IB Diploma Chemistry HL · Reactivity 2.2 & 2.3 — rate and extent of reaction
Mini-Lesson
How fast, how far — rate and equilibrium
This mini-lesson covers Reactivity 2.2 & 2.3: rate of reaction and collision theory, the factors that change rate, dynamic equilibrium, Kc and Le Chatelier's principle. This HL lesson also builds in the Additional Higher Level (AHL) material.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Reactivity 2.2
Collision theory
Reactions happen when particles collide with enough energy (≥ the activation energy) and the correct orientation. More frequent, more energetic successful collisions = faster rate.
Reactivity 2.2
Factors affecting rate
Concentration / pressure ↑ → more frequent collisions.
Surface area ↑ (powder) → more contact.
Temperature ↑ → particles faster and more have ≥ Ea.
Catalyst → provides a lower-Ea pathway (not used up).
Quick check
Speeding it up
?Which change does NOT increase the rate of a reaction between a solid and an acid?
Calculate
Temperature rule of thumb
1As a rule of thumb the rate roughly doubles for each 10 °C rise. By roughly what factor does the rate increase for a 30 °C rise?
×
Doubling three times: 2 × 2 × 2 = 2³.
Reactivity 2.3
Dynamic equilibrium
In a closed system a reversible reaction reaches dynamic equilibrium: the forward and reverse reactions continue at equal rates, so concentrations stay constant (but are not equal).
Reactivity 2.3
The equilibrium constant Kc
For aA + bB ⇌ cC + dD:
Kc = [C]ᶜ[D]ᵈ ÷ [A]ᵃ[B]ᵇproducts over reactants, each raised to its coefficient
A large Kc means products are favoured; a small Kc means reactants are favoured.
↑ pressure → shifts toward the side with fewer gas moles.
↑ temperature → shifts in the endothermic direction.
A catalyst speeds both directions equally — it does not move the position or change Kc.
Quick check
Predict the shift
?For N₂ + 3H₂ ⇌ 2NH₃, increasing the pressure shifts the equilibrium:
Quick check
What a catalyst does
?Adding a catalyst to a reaction at equilibrium:
AHL — Reactivity 2.2 & 2.3
Rate laws & Kc calculations
AHL introduces the rate law rate = k[A]ᵐ[B]ⁿ, where m and n are the orders found from experiment (not the equation). The overall order is m + n. AHL also calculates Kc from an ICE table and links equilibrium to Gibbs energy through ΔG° = −RT ln K.
AHL check
Overall order
?A reaction has rate = k[A][B]². What is its overall order?
Sort it
Which idea?
Tap a phrase, then the concept it belongs to.
💥 Collision theory
⚖️ Equilibrium feature
🔄 Le Chatelier response
Match it
Match change to effect
Tap an item on the left, then its match on the right.
Change
Effect
Recap
The big ideas to know
Rate: collisions with energy ≥ Eₐ and correct orientation; conc, SA, temp, catalyst