Why does a reaction with a high activation energy need heating before it will start?

Heating gives the particles more energy so that more of their collisions reach the activation energy.

Does an exothermic reaction still need activation energy?

Yes, because energy must first be supplied to break the bonds in the reactants before any energy can be released.

What happens to colliding particles that do not have the activation energy?

They simply bounce apart again without reacting.

Explain in terms of activation energy why most reactions go faster when the temperature is raised.

A greater proportion of the collisions has energy equal to or above the activation energy, so more of them are successful.

OCR GCSE Chemistry A J248

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions Ionic equations, and state symbols The mole and the Avogadro constant Conservation of mass, and changes of mass in open systems Stoichiometry, and calculating reacting masses Exothermic and endothermic reactions, and reaction profiles Activation energy, and bond energy calculations Oxidation and reduction, by oxygen and by electrons Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids Dilute and concentrated, weak and strong The pH scale, and hydrogen ion concentration pH as a factor of ten, and measuring pH Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures Nanoparticles: size and surface area to volume Uses and risks of nanoparticulate materials

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Biological methods of metal extraction Rate against yield in industry, and reaction conditions The Haber process and fertiliser production Nitrogen, phosphorus and potassium in agriculture Life cycle assessment Recycling, and the decisions behind it Alloys, and the corrosion of metals Comparing glass, ceramics, polymers, composites and metals Functional groups and homologous series Predicting the products of organic reactions Addition polymerisation Condensation polymerisation, and making a polymer DNA and other natural polymers Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking Chemical cells and fuel cells The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Concentration in mol/dm³, and titration Titration calculations, and the mass of a solute Molar gas volume and the volumes of gases Theoretical mass and percentage yield Atom economy, and choosing a reaction pathway Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Changing the conditions of an equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state Limitations of the particle model How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Transition metals Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine Tests for aqueous cations and anions Flame tests, and identifying metal ions Identifying a species from test results Instrumental methods of analysis

Chemical Reactions

Activation energy, and bond energy calculations

3 things to know
Paper 3

Overview

What activation energy is, why heating, a spark or a flame starts a reaction, why bond breaking is endothermic and bond making exothermic, and calculating energy changes from bond energies.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Using activation energy, explain the spark for methane and air, the effect of heating, bond breaking, and why a candle keeps burning.

Key term

What is activation energy?

Key term

Why does a mixture of methane and air not react until a spark is applied?

Questions from this subtopic

Each one with the model answer underneath.

1
What is activation energy?
Model answer
The minimum energy that colliding particles must have for a reaction to take place.
2
Why does a reaction with a high activation energy need heating before it will start?
Model answer
Heating gives the particles more energy so that more of their collisions reach the activation energy.
3
Why does a mixture of methane and air not react until a spark is applied?
Model answer
The particles do not have the activation energy until the spark supplies it.
4
Does an exothermic reaction still need activation energy?
Model answer
Yes, because energy must first be supplied to break the bonds in the reactants before any energy can be released.
5
What happens to colliding particles that do not have the activation energy?
Model answer
They simply bounce apart again without reacting.
6
Explain in terms of activation energy why most reactions go faster when the temperature is raised.
Model answer
A greater proportion of the collisions has energy equal to or above the activation energy, so more of them are successful.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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AThe products sit at a higher energy level than the reactants.

BThe activation energy, the energy that must be supplied before the reaction can happen.

CThe reactants, the products, the activation energy and the overall energy change.

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