A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Theory and principles
CSCA Chemistry · Lesson 14 of 21Reaction rates
Vitesse de réaction · سرعة التفاعل
▶ Watch the video lesson (A7LAM students)
Reactions happen when particles collide with enough energy. Anything that makes useful collisions more frequent speeds the reaction up.
Key ideas
- Rate v = Δc/Δt, usually in mol/(L·s).
- Higher concentration or pressure → more collisions → faster.
- Higher temperature → more particles have the activation energy → much faster.
- A catalyst lowers the activation energy and is not used up.
Formulas to know
- v = Δc / Δt
- N₂ + 3H₂ → 2NH₃: v(N₂) : v(H₂) : v(NH₃) = 1 : 3 : 2
- Powder reacts faster than a lump (larger surface)
Worked example
[A] falls from 2.0 to 1.2 mol/L in 4 s. Average rate?
- Change: Δc = 2.0 − 1.2 = 0.8 mol/L
- Time: Δt = 4 s
- Divide: v = 0.8 / 4 = 0.2 mol/(L·s)
Answer: 0.2 mol/(L·s)
Practice questions
1. N₂ + 3H₂ → 2NH₃ with v(N₂) = 0.1 mol/(L·s). v(H₂)?
0.1 mol/(L·s) · 0.2 mol/(L·s) · 0.3 mol/(L·s) ✓ · 0.033 mol/(L·s)
Ratio 1 : 3.
2. A catalyst speeds up a reaction by:
raising the temperature · lowering the activation energy ✓ · increasing the concentration · shifting the equilibrium
It offers an easier path.
3. Which reacts fastest with the same acid?
A marble lump at 20 °C · Marble powder at 20 °C · A marble lump at 40 °C · Marble powder at 40 °C ✓
Larger surface and higher temperature.