GCSE AQA Chemistry - Chemical Changes

Acids

Produce H+ ions in an aqueous solution.

Bases

Neutralise acids to form a salt and water.

Alkalis

Bases that are soluble in water and produce OH- ions in aqueous solutions.

pH probe

Determines pH electronically

Universal indicator

Changes colour depending on whether the solution is acid, alkaline or neutral.

Titration

Allows you to find exactly how much acid is required to neutralise a quantity of alkali.

What can a titration be used to work out?

The concentration of the acid or alkali.

Where is a pipette and pipette filler used in titration?

To add a set volume of acid or alkali to a conical flask.

How is a burette used in titration? (2)

• Using a funnel, some acid/alkali of known concentration is added to the burette

• Acid/alkali is added to the conical flask a bit at a time until there is a colour change.

What colour is phenolphthalein in alkalis?

Pink

What colour is phenolphthalein in acids or neutral solutions?

Colourless

Strong acids

Completely ionise in aqueous solutions.

Weak acids

Partially ionise in aqueous solutions.

pH

A measure of the concentration of H+ ions in the solution.

As pH decreases by one unit…

Hydrogen ion concentration of the solution increases by a factor of 10.

Concentration of an acid

The amount of acid in a certain volume of water.

Acid + Metal Oxide →

Salt + Water

Acid + Metal Hydroxide →

Salt + Water

Acid + Metal Carbonate →

Salt + Water + Carbon Dioxide

Making soluble salt/copper sulfate crystal - step 1

Neutralisation - insoluble copper oxide (base) is added in excess to gently warmed sulfuric acid and stirred until no more reacts.

Making soluble salt/copper sulfate crystal - step 2

Filtration - Excess copper oxide is filtered out

Making soluble salt/copper sulfate crystal - step 3

Evaporation - The solution is gently heated under a water bath to evaporate the water. Stop heating when the first crystals form at the edge of the solution.

Making soluble salt/copper sulfate crystal - step 4

Crystallisation - The rest of the water evaporates off slowly, and then any excess of the solution can be removed by dabbing between filter paper.

The reactivity series (10)

• Potassium

• Sodium

• Lithium

• Calcium

• Magnesium

• Carbon

• Zinc

• Iron

• Hydrogen

• Copper

Metal + Acid →

Salt + Hydrogen

How can we measure the reactivity of a metal based on it’s reaction with an acid?

• The speed of the reaction - rate at which bubbles of hydrogen are given off

• The temperature change

Metal + Water →

Metal Hydroxide + Hydrogen

Oxidation (2)

• Gain of oxygen

• Loss of electrons

Reduction (2)

• Removal of oxygen

• Gain of electrons

How are metals less reactive than carbon extracted?

By reduction with carbon.

How are metals more reactive than carbon extracted?

By using electrolysis.

Redox reaction

A reaction in which one substance is reduced and another is oxidised.

Displacement

A more reactive metal displacing a less reactive metal from it’s compound.

Ionic equation

An equation that only shows the reactants that react and the products that form.

Spectator ions

Ions that are unchanged in a reaction.

Electrolysis

The process of breaking down a substance using an electric current.

Why is it that the ionic compound must be molten or aqueous to be electrolysed?

So that the ions can move freely and conduct electricity.

Cathode

Where positive metal ions are reduced to the element.

Anode

Where negative metal ions are reduced to the element.

Cryolite

Mixed with aluminium oxide to lower it’s melting point.

Describe electrolysis of aluminium oxide

• Positive Al³⁺ ions are attracted to the negative electrode where they are reduced and sink to the bottom of the tank.

• Negative O^2- ions are attracted to the positive electrode where they are oxidised and combine to form O₂ molecules.

What will be produced at the cathode in electrolysis of an aqueous solution?

• If the metal ions are more reactive than hydrogen, hydrogen gas will be produced

• If the metal ions are less reactive than hydrogen, the metal will be produced

What will be produced at the anode in electrolysis of an aqueous solution?

• If halide ions are present, chlorine, bromine or iodine will be formed

• If no halide ions are present, the OH^- ions are discharged and oxygen will form.

Describe electrolysis of copper (III) sulfate

• Copper metal is produced and coats the electrode

• Oxygen is produced and can be seen as bubbles

Half equations

An equation that shows how electrons are transferred when a substance is oxidised or reduced.