Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Read the equation
Identify reactants and products, state symbols, gas coefficients and the energy direction. Avoid predicting a pressure effect before counting gas moles or a temperature effect before identifying the endothermic direction.
Name the imposed change
Decide whether concentration, pressure, temperature or catalyst has changed. Each has a different rule. If more than one changes, analyse them separately before combining conclusions.
State the response
Write which direction is favoured and why it opposes the change. Then name the substance whose equilibrium yield rises or falls. Do not merely write ‘moves right’ without context.
Separate rate from yield
Pressure and temperature often affect both rate and equilibrium position. Catalysts affect rate only. Concentration changes can cause an immediate rate change and a new equilibrium position. Examination questions reward clear separation.
Practical or data skill
Complete a table of condition changes for several supplied equilibria, with columns for immediate rate effect, equilibrium shift and product yield.
Examination tip
Use the information provided; do not assume every forward reaction is exothermic.
Review questions and suggested answers
Question 1
What must be known for a temperature prediction?
Suggested answer
Which direction is endothermic or the sign of Delta H.
Question 2
What must be counted for a pressure prediction?
Suggested answer
Moles of gaseous species on each side.
Question 3
Which factor changes rate but not position?
Suggested answer
A catalyst.
