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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R2.2 - How fast? The rate of chemical change

2.2.1 Rate of Reaction 2.2.2 Collision Theory 2.2.3 Factors Affecting Reaction Rate 2.2.4 Activation Energy and Temperature 2.2.5 Catalyst and Activation Energy 2.2.6 Reaction Mechanism and Intermediates (AHL) 2.2.7 Energy Profile and Rate Determining Step (AHL) 2.2.8 Molecularity in Reaction Mechanism (AHL) 2.2.9 Rate Equations and Experimental Data (AHL) 2.2.10 Reaction Orders and Graphs (AHL) 2.2.11 Rate Constant, K (AHL) 2.2.12 Arrhenius Reaction and Temperature (AHL) 2.2.13 Arrhenius Factor and Activation Energy (AHL)

Rate of Reaction

Specification Reference R2.2.1

Quick Notes:

  • The rate of reaction is the change in concentration of a reactant or product per unit time.
  • Units are usually mol dm⁻³ s⁻¹.
  • Can also be measured by changes in mass or volume over time.
  • Rate can be determined by:
    • Calculating the gradient (slope) of a concentration–time graph.
    • Drawing a tangent to a curve at a point to estimate the instantaneous rate.

Full Notes:

Definition of Rate of Reaction

The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time.

Rate = change in concentration / time

Units: mol dm⁻³ s⁻¹ (for concentration), g s⁻¹ (for mass), or cm³ s⁻¹ (for gas volume).

Measuring Reaction Rate

Rates can be measured by observing how a property changes with time:

Concentration–time graph

Concentration–time graphs give useful information about the rate of a reaction and how it changes during the reaction.

IB Chemistry concentration–time graphs showing different slopes where gradient equals rate of reaction and steeper slopes indicate faster reactions.

Gradient = rate of reaction

Note - concentration can refer to reactant or product. If reactant concentration change is measured, a steep downward curve represents a fast rate of reaction (as reactant concentration decreases as reaction occurs). The opposite is true if product concentration change is measured (an upward curve is observed).

To calculate instantaneous rate:

An instantaneous rate is a rate at an exact moment in time. We can use a concentration-time graph find an instantaneous rate.

IB Chemistry tangent drawn to a concentration–time curve to estimate instantaneous rate from the gradient at a point.

Other graph types:
Volume–time or mass–time graphs can also be used. Tangents and gradients still represent the rate.

Example - Measuring Initial Rate of Reaction

Measure initial rate of hydrogen gas production when magnesium reacts with HCl at different concentrations.

Reaction: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Apparatus and Reagents:

Method Summary:

IB Chemistry gas collection setup for magnesium with hydrochloric acid using a conical flask, bung and gas syringe.

Analysis:

IB Chemistry rate comparison graph of hydrogen volume versus time for two hydrochloric acid concentrations with tangents at t = 0 to find initial rates.
  1. Plot volume of H2 (y-axis) vs time (x-axis)
  2. Draw lines of best fit and tangents at t = 0
  3. Calculate gradients = rate
  4. Compare rates for both acid concentrations

Summary

Linked Course Questions

Tool 1, 3, Inquiry 2 – Linked Course Question

Concentration changes in reactions are not usually measured directly. What methods are used to provide data to determine the rate of reactions?

Concentration changes are often measured indirectly by tracking a physical property that changes during the reaction. Common methods include:

  • Change in mass (e.g. when a gas is released)
  • Change in volume of gas produced (using a gas syringe or inverted burette)
  • Change in colour or absorbance (using colorimetry or spectrophotometry)
  • Change in pH (for reactions involving acids or bases)
  • Change in conductivity (if ions are produced or consumed)

These measurements are taken at regular time intervals to calculate the rate of reaction from the change over time.


Tool 1 – Linked Course Question

What experiments measuring reaction rates might use time as i) a dependent variable ii) an independent variable?

In reaction rate experiments, time can be used as:

  • i) A dependent variable when you measure how long it takes for a specific event to occur — e.g. the time for a cross to disappear in a reaction producing a precipitate, or the time for a colour change to occur.
  • ii) An independent variable when you take measurements at regular time intervals to track how a property (like mass, volume of gas, or absorbance) changes over time. This is the most common approach in rate experiments.