Equilibrium II
Notes for every section, eight new exam-style questions with full worked solutions, and online recall cards for your review days. Nothing here is copied from the book.
Learn it
Read one section at a time, then answer its check question before you open the answer.
11.1 Values and units of Kc
A large Kc (much more than 1) means the equilibrium lies to the right: mostly products. A small value means mostly reactants.
Units: write mol dm−3 for every concentration in the expression and cancel.
- H2 + I2 ⇌ 2HI: (mol dm−3)2 ÷ (mol dm−3)2 = no units.
- N2 + 3H2 ⇌ 2NH3: (mol dm−3)2 ÷ (mol dm−3)4 = mol−2 dm6.
- PCl5 ⇌ PCl3 + Cl2: mol dm−3.
Check yourself: What are the units of Kc for 2NO2 ⇌ N2O4?
(mol dm−3) ÷ (mol dm−3)2 = mol−1 dm3
11.2 Calculating Kc from experimental data
The notes for this section are part of Premium. See Premium
11.3 Finding Kc by experiment: esterification
The notes for this section are part of Premium. See Premium
11.4 Mole fractions and partial pressures
The notes for this section are part of Premium. See Premium
11.5 The equilibrium constant Kp
The notes for this section are part of Premium. See Premium
11.6 Temperature changes the value of K
The notes for this section are part of Premium. See Premium
11.7 Changes that do not alter K
The notes for this section are part of Premium. See Premium
Extra worked questions
Cover the solution, answer on paper, then mark yourself against each step. Each step shows the mark it earns.
All 8 worked solutions are free for members. The first one is open to everyone.
Units of Kc
Give the units of Kc for (a) N2(g) + 3H2(g) ⇌ 2NH3(g) and (b) PCl5(g) ⇌ PCl3(g) + Cl2(g).
Try it first, then show the worked solution
| (a) | (mol dm−3)2 ÷ (mol dm−3)4 = mol−2 dm6 | 1 |
|---|---|---|
| (b) | (mol dm−3)2 ÷ mol dm−3 = mol dm−3 | 1 |
Answer: (a) mol−2 dm6 (b) mol dm−3
Examiner tip: Write the units into the expression and cancel; never guess.
Kc for hydrogen iodide
At equilibrium at a fixed temperature, [H2] = 0.020 mol dm−3, [I2] = 0.020 mol dm−3 and [HI] = 0.140 mol dm−3. Calculate Kc for H2(g) + I2(g) ⇌ 2HI(g).
Join free to see the full worked solution. Join free or log in.
Phosphorus pentachloride
0.500 mol of PCl5 is heated in a sealed 2.00 dm3 flask. At equilibrium 0.200 mol of Cl2 is present. PCl5(g) ⇌ PCl3(g) + Cl2(g). Calculate Kc.
Join free to see the full worked solution. Join free or log in.
Kp for dinitrogen tetroxide
An equilibrium mixture at a total pressure of 100 kPa contains 0.60 mol N2O4 and 0.80 mol NO2. N2O4(g) ⇌ 2NO2(g). Calculate Kp.
Join free to see the full worked solution. Join free or log in.
Esterification equilibrium
1.00 mol of ethanoic acid and 1.00 mol of ethanol are mixed with a little acid catalyst and left to reach equilibrium. 0.333 mol of ethanoic acid remains. Calculate Kc for CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O.
Join free to see the full worked solution. Join free or log in.
What changes Kc?
For A(g) + B(g) ⇌ 2C(g), ΔH is positive. State the effect on Kc of (a) adding more A, (b) adding a catalyst and (c) raising the temperature.
Join free to see the full worked solution. Join free or log in.
Kp for hydrogen iodide
1.00 mol H2 and 1.00 mol I2 are heated in a sealed container. At equilibrium there is 1.56 mol HI and the total pressure is 100 kPa. H2(g) + I2(g) ⇌ 2HI(g). Calculate Kp and state the effect on Kp of doubling the total pressure.
Join free to see the full worked solution. Join free or log in.
Kp for sulfur trioxide
2.00 mol SO2 and 1.00 mol O2 reach equilibrium at a total pressure of 200 kPa: 2SO2(g) + O2(g) ⇌ 2SO3(g). 1.60 mol SO3 is present. Calculate Kp with units.
Join free to see the full worked solution. Join free or log in.
Video walkthroughs
Narrated walkthroughs of exam-style questions from this topic of the book, showing where each mark comes from. Try the question first, then watch. Captions are available in the player.
Calculating Kc for hydrogen iodide
Kp for the Haber equilibrium
This walkthrough is part of Premium. See Premium
Kc for ester hydrolysis by titration
This walkthrough is part of Premium. See Premium
Kp for dinitrogen tetroxide
This walkthrough is part of Premium. See Premium
How temperature changes Kc
This walkthrough is part of Premium. See Premium
Equilibrium amount from Kc
This walkthrough is part of Premium. See Premium
Premium includes a walkthrough video for the key questions in every topic. Each one is narrated step by step, with captions.
Recall cards
Answer out loud or on paper before you turn the card. Only a complete answer counts as "Got it".
These are 3 of 18 recall cards. Premium members get the full deck for every topic, ready for each review day in their plan.
Your review days
Count from your Day 0, the day you finish learning this topic. Your plan schedules these for you.
- +1 Recall cards (these online cards, or the cards in the book)
- +3 Recall cards and Q1 to Q4
- +7 Recall cards and Q5 to Q8
- +14 Recall cards and Q9 to Q12
- +30 Full chapter test, timed
All questions on this page are original and are not in the book. This site is independent and is not endorsed by AQA or Pearson Edexcel.