College Prep Chemistry
Core Lab #16
50 points
Establishing Equilibrium
Objectives:
1. To be able to determine when a system is at equilibrium.
2. To compare the quantity of reactants and products at equilibrium.
3. To compare equilibrium when starting with reactants and starting with products.
4. To be able to relate Keq to a system in equilibrium
Hypothesis:

Sketch a graph that shows how the volumes of the reactants and products change until
equilibrium is reached. The reactants start at 25.0 mL and the products start at 0.0 mL.
Materials:
2 25-mL graduated cylinder
2 drinking straws with different diameters
Water (food coloring)
Procedure:
1. Label one 25-mL graduated cylinder “Reactants” and the other “Products.” Label one
straw “Forward” and the other straw “Reverse.” Be sure the two straws are different
diameters.
2. Add one drop of food coloring to the reactant cylinder. Then fill the cylinder with water
to the 25.0 mL mark.
3. Place the forward straw into the reactant cylinder so that it touches the bottom. Place
the reverse straw in the empty product cylinder.
4. Put your finger over the top of the forward straw so that liquid is trapped in the straw.
Transfer the colored water in the straw to the product cylinder.
5. Now put your finger over the reverse straw and transfer colored liquid from the product
cylinder to the reactant cylinder.
6. Record the volume of each cylinder in the data table on the next page. Each time you
transfer liquid from the reactant cylinder to the product cylinder and back from the
product cylinder to the reactant cylinder - equals one transfer.
7. Repeat steps 4, 5, and 6 until the volume in the cylinders remains constant for at least 5
readings.
8. Repeat the entire experiment, but this time begin with 25.0 mL of colored water in the
product cylinder and nothing in the reactant cylinder. Use the same straws for the
forward and reverse reactions.
Data:
Table 1: ___________________________________________
Volume
Trial 1
Transfer
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Reactant cylinder (mL)
Product cylinder (mL)
Table 2: (same table for Trial 2 but Trial 0 = 0.0 mL reactants, 25.0 mL for products)
Observations:
1. Was the larger straw used for the forward or reverse reaction?
2. At equilibrium, was there a greater volume of products or reactants? What does this tell
you about the size of the equilibrium constant?
3. How does the effect on equilibrium volumes between the two graphs compare?
Analysis:


Create a graph for each table. (two graphs total)
x-axis will be the transfer # and the y-axis will be volume of reactants & products
(there will be two lines on each graph, don’t forget to include a key)
Results:



Report time at which equilibrium was reached for each trial.
Report quantities of reactants and products at equilibrium.
Sentence format
Conclusion:

See rubric for guidelines
RST:
There are numerous chemical reactions that play a role in the formation of photochemical smog.
One component of smog is nitrogen dioxide as seen in the reversible reaction below.
2NO(g) + O2(g) ↔ 2NO2(g)
Nitrogen monoxide, a reactant, is a component of engine exhaust. Photochemical smog forms most
often in and around cities with significant automobile traffic.
In this lab, you modeled how the concentration of a system reaches equilibrium. You will analyze
your data and results (Source A), the photo on page 592 in the text (Source B), and the paragraph
above (Source C) to gather information and formulate a written explanation about why the smog
levels in a city will vary depending on the temperature.
College Prep Chemistry
Core Lab #16
50 points
Establishing Equilibrium Rubric
Format
5 points





Written in pencil (1)
Underline appropriate titles (1)
Use appropriate units throughout and significant digits (1)
Title all tables, graph, and figures (1)
Tables and graphs are drawn with straight edge (1)
Hypothesis / Introduction
10 points



Hypothesis sketched appropriately (2)
Explain how equilibrium was modeled in this lab. (3)
o Include appropriate definitions.
How will each objective be met by conducting this lab? (5).
Data /Analysis
11 points



Table1 & 2 completed appropriately (4)
Graphs sketched and labeled (sketched from calculator) appropriately (4)
Observation questions answered correctly and in sentences. (3)
Results
5 points



Report time at which equilibrium was reached for each trial.
Report quantities of reactants and products at equilibrium.
Sentence format
Conclusion
9 points


Defend the hypothesis use data support (6)
Conclusion quality: well organized and cohesive and contains no mechanical/grammatical
errors. (3)
o Well organized and cohesive but contains grammatical or mechanic (2)
o Somewhat organized but does not contain any errors (2)
o Somewhat organized with some errors (1)
o Disorganized with errors (0)
RST
10 points
 Thesis statement (2)
 Comprehension of key ideas and details (3)
o Cites convincing evidence to support analysis, showing full comprehension of
complex ideas expressed in the texts provided.
 Development of ideas and organization (2)
o Student response addresses the prompt and provides effective development of the
claim by using clear and convincing reasoning, details, and evidence.
 Appropriate integration of source material/information (3)
o Student response utilizes information in a way where the sources “speak with one
another” are used adequately to support the student’s claims and lab conclusions.