Chapter 8: Groups 2 + 7, Qualitative Analysis

what are Group 2 sometimes called?

alkaline earth metals

Group 2 elements often act as …. agents

reducing → they become oxidised by losing the 2 outer electrons in the outer s subshell

Group 2 metal + oxygen →

MO → a metal oxide

magnesium + oxygen appearance:

brilliant white flame

group 2 metal + water →

M(OH)₂ + H₂

the reactivity of group 2 … going down the group

increases

in the reaction between a group 2 metal and water, are all hydrogen atoms reduced?

no

metal + acid →

salt + hydrogen

ionisation energies … down Group 2

decrease

• attraction between nucleus+ outer electrons decreases

• as a result of the increasing atomic radius + increasing shielding

Group 2 oxide + water →

metal hydroxide + water

• alkaline solution

• CaO(s) + H₂O(l) → Ca²⁺ (aq) + 2OH^-(aq)

group 2 hydroxides are only slightly soluble in water. when the solution is saturated, further metal + hydroxidei ons

• 2OH^-(aq) + 2OH^-(aq) → Ca(OH)₂(s)

the solubility of hydroxides in water [does what] going down the group

increases → resulting solutions are more alkaline

going down the group, the [what] of metal does [what]:

• reactivity

• ionisation energies

• solubility of hydroxides

• reactivity increases

• i.e. decreases

• solubility of hydroxides increases

what do farmers add to fields to increase the pH of soil?

Ca(OH)₂

which group 2 compound is used as an antacid?

Mg(OH)₂ → ‘milk of magnesia’ → suspension of white magnesium hydroxide in water

or

CaCO₃ (but causes gas)

how are group 7 elements found on Earth?

stable halide ions in seawater + in compounds like NaCl

at RTP, how do Group 7 elements exist?

diatomic molecules

appearance of group 7 elements individually at RTP

group 7 elements act as … agents

oxidising

group 7 displacement reactions:

if a more reactive halogen is added,

• a reaction occurs → halogen displaces halide (e.g. Cl₂ + 2NaBr → NaCl + Br₂)

• solution changes colour

colours of iodine, bromine, and chlorine in (and not in) cyclohexane:

chlorine: pale green → pale green

iodine: brown → deep violet

bromine: brown → brown

fluorine + astatine:

• fluorine reacts with almost any substance in comes in contact with

• astatine decays rapidly → predicted to be least reactive

trend in reactivity of halogens

Fluorine = strongest oxidising agent

weaker oxidising agents as you go down

what is a disproportionation reaction?

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

chlorine + water (disproportionation)

c.w.c.h. (can’t want, can’t have!)

chloric acid works as a weak bleach → by adding indicator, the indicator will first turn red (acid) before going white because of the bleaching effect

cold, dilute, aqueous sodium hydroxide + chlorine (disproportionation):

C.S.S.S.W

(Charles Second Seriously Sells Water)

NaClO = bleach

pros + cons of chlorine use:

• kills bacteria in water

• toxic gas

• chlorinated hydrocarbons are carcinogenic

test for carbonates:

1. add dilute nitric acid to substance being tested

2. bubbles present prove that the substance could be a carbonate

3. bubble gas through limewater (saturated aqueous Ca(OH)₂)

4. carbon dioxide reacts to form a white precipitate, CaCO₃ → lime water turns milky

reaction for the testing of the gas:

CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

test for sulphates:

• add Barium Chloride or Barium Nitrate to solution

• more likely Ba(NO₃)₂ → if we don’t know whether the substance is a chloride!

• white precipitate is formed

test for halides:

1. add aqueous silver nitrate, AgNO₃ to aq. solution of halide

2. precipitate will be different colours

THEN

3. add dilute ammonia + then conc. ammonia → the different compounds have different solubilities in ammonia!!

positive test results for halides (when AgNO₃ is added)

1. Chloride = white prec.

2. Bromide = cream prec.

3. Iodide = yellow prec.

solubilities of silver halides in ammonia

chloride = soluble in DILUTE NH₃

bromide = soluble in CONC. NH₃

iodide = insoluble

what order do the anion tests need to be carried out in?

1. carbonate → definitely/ definitely not a carbonate

2. sulphate → barium carbonate could be mistaken for barium sulphate

3. halide → silver carbonate/silver sulphate could be mistaken for silver chloride

test for ammonium ions:

1. add aqueous sodium hydroxide (NaOH) to ammonium ion solution

2. heat → ammonia gas is produced

3. damp indicator paper will turn blue