Chemical Equilibrium: How do changes
affect the equilibrium point of a reaction?
Introduction:
It is often useful to think of a reaction as a process that consists of
two parts acting in opposite directions. From this view, a reaction begins with
all reactants and no products. The reactants then begin to interact with each
other and transform into products. The rate at which the reactants transform
into products will begin to decrease over time as the concentration of the
reactant decreases. At this point, some of the products will begin to revert
back into reactants. The rate at which the products revert back into reactants
will increase as the concentration of the product increases. There is a point,
as a result, where the forward and reverse components of a reaction are
happening at equal rates. This point is called chemical equilibrium. At
equilibrium, the rates of the forward and reverse components of the reaction
are equal but the concentrations of reactants and products are not.
Chemical equilibrium, therefore,
can be defined as the point in a reaction where the rate at which reactants
transform into products is equal to the rate at which products revert back into
reactants. The equilibrium point of a chemical reaction occurs when the
amount or concentration of the products and reactants in a closed system is
stable. Chemists use a specific property, such as color, concentration, or
density, to determine when a reaction is in equilibrium. It is important to
note, however, that chemists view the state of chemical equilibrium as dynamic
because reactants continue to transform into products and products continue to
revert back into reactants even though the amount of reactants and products in
the closed system is stable.
The equilibrium point of a
reaction can change because chemical equilibrium is not static. There are a
number of different factors that can change the equilibrium point of a reaction
by changing the rate at which reactants transform into products or by changing
the rate at which products revert back into the reactants. These factors
include a change in temperature, pressure, reactant concentration, and product
concentration. When any of these factors are changed, the equilibrium point of
the reaction will move and the concentration of products and reactants in the
system at the new equilibrium point will be different.
To control the amount of product or reactant present at the equilibrium
point of a reaction in a closed system, chemists need to understand how various
factors affect chemical equilibrium and why these various factors change the
equilibrium point of a reaction. You will therefore explore how three specific
factors affect the equilibrium point of chemical reaction. You will then
develop a conceptual model that you can use to explain your observations and
predict how the equilibrium point of a different reaction will change when the
equilibrium point is disturbed by changing these same three factors.
Your task:
Determine how changes in temperature and the addition
of extra reactant and product affect the equilibrium point of the reaction
between iron(III) nitrate and potassium thiocyanate. The guiding question of
this investigation is: Why do changes
in temperature, reactant concentration, and product concentration affect the
equilibrium point of a reaction?
1. Explain what is the goal of the experiment; what are
you trying to show?
Materials:
Consumables:
Equipment:
-iron (III)
nitrate, Fe (NO3)3 -9
test tubes
-potassium
thiocyanate, KSCN -test
tube rack
-copper (II)
chloride, CuCl2 -graduated
cylinder (10 mL)
-sodium
chloride, NaCl -pipettes
-silver
nitrate, AgNO3 -beaker
(250 mL)
-water -beakers
(250 mL) for hot and cold water baths
-ice -thermometer
-hot
plate
Procedure:
The first step in developing your model is to design and carry out three
experiments. The goal of the first experiment will be to determine how a change
in reactant concentration affects the equilibrium point of a reaction. The goal
of the second experiment will be to determine how a change in product
concentration affects the equilibrium point of a reaction. The goal of the
third experiment will be to determine how temperature affects the equilibrium
point of a reaction. For these three experiments, you will focus on the
reaction of iron(III) nitrate and potassium thiocyanate. Iron(III) ions react
with thiocyanate ions to form FeSCN2+ complex ions according to the
following reaction:
Fe3+
(aq) + SCN- (aq) ↔
FeSCN2+ (aq)
yellow colorless orange-red
To determine what type of data you need to collect,
think about the following questions:
• What type of measurements or observations will you need
to record during each experiment?
• When will you need to make these measurements or
observations?
To determine how you will collect the data, think
about the following questions:
• What will serve as your independent variable in each
experiment?
• How will you change the independent variable in each
experiment?
• What types of comparisons will you need to make?
• What will you do to reduce measurement error?
• How will you keep track of the data you collect and how
will you organize it?
To determine how you will analyze the data, think
about the following questions:
• What type of calculations will you need to make?
• What type of graph could you create to help make sense
of your data?
2. Write your procedure as a group of what steps you will
take to conduct the each of the three experiments. Label them experiment 1, 2
and 3. Detail the steps that you will take in each experiment.
3. Write out data tables for each of your three experiments including
any measurements or observations that you have made.
4. Explain in your own words what changes each of your experiments
caused to the equilibrium point of the reaction. (Explain using shifts in the
reaction, if it produced reactants or products, changes in colors and so forth.
5. Modify your models (of the three equilibrium points) from the beginning
of the laboratory and revise it to show what the effect your experimental
factors had on the reaction. (Use the paragraph below to help you with this
model)
Once you have carried out your three experiments, your group will need to
develop a conceptual model. This conceptual model will need to be able to
provide an underlying reason for your findings about the effect of temperature,
changes in reactant concentration, and changes in product concentration on the
equilibrium point of a reaction. Your model should also include an explanation
of what is happening at the submicroscopic level between and within molecules
during a reaction. The collision theory of reaction rates and the concept of
chemical equilibrium should serve as the theoretical foundation for your model.
The last step in this investigation is to test your model. To accomplish
this goal, you can use a different reaction to determine if your model leads to
accurate predictions about how the equilibrium point changes in response to
different factors. If you can use your model to make accurate predictions about
how the equilibrium point of a different reversible reaction changes, then you
will be able to generate the evidence you need to convince others that the
conceptual model you developed is valid.
You can use the reversible
formation of copper(II) complexes to test your model. When copper(II) chloride
(CuCl2) is dissolved in water, two different solutes are present in
the solution. These solutes include Cu2+ ions and Cl–
ions. These solutes interact with water molecules to form two different complex
ions. One complex ion is Cu(H2O)62+ and the
other is CuCl42–. The reversible equation for the formation
of the two complex ions is
Cu (H2O)62+
(aq) + 4Cl- (aq) ↔
CuCl42- (aq)
+ 6H2O (l)
blue
green
You can change the equilibrium point by adding NaCl, or AgNO3
or by changing the temperature of the solution. To change the concentration of
the reactants or the products, simply add 2 ml of the copper(II) chloride
solution to a test tube and then add up to eight drops of NaCl or AgNO3.
The addition of NaCl will increase the number of Cl– ions in the system. The
addition of AgNO3, in contrast, will decrease the number of Cl– ions
in the system (because the Ag+ ions react with Cl– ions to form AgCl). To
change the temperature of the system, use a hot-water bath or an ice bath.
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What are you adding:
(reactant/product)
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Prediction (color):
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Result (color):
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Addition of NaCl
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Addition of AgNO3-
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6. How does the copper chloride experiment
compare to the FeSCN reaction? What is similar (how did they shift the same
way?)? Could you predict what could change in the copper chloride from your
previous observations?
7. What is important to keep in mind when
designing your own experiment?
8. What would you do differently if you
would conduct the experiments and design the procedure again?
