Group 7 elements

Question 1

key facts

Answer 1

• MOST REACTIVE non-metallic group
• In nature they exist in the form of their stable halide ions either dissolved in the sea or combined with K+ or Na+ forming solid deposits.
• At RTP, they exist as DIATOMIC molecules
• Have 7 electrons in their outer shell
• Electron configuration ends in ns2 np5 where n is the highest shell number or energy level.

Question 2

The MP & BP …

Answer 2

INCREASES down the group:

• The STRENGTH of the LONDON FORCES INCREASES.
• This is due to the INCREASE in the number of ELECTRONS in the diatomic molecules going down the group.
• This leads to LARGER TEMPORARY DIPOLES.
• MORE ENERGY is required to separate the molecules.

Question 3

Consequence of the trend in boiling point

Answer 3

The PHYSICAL STATES of the elements changes from a GAS to SOLID down the group.

Fluorine ~ pale yellow GAS
Chlorine ~ pale green GAS
Bromine ~ red-brown LIQUID
Iodine ~ shiny grey-black SOLID
Astatine ~ never been seen (radioactive)

Question 4

TREND in ELECTRONEGATIVITY

Answer 4

DECREASES down the group.

This is because the following INCREASE:
- Radius of the atom
- Distance between bonding electrons and nucleus
- Number of electron shells
- Nuclear charge

Therefore:
The ATTRACTION of the nucleus for the bonding pair of electrons DECREASES and therefore also the electronegativity.

Question 5

What type of AGENT are halogens and the TREND in the STRENGTH of the agent down the group.

Answer 5

OXIDISING AGENTS:
- halogens have seven electrons in the outer shell so want to gain one electron.
- This gain in electrons is called REDUCTION so they are behaving as oxidising agents.

OXIDISING STRENGTH:
- The ability of a halogen to attract an electron to form a halide ion.
- DECREASES down the group due to same reasoning as trend in electronegativity.

Question 6

DISPLACEMENT (redox) reactions

Answer 6

• A MORE REACTIVE halogen will oxidise and displace the halide ion of a less reactive halogen.

Chlorine ~ bromide or iodide
Bromine ~ iodide

Question 7

COLOURS of halogens in SOLVENTS

Answer 7

Chlorine:
water ~ pale green
cyclohexane ~ pale green

Bromine:
water ~ orange
cyclohexane ~ orange

Iodine :
water ~ brown
cyclohexane ~ violet/purple

*cyclohexane (organic solvent containing C&H)

Question 8

Reaction of: Chlorine & Sodium bromide

Observations before and after adding cyclohexane with reasoning

Answer 8

Chlorine + sodium bromide ——– sodium chloride + bromine

Observation before ~ ORANGE:
BROMINE is less reactive than CHLORINE

Observation after ~ ORANGE colour in cyclohexane layer :
Dure to NON-POLAR bromine being more soluble in cyclohexane.

• Chlorine OXIDSES and DISPLACES
  bromide ions.
• The redox reaction shows chorine is more reactive than bromine.

Question 9

DISPROPORTIONATION reaction

Answer 9

A type of REDOX reaction in which an elements is BOTH oxidised and reduced.

Question 10

Use 1 of CHLORINE

Answer 10

WATER PURIFICATION ~ adding chlorine to water to kill bacteria and make it safe to drink.

Cl2 + H2O —– HClO(chloric acid) + HCl

• Chlorine is both oxidised and reduced so is a disproportionation reaction.
• The product mixture turns blue litmus paper red indicating the products are ACIDIC.

Question 11

BENEFITS and RISKS of using chlorine in water treatment

Answer 11

BENEFITS:
- Bacteria in water are killed by the reactive oxygen atoms produced by the slow decomposition of chloric (I) acid:

HClO —– HCl + O

RISKS:
- Chlorine gas is TOXIC.
- Chlorinated hydrocarbons are
carcinogenic and can form when alkanes
react with chlorine to form a
CHLORALKANE

Question 12

Uses 2 of CHLORINE

Answer 12

MAKING HOUSEHOLD BLEACH :
- Cold dilute sodium hydroxide is added to Cl2 at room temperature due to Cl2 being slightly soluble.

Cl2 + 2NaOH —— NaCl + NaClO + H2O

• Chlorine is both oxidised and reduced so it is a disproportionation reaction.
• Household bleach is a solution of SODIUM CHLORIDE and SODIUM CHLORATE in a 1:1 molar ratio.
• The chlorate ion kills bacteria
• The product mixture is used to make
  STRONGER commercial bleach which is
  CHLORIC(I) acid or HYPOCHLOROUS acid,
  HClO

Question 13

They form..

Answer 13

• SIMPLE MOLECULAR structures
• Held together by LONDON FORCES between the diatomic molecules , X2.

Question 14

Stability & uses of HALIDE IONS

Answer 14

STABILITY:
- Stable, unlike their elements
- They have a full outer shell of electrons

USES:
Sodium chloride NaCl ~
- used as a common salt

Sodium fluoride NaF ~
- used in toothpaste to prevent tooth decay

Calcium Fluoride CaF2 ~
- uses to make lenses which focus IR light

Question 15

How do we test for halide ions ?

Answer 15

Use PRECIPITATION reactions:

• Precipitates can be formed when two AQUEOUS solutions are mixed together.
• A SOLID is formed as a result of a chemical reaction

Question 16

METHOD for testing for halide ions

Answer 16

1 ~ DISSOLVE unknown halide salt in water

2 ~ Add aqueous SILVER NITRATE (AgNO3)

3 ~ A SILVER HALIDE PRECIPITATE , AgX(s) , forms whose colour depends upon the halide ion present.

Question 17

Halide ions test equations & results for:
- Chloride
- Bromide
- Iodide

Answer 17

CHLORIDE:
AgNO3(aq) + NaCl(aq)— AgCl(s) + NaNO3(aq)

Ag+ (aq) + Cl- (aq) —- AgCl(s)

Colour of precipitate ~ WHITE
Addition of NH3 ~ precipitate is SOLUBLE in DILUTE ammonia.

BROMIDE:
AgNO3(aq) + NaBr(aq)— AgBr(s) + NaNO3(aq)

Ag+ (aq) + Br-(aq) —— AgBr (s)

Colour of precipitate ~ CREAM
Addition of NH3 ~ precipitate is SOLUBLE in CONCENTRATED ammonia.

IODIDE:
AgNO3(aq) + NaI(aq) — AgI(s) + NaNO3(aq)

Ag+ (aq) + I- (aq) —– AgI (s)

Colour of precipitate ~ YELLOW
Addition of NH3 ~ precipitate is INSOLUBLE in CONCENTRATED ammonia.