Explain how a catalyst increases the rate of a reaction.

It provides an alternative reaction pathway that has a lower activation energy.

What does a reaction profile show?

It shows how the energy of the substances changes as the reaction proceeds, with the reactants on the left, the products on the right and a peak between them.

Does a catalyst change the overall energy change of a reaction?

No, the overall energy change is unaltered; only the activation energy is lowered.

OCR GCSE Combined Science Chemistry J250

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions State symbols in equations Conservation of mass, and changes of mass in open systems Exothermic and endothermic reactions, and reaction profiles Activation energy and why reactions need it Oxidation and reduction in terms of oxygen Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids The pH scale, acidity and alkalinity Measuring pH with indicators and pH meters 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

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Life cycle assessment Recycling, and the decisions behind it Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking 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 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 Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state 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 Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine

Monitoring and Controlling Chemical Reactions

Activation energy, and enzymes as catalysts

3 things to know
Paper 4 (Chemistry)

Overview

How a catalyst gives a pathway with a lower activation energy, reaction profiles for catalysed reactions, why lowering the activation energy speeds a reaction, calculating activation energies and energy changes from profiles, what enzymes are and why organisms need them, denaturing by heat and pH, enzyme specificity, uses in brewing and washing powders, and how enzymes compare with metal catalysts.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe how the reaction profile for a catalysed reaction differs from that of the same reaction without a catalyst.

Key term

Explain why lowering the activation energy makes a reaction faster.

Key term

A reaction profile shows the reactants at 100 kJ/mol, the peak at 180 kJ/mol and the products at 40 kJ/mol. Calculate the activation energy and the overall energy change.

Questions from this subtopic

Each one with the model answer underneath.

1
Explain how a catalyst increases the rate of a reaction.
Model answer
It provides an alternative reaction pathway that has a lower activation energy.
2
What does a reaction profile show?
Model answer
It shows how the energy of the substances changes as the reaction proceeds, with the reactants on the left, the products on the right and a peak between them.
3
Describe how the reaction profile for a catalysed reaction differs from that of the same reaction without a catalyst.
Model answer
The peak is lower because the activation energy is smaller, while the energy levels of the reactants and the products are exactly the same.
4
Does a catalyst change the overall energy change of a reaction?
Model answer
No, the overall energy change is unaltered; only the activation energy is lowered.
5
Explain why lowering the activation energy makes a reaction faster.
Model answer
A greater proportion of the colliding particles then have enough energy to react, so a larger fraction of collisions are successful.
6
A reaction profile shows the reactants at 100 kJ/mol, the peak at 180 kJ/mol and the products at 40 kJ/mol. Calculate the activation energy and the overall energy change.
Model answer
The activation energy is 180 − 100 = 80 kJ/mol, and the overall energy change is 40 − 100 = −60 kJ/mol, so the reaction is exothermic.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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BKI, because potassium loses one electron and iodine gains one.

CCaCl₂, because a calcium atom loses two electrons and each chlorine atom gains one.

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