Equilibrium: Three Stooges in Chemical Reactions
Amina Khalifa El-Ashmawy, Ph.D. and Gezahegn Chaka, Ph.D.
Collin College
Department of Chemistry
Objectives:
• Determine K for an equilibrium reaction based on experimental data
• Design a series of experiments that demonstrate Le Châtelier’s Principle
• Write chemical equations representing shifts in equilibrium based on experimental
observations
Introduction:
Imagine an old black and white television comedy like the Three Stooges. They are deserted on
an island. One person is gathering berries and placing them in a hat while another person is
sitting there eating the berries. This process continues until the first one gets irritated and
pokes the other in the eyes. My, how funny those simple comedies were!
During rush hour on the highway, you find that at each exit and entrance, there is the same
number of cars exiting and entering such that the number of cars remaining on the highway is
constant. This is similar to filling water in a bottle that has a leak. No matter how much water
you put in it, it never gets filled.
Opposing processes that occur at the same rate happen not only on the macroscopic level but
also at the molecular level in chemical reactions. Many reactions are reversible. For example,
you can have nitrogen reacting with hydrogen to produce ammonia.
N2(g) + 3H2(g) → 2NH3(g)
(1)
Under certain conditions, the reverse reaction will take place.
2NH3(g) → N2(g) + 3H2(g)
(2)
If we enter ammonia gas in a sealed vessel at a certain temperature and allow it to react,
reaction (2) will proceed. Once enough nitrogen and hydrogen are produced, they will react
according to reaction (1) to produce ammonia. We now have the same situation as with rush
hour traffic where the reactants are reacting as quickly as they are being produced by the
reverse reaction. Effectively at this point, we observe no change in concentration of all species
in the reaction. At the molecular level, however, there is constant reaction taking place in both
directions. At this point, when the rate of the forward reaction is the same as the rate of the
Copyright ©2017 Amina K. El-Ashmawy and Gezahegn Chaka
reverse reaction, the system is considered to be at equilibrium. The process can be written as
one reaction with opposing half arrows as follows:
N2(g) + 3H2(g) ⇄ 2NH3(g)
(3)
In the combined reaction (3), the “reactants” and “products” are those for the forward reaction
as written.
Equilibrium Constant Expression and the Reaction Quotient:
Since there is no change in concentration for reactants or products at chemical equilibrium, we
can write an expression of the quotient of product over reactant concentrations, which will be
constant.
For the general reaction aA + bB ⇄ cC + dD, where a, b, c, and d are the stoichiometric
coefficients for the balanced equation, the specific equilibrium constant expression is:
For reaction (3) above, the equilibrium constant expression would be
If the reaction is not yet at equilibrium, we can still write the quotient of product over reactant
concentrations at any point in the reaction. However, it is called the reaction quotient, Q, and
not the equilibrium constant.
Chemical Information Obtained from the Equilibrium Constant:
When the value of the equilibrium constant is large, say much greater than 1, the numerator in
the quotient is far greater than the denominator. From a chemistry standpoint, this means that
the concentration of the products is greater than the concentration of reactants. Hence, for that
chemical reaction at the given temperature, the products (or forward reaction) are favored. The
reverse reaction is minimal compared to the forward reaction.
Copyright ©2017 Amina K. El-Ashmawy and Gezahegn Chaka
When the value of the equilibrium constant is small, the reactants, or reverse reaction, are
favored at that temperature. The forward reaction is minimal compared to the reverse reaction.
It is important to know that the equilibrium constant value is dependent on temperature. If
temperature changes, the value of the equilibrium constant will change depending on whether
the reaction is endothermic or exothermic.
Putting Stress on the Equilibrium System:
Consider a reaction that has reached equilibrium. What happens if the system is disturbed, say,
a product is removed? Since we changed the concentration of one of the species in the reaction,
the ratio of product to reactant concentrations is no longer what it needs to be for equilibrium
at that temperature.
Let’s use the reaction quotient to help us figure out what will happen.
When we remove a product, the reaction quotient value will be smaller than the value of the
equilibrium constant. To reestablish equilibrium, the system must shift to make more
products. In other words, the forward reaction rate will increase to produce products faster
than the reverse reaction is occurring. This causes the system to go back to equilibrium, regain
the necessary product to reactant concentration quotient. This shift to reestablish equilibrium
is an example of Le Châtelier’s Principle, which states that when a stress is placed on a
system at equilibrium the system will shift to relieve the stress and reestablish equilibrium.
Possible forms of stress include change in concentration of one or more of the species in
reaction, change in pressure, and change in volume.
Change in temperature will affect the equilibrium position. When a reaction at equilibrium is
heated, the forward reaction rate will be affected differently than the reverse reaction rate. The
system will reach a point where the concentrations of all species remain constant, but the value
of the equilibrium constant ends up changing since the concentrations of reactants and
products shifted entirely. For an endothermic reaction, increasing temperature will cause the
forward reaction to produce more products before equilibrium is reached. The value of K will
increase with increased temperature. For an exothermic reaction, increasing temperature will
cause the reverse reaction to produce more reactants before equilibrium is reached. The value
of K will decrease with increased temperature.
The Experiment:
Part 1. Determination of an equilibrium constant at room temperature
The equilibrium constant, K, can be determined for the system
Fe3+(aq) + SCN-(aq) ⇄ FeSCN2+(aq)
Copyright ©2017 Amina K. El-Ashmawy and Gezahegn Chaka
by determining the concentrations of Fe3+, SCN-, and FeSCN2+ present in a mixture at
equilibrium. One method of determining the concentration of FeSCN2+ is by absorbance
measured at 468 nm. According to Beer’s law, the absorbance of a colored solution is directly
proportional to the concentration of the colored species. Therefore, A = εbC where A is
absorbance, ε is the molar absorptivity of FeSCN2+ (7260 M-1cm-1), b is the [reactants]
[products] path length in cm (can be measured), and C is the molar concentration of FeSCN2+.
The molar concentration of FeSCN2+ can be calculated as C = A/εb and the equilibrium
concentrations of Fe3+ and SCN- can be calculated by applying the principle of chemical
equilibrium. (HINT: Consider setting up a RICE table.)
Design an experiment to calculate the value of the equilibrium constant for Fe 3+(aq) + SCN(aq) ⇄ FeSCN2+(aq) system using 2.0 x10-3 M iron (III) nitrate (in 1.0 M HNO3) and 2.0 x10-3
M potassium thiocyanate solutions. Prepare a 10.0 mL solution of iron (III) thiocyanate
complex by mixing measured volumes of the 2.0 x10-3 M iron (III) nitrate and 2.0 x10-3 M
potassium thiocyanate solutions. Measure the absorbance of the resulting equilibrium mixture
at 468 nm and calculate the value of the equilibrium constant, K.
Repeat two more times and calculate the average value and average deviation for the
equilibrium constant.
Part 2. Demonstration of the Le Châtelier’s Principle
Design a series of experiments that demonstrate the Le Châtelier’s Principle using the reaction:
Fe3+ + SCN- ⇄ FeSCN2+, and the following reagents and conditions: 0.05 M solutions of
Fe(NO3)3, KSCN, AgNO3, HCl, Hg(NO3)2, Na3PO4, Na2C2O4, NaNO3, KNO3, solid NaF, ice bath,
and water bath.
Determine which reagents or reaction conditions shift the equilibrium to the right (products)
and to the left (reactants) as well as those that do not cause a shift.
Prelab Questions:
1. Does it matter what initial concentration of reactants and products are used to
determine the equilibrium constant?
2. What equipment, materials or glassware do you need to carry out this experiment?
3. How do you know that equilibrium is reestablished after a stress is applied to the
system?
4. When is the system or reaction said to be at equilibrium?
5. Provide your own example of Le Châtelier’s Principle applied in an area other than
chemistry.
Copyright ©2017 Amina K. El-Ashmawy and Gezahegn Chaka
6. Which reagents or reaction conditions listed above can change the value of the
equilibrium constant?
Critical Data and Discussion to Include in the Lab Report:
• Provide explanation, including balanced chemical equations, for the reactions involving
reagents or reaction conditions that caused a shift towards products.
• Provide explanation, including balanced chemical equations, for the reactions involving
reagents or reaction conditions that caused a shift towards reactants.
• Provide explanation for the reagents or reaction conditions that caused no shift in the
equilibrium.
• Explain whether the reaction is endothermic or exothermic based on observations and
chemical equation.
• References cited
Reference:
F. Nyasulu and R. Barlag, Journal of Chemical Education, Vol. 88, No. 3, March 2011.
Copyright ©2017 Amina K. El-Ashmawy and Gezahegn Chaka