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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

3.2.2 Reaction Rates

CatalystsSimple collision theoryThe Boltzmann distribution

Collision Theory

Specification Reference 3.2.2 (a)–(b)

Quick Notes

  • Rate of reaction is the change in concentration of a reactant or product per unit time.
  • Collision theory states that for a reaction to occur, particles must collide with sufficient energy (activation energy, Ea).
    • Most collisions between reactant particles don’t lead to a reaction as they lack enough energy.
  • Effect of Concentration and Pressure:
    • Higher concentration or gas pressure increases collision frequency.
    • More collisions per second = higher reaction rate.
  • Calculating Reaction Rate:
    • Rate = gradient of graph (change in y ÷ change in time).
    • Tangent to curve gives rate at a specific time.

Full Notes

The rate of a reaction measures how quickly reactants are converted into products. It is calculated as:

Rate of reaction = Change in concentration ÷ Time Rate = Δ[Reactant or Product] ÷ Δt

Units: mol dm−3 s−1

Collision Theory

According to collision theory, for a reaction to occur:

OCR (A) A-Level Chemistry reaction profile diagram showing activation energy required for a successful collision.

If collision energy is lower than activation energy (Ea), the reaction does not occur.

Increasing temperature, concentration, surface area, or adding a catalyst increases the number of successful collisions per second, speeding up a reaction.

Effect of Concentration and Pressure

Reaction rate depends on how often particles collide with enough energy.

Concentration effect on rate:

Increased concentration means more particles in a given volume, resulting in more frequent collisions, giving a faster reaction.

OCR (A) A-Level Chemistry diagram showing that increasing concentration increases collision frequency and reaction rate.

Pressure effect on rate:

Increased pressure (in gases) means particles are pushed closer together, resulting in an increased frequency of collisions and a faster reaction.

OCR (A) A-Level Chemistry diagram showing that increased gas pressure increases particle collisions and reaction rate.

This is all about increasing collision frequency. However, only successful collisions (with sufficient energy and correct orientation) cause reactions.

Calculating the Rate of a Reaction

There are two main methods used at this level to calculate the rate of a reaction:

Summary