Unit Overview

Reactivity 2: Energetics and Kinetics

IB Chemistry SL· 5 min read 📊 15-20% of overall IB Chemistry SL exam

1. Unit at a Glance

This unit is split into two connected thematic strands: energetics (first four sub-topics) and kinetics (last three sub-topics). We start with experimental measurement of heat transfer, build up to enthalpy calculations, then connect enthalpy and entropy to predict whether a reaction will occur spontaneously.

After mastering energetics, we shift to how fast reactions proceed. We start with the foundational collision theory, then move to quantitative rate expressions, connect rate laws to reaction mechanisms, and finish with calculating activation energy from experimental data. Concepts build incrementally, so we recommend working through sub-topics in the order listed below.

Core sub-topics in this unit:

Temperature change and heat capacity

Learn to measure heat transfer in reactions using simple calorimetry.

★★⏱ 8 min

Enthalpy of reaction

Understand enthalpy change for different reaction types and calculate it from bond energies.

★★⏱ 7 min

Hess's law

Apply Hess's law to calculate enthalpy changes that cannot be measured directly.

★★★⏱ 9 min

Entropy and spontaneity

Relate entropy and Gibbs free energy to the spontaneity of a reaction.

★★★★⏱ 10 min

Collision theory and reaction rates

Explain how concentration, temperature, and catalysts affect reaction rate.

★★⏱ 7 min

Rate expression and reaction mechanism

Determine reaction orders and rate laws, and connect them to reaction mechanisms.

★★★★⏱ 11 min

Activation energy

Calculate activation energy using the Arrhenius equation and experimental rate data.

★★★⏱ 8 min

2. Common Pitfalls

Wrong move:

Mixing up the sign of $Δ H$ for exothermic and endothermic reactions

Why:

Students often confuse the perspective of the system vs the surroundings

Correct move:

$Δ H$ is negative for exothermic (system releases heat) and positive for endothermic (system absorbs heat)

Wrong move:

Ignoring stoichiometry when calculating molar enthalpy change

Why:

Enthalpy is an extensive property that scales with the amount of reactant

Correct move:

Always scale your calculated $Δ H$ to match the molar quantity in the balanced reaction equation

Wrong move:

Assuming spontaneous reactions are always fast

Why:

Spontaneity is a thermodynamic property, not a kinetic one

Correct move:

A spontaneous reaction can be extremely slow if it has a very high activation energy

3. Quick Reference Cheatsheet

Concept / Formula	Key Description
$q = m c Δ T$	Heat change calculation for calorimetry
$Δ H = H_{p r o d u c t s} - H_{r e a c t an t s}$	Enthalpy change of a reaction
Hess's Law	Total enthalpy change = sum of enthalpy changes for all steps
$Δ G = Δ H - T Δ S$	Gibbs free energy: $Δ G < 0$ = spontaneous reaction
$Δ S_{u ni v er se} > 0$ for spontaneous change	Second law of thermodynamics
Rate = $k [A]^{m} [B]^{n}$	General rate expression, $m, n$ = reaction orders
$k = A e^{- E_{a} / R T}$	Arrhenius equation relating $k$ to activation energy

What's Next

Start with the first sub-topic of this unit to learn about calorimetry and heat transfer, the experimental foundation for all subsequent energetics calculations. Concepts build incrementally, so work through each sub-topic in order. Once you complete all sub-topics in this unit, you will be ready to move on to the next unit on organic chemistry reactivity.