What is a reversible reaction?

It is a reaction in which the products can react together to re-form the original reactants.

If the forward direction of a reversible reaction is exothermic, what can be said about the reverse direction?

It is endothermic, and it takes in exactly the same amount of energy as the forward reaction gives out.

Describe what happens when blue hydrated copper(II) sulfate crystals are heated, and how the change can be reversed.

They turn white as water is driven off to leave anhydrous copper(II) sulfate, and adding water turns the white solid blue again.

Describe what happens when ammonium chloride is heated and then allowed to cool.

On heating it breaks down into ammonia and hydrogen chloride gases, and on cooling the two gases recombine to form solid ammonium chloride again.

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

Reversible reactions and dynamic equilibrium

3 things to know
Paper 4 (Chemistry)

Overview

What a reversible reaction is and its symbol, why the reverse of an exothermic reaction is endothermic, heating hydrated copper(II) sulfate and testing for water, heating ammonium chloride, the nitrogen and hydrogen that form ammonia, what dynamic equilibrium and a closed system are, what the position of equilibrium means, and how the forward and reverse rates change until equilibrium is reached.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What symbol is used in an equation to show that a reaction is reversible?

Key term

How is anhydrous copper(II) sulfate used as a test for water?

Key term

Name the two gases that combine reversibly to make ammonia.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a reversible reaction?
Model answer
It is a reaction in which the products can react together to re-form the original reactants.
2
What symbol is used in an equation to show that a reaction is reversible?
Model answer
Two half arrows pointing in opposite directions, written ⇌.
3
If the forward direction of a reversible reaction is exothermic, what can be said about the reverse direction?
Model answer
It is endothermic, and it takes in exactly the same amount of energy as the forward reaction gives out.
4
Describe what happens when blue hydrated copper(II) sulfate crystals are heated, and how the change can be reversed.
Model answer
They turn white as water is driven off to leave anhydrous copper(II) sulfate, and adding water turns the white solid blue again.
5
How is anhydrous copper(II) sulfate used as a test for water?
Model answer
A few drops of the liquid are added to the white solid, and it turns blue if water is present.
6
Describe what happens when ammonium chloride is heated and then allowed to cool.
Model answer
On heating it breaks down into ammonia and hydrogen chloride gases, and on cooling the two gases recombine to form solid ammonium chloride again.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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AMetals are on the left and in the centre, and non-metals are on the right-hand side.

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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