chemical changes

Electrolysis

Using electricity to break a compound apart.

Electrolyte

A liquid that can carry electricity (molten or dissolved).

Anode

Positive electrode (loses electrons).

Cathode

Negative electrode (gains electrons).

OIL RIG

Oxidation Is Loss, Reduction Is Gain.

Reduction

Gain of electrons.

Oxidation

Loss of electrons.

At the cathode (aqueous)

If metal is less reactive than hydrogen → metal forms, if more reactive → hydrogen gas forms.

At the anode (aqueous)

If halide present → halogen forms, if not → oxygen forms.

Hydrogen equation

2H^+ + 2e^- → H_2.

Chlorine equation

2Cl^- → Cl_2 + 2e^-.

Oxygen equation

4OH^- → O_2 + 2H_2O + 4e^-.

Molten electrolysis

Only metal + non-metal (no water involved).

Molten cathode

Metal is made.

Molten anode

Non-metal is made.

Aluminium ore

Aluminium oxide (Al₂O₃).

Why cryolite is used

Makes melting easier (uses less energy).

Aluminium cathode

Al^{3+} + 3e^- → Al.

Aluminium anode

2O^{2-} → O_2 + 4e^-.

Displacement reaction

More reactive metal pushes out less reactive metal.

Example of displacement reaction

Fe + CuSO_4 → FeSO_4 + Cu.

Reactivity series

List of metals from most to least reactive.

More reactive metals

Lose electrons easier.

Metal + acid

Salt + hydrogen.

Metal + water

Metal hydroxide + hydrogen.

Metal oxide + acid

Salt + water.

Acid + carbonate

Salt + water + carbon dioxide.

Extraction below carbon

Use carbon.

Extraction above carbon

Use electrolysis.

Iron equation

Fe_2O_3 + 3CO → 2Fe + 3CO_2.

Acid

Makes H⁺ ions.

Alkali

Makes OH⁻ ions.

Neutralisation

Acid + base → salt + water.

Ionic equation for neutralisation

H^+ + OH^- → H_2O.

Strong acid

Fully breaks apart in water.

Weak acid

Only partly breaks apart.

pH scale

0–14; Acid: below 7, Neutral: 7, Alkali: above 7.

pH rule

Each number = 10× stronger/weaker.

Titration

Used to find concentration.

Steps of titration

1. Add alkali to flask; 2. Add indicator; 3. Add acid slowly; 4. Stop at colour change.

Indicators

Litmus = red (acid), blue (alkali); Methyl orange = red (acid), yellow (alkali); Phenolphthalein = colourless (acid), pink (alkali).

Making salt steps

Making a soluble salt (using an insoluble base) :

1. Add base (e.g. copper oxide) to acid (e.g. sulfuric acid)

2. Keep adding until some base is left (this means acid is used up)

3. Filter to remove the extra solid

4. Heat the solution to evaporate some water

5. Leave to cool → crystals form

6. Dry the crystals

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Key idea :

Add excess base → makes sure ALL acid is reacted

Gas tests

Hydrogen = squeaky pop; Oxygen = relights glowing splint; Chlorine = bleaches paper.

Memory trick for oxidation and reduction

OIL RIG = Oxidation Loss, Reduction Gain.

Memory trick for electrodes

PANIC = Positive Anode, Negative Is Cathode.