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Physical Sciences P2 Learner’s GuideExam School 2012
www.learnxtra.co.za
CHEMICAL EQUILIBRIUM
STUDY NOTES
What is Equilibrium?
Reactions that take place in both the forward and reverse directions simultaneously are
called reversible reactions.
Observable macroscopic changes stop, while microscopic changes continue as reactants
change to products, and products change back into reactants.
When the rate of the forward reaction equals the rate of the reverse reaction, we say a state
of dynamic equilibrium has been reached.
Le Chatelier’s Principle
If the conditions of an equilibrium system are changed by changing temperature, pressure
or concentration, a process takes place which tends to oppose the effect of the change
An equilibrium may be disturbed by changing any one (or more) of the
factors for the equilibrium.
Temperature
Concentration (gases and solutions)
Pressure (gases only)
Equilibrium Constant
If we look at the following GENERAL equation
aA + bB cC + dD
The expression for the Equilibrium constant – Kc will be as follows:
c d
[ C] [ D]
Kc = a b
[A] [ B]
If A, B, C or D are solids or pure liquids, they must be LEFT OUT of the Kc expression.
When you answer exam questions do not use the general expression. Write down the Kc
expression using the given balanced chemical equation.
When Kc has a high value, there will be proportionally more of the substance on the product
side - we say the equilibrium lies to the product side (vice versa for a low value).
Only temperature alters the Kc value for a specific reaction.
If pressure or concentration is changed, the system adjusts the product and reactant
concentrations in such a way that Kc stays exactly the same (on condition the
temperature does NOT change).
Mindset Learn Xtra Exam School is brought to you by Page 36
, Physical Sciences P2 Learner’s Guide
Exam School 2012
www.learnxtra.co.za
CHEMICAL EQUILIBRIUM QUESTIONS
Question 1 (Adapted from Nov 2011 Paper 2, Question 7)
1.1 The industrial preparation of hydrogen gas is represented by the equation below.
CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g) ∆H > 0
The reaction reaches equilibrium at 1 000 °C in a closed container.
1.1.1 State Le Chatelier's principle. (3)
1.1.2 How will an increase in pressure at 1 000 °C (by decreasing the volume) affect the
yield of hydrogen gas? Write down only INCREASES, DECREASES OR NO
EFFECT. Explain the answer. (3)
1.1.3 Give TWO reasons why high temperatures are used for this reaction. (2)
1.2 Study the reversible reaction represented by the balanced equation below.
H2(g) + CO2(g) ⇌ H2O(g) + CO(g)
Initially x moles of H2(g) is mixed with 0,3 moles of CO2(g) in a sealed 10 dm3 container. When
equilibrium is reached at a certain temperature, it is found that 0,2 moles of H2O(g) is present.
The equilibrium constant (Kc) for the reaction at this temperature is 4.
1.2.1 Calculate the initial number of moles of H2(g), x, that was in the container. (8)
1.2.2 The reaction is now carried out at a much higher temperature. It is found that Kc
decreases at this higher temperature.
Is this reaction exothermic or endothermic? Explain the answer. (3)
[19]
Question 2
Consider the following equilibrium reaction:
N2 (g) + 3 H2(g) 2NH3 (g) H< 0
9 mol of N2 and 15 mol of H2 are pumped into a 500cm3 container at room temperature.
The temperature of the gas mixture is now raised to 405°C resulting in 8 mol NH3 being present at
equilibrium.
Calculate the value of Kc at 405°C [6]
Mindset Learn Xtra Exam School is brought to you by Page 37
, Physical Sciences P2 Learner’s Guide
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Question 3
Consider the following reaction:
2SO2 (g) + O2 (g) 2SO3 (g) H < 0
A graph of the AMOUNT of SO3 (g) was plotted against time as shown below:
3.1 How does the rate of the forward reaction compare to the rate of the reverse
reaction during the following intervals?:(Write down only GREATER THAN, EQUAL
TO or LESS THAN.)
3.1.1 OA
3.1.2 BC
3.1.3 DE
(3)
3.2 Initially 8,0 moles of SO2 (g) and x moles of O2 (g) are placed in a 2,0 dm3 empty container
and sealed at a specific temperature. At equilibrium 6,0 moles of SO3 (g) are present in the
container. If the KC value of the above equilibrium at this temperature is 9, calculate x, that is, the
initial amount of O2 (g) that was placed in the container. (6)
3.3 If the changes in the graph from B to D are due to changes in the TEMPERATURE,
at which points (B, C or D) will the temperature be the lowest? (1)
3.4 Give an explanation for the answer to 3.3.
(2)
3.5 At which point (B, C or D) will the KC value be the greatest?
(1)
3.6 Give an explanation for the answer to 2.5.
(2)
3.7 If the changes in the graph from B to D are due to PRESSURE changes, at which
point (B, C or D) will the pressure be the lowest?
(1)
3.8 Give an explanation for the answer to 3.7.
(2)
[18]
Mindset Learn Xtra Exam School is brought to you by Page 38
, Physical Sciences P2 Learner’s Guide
Exam School 2012
www.learnxtra.co.za
ELECTROCHEMISTRY
STUDY NOTES
DIRECT TRANSFER OF ELECTRONS
Example: Zn + CuSO4 → ZnSO4 + Cu
Blue clear brown
Observations: a small Zn plate placed in a blue copper sulphate solution gets covered with a
brown precipitate of copper and, eventually, the solution turns clear, and the clear solution is zinc
sulphate.
Reaction in ionic form: Zn + Cu2+ + SO42- → Zn2+ + SO42- + Cu
Oxidation half : Zn → Zn2+ + 2e- (Oxidation is a loss of electrons- OIL)
Reduction half: Cu2+ + 2e- → Cu (Reduction is a gain of electron –RIG)
2+ 2+
Nett equation: Zn + Cu → Zn + Cu
THE VOLTAIC OR GALVANIC CELL
Chemical potential energy Electrical potential energy
Example: Zn/Cu Cell:
Zinc half cell: Zinc electrode placed in a beaker containing a solution of zinc ions
Copper half cell: Copper electrode placed in a beaker containing a solution of copper ions
Salt bridge: Tube filled with ions of a very soluble salt (KNO3). Completes the internal circuit and
maintains electrical neutrality in the electrolyte solutions.
External circuit: Connecting wires that join the Zn electrode to the Cu electrode (can include a
voltmeter).
Anode – where oxidation occurs Cathode – where reductions occurs
Cell Notation: Zn(s) / Zn (1mol·dm-3) // Cu2+ (1mol·dm-3) / Cu(s)
2+
STANDARD ELECTRODE POTENTIALS: THE TABLE
o The Table gives standard Emf values and these values were all measured under standard
conditions.
o A reference electrode (standard hydrogen half-cell) was chosen, and all values are
measured relative to this.
o Calculate Emf or E0cell using the equation E0cell = E0cathode - E0anode
o E0cell for galvanic cells is positive ( spontaneous reactions)
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