# Reactivity 2: Energetics and Kinetics

> IB Chemistry SL · IB SL Chemistry
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u5-overview/
> Weight: 15-20% of overall IB Chemistry SL exam

This unit explores two core pillars of chemical reactivity: energy transfer during reactions and reaction rate. You will learn to predict reaction spontaneity and speed, concepts critical for industrial, environmental, and biological chemistry.

**Prerequisites:** Basic stoichiometric calculations; Fundamentals of chemical bonding and reaction types

## Learning objectives

- Calculate energy changes in chemical reactions from experimental data and Hess's Law
- Predict reaction spontaneity using enthalpy, entropy, and Gibbs free energy
- Explain factors affecting reaction rate using collision theory
- Derive and use rate expressions from experimental data and reaction mechanisms
- Calculate activation energy from rate constant data

## 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](https://www.owlsprep.com/study/ib-chemistry-sl-u5-temperature-change-and-heat-capacity/) — Learn to measure heat transfer in reactions using simple calorimetry.
- [Enthalpy of reaction](https://www.owlsprep.com/study/ib-chemistry-sl-u5-enthalpy-of-reaction/) — Understand enthalpy change for different reaction types and calculate it from bond energies.
- [Hess's law](https://www.owlsprep.com/study/ib-chemistry-sl-u5-hess-s-law/) — Apply Hess's law to calculate enthalpy changes that cannot be measured directly.
- [Entropy and spontaneity](https://www.owlsprep.com/study/ib-chemistry-sl-u5-entropy-and-spontaneity/) — Relate entropy and Gibbs free energy to the spontaneity of a reaction.
- [Collision theory and reaction rates](https://www.owlsprep.com/study/ib-chemistry-sl-u5-collision-theory-and-reaction-rates/) — Explain how concentration, temperature, and catalysts affect reaction rate.
- [Rate expression and reaction mechanism](https://www.owlsprep.com/study/ib-chemistry-sl-u5-rate-expression-and-reaction-mechanism/) — Determine reaction orders and rate laws, and connect them to reaction mechanisms.
- [Activation energy](https://www.owlsprep.com/study/ib-chemistry-sl-u5-activation-energy/) — Calculate activation energy using the Arrhenius equation and experimental rate data.

## Common pitfalls

- **Wrong:** Mixing up the sign of $ΔH$ for exothermic and endothermic reactions
  - Why it fails: Students often confuse the perspective of the system vs the surroundings
  - Correct: $ΔH$ is negative for exothermic (system releases heat) and positive for endothermic (system absorbs heat)
- **Wrong:** Ignoring stoichiometry when calculating molar enthalpy change
  - Why it fails: Enthalpy is an extensive property that scales with the amount of reactant
  - Correct: Always scale your calculated $ΔH$ to match the molar quantity in the balanced reaction equation
- **Wrong:** Assuming spontaneous reactions are always fast
  - Why it fails: Spontaneity is a thermodynamic property, not a kinetic one
  - Correct: A spontaneous reaction can be extremely slow if it has a very high activation energy

## Cheatsheet

| Concept / Formula | Key Description |
| --- | --- |
| $q = mc\Delta T$ | Heat change calculation for calorimetry |
| $ΔH = H_{products} - H_{reactants}$ | Enthalpy change of a reaction |
| Hess's Law | Total enthalpy change = sum of enthalpy changes for all steps |
| $ΔG = ΔH - TΔS$ | Gibbs free energy: $ΔG < 0$ = spontaneous reaction |
| $ΔS_{universe} > 0$ for spontaneous change | Second law of thermodynamics |
| Rate = $k[A]^m[B]^n$ | General rate expression, $m,n$ = reaction orders |
| $k = A e^{-E_a/RT}$ | 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.

- [First Sub-Topic: Temperature change and heat capacity](https://www.owlsprep.com/study/ib-chemistry-sl-u5-temperature-change-and-heat-capacity/)
- [Next Unit: Reactivity 3: Organic Chemistry](https://www.owlsprep.com/study/ib-chemistry-sl-u6-overview/)
- [Enthalpy of reaction](https://www.owlsprep.com/study/ib-chemistry-sl-u5-enthalpy-of-reaction/)

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