8 Reactivity trends

what do grp2 metals react w water to form

an alkaline hydroxide and hydrogen gas

how does reactivity change down grp2

reactivity increases down grp2

(Be least reactive

Ra most reactive)

what is produced when a metal reacts w a dilute acid

salt and hydrogen gas

why does reactivity increase down grp2

• the atoms of grp2 elements react by losing their 2 valence electrons to form +2 ions

• the formation of +2 ions from gaseous atoms requires the input of two ionisation energies

• ionisation energies decrease down the group because the nuclear attraction of the outer electrons decreases

• this is due to increasing atomic radius and increasing electron shielding down the group

how does solubility change down grp2 hydroxides

solubility of the hydroxides in water increases down the group therefore the resulting solutions contain more OH-(aq) ions and are more alkaline

what grp2 compound is used in agriculture and what for

• calcium hydroxide Ca(OH)₂

• increase the pH of acidic soils (neutralises acid in soil forming water)

what are grp2 compounds used for in medicine

antacids, treating acid indigestion

what is ‘milk of magnesia’ and the benefits

a suspension of white magnesium hydroxide Mg(OH)₂ in water

• no gas produced

• mostly insoluble (won’t harm body on way down)

reaction of grp2 metal + water(g)

produces metal oxide and hydrogen gas

what else can be used as an antacid and why isn’t it used as often

calcium carbonate CaCO₃ but it produces carbon dioxide :(

what structure do halogens have in their solid state

they form lattices with simple molecular structures

trend in bpt down grp7 and why

bpt increases down grp

• more electrons

• stronger London forces

• more energy required to break IMF

• bpt increases

trend in reactivity down grp7 and why

reactivity decreases down grp

• atomic radius increases

• more electron shielding

• less nuclear attraction to attract an electron from another species

• reactivity decreases

what does fluorine look like at rtp

pale yellow gas

what does chlorine look like at rtp

pale green gas

what does bromine look like at rtp

red-brown liquid

what does iodine look like at rtp

shiny grey-black solid

what does chlorine solution in water and cyclohexane look like

in water: pale green

in cyclohexane: pale green

what does bromine solution look like in water and in cyclohexane

in water: orange

in cyclohexane: orange

what does iodine solution look like in water and cyclohexane

in water: brown

in cyclohexane: violet

halogen-halide displacement reactions

• a solution of each halogen is added to aqueous solutions of the other halides

• a reaction takes place, the halogen displacing the halide from solution if more reactive

• solution changes colour

what needs to be added to halogen-halide displacement reactions to distinguish the results and why

• solutions of iodine and bromine in water appear a similar orange-brown colour depending on conc

• add an organic non-polar solvent e.g. cyclohexane and shake

• non-polar halogens dissolve more readily in cyclohexane than in water

• in cyclohexane the colours are much easier to tell apart - iodine being a violet

what are the results of the halogen displacement reactions

• chlorine has reacted with both Br- and I-

• bromine has reacted with I- only

• iodine has not reacted at all (least reactive)

which is the strongest oxidising agent in the halogens

fluorine

oxidising agent def

a reagent that oxidises (takes electrons from) another species

disproportionation

a redox reaction in which the same element is both oxidised and reduced

what is chlorine used for

water purification

formula for chloric (I) acid

HClO

formula for chlorate (I) ions

ClO-

reaction of chlorine with cold, dilute aqueous sodium hydroxide

what is NaClO used for

household bleach

benefits and risks of chlorine use

• kills bacteria, water safe to drink

• chlorine extremely toxic gas

• is a respiratory irritant in small concs., large concs. can be fatal

how are chlorinated hydrocarbons formed and what’s their risk

• when chlorine in drinking water reacts with organic hydrocarbons e.g. methane formed from decaying vegetation

• suspected to cause cancer

carbonate test

• Add HNO₃ until bubbles stop

  • Removes CO₃^2- ions

  • Gas turns lime water cloudy

sulphate test

• Add Ba(NO₃)₂

• White ppt formed in SO₄^2- present

• Filter to remove BaSO₄ ppt

halide test

• Add AgNO₃

• Confirm with aqueous ammonia

White Cl^- dissolves in dilute NH₃

Cream Br^- dissolves in concentrated NH₃

Yellow I^- doesn't dissolve

test for ammonium ion NH₄⁺

• Add NaOH(aq)

• Ammonia gas NH₃ released

  • No bubbles as very soluble in water

• Warm the mixture

• Test with damp (so NH₃ can dissolve) red litmus paper -> goes blue

• If there's ammonia given off, there are ammonium ions.

what order should you test

1. carbonate

2. sulphate

3. halides