CHEM10007 · Fundamentals Of Chemistry
Reaction Kinetics
How fast reactions go and what changes that speed. You define reaction rate as a concentration change per unit time, identify the factors affecting rate (concentration, temperature, surface area, catalyst), and explain rate using collision theory and the activation energy barrier. Catalysts lower Ea by offering an alternative pathway, shown on a reaction-profile diagram.
What this chapter covers
- 01Reaction rate = change in concentration per unit time, rate = −Δ[reactant]/Δt = +Δ[product]/Δt
- 02Stoichiometric scaling of rates between reactants and products
- 03Factors affecting rate: concentration, temperature, surface area, catalyst
- 04Collision theory: effective collisions need correct orientation and energy ≥ Ea
- 05Activation energy Ea as the energy barrier to reaction
- 06Temperature effect: higher T means more molecules exceed Ea, so faster reaction
- 07Catalysts: lower Ea via an alternative pathway and are not consumed
- 08Reaction-profile diagrams with and without a catalyst
Average reaction rate from concentration–time data
- 1 mark — correct rate expressionrate = −Δ[N2O5]/Δt = −(0.090 − 0.150)/50.
- 1 mark — numerical raterate = −(−0.060)/50 = 1.2 × 10−3 M s−1.
- 1 mark — temperature/E<sub>a</sub> explanationAt lower temperature, fewer molecules have energy ≥ Ea, so there are fewer effective collisions per second; the reaction is slower and the concentration changes less over the same time.
Key terms
- Reaction rate
- The change in concentration of a reactant or product per unit time, with a stoichiometric factor so all species give the same overall rate.
- Activation energy (Ea)
- The minimum energy a collision must have, with the correct orientation, for a reaction to occur — the barrier on a reaction-profile diagram.
- Collision theory
- The model that reactions occur only when particles collide with sufficient energy (≥ Ea) and the correct orientation.
- Catalyst
- A substance that speeds a reaction by providing an alternative pathway with lower Ea; it is not consumed overall.
- Reaction profile
- A diagram of energy versus reaction progress showing reactants, the activation-energy barrier, the transition state and products.
Reaction Kinetics FAQ
Why does raising the temperature speed up a reaction?
Higher temperature widens the energy distribution so a larger fraction of molecules have energy at or above Ea. There are more frequent and more energetic effective collisions per second, increasing the rate.
How does a catalyst work without being used up?
A catalyst offers an alternative reaction pathway with a lower activation energy, so more collisions succeed. It is regenerated by the end of the mechanism, so it does not appear in the overall balanced equation.
Why is the rate of consumption written with a minus sign?
A reactant's concentration decreases, so Δ[reactant] is negative. Placing a minus sign in front makes the reported rate a positive quantity, matching the positive rate of product formation.
Exam move
Be able to compute an average rate from concentration–time data and to give a crisp collision-theory explanation for each rate factor, since kinetics questions are usually short. Practise sketching reaction profiles with and without a catalyst, labelling Ea and the transition state. Connect the temperature argument here to the equilibrium and weak-acid chapters, where the same energy-distribution reasoning reappears.
