Making Changes

Topic overview

The topic can be conveniently split into six interrelated sections

Oxidation/Reduction

Salts

Preparation of gases

Metal extraction

Reactions of carbonates

Hydration

Oxidation & Reduction

A substance is oxidised when

It gains oxygen

Or loses hydrogen

2Cu + O2 2CuO Copper (Cu) is oxidised

A substance is reduced when

It gains hydrogen

Or loses oxygen

CuO + H2 Cu + H2O Copper oxide (CuO) is reduced

Salts

Salts

•Are formed when a metal atom replaces the hydrogen atom(s) in an acid

•Are usually soluble

The acids that are needed to be known about are

•Sulphuric acid - H2SO4 - makes sulphates

•Hydrochloric acid - HCl - makes chlorides

•Nitric Acid - HNO3 - makes nitrates

Metal oxide + acid metal salt + water

Magnesium Oxide + sulphuric Acid magnesium sulphate + water

MgO + H2SO4 MgSO4 + H2O

Metal Hydroxide + acid metal salt + water

Sodium Hydroxide + Hydrochloric acid sodium chloride + water

NaOH + HCl NaCl + H2O

Metal carbonate + acid metal salt + water + carbon dioxide

Calcium carbonate + nitric acid calcium nitrate + water + carbon dioxide

CaCO3 + HNO3 Ca(NO3)2 + H2O + CO2

Neutralisation

When an acid is added to an alkali

Equal amounts of the acid and alkali react together

Technique for measuring amounts is called titration

Titration can be used to measure concentrations of alkaline solutions and acids

Preparation of oxygen

Decomposition of hydrogen peroxide in the presence of manganese dioxide catalyst

Hydrogen peroxide oxygen + water

H2O2 O2 + H2O

Preparation of hydrogen

React a metal with acid

Magnesium + Hydrochloric acid magnesium chloride + hydrogen

Mg + 2HCl MgCl2 + H2

Collection of gases

Upward displacement of water – suitable for a low density, insoluble gas e.g. oxygen or hydrogen

Downward displacement of air – suitable for a dense, soluble gas e.g. chlorine

Gas syringe – suitable for soluble gases, e.g. ammonia or carbon dioxide

Tests for common gases

Oxygen - relights a glowing spill

Hydrogen - “pop” test with a lit spill

Carbon dioxide - turns limewater cloudy (or milky)

Ammonia - forms a white smoke with hydrochloric acid vapour

Chlorine - bleaches moist indicator paper

Metal extraction

An ore is any rock containing commercially useful amounts of a metal

Metals often found as oxides or sulphides

The method used to extract a metal depends upon the reactivity of that metal

Unreactive metals (below hydrogen in the reactivity series) e.g. gold, platinum

Often found un-combined (or “native”) in the ore

Ore simply broken up and the pure metal extracted

Sometimes simple heating of the ore is required to extract the metal

Metals in the middle of the reactivity series (above hydrogen, below carbon)

Metal compound converted to oxide if not already

Heated with carbon, the metal is displaced by the carbon

Carried out in a blast furnace

Reactive metals (above carbon in the reactivity series)

Extraction is more difficult

Either displacement using a more reactive metal (Titanium Chloride + sodium titanium + sodium chloride)

Or use electrolysis (e.g. aluminium or sodium)

Carbonates

Decompose on heating (thermal decomposition) to give the oxide and carbon dioxide

Calcium carbonate Calcium Oxide + carbon dioxide

CaCO3 CaO + CO2

Hydrogen carbonates

Decompose readily on heating to give the oxide, carbon dioxide and water

Sodium hydrogen carbonate sodium oxide + carbon

dioxide + water

Hydration

When a substance has water present it is said to be hydrated, If the water is removed it is dehydrated or anhydrous

E.g. anhydrous copper sulphate is a pale blue/grey colour, addition of a small amount of water to the anhydrous salt turns it into the dark blue hydrated salt.

Anhydrous copper sulphate + water hydrated copper sulphate

CuSO4 + 5H2O CuSO4.5H2O Dehydration is the reverse, for example heating the hydrated copper sulphate will give the anhydrous form and water