chapter 8: Reactivity trends

Why are elements at the bottom of group 2 the most reactive?

Because they lose the two outer electrons the easiest

What happens to ionisation energy down group 2?

It decreases

Why does ionisation energy decrease down group 2?

Because of more shielding, larger atomic radius and electrons are lost easier

What does it mean in terms of reduction-oxidation when group 2 elements change from 0 → +2?

The group 2 element has been oxidised

2Ca(s) + O₂ (g) → 2Cao(s)

Calcium burning in air to produce calcium oxide

What do metal oxides react in water to form?

Metal hydroxides

Why does Magnesium oxide struggle to react with water? (The exception)

Because magnesium oxide isn’t very soluble

CaO(s) + H₂O (l) → Ca²⁺ (aq) + 2OH-

Calcium oxide reacting with water to form calcium hydroxide (it dissolves in water to form calcium ions an hydroxide ions)

Why can’t Berylium dissolve?

Because it is insoluble

What happens to group 2 metal oxides solubility down the group?

It increases

What forms when group 2 metals react with water?

Metal hydroxide solutions and hydrogen gas

Ca(s) + 2H₂O (l) → Ca²⁺ (aq) + 2OH- + H₂(g)

Calcium reacting with water to form calcium hydroxide and hydrogen

What happens down group 2 reactivity when they react with water?

It increases

What are group 2 elements also called?

alkali earth elements

Why are group 2 elements also called alkali earth elements?

They form alkaline (basic) oxides and hydroxides,

What is disproportionation?

When a single element is simultaneously oxidised and reduced

What’s an example of when disproportionation happens?

When halogens react with cold dilute alkali solutions like hydroxide

X₂ + 2NaOH → NaXO + NaX + H₂O

O +1 -1 = Oxidation no of X

Equation showing disproportionation

What is the process of making bleach?

Chlorine gas is reacted with cold dilute aqueous sodium hydroxide which produces sodium chlorate(I)

Why is the process of making bleach a disproportionation reaction?

Because the chlorine undergoes disproportionation

What is formed when chlorine undergoes disproportionation when reacting with water?

Hydrochloric acid and chloric (I) acid

Cl₂ + H₂O ⇌ HCl + HClO-

O -1 +1

Example of chlorine undergoing disproportionation when reacting with water

What does the product of chlorine and water- chloric (I) acid, ionise into?

Chloric (I) acid ionises to make chlorate(I) ions (also called hypochlorite ions)

Why is chlorine added to water?

The chloric acid formed reacts with more water to release the chlorate(I) ions. This is why chlorine is added to water, the chlorate ion formed kills the bacteria

How to make water safe to drink with chlorine

Chlorine is added to drinking water as the chlorate(I) ions is produced which kills the bacterias and make it safe to drink

What’s a negative of using chlorine to treat water?

Chlorine is toxic and can cause chemical burns. It reacts with organic compounds to form chlorinated hydrocarbons which can be carcinogenic

What are alternative to treat water instead of chlorine?

Ozone. It’s great at killing microorganisms.

UV light also kills microorganisms my damaging DNA

What’s a downside of using Ozone to treat water?

It’s too expensive and has a short half life- meaning the treatment isn’t permanent

What’s a downside of using UV light to treat water instead of chlorine?

It’s ineffective in cloudy water

Why does boiling point increase down the halogen group?

Because the molecules are larger so the induced dipole-dipole forces is larger. Also, they have more electrons so intermolecular forces are stronger

What do halogens turn into when they are reduced?

Halide ions

Why are halogens good oxidising agents?

Because they are good at getting reduced

Why does reactivity increase up the halogen group?

Reactivity increases since atomic radius decreases and there is less shielding so the attraction from the nucleus to an electron is stronger

What does a displacement reaction test in halogen?

The halogens relative oxidising strengths

what happens during a displacement reaction?

More reactive halogens will oxidise and displace the halide ions of less reactive halogens

What will the colour change be when you mix bromine water with a potassium iodide solution as the bromine displaces the iodide ions?

From orange to brown.

Iodine water is brown and bromine water is orange

How do you avoid confusing the halogens during a displacement reaction?

Add an organic solvent (like hexane) to the end mixture so that the halogen that’s present will dissolve in the solvent and form a layer

What colour change will happen with the presence of iodine when you add an organic solvent?

Pink

What colour change will happen with the presence of bromine when you add an organic solvent?

Orange/red

What colour change will happen with the presence of chlorine when you add an organic solvent?

A pale yellow/green