8.1(3.1.2) - Reactivity Trends

Alkaline earth metals

Oxides, as are very reactive E.g. CaCO3

Redox reactions They are seen as reducing agents They reduce another species in the reaction

All react with oxygen to form a metal oxide E.g. Magnesium reacts with oxygen in the air Burns with a very bright white light Forms magnesium oxide

Each Mg increases by +2 - (2Mg so +4) Each O decreases by -2 - (O2 so -4)

React with water to form an alkaline hydroxide and hydrogen gas. Water and Mg reacts slowly Reaction becomes more vigorous with metals further down the group Therefore reactivity increases down the group

E.g Sr + 2H2O —> Sr(OH)2 + H2 Sr increases by 2(+2) Two H decrease by 1 forming H2 Two H don’t change - forming Sr(OH)2

Metal + acid —> salt + hydrogen Mg - +2 so is oxidised 2H in HCl - each H decreases by 1 (-2 in total)

Reactivity increases down the group Ionisation energy decreases down the group

Ionisation energies decrease because: Atomic radius and shielding increases So attraction to nucleus decreases Reactivity increases because ionisation energy decreases when forming 2+ ions/losing electrons more readily to react.

Cl2(aq) + 2NaOH(aq) —> NaClO(aq) +NaCl(aq) + H2O(l) Conditions required: • Dilute aqueous sodium hydroxide • cold conditions

Where an element in a reaction is both oxidised and reduced

-Reactivity decreases down the group Explanation: -Atomic radius increases -More inner shells so shielding increases -Less nuclear attraction to capture an electron -Reactivity decreases

Bromine - Orange/Yellow Chlorine - pale yellow Iodine - Brown in solution and purple when a gas

H

-Boiling and melting point increase down the group Explanation: • Elements have more electrons down the group • So they have more/stronger London Forces • More energy is required to break more/stronger intermolecular forces • So boiling point increases

A redox reaction is a reaction in which one element is reduced and another element is oxidised

Loss of electrons

Gain of electrons

-Add solution of each halogen to aqueous solutions of the other halides -e.g. add Cl2 to solutions of NaI and NaBr -If the halogen added is more reactive than the halide present then: -A reaction takes place, the halogens displaces the glide from solution -The solution changes colour -Colours in solutions for halogens are as follows: -Iodine is brown -Bromine is orange -Chlorine is pale green -Add a solution of cyclohexane to each test tube and shake the mixture fully tell them apart The colours of the halogens in solution of cyclohexane are as follows: -Chlorine - pale green -Bromine - orange -Iodine - violet -Then displacements can be shown by presenting reactions as ionic equations - more reactive halogen displaces present halide ion

Pros: • Used in water treatment • Helps kill bacteria in drinking water(makes it drinkable and safe) and in swimming pools Cons: -Chlorine is toxic to humans(in larger quantities) - is an irritant

• Test for carbonates • Test for sulfates • test for halides • Test for ammonium ions

• Add dilute nitric acid to the solid or solution to be tested in the test tube • If bubbles are produced, the unknown compound could be a carbonate • To prove the gas is CO2: • Bubble the gas through lime waterCa(OH)2 • CO2 reacts to form a fine white precipitate of calcium carbonate • This turns the lime water milky -CO2(g) + Ca(OH)2(aq) —> CaCO3(s) + H20(l)

• Add barium chloride • A white precipitate of barium sulfate forms(BaSO4 is insoluble) • Ionic equation shown: • Ba2+(aq) + SO4^2-(aq) —BaSO4