calculate concentrations
and solubilities.

An equilibrium constant is the ratio of product concentrations to reactant concentrations
at equilibrium.

The equilibrium constant is represented by Keq.

When the equilibrium constant is greater than one, formation of products is favored.
When it is less than one, formation of reactants is favored.

Sample Problem How many significant figures are in 23500?

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Sample Problem How many significant figures are in 0.00075?

Now it’s your turn to identify significant figures in the numbers below.

1. 0.0145009

2. 34.9

3. 600.0

4. 200.000

Significant Figures in Calculations

Multiplication or Division The number of significant figures in the answer is the same
as that in the quantity with the smallest number of significant figures.

For example, suppose you divide 25.624 grams by 25 mL. The answer should have only
2 significant figures because 25 mL has the smallest number of significant figures. The
answer is 1.0 g/mL, not 1.0000 g/mL or 1.000 g/mL.

Addition and Subtraction The number of decimal places in the answer is equal to the
number of decimal places in the quantity with the smallest number of decimal places.

For example, suppose you add 14.16 + 3.2. You get 17.36, but this is not the final answer.
Since the smallest number of decimal places is 1, you have to round to get only 1 decimal
place. The correct answer is 17.4.

Now it’s your turn to do some calculations with significant figures. Be sure that each answer is
expressed to the correct number of significant figures.

5. 46.6 kg + 5.72 kg =

6. 4.62 × 0.19 =

For questions 7−9, write the final answer and explain your reasoning.

7. 22.37 cm × 3.10 cm × 85.75 cm = 5946.50525 cm3

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8. 91.68 mL − 19.1 mL = 72.50 mL

9. 1.473 ÷ 2.6 = 0.5665

After reading Lesson 18.3, answer the following questions.

Reversible Reactions

10. What happens in a reversible reaction?

11. Is the following sentence true or false? Chemical equilibrium is a state in which the
forward and reverse reactions take place at different rates.

12. The equilibrium position of a reaction is given by the relative of
the system’s components at equilibrium.

13. Fill in the missing labels on the diagram below with either the words at equilibrium
or not at equilibrium. At equilibrium, how many types of molecules are present in the
mixture?

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14. Use Figure 18.13 to answer these questions.

a. Which graph, left or right, shows an initial concentration of 100% SO3 and no SO2?

b. Compare the initial concentrations of the substances shown in the other graph.

c. What is the favored substance at equilibrium? How can you tell?

Factors Affecting Equilibrium: Le Châtelier’s Principle

15. What is Le Châtelier’s principle?

16. Circle the letters of the terms that complete the sentence correctly. Stresses that upset
the equilibrium of a chemical system include changes in .

17. When you add a product to a reversible chemical reaction, the reaction is always pushed
in the direction of . When you remove a product, the reaction is
pulled in the direction of .

18. Is the following sentence true or false? Increasing the temperature of a chemical
reaction causes the equilibrium position of a reaction to shift in the direction that
absorbs heat.

19. How does increasing pressure affect a chemical system?

20. Decreasing the pressure on the system shown in Figure 18.16 results in a shift of
equilibrium to favor .

Equilibrium Constants

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

22. What are the exponents in the equilibrium-constant expression?

23. What do the square brackets indicate in the equilibrium-constant expression?

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24. Is the following sentence true or false? The value of Keq for a reaction depends on the
temperature.

25. A value of Keq greater than one means that are favored over
. A value of Keq less than one means that
are favored over .

18.4 Solubility Equilibrium

Equilibrium between ions in solution and undissolved compounds
reflect the solubility of an ionic compound.

Lesson Summary

The Solubility Product Constant The solubility product constant of an ionic compound
reflects the number of ions the compound forms in solution.

Even ionic compounds that appear to be insoluble produce some ions in solution.

The solubility product constant, Ksp, reflects only the concentrations of the dissolved
ions raised to the power that is the coefficient of that ion in the equation.

Ksp = [A+]a[ B−]b

The Common Ion Effect The common ion effect is a shift in equilibrium that happens
because the concentration of an ion that is part of the equilibrium is changed.

A common ion is an ion that is found in more than one compound in solution.

The addition of a common ion to a solution reduces the solubility of the dissolved
compound.

A precipitate forms if the concentration of two ions in solution is greater than the Ksp
for the compound formed from the ions.

After reading Lesson 18.4, answer the following questions.

The Solubility Product Constant

1. What is the solubility product constant (Ksp)?

2. Look at Table 18.1. Which ionic compounds are exceptions to the general insolubility of
carbonates, phosphates, and sulfites?

3. Look at Table 18.2. Which salt is more soluble in water, silver bromide (AgBr) or silver
chromate (Ag2CrO4)?

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