Chemistry 2
Chapter 18 Chemical Equilibrium
pp 589-612 Note: We are skipping Section 4.
Review Assignment: P 627 (1-10)
Lab: # Establishing Equilibrium
Objectives. Answer objectives on this paper.
1. Describe a reversible reaction.
2. Define chemical equilibrium and explain how it is achieved. Tell how the rates of the
forward and reverse reactions change throughout a reaction.
3. Determine the equilibrium constant for a given reaction.
• Write the equilibrium expression for the reaction aA + bB <==> cC + dD
4.
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•
•
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Write the equilibrium expression for the oxidation of hydrogen to form water vapor.
•
Write the equilibrium expression for the when dinitrogen tetroxide is produced from
nitrogen dioxide.
Analyze the extent of a reaction from its equilibrium constant.
If Keq = 0.005, will the reactants or the products be favored?
If Keq = 500, will the reactants or the products be favored?
If Keq = 1, will the reactants or the products be favored?
5. Define reaction quotient and use it to determine if the reaction is at equilibrium. Note:
This is not in the text.
•
For the reversible reaction where nitrogen and hydrogen form ammonia, Keq is 0.105.
Calculate Q for the following concentrations: [NH3] = 0.15M; [N2] = 0.0020 M; and [H2] =
0.10 M. Then tell in which direction the reaction will proceed.
6. Explain and apply LeChatelier’s Principle.
7. Describe how changes in concentration, pressure, and temperature affect a reaction at
equilibrium.
8. Relate the Haber process to LeChatelier’s principle.
9. Discuss conditions under which reactions go to completion.
10. Explain the concept of acid-ionization constants and write acid-ionization equilibrium
expressions. Write acid ionization expressions for hydrofluoric acid and nitrous acid.
11. Calculate the pH, pOH, [H3O+], and [OH- ] of a weak acid or weak base using equilibrium
concentrations and Ka or Kb .
Examples: (a) Calculate the pH of a 0.10 M solution of acetic acid. Ka = 1.8 x 10-5
(b) Calculate the pH of a 0.15 M solution of ammonia. Kb = 1.8 x 10 –5
12. Explain how buffers work. Give 2 examples of buffers.
13. Compare cation and anion hydrolysis.
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For each reaction the follows fill out the table.
Conditions:
a. Temperature is increased.
b. Pressure is decreased.
c. Catalyst is added.
d. Concentration of underlined substance is increased.
Note: Assume all species are gases unless otherwise noted.
Homogeneous
What happens to K if
What happens to
direction of reaction if
(1)
(Use ↑, ↓or 0)
or
(Use ←, →, or 0)
Heterogeneous Keq expression
a
b
c
d
a
b
c
d
(2)
1.
2.
3.
4.
5.
6.
7.
8.
9.
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Lab: Establishing Equilibrium
Problem: How can you determine when a system is at equilibrium?
Materials (per group)
2 - identical 25-mL graduated cylinders
2 drinking straws with different diameters
food coloring (optional)
Either graph paper and pencil or Logger-Pro to process data
Procedure:
1. Label one cylinder “R” for reactants and one cylinder “P” for products. Label one straw
“F” for forward and one straw “R” for reverse.
2. If you want to, add a few drops of food coloring to the reactant cylinder. Fill cylinder to the
25.0 mL mark. Record volumes in your data table. Call this table Data Table 1.
3. Place the forward straw in the reactant cylinder and the reverse straw in the empty product
cylinder.
4. Place your finger over the top of the forward straw. Transfer the water to the product
cylinder.
5. Put your finger over the reverse straw and transfer liquid back to the reactant cylinder.
6. Record the volume of each cylinder in your data table. Each time you complete both
transfers (to product and to reactant) record your data.
7. Repeat steps 4, 5, and 6 until the volumes in both cylinders remain constant for at least 5
readings. Call this data table Data Table 2.
8. Repeat the entire experiment, but this time begin with the 25.0 mL of water in the product
cylinder and nothing in the reactant cylinder. Leave the straws in their respective
cylinders.
Observations and Data Table This table may have more than 30 transfers. You will need two of
tables that are similar to this.
Transfer
Volume
#
Reactant
Product
Cylinder
Cylinder
(mL)
(mL)
0
1
2
3
Post Lab questions:
1. Did you use the larger straw for the forward or reverse reaction?
2. At equilibrium, was there a greater volume of products or reactants?
3. What was the effect on equilibrium volumes when you started with no reactants and 25.0
mL of products?
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4. Did it take longer (more transfers to achieve equilibrium) for one of the experiments? If so,
tell whether it was the forward reaction or the reverse reaction and tell which straw
diameter was larger. Example: It took more transfers to achieve the ______ (forward or
reverse) reaction. The ______ (forward or reverse) straw was __________ (larger or
smaller) than the other straw.
5. Graph your results for both experiments. If you use Logger Pro, be sure to label graphs
correctly and tape them in the lab book on the opposite side of your lab notebook. If you
use graph paper, be sure to label your graphs, print them, and tape them in the opposite side
of the lab book.
6. On each graph, draw a line from the x-axis upward where equilibrium was established.
Clearly identify to the right of this vertical line which line you graphed represents products
and which represents reactants. Also indicate whether the forward straw was larger or
smaller in diameter than the reverse straw on each graph.
7. Write an equilibrium expression for your first graph and tell whether it is >, <. or = 1.
Explain how you determined this.
8. What variable determined whether there was a greater volume of reactants or products at
equilibrium?
Prelab Questions
1. What is a reversible reaction?
2. Describe chemical equilibrium.
3. Do the forward and reverse reactions of a reversible reactions stop when the equilibrium is
reached? Explain.
4. Below is a graph where the equilibrium concentrations of products and reactants are equal.
Draw a similar graph where the equilibrium concentrations of products are less than those
of the reactants. Would Keq be >, <, or = 1 for the graph you drew?
5. What information does the equilibrium constant reveal about a reversible reaction?
6. What is the difference between heterogeneous and homogeneous equilibria? Write one
equation showing heterogeneous equilibrium and one equation showing homogeneous
equilibrium.
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