Unit notes and practice questions
Everything here is for unit 5 of the Grade 11 Chemistry textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 11 Chemistry page. Open one at a time.
When you use it
Use to predict how a change in pressure or volume shifts the position of a gaseous equilibrium system.
Watch out
Increasing pressure shifts the reaction toward the side with fewer gas moles. Adding an inert gas at constant volume produces no shift.
Drafted from Grade 11 Chemistry, pages 222-263, then checked twice before it went up
When you use it
Use when formulating equilibrium constant expressions for heterogeneous systems involving multiple physical states.
Watch out
Never include pure solids (s) or pure liquids (l) in KC or KP expressions because their concentrations are constant.
Drafted from Grade 11 Chemistry, pages 222-263, then checked twice before it went up
When you use it
Use when converting between the concentration equilibrium constant and the pressure equilibrium constant for gas phase reactions.
Watch out
Calculate Delta n using gaseous species only. When the number of moles of gaseous products equals reactants, Delta n is zero and KP equals KC.
Drafted from Grade 11 Chemistry, pages 222-263, then checked twice before it went up
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
Which statement is correct for a reaction that has reached equilibrium?
Answer: B The rate of the forward reaction is equal to the rate of the reverse reaction.
Why
At equilibrium the forward and reverse reactions still happen, but they happen at the same rate. The concentrations stay constant. They are not necessarily equal, so option A is wrong, and the reaction has not stopped, so option C is wrong.
Question 2
In a chemical equation, what does a double arrow (⇌) show?
Answer: B The reaction can go in both the forward and the reverse direction.
Why
A double arrow means the reaction is reversible. Reactants form products (forward reaction), and products form reactants again (reverse reaction). A single arrow is used for irreversible reactions that go in one direction only.
Question 3
For the reaction CaCO3(s) ⇌ CaO(s) + CO2(g), which expression is the correct KC?
Answer: A K<sub>C</sub> = [CO<sub>2</sub>]
Why
Pure solids and pure liquids are not included in the KC expression. CaCO3 and CaO are both solids, so only the gas CO2 appears in the expression.
Question 4
A reaction goes essentially to completion if its equilibrium constant is greater than what value?
Answer: A 10<sup>10</sup>
Why
The chapter says a reaction goes essentially to completion if KC or KP is greater than 1010. It does not go at all if the value is less than 10-10.
Question 5
At a certain temperature, KC = 13.8 for CO(g) + Cl2(g) ⇌ COCl2(g). A mixture has [CO] = 2.5 mol L-1, [Cl2] = 1.2 mol L-1 and [COCl2] = 5.0 mol L-1. What is the reaction quotient QC, and which way will the reaction go to reach equilibrium?
Answer: A Q<sub>C</sub> = 1.67, so the reaction goes forward (to the right).
Why
QC = [COCl2] / ([CO][Cl2]) = 5.0 / (2.5 × 1.2) = 1.67. Since QC is less than KC (13.8), the reaction must go forward to make more product and reach equilibrium.
Question 6
For the decomposition of CaCO3 at 1273 K, the pressure of CO2 in equilibrium is KP = 3.87 atm. Use R = 0.082 L atm K-1 mol-1. Which value is closest to KC? (Use KC = KP / RT, because the change in moles of gas is +1.)
Answer: A 0.037 M
Why
RT = 0.082 × 1273 = 104.4. Then KC = 3.87 / 104.4 = 0.037 M. Dividing by RT is the right step here because KP = KC(RT)Δn with Δn = +1.
Question 7
A fertiliser plant in Addis Ababa makes ammonia by the Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g). The manager wants more ammonia at equilibrium. Which change, at constant temperature, will increase the yield of NH3?
Answer: A Increase the pressure by decreasing the volume of the vessel.
Why
There are 4 mol of gas on the left and 2 mol on the right, so the forward reaction makes fewer gas molecules. Increasing the pressure shifts the equilibrium toward fewer gas molecules, which is the forward direction. A catalyst speeds up both reactions but does not change the yield.
Question 8
In the Contact process, the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g) is exothermic. If the temperature at equilibrium is lowered, what happens to the yield of SO3 according to Le Chatelier's principle?
Answer: A The yield of SO<sub>3</sub> increases.
Why
Because the forward reaction is exothermic, it gives out heat. Lowering the temperature removes heat, so the system shifts in the direction that makes heat, which is the forward direction. More SO3 is made. In real plants a compromise temperature of about 450 °C is used, because lower temperatures make the reaction too slow.
Question 9
Reactions can be added to give an overall reaction. For ½N2(g) + ½O2(g) + ½Br2(g) ⇌ NOBr(g), the equilibrium constant of NO(g) + ½Br2(g) ⇌ NOBr(g) is K2 = 1.4, and the equilibrium constant of ½N2(g) + ½O2(g) ⇌ NO(g) is K1' = 6.5 × 10-16. What is the KC of the overall reaction?
Answer: A 9.1 × 10<sup>-16</sup>
Why
When reactions are added, the overall K is the product of the individual K values, not the sum. So Koverall = K2 × K1' = 1.4 × 6.5 × 10-16 = 9.1 × 10-16. Adding the numbers (option C) is a common mistake.
Question 10
At 430 °C, KC = 54.3 for H2(g) + I2(g) ⇌ 2HI(g). A 3.5 L vessel contains 0.0218 mol H2, 0.0145 mol I2 and 0.0783 mol HI. Which statement is correct?
Answer: A Q<sub>C</sub> is about 19.4, so the reaction goes forward to form more HI.
Why
First find the concentrations by dividing by 3.5 L: [H2] = 0.00623, [I2] = 0.00414, [HI] = 0.02237 mol L-1. Then QC = (0.02237)2 / (0.00623 × 0.00414) = about 19.4. Since QC is less than KC (54.3), the reaction goes forward.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 5 runs from page 222 to page 263 of the Ministry of Education edition.
Ministry of Education edition, English, 342 pages
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