Making Changes

1. Making Changes Topic overview

2. The topic can be conveniently split into six interrelated sections • Oxidation/Reduction • Salts • Preparation of gases • Metal extraction • Reactions of carbonates • Hydration

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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)

12. 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

13. 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