# R2: How much / how fast / how far?

> IB Chemistry HL · IB Chemistry HL
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u5-overview/
> Weight: n/a

This core unit connects three foundational pillars of chemistry: quantitative reaction stoichiometry, reaction kinetics (how fast), and chemical equilibrium (how far). It builds core knowledge required for all subsequent IB Chemistry topics.

**Prerequisites:** Basic atomic structure, chemical formulas, and balanced chemical equations

## Learning objectives

- Calculate quantitative relationships between reactants and products for all reaction types
- Explain factors that influence the rate of chemical reactions using collision theory
- Describe the nature of dynamic equilibrium and apply equilibrium principles to different reaction systems
- Solve higher level extension problems for stoichiometry, kinetics and equilibrium

## 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](https://www.owlsprep.com/study/ib-chemistry-hl-u5-stoichiometric-relationships/) — Introduces the mole concept and calculates mass, mole, and gas volume relationships
- [Limiting and excess reactants](https://www.owlsprep.com/study/ib-chemistry-hl-u5-limiting-and-excess-reactants/) — Identifies limiting reactants and calculates percent yield for chemical reactions
- [Reaction rate and rate expressions](https://www.owlsprep.com/study/ib-chemistry-hl-u5-reaction-rate-and-rate-expressions/) — Defines reaction rate and explains how to measure rate experimentally
- [Collision theory](https://www.owlsprep.com/study/ib-chemistry-hl-u5-collision-theory/) — Explains how temperature, concentration and catalysts affect reaction rate
- [Dynamic equilibrium](https://www.owlsprep.com/study/ib-chemistry-hl-u5-dynamic-equilibrium/) — Describes reversible reactions and the nature of dynamic equilibrium
- [The equilibrium constant](https://www.owlsprep.com/study/ib-chemistry-hl-u5-the-equilibrium-constant/) — Writes equilibrium constant expressions for homogeneous reaction systems
- [Solubility equilibria](https://www.owlsprep.com/study/ib-chemistry-hl-u5-solubility-equilibria/) — Applies equilibrium principles to sparingly soluble ionic compounds
- [AHL: Advanced stoichiometry and titration calculations](https://www.owlsprep.com/study/ib-chemistry-hl-u5-ahl-advanced-stoichiometry-and-titration/) — Solves complex titration problems including back titration calculations
- [AHL: Rate laws and reaction order](https://www.owlsprep.com/study/ib-chemistry-hl-u5-ahl-rate-laws-and-reaction/) — Calculates reaction order, rate constants, and half-lives from experimental data
- [AHL: Activation energy and Arrhenius equation](https://www.owlsprep.com/study/ib-chemistry-hl-u5-ahl-activation-energy-and-arrhenius/) — Uses the Arrhenius equation to relate temperature to rate constant
- [AHL: Extended Le Chatelier's principle](https://www.owlsprep.com/study/ib-chemistry-hl-u5-ahl-extended-le-chatelier-s/) — Predicts how changing conditions affect equilibrium position and $K_c$
- [AHL: Reaction quotient](https://www.owlsprep.com/study/ib-chemistry-hl-u5-ahl-reaction-quotient/) — Uses reaction quotient to predict the direction of a reaction shift

## Common pitfalls

- **Wrong:** Confusing reaction rate with reaction order in kinetics
  - Why it fails: Rate changes with reactant concentration but reaction order is a constant for a given reaction
  - Correct: Remember rate is the change in concentration over time, while order is the exponent in the rate law expression
- **Wrong:** Forgetting to apply balanced equation coefficients to mole ratios in stoichiometry
  - Why it fails: Skipping this step leads to incorrect limiting reactant and yield calculations
  - Correct: Always write the mole ratio explicitly from the balanced equation before solving problems
- **Wrong:** Confusing the reaction quotient $Q$ with the equilibrium constant $K_c$
  - Why it fails: Both use the same expression format but rely on different concentration values
  - Correct: $K_c$ uses only equilibrium concentrations, while $Q$ uses concentrations at any non-equilibrium point

## Cheatsheet

| Concept / Formula | Key Use Case |
| --- | --- |
| $n = \frac{m}{M}$ | Calculate moles from mass and molar mass |
| Percent Yield = $\frac{actual\ yield}{theoretical\ yield} \times 100\%$ | Calculate reaction efficiency |
| Average Rate = $\frac{\Delta [product]}{\Delta t}$ | Calculate average reaction rate from experimental data |
| $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ for $aA + bB \rightleftharpoons cC + dD$ | Write equilibrium constant expressions |
| Rate Law: $rate = k[A]^m[B]^n$ | Relate reactant concentration to reaction rate |
| Arrhenius Equation: $k = A e^{-\frac{E_a}{RT}}$ | Relate activation energy to rate constant and temperature |
| $Q = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ | Predict direction of reaction shift when $Q \neq K_c$ |
| $K_{sp} = [M^{n+}]^a[X^{m-}]^b$ | 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.

- [Stoichiometric relationships](https://www.owlsprep.com/study/ib-chemistry-hl-u5-stoichiometric-relationships/)
- [Next Unit: Acids and Bases](https://www.owlsprep.com/study/ib-chemistry-hl-u6-overview/)
- [Limiting and excess reactants](https://www.owlsprep.com/study/ib-chemistry-hl-u5-limiting-and-excess-reactants/)

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