Chemistry Topic 4 - Chemical Changes

Oxidation in terms of oxygen

The gain of oxygen by a substance

Reduction in terms of oxygen

The loss of oxygen from a substance

Oxidation in terms of electrons (HT only)

The loss of electrons

Reduction in terms of electrons (HT only)

The gain of electrons

Reactivity of a metal depends on

Its tendency to form positive ions by losing its outer shell electrons

Order of the reactivity series

Potassium / Sodium / Lithium / Calcium / Magnesium / Aluminium / Carbon / Zinc / Iron / Hydrogen / Copper / Silver / Gold

Metals that react violently with cold water

Potassium / Sodium / Lithium / Calcium

Products of a metal reacting with water

Metal hydroxide and hydrogen gas

Metals that react with dilute acids but not water

Magnesium / Zinc / Iron

Products of a metal reacting with an acid

A salt and hydrogen gas

Why Copper does not react with dilute acids

It is less reactive than hydrogen

Displacement reaction

A reaction where a more reactive metal pushes out and replaces a less reactive metal from its compound

Native metals

Unreactive metals like gold found in the Earth's crust as pure elements

Metal ore

A rock that contains enough metal compound to make extraction economically worthwhile

Extraction method for metals less reactive than carbon

Reduction by heating with carbon

Why carbon can extract iron from iron oxide

Carbon is more reactive than iron so it displaces and reduces it

Extraction method for metals more reactive than carbon

Electrolysis of molten compounds

Displacement reaction in terms of redox (HT only)

The more reactive metal is oxidised and the less reactive metal ion is reduced

Acid definition in terms of ions

A substance that releases hydrogen ions (H+) in aqueous solutions

Alkali definition in terms of ions

A soluble base that releases hydroxide ions (OH-) in aqueous solutions

Base definition

A chemical substance that neutralises an acid to form a salt and water (usually metal oxides or hydroxides)

Salt produced by Hydrochloric acid (HCl)

A metal chloride

Salt produced by Sulfuric acid (H2SO4)

A metal sulfate

Salt produced by Nitric acid (HNO3)

A metal nitrate

Product of an acid reacting with a metal oxide

A salt and water

Product of an acid reacting with a metal hydroxide

A salt and water

Product of an acid reacting with a metal carbonate

A salt / water / and carbon dioxide gas

Indicator used to view the full pH scale

Universal indicator

pH value of a neutral solution

pH 7

pH value of a strong acid

pH 0 to 3

pH value of a strong alkali

pH 11 to 14

Ionic equation for a neutralisation reaction

H+ (aq) + OH- (aq) -> H2O (l)

Soluble salt preparation step 1

Add excess insoluble metal oxide or carbonate to the acid until no more dissolves

Soluble salt preparation step 2

Filter the mixture to remove the unreacted excess insoluble solid base

Soluble salt preparation step 3

Heat the remaining solution gently in an evaporating dish to crystallise the salt

Strong acid definition

An acid that completely ionises/splits into ions in an aqueous solution

Examples of strong acids

Hydrochloric acid / Nitric acid / Sulfuric acid

Weak acid definition

An acid that only partially ionises in an aqueous solution

Examples of weak acids

Ethanoic acid / Citric acid / Carbonic acid

pH relationship to hydrogen ion concentration

As the pH decreases by 1 unit / the hydrogen ion concentration increases by a factor of 10

Concentrated vs Dilute acid

Concentrated has a large amount of acid solute per volume / dilute has a small amount of acid solute per volume

Titration purpose

To accurately measure the volumes of acid and alkali that react together completely

Indicator for titration showing a sharp color change

Phenolphthalein or Methyl orange

Phenolphthalein color change from acid to alkali

Colorless in acid to pink in alkali

Methyl orange color change from acid to alkali

Red in acid to yellow in alkali

Why universal indicator cannot be used in a titration

It changes color gradually rather than having a single sharp end-point color change

Electrolysis definition

The splitting up of an ionic compound using an electric current passed through an electrolyte

Electrolyte

A molten liquid or aqueous solution containing free-moving ions that conducts electricity

Cathode

The negative electrode that attracts positive ions (cations)

Anode

The positive electrode that attracts negative ions (anions)

Process occurring at the cathode (HT only)

Reduction because positive ions gain electrons to form neutral atoms

Process occurring at the anode (HT only)

Oxidation because negative ions lose electrons to form neutral atoms

Why solid ionic compounds cannot be electrolysed

Their ions are locked in a rigid lattice and cannot move to carry charge

Aluminium extraction electrolyte mixture

Aluminium oxide dissolved in molten cryolite

Why cryolite is used in aluminium extraction

It lowers the melting point of the aluminium oxide to save huge amounts of thermal energy and cost

Material used for electrodes in aluminium extraction

Carbon (graphite)

Why the anode must be replaced regularly in aluminium extraction

The oxygen gas produced reacts with the hot carbon anodes to form carbon dioxide gas / wearing them away

Rule for the cathode product in aqueous electrolysis

Hydrogen gas is produced if the metal is more reactive than hydrogen

Rule for the anode product in aqueous electrolysis

Oxygen gas is produced unless halide ions (Cl- / Br- / I-) are present / in which case the halogen gas forms

Half equation for hydrogen gas formation at the cathode (HT only)

2H+ + 2e- -> H2

Half equation for chlorine gas formation at the anode (HT only)

2Cl- -> Cl2 + 2e-

Half equation for oxygen gas formation from hydroxide ions (HT only)

4OH- -> O2 + 2H2O + 4e-

Three products from the electrolysis of aqueous sodium chloride (brine)

Hydrogen gas / Chlorine gas / and Sodium hydroxide solution

Industrial use of the sodium hydroxide from brine electrolysis

Used in the manufacture of soap and bleach