General orrganic and biological chemistry structures off liffe 5th by karen timberlake10 worked examples

Sample Problem 10.1 Factors That Affect the Rate of Reaction
Indicate whether the following changes will increase, decrease, or have no effect on the rate of reaction:
a. increasing the temperature
b. increasing the number of reacting molecules
c. adding a catalyst

Solution
a. A higher temperature increases the kinetic energy of the particles, which increases the number of collisions
and makes more collisions effective, causing an increase in the rate of reaction.
b. Increasing the number of reacting molecules increases the number of collisions and the rate of the reaction.
c. Adding a catalyst increases the rate of reaction by lowering the activation energy, which increases the number
of collisions that form product.

Study Check 10.1
How does using an ice blanket on a patient affect the rate of metabolism in the body?

Answer
Lowering the temperature will decrease the rate of metabolism.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.2 Reaction Rates and Equilibrium
Complete each of the following with equal or not equal, faster or slower, change or do not change:
a. Before equilibrium is reached, the concentrations of the reactants and products ______.
b. Initially, reactants placed in a container have a _________ rate of reaction than the rate of reaction of the
products.
c. At equilibrium, the rate of the forward reaction is _________ to the rate of the reverse reaction.

Solution
a. Before equilibrium is reached, the concentrations of the reactants and products change.
b. Initially, reactants placed in a container have a faster rate of reaction than the rate of reaction of the products.
c. At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.

Study Check 10.2
Complete the following statement with change or do not change:
At equilibrium, the concentrations of the reactants and products _________.

Answer
At equilibrium, the concentrations of the reactants and products do not change.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.3 Writing Equilibrium Constant Expressions
Write the equilibrium constant expression for the following reaction:

Solution
Step 1

Write the balanced chemical equation.

Step 2

Write the concentrations of the products as the numerator and the reactants as the denominator.

Step 3

Write any coefficient in the equation as an exponent.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.3 Writing Equilibrium Constant Expressions
Continued

Study Check 10.3
Write the equilibrium constant expression for the following reaction:

Answer

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.4 Calculating an Equilibrium Constant
The decomposition of dinitrogen tetroxide forms nitrogen dioxide.

What is the numerical value of Kc at 100 °C if a reaction mixture at equilibrium contains 0.45 M N2O4 and
0.31 M NO2?

Solution
Step 1

State the given and needed quantities.

Step 2

Write the Kc expression for the equilibrium.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.4 Calculating an Equilibrium Constant
Continued
Step 3

Substitute equilibrium (molar) concentrations and calculate Kc.

Study Check 10.4
Calculate the numerical value of Kc if an equilibrium mixture contains 0.040 M NH3, 0.60 M H2, and 0.20 M N2.

Answer
Kc = 27

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.5 Calculating Concentration Using an
Equilibrium Constant
For the reaction of carbon dioxide and hydrogen, the equilibrium concentrations are 0.25 M CO 2, 0.80 M H2, and
0.50 M H2O. What is the equilibrium concentration of CO(g)?

Solution
Step 1

State the given and needed quantities.

Step 2

Write the Kc expression for the equilibrium and solve for the needed concentration.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.5 Calculating Concentration Using an
Equilibrium Constant
Continued
We rearrange Kc to solve for the unknown [CO] as follows:
Multiply both sides by [CO2][H2].

Divide both sides by [H2O].

Step 3

Substitute the equilibrium (molar) concentrations and calculate the needed concentration.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.5 Calculating Concentration Using an
Equilibrium Constant
Continued

Study Check 10.5
When the alkene ethene (C2H4) reacts with water vapor, the alcohol ethanol (C2H5OH) is produced. If an equilibrium
mixture contains 0.020 M C2H4 and 0.015 M H2O, what is the equilibrium concentration of C2H5OH? At 327
°C, the Kc is 9.0 × 103.

Answer
[C2H5OH] = 2.7 M

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Sample Problem 10.6 Using Le Châtelier’s Principle
Methanol, CH3OH, is finding use as a fuel additive. Describe the effect of each of the following changes on the
equilibrium mixture for the following reaction:
2CH3OH(g) + 3O2(g)
a.
b.
c.
d.
e.

2CO2(g) + 4H2O(g) + 1450 kJ

adding more CO2
adding more O2
increasing the volume of the container
increasing the temperature
adding a catalyst

Solution
a. When the concentration of the product CO2 increases, the equilibrium shifts in the direction of the reactants.
b. When the concentration of the reactant O2 increases, the equilibrium shifts in the direction of the products.
c. When the volume increases, the equilibrium shifts in the direction of the greater number of moles of gas, which is
the products.
d. When the temperature is increased for an exothermic reaction, the equilibrium shifts in the direction of the reactants
to remove heat.
e. When a catalyst is added, there is no change in the equilibrium mixture.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 10.6 Using Le Châtelier’s Principle
Continued

Study Check 10.6
Describe the effect of each of the following changes on the equilibrium mixture for the following reaction:
2HF(g) + Cl2(g) + 357 kJ

2HCl(g) + F2(g)

a. adding more Cl2
b. decreasing the volume of the container
c. increasing the temperature

Answer
a. When the concentration of the reactant Cl2 increases, the equilibrium shifts in the direction of the products.
b. There is no change in equilibrium mixture because the moles of reactant are equal to the moles of product.
c. When the temperature for an endothermic reaction increases, the equilibrium shifts to remove heat, which is in the
direction of the products.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.