State the two requirements of collision theory for a reaction to occur between two particles.

The particles must actually collide with each other, and the collision must have at least the activation energy. In addition, for anything other than the simplest species, the particles must be correctly orientated when they meet.

What is meant by the rate of a chemical reaction?

The rate of reaction is the change in the concentration of a reactant or a product per unit time; it measures how quickly reactants are used up or products are formed.

Why does increasing the pressure of a gaseous reaction mixture speed it up?

Compressing a gas forces the same number of molecules into a smaller volume, which is equivalent to raising the concentration. The molecules collide more frequently, so the frequency of successful collisions rises and the rate increases.

Why does powdered calcium carbonate react with dilute hydrochloric acid much faster than the same mass of large marble chips?

Grinding the solid into a powder greatly increases the total surface area exposed to the acid. More of the carbonate is available for the acid particles to collide with at any instant, so successful collisions happen more frequently and the rate rises.

WJEC AS-Level Chemistry 1410QS

Formulae and Equations

Topic overview Charges on common ions Building formulae for ionic and molecular compounds Assigning oxidation numbers Using oxidation numbers to follow redox changes Balancing equations and using state symbols Ionic and half equations

Basic Ideas About Atoms

Topic overview Radioactive decay and nuclear equations Alpha, beta and gamma radiation Half-life, dating and the uses of radioisotopes: half-life of radioactive decay Half-life, dating and the uses of radioisotopes: adverse consequences for living cells Ionisation energies and electronic structure Orbitals and the filling of sub-shells Atomic spectra and the energy of light: atomic emission spectrum of the hydrogen atom Atomic spectra and the energy of light: relationship between energy and frequency Atomic spectra and the energy of light: significance of the frequency

Chemical Calculations

Topic overview Relative masses and the mass spectrometer: relative atomic Relative masses and the mass spectrometer: how a mass spectrometer works Relative masses and the mass spectrometer: molecular ion The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectra The mole, the Avogadro constant and molar mass: moles and percentage composition by mass Empirical and molecular formulae Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3 Concentration, solubility and titration calculations: stoichiometry and the limiting reagent Concentration, solubility and titration calculations: titration calculations and estimating a titre Gas volumes and the ideal gas equation Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield Yield, atom economy and experimental uncertainty: how to estimate the percentage error

Bonding

Topic overview Ionic bonds and covalent bonds: ionic bonding in terms of ion formation Ionic bonds and covalent bonds: covalent bonding and forces Electronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarity Electronegativity, polarity and in-between bonds: forces Forces between molecules Hydrogen bonding Shapes of molecules and ions: VSEPR principle and its use Shapes of molecules and ions: bond angles associated with linear

Solid Structures

Topic overview Ionic crystals: sodium chloride and caesium chloride Giant covalent structures: diamond and graphite Simple molecular solids: iodine and ice Metals and the electron sea Using structure to explain physical properties

The Periodic Table

Topic overview How the Periodic Table is organised Oxidation and reduction as electron transfer Trends across periods and down groups Group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid Group 2: reactions, trends and the reasons behind them — trends down the group Group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates Group 2: reactions, trends and the reasons behind them — oxides and hydroxides Identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide Identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid Group 7: the halogens — physical properties and the trend in volatility Group 7: the halogens — reactions of the halogens with metals Group 7: the halogens — displacement reactions and the trend Group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium

Simple Equilibria and Acid-Base Reactions

Topic overview Reversible reactions and dynamic equilibrium Le Chatelier's principle and shifting the position of equilibrium The equilibrium constant Kc Acids, bases, salts and neutralisation pH, strong acids and weak acids Acid-base titrations and their calculations

Thermochemistry

Topic overview Enthalpy changes and their definitions Calculating enthalpy changes from experimental measurements Determining enthalpy changes in the laboratory Hess's law and energy cycles Average bond enthalpies

Rates of Reaction

Topic overview Collision theory and the conditions that change rate Measuring rate and relating it to concentration Energy profiles and activation energy Temperature and the Boltzmann distribution Catalysts and how they work Following reactions in the laboratory

The Wider Impact of Chemistry

Topic overview Social and economic consequences of chemical manufacture and energy supply Environmental consequences of chemical processes and energy production Green chemistry and sustainable practice

Organic Compounds

Topic overview Drawing and reading organic structures Naming organic compounds Chain length, functional groups and physical properties Structural isomerism Electrophiles, nucleophiles, radicals and bond fission

Hydrocarbons

Topic overview Alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits Alkane bonding, combustion and fossil fuels: combustion calculations and acid rain Radical substitution of alkanes The C=C bond and why alkenes are reactive E–Z isomerism in alkenes Electrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic addition Electrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water test Electrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bond Hydrogenation and addition polymers: conditions required Hydrogenation and addition polymers: nature of addition polymerisation

Halogenoalkanes

Topic overview Nucleophilic substitution by hydroxide ions Elimination to form alkenes Reactivity of the carbon–halogen bond and the silver nitrate test Uses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCs Uses of halogenoalkanes, CFCs and the ozone layer: radical mechanism Uses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCs Heating under reflux in the laboratory

Alcohols and Carboxylic Acids

Topic overview Two routes to ethanol: hydration and fermentation — hydration of ethene Two routes to ethanol: hydration and fermentation — fermentation Two routes to ethanol: hydration and fermentation — comparing the two routes on rate Classifying alcohols and dehydrating them Oxidising alcohols to aldehydes, ketones and acids Chemical tests for alcohols and carboxylic acids Carboxylic acids, bases and salts Making an ester and separating it by distillation: esterification Making an ester and separating it by distillation: carrying out the preparation Making an ester and separating it by distillation: separating and purifying Making an ester and separating it by distillation: yield

Instrumental Analysis

Topic overview Reading a mass spectrum Infrared spectra and functional groups NMR spectra and atomic environments Putting the three techniques together

Practical Skills and Techniques

Topic overview Measuring mass, volume, time and temperature Standard solutions and titration technique Indicators and acid–base titration curves Heating, refluxing, distilling and purifying: Heating Heating, refluxing, distilling and purifying: purifying Melting temperature, chromatography and pH measurement Electrochemical cells, rate methods and working safely: Electrochemical cells Electrochemical cells, rate methods and working safely: rate methods and working safely

Planning, Analysis and Evaluation

Topic overview Planning an investigation and controlling variables Recording observations and presenting data Graphs, gradients and processing data Errors, uncertainty, accuracy and precision Evaluating a method and suggesting improvements Research and referencing

Core Experiments and Their Chemistry

Topic overview Gravimetric and acid–base analysis Enthalpy and rate experiments: Enthalpy Enthalpy and rate experiments: rate experiments Redox titrations and electrochemical cells Equilibrium constants and following a titration with a pH probe Organic synthesis, separation and identification: Organic synthesis Organic synthesis, separation and identification: identification tests and chromatography

Mathematical Skills in Chemistry

Topic overview Units, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy units Units, standard form and proportion in chemical calculation: standard form Units, standard form and proportion in chemical calculation: proportion Units, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and units Significant figures, means and uncertainty in experimental data: significant figures in multiplication Significant figures, means and uncertainty in experimental data: relative atomic mass as a weighted mean Significant figures, means and uncertainty in experimental data: absolute and percentage uncertainty Symbols, rearranging equations and logarithms: chemical symbols and what each one asserts Symbols, rearranging equations and logarithms: rearranging an equation Symbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pH Symbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulae Graphs: plotting, gradients, intercepts and tangents — reading a spectrum Graphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphs Graphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangents Graphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphs Shape, symmetry and three-dimensional structure: Shape Shape, symmetry and three-dimensional structure: symmetry

Rates of Reaction

6 subtopics
17 things to know
AS Unit 2

What Rates of Reaction covers

Rates of Reaction is examined on AS Unit 2 of WJEC Chemistry, Energy, Rate and Chemistry of Carbon Compounds. 104 recall questions across 6 subtopics.

Where it is assessed

AS Unit 2

Energy, Rate and Chemistry of Carbon Compounds.

Check the WJEC specification ↗

The 6 subtopics

Each one has its own page. WJEC AS Chemistry students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why does only a very small fraction of the collisions in a reaction mixture lead to reaction?

Key term

Define the rate of reaction, and use collision theory to explain why most collisions fail and why higher concentration speeds up a reaction.

Key term

Explain, using collision theory, why increasing the concentration of a reactant in solution increases the rate of reaction.

Key term

Why is a graph of the volume of gas against time steepest at the beginning and gradually flatter thereafter?

Key term

Define activation energy, give the axes of an energy profile, and explain why even an exothermic reaction needs energy to start.

Process

For an exothermic reaction, is the activation energy of the forward reaction larger or smaller than that of the reverse reaction? Explain.

Key term

Explain the relationship between the size of the activation energy and the rate of a reaction at a given temperature.

Key term

Describe how the shape of the Boltzmann distribution curve changes when the temperature is raised.

Process

Describe how the Boltzmann distribution is used to explain the effect of a catalyst.

Key term

What is meant by a heterogeneous catalyst?

Key term

Explain how a solid catalyst speeds up a reaction between two gases.

Key term

Why is using a catalyst usually cheaper for a manufacturer than simply raising the temperature?

Key term

Describe how a gas collection method can be used to follow the rate of a reaction that produces a gas.

Key term

Describe how the reaction between sodium thiosulfate solution and dilute hydrochloric acid is used to study the effect of concentration on rate.

Key term

What is an iodine clock reaction?

Key term

Why is the initial rate the most reliable rate to use when comparing experiments?

Numbers

A reaction produces 48 cm³ of gas in the first 30 s. Calculate the mean rate of reaction over that period.

Take Rates of Reaction further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

Rate of reaction vs Activation energy
Rate of reaction: the change in concentration of a reactant or product in unit time. Activation energy: the minimum energy a colliding pair must have for a reaction to occur.

Questions from this topic

Each one with the model answer underneath.

1
State the two requirements of collision theory for a reaction to occur between two particles.
Model answer
The particles must actually collide with each other, and the collision must have at least the activation energy. In addition, for anything other than the simplest species, the particles must be correctly orientated when they meet.
2
Why does only a very small fraction of the collisions in a reaction mixture lead to reaction?
Model answer
Most collisions are between particles whose combined energy is below the activation energy, so the existing bonds cannot be broken and the particles simply bounce apart. Only the small proportion of collisions that are energetic enough, and correctly orientated, produce products.
3
What is meant by the rate of a chemical reaction?
Model answer
The rate of reaction is the change in the concentration of a reactant or a product per unit time; it measures how quickly reactants are used up or products are formed.
4
Explain, using collision theory, why increasing the concentration of a reactant in solution increases the rate of reaction.
Model answer
A higher concentration means more reactant particles in the same volume, so the particles are closer together. Collisions therefore happen more often, and since the same proportion of collisions is successful, more successful collisions occur per second and the rate rises.
5
Why does increasing the pressure of a gaseous reaction mixture speed it up?
Model answer
Compressing a gas forces the same number of molecules into a smaller volume, which is equivalent to raising the concentration. The molecules collide more frequently, so the frequency of successful collisions rises and the rate increases.
6
Why does powdered calcium carbonate react with dilute hydrochloric acid much faster than the same mass of large marble chips?
Model answer
Grinding the solid into a powder greatly increases the total surface area exposed to the acid. More of the carbonate is available for the acid particles to collide with at any instant, so successful collisions happen more frequently and the rate rises.

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Bonding

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Rates of Reaction revision questions

Is Rates of Reaction examined at AS in WJEC Chemistry?: Yes. It sits in AS Unit 2, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Rates of Reaction cover in WJEC AS-Level Chemistry?: 6 subtopics: collision theory and the conditions that change rate, measuring rate and relating it to concentration, energy profiles and activation energy, temperature and the boltzmann distribution, catalysts and how they work and following reactions in the laboratory. Each one has its own page with questions and model answers.
How is Rates of Reaction organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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