Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Opposing thermal change
Increasing temperature favours the endothermic direction because that direction absorbs thermal energy. Decreasing temperature favours the exothermic direction because that direction releases thermal energy.
Using Delta H
If the forward reaction has negative Delta H, it is exothermic. Heating shifts equilibrium in reverse; cooling shifts it forward. If forward Delta H is positive, the pattern reverses.
Rate effect
Higher temperature increases both forward and reverse rates by increasing kinetic energy and successful collisions. Equilibrium establishes faster, even if the equilibrium yield of the desired product decreases.
Industrial compromise
An industrial temperature balances rate and yield. A low temperature may give excellent equilibrium yield but an uneconomically slow reaction. A catalyst can improve rate without sacrificing yield.
Practical or data skill
Use reversible-reaction data at several temperatures to identify whether the forward direction is exothermic or endothermic.
Examination tip
Temperature is the equilibrium factor that requires knowing the reaction’s energy direction.
Review questions and suggested answers
Question 1
What direction is favoured by heating?
Suggested answer
The endothermic direction.
Question 2
If the forward reaction is exothermic, what does cooling do?
Suggested answer
It shifts equilibrium forward.
Question 3
Does higher temperature always increase product yield?
Suggested answer
No. It increases rate but may shift equilibrium away from an exothermic product.
