1. Can reaction order be zero?
- No
- Yes
- Only for a catalyst
Check the answer and reasoning
Yes
A measured rate can be independent of a species concentration over the studied range.
CHEM / A2 level
Rate equations and mechanisms
A rate equation is determined experimentally rather than read automatically from the overall balanced equation. Reaction order describes how rate depends on a concentration.
Learning objectives
- Explain rate equations and mechanisms using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
A rate equation is determined experimentally rather than read automatically from the overall balanced equation. Reaction order describes how rate depends on a concentration.
Rate constants depend on temperature, and units depend on overall order. A mechanism must be consistent with the rate law and reaction stoichiometry, but supporting evidence rarely makes it uniquely proven.
Equations and relationships
m and n are experimental orders.
Worked example
Doubling [A] multiplies rate by 4 with other factors fixed. What is the order in A?
Follow the solution
The concentration factor gives 2^m=4, so the experimental order m=2.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Use the evidence activity below; no exact PhET model is claimed.
Check your understanding
2. Doubling [A] multiplies rate by 4 with other factors fixed. What is the order in A?
- 1
- 4
- 2
Check the answer and reasoning
2
The concentration factor gives 2^m=4, so the experimental order m=2.
3. Does the overall equation always reveal the rate-determining step?
- No
- Yes
- Only for gases
Check the answer and reasoning
No
Different multi-step mechanisms can produce the same overall stoichiometry.