Unit Overview

R2: How much / how fast / how far?

IB Chemistry HL· 6 min read 📊 n/a

1. Unit at a Glance

This unit follows the natural order chemists use to analyze any reaction: first we calculate how much reactant is consumed and how much product is formed (stoichiometry), then we examine how fast the reaction proceeds (kinetics), and finally we determine how far the reaction will go before reaching a steady state (equilibrium).

Core standard level (SL) content is covered first for each theme, followed by additional higher level (AHL) extensions that deepen conceptual understanding and problem-solving skills for exam success.

Below are all sub-topics in this unit:

Stoichiometric relationships

Introduces the mole concept and calculates mass, mole, and gas volume relationships

★★⏱ 10 min

Limiting and excess reactants

Identifies limiting reactants and calculates percent yield for chemical reactions

★★⏱ 8 min

Reaction rate and rate expressions

Defines reaction rate and explains how to measure rate experimentally

★★⏱ 7 min

Collision theory

Explains how temperature, concentration and catalysts affect reaction rate

★★⏱ 7 min

Dynamic equilibrium

Describes reversible reactions and the nature of dynamic equilibrium

★★★⏱ 6 min

The equilibrium constant

Writes equilibrium constant expressions for homogeneous reaction systems

★★★⏱ 8 min

Solubility equilibria

Applies equilibrium principles to sparingly soluble ionic compounds

★★★⏱ 8 min

AHL: Advanced stoichiometry and titration calculations

Solves complex titration problems including back titration calculations

★★★★⏱ 12 min

AHL: Rate laws and reaction order

Calculates reaction order, rate constants, and half-lives from experimental data

★★★★⏱ 12 min

AHL: Activation energy and Arrhenius equation

Uses the Arrhenius equation to relate temperature to rate constant

★★★★⏱ 10 min

AHL: Extended Le Chatelier's principle

Predicts how changing conditions affect equilibrium position and $K_{c}$

★★★⏱ 8 min

AHL: Reaction quotient

Uses reaction quotient to predict the direction of a reaction shift

★★★⏱ 7 min

2. Common Pitfalls

Wrong move:

Confusing reaction rate with reaction order in kinetics

Why:

Rate changes with reactant concentration but reaction order is a constant for a given reaction

Correct move:

Remember rate is the change in concentration over time, while order is the exponent in the rate law expression

Wrong move:

Forgetting to apply balanced equation coefficients to mole ratios in stoichiometry

Why:

Skipping this step leads to incorrect limiting reactant and yield calculations

Correct move:

Always write the mole ratio explicitly from the balanced equation before solving problems

Wrong move:

Confusing the reaction quotient $Q$ with the equilibrium constant $K_{c}$

Why:

Both use the same expression format but rely on different concentration values

Correct move:

$K_{c}$ uses only equilibrium concentrations, while $Q$ uses concentrations at any non-equilibrium point

3. Quick Reference Cheatsheet

Concept / Formula	Key Use Case
$n = \frac{m}{M}$	Calculate moles from mass and molar mass
Percent Yield = $\frac{a c t u a l y i e l d}{t h eor e t i c a l y i e l d} \times 100%$	Calculate reaction efficiency
Average Rate = $\frac{Δ [ p r o d u c t ]}{Δ t}$	Calculate average reaction rate from experimental data
$K_{c} = \frac{[ C ] ^{c} [ D ] ^{d}}{[ A ] ^{a} [ B ] ^{b}}$ for $a A + b B ⇌ c C + d D$	Write equilibrium constant expressions
Rate Law: $r a t e = k [A]^{m} [B]^{n}$	Relate reactant concentration to reaction rate
Arrhenius Equation: $k = A e^{- \frac{E _{a}}{R T}}$	Relate activation energy to rate constant and temperature
$Q = \frac{[ C ] ^{c} [ D ] ^{d}}{[ A ] ^{a} [ B ] ^{b}}$	Predict direction of reaction shift when $Q \neq = K_{c}$
$K_{s p} = [M^{n +}]^{a} [X^{m -}]^{b}$	Calculate solubility product for sparingly soluble salts

What's Next

Start with the first sub-topic of this unit to build your foundational understanding of stoichiometry, the first core theme of this unit. Work through each sub-topic in order, as concepts build sequentially throughout the unit. Once you complete all sub-topics in this unit, you will move on to the next unit covering acids and bases.