1. Which describes a solution that contains a system at equilibrium? One in which the color
of the solution is changing slowly, or one in which the color is not changing. Explain your
answer.
A solution that contains a system at equilibrium is one in which the colour of the solution is
not changing. At equilibrium the concentrations of reactants and products will not change
because the rates of forward and reverse reactions become equal. At this state, the physical
properties such as density, state and color become constant. This shows that the solution in
which color is not changing is at equilibrium and indicating that there is no net change in the
system.
2. The following equilibrium is established when copper ions and bromide ions are placed in
solution.
heat + Cu (H2O)6+2(aq) + 4 Br-(aq) ------ 6 H2O (l) + CuBr4-2 (aq)
The tube on the left contains only copper sulfate dissolved in solution. The tube on the right
is the result of adding some potassium bromide solution. Given that the Cu (H2O)6+2 ion is
blue and that the CuBr4-2 ion is green, answer the questions below.
a) What happened to the concentration of each of the ions when the KBr was added?
When KBr is added, more Br- is produced hence disturbing the equilibrium. As a result, the
forward reaction is favoured.
So, the concentration of Br- and CuBr4-2 will be increased
The concentration of Cu (H2O)6+2 will get decreased
b) Explain why the solution changed color.
The color of the solution changed as the Br^- ions were added on the reactants side. So, the
concentration of blue Cu (H2O)6+2 ions decreased, while that of green CuBr4-2 ions will get
increased and therefore the change in color of the solution will be observed with naked
eyes.
c) Would the tube feel hot or cold, or the same, when the KBr was added?
Adding KBr favours’ forward reaction. The tube will feel cold because heat will be used up.
3. Consider the following equilibrium: Mg (OH)2(s) - Mg+2(aq) + 2 OH- (aq)
The tube on the left contains Mg (OH)2(s) and water. A chemical has been added to cause
the change shown in the tube on the right. Suggest a possibility for what chemical could
have been added. Explain your answer.
The removal of OH- and Mg2+ from the reaction would make it proceed to the right. In this
case, adding an acid will remove the OH-. As a result, magnesium hydroxide will be forced to
dissolve to replace some of the OH- which had been removed by the acid. This is according
to Le Chatlier's principle which states that if a system of equilibrium is subjected to some
changes, the equilibrium will be shifted towards the side as a way of opposing the change.
4. Methanol has the formula CH3OH and can be produced by the reaction of carbon
monoxide with hydrogen gas as follows: CO + 2 H2CH3OH + heat
In an attempt to maximize the yield of methanol (amount of methanol produced), a chemist
would try to shift the equilibrium as far to the right as possible. Which of the following
would accomplish this? Explain your answer.
(a) heating the mixture
No change in the equilibrium
(b) adding an excess of carbon monoxide
The equilibrium shifts to the right because of increasing the product side.
(c) removing the methanol as it is formed
Yes, the reaction shifts to the right as it seeks to reach equilibrium (due to the removed
methanol formed)
(d) adding a substance that reacts with carbon monoxide
No change in the equilibrium
C. Post lab Questions: Answer these questions after completing the experiment using your
observations to help.
You can either write your answers using your MS-word editor (or equivalent), in the relevant
spaces below OR print this page, write your answers by hand, and take a picture of your
answers to submit on Canvas.
1. Explain the effect of AgNO3 on the cobalt system (experiment I). What did you observe
and why?
AgNO3 will provide Ag+ which will react with Cl- from the original reaction and form AgCl,
which is a white precipitate. Therefore, the equilibrium will be pushed to the right.
2. Predict what would happen if the ammonium system (experiment II) was heated. What
would you observe and why?
The equilibrium would shift to the right so as to dissipate the heat. This makes the
concentration of H2O and that of NH3 to increase, while that of OH- and NH4- to decrease.
3. Predict what would happen if you cooled the iron thiocyanate system (experiment III).
What would observe and why?
Cooling iron thiocyanate systems would shift the equilibrium to the right. The concentration
of the product would be higher, while that of the reactants would be lowered. Cooling
means a heat deficit is created, and therefore the forward reaction would be favoured to
replace the lost heat.
4. When the chromate system (experiment IV) is heated the tube is observed to go pale
yellow. What does this tell you about the enthalpy of the forward reaction as written
below?
2 CrO4-2 + 2 H+ ---- Cr2O7-2 + H2O
In this case, the enthalpy of the reaction would be positive.
5. For experiment V., when the pressure of the tube containing gaseous NO2 and N2O4 is
increased the tube goes colourless. Explain why.
An increase in pressure favours’ the forward reaction. Increasing pressure shifts the reaction
to the side with a smaller number of moles. N2O4 has less moles as compared to NO2.
6. Consider the following equilibrium. Cu+2(aq) + 4 NH3(aq) Cu (NH3)4+2 (aq)
Look at the images of a test tube below. The copper ion in solution is light blue. The Cu
(NH3)4+2 ion is a deep blue. The tube on the left contains only copper ions.
(a) Identify what chemical was added to the tube on the left to produce the results shown in
the centre tube. Explain your answer in terms of how this affected the equilibrium.
The chemical added was excess ammonia. Addition of one of the reactants shifts the
equilibrium towards the right side.
(b) Identify what chemical may have been added to the centre tube to produce the results
shown in the tube on the right. Explain your answer.
The chemical added is EDTA. The chemical forms (Cu (EDTA)2-, which is pale blue.
7. The barium ion, Ba2+, is toxic to humans. However, barium sulfate is commonly used as
an image enhancer for gastrointestinal x-rays. What does this imply about the position of
the equilibrium shown here: BaSO4(s) Ba+2(aq) + SO4-2(aq)
Explain your answer.
Since barium sulfate is non-toxic, the equilibrium should always be towards the left. Ba2+ is
toxic and this means that if the equilibrium shifts to the right, Ba 2+ would lead to death of
people.
8. Haemoglobin (Hb) and oxygen gas form a complex (HbO2) that carries oxygen throughout
the human body. Unfortunately, carbon monoxide also binds to haemoglobin so that an
equilibrium is established as follows: HbO2 + CO HbCO + O2
Carbon monoxide poisoning occurs when the concentration of HbO2 in the blood is
reduced.
The first aid for a person suffering from carbon monoxide poisoning is to (1) move them to
an area of fresh air, and (2) administer oxygen. Using the principles of equilibrium, explain
how each of these helps to restore the HbO2concentration.
The administration of oxygen leads to an increase of the level of oxygen in the body and
therefore shifting the reaction to the reactants side. Also, removing them in areas with fresh
air helps in reducing the CO in the system hence shifting the reaction to the reactants side.