group 7 - halogens

Question 1

what is the name of the group 7

Answer 1

the halogens

Question 2

how do the colours of the halogens vary

Answer 2

get darker going down the group

Question 3

what is the appearance of F2

Answer 3

yellow gas
very reactive
toxic

Question 4

what is the appearance of Cl2

Answer 4

green gas
very reactive
toxic

Question 5

what is the appearance of Br2

Answer 5

orange liquid
very reactive
toxic
often in solution as aqueous bromine water

Question 6

what is the appearance of I2

Answer 6

grey crystals
reactive
toxic
easily turns into a purple vapour upon heating

Question 7

how does the volatility change down group 7

Answer 7

Going down the group, the boiling point of the elements increases which means that the volatility of the halogens decreases
This means that fluorine is the most volatile and iodine the least volatile

Question 8

how does the bond strength change down the group

Answer 8

it decreases
Bond enthalpy is the energy needed to break one mole of covalent bonds
The higher the bond enthalpy, the stronger the bond
An exception to this is fluorine which has a smaller bond enthalpy than chlorine and bromine
Fluorine is so small that when two atoms of fluorine get together their lone pairs get so close that they cause significant repulsion counteracting the attraction between the bonding pair of electrons and two nuclei

Question 9

what intermolecular forces exist between the halogens

Answer 9

van der Waals
The halogens are simple molecular structures with weak van der Waals’ forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces

Question 10

how does the strength of the VDW change down the group?

Answer 10

The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces
Therefore, the larger the molecule the stronger the van der Waals’ forces between molecules

Question 11

what is the trend in melt + boiling points

Answer 11

increase
-bigger molecules
-more VDW
-higher melt/boil
more energy required to break forces

Question 12

what is the trend in electronegativity?

Answer 12

electronegativity of an atom refers to how strongly it attracts electrons towards itself in a covalent bond
decrease
Going down the group, the atomic radii of the elements increase which means that the outer shells get further away from the nucleus
An ‘incoming’ electron will therefore experience more shielding from the attraction of the positive nuclear charge
The halogens’ ability to accept an electron (their oxidising power) therefore decreases going down the group

Question 13

what is the trend in 1 IE

Answer 13

decrease
atoms get bigger
more shielding
weaker attraction from nuclear to electron in the outer shell
so less energy is needed to remove one mole of electrons from the outer shell of gaseous atom/ion

Question 14

what is the oxidising power of the group

Answer 14

decrease
-more shells of electrons
weaker attraction bwt outer shell + nucleus
-harder to attract/gain an extra electron to gain a full outer shell

Question 15

how do the halogens act as oxidising agents

Answer 15

Halogens react with metals by accepting an electron from the metal atom to become an ion with 1- charge
Eg. Ca (s) + Cl2 (g) → CaCl2 (s) consisting of Ca2+ and 2Cl- ions

Halogens are therefore oxidising agents:
Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases)
Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases)

Question 16

reactions of halogen with halide

Answer 16

Cl2 with Br- = orange solution
Cl2 with I- = brown solution
Br2 with Cl- = no visable change
Br2 with I- = brown solution
I2 with Cl- and Br- = no visible change

Question 17

how do the halogens act as reducing agents

Answer 17

Halide ions can also act as reducing agents and donate electrons to another atom
The halide ions themselves get oxidised and lose electrons
The reducing power of the halide ions increases going down the group
This trend can be explained by looking at the ionic radii of the halide ions
Going down the group, the halide ions become larger
The outermost electrons get further away from the nucleus
The outermost electrons also experience more shielding by inner electrons
As a result of this, the outermost electrons are held less tightly to the positively charged nucleus
Therefore, the halide ions lose electrons more easily going down the group and their reducing power increases

Question 18

what is thermal stability

Answer 18

Thermal stability refers to how well a substance can resist breaking down when heated
A substance that is thermally stable will break down only at high temperatures

Question 19

how does the thermal stability change going down the group- hydrogen halides

Answer 19

The hydrogen halides formed from the reaction of halogen and hydrogen gas decrease in thermal stability going down the group
The decrease in thermal stability can be explained by looking at the bond energies of the hydrogen-halogen bond
Going down the group, the atomic radius of the halogens increases
The overlap of its outer shell with a hydrogen atom therefore gives a longer bond length
The longer the bond, the weaker it is, and the less energy required to break it
As the bonds get weaker, the hydrogen halogens become less stable to heat going down the group

Question 20

how can u test for the reducing power of the halide ion

Answer 20

react with H2SO4
Cl- does not reduce
Br- reduce from S (+6) to S(+4)
I- reduce from S(+6) to S (-2)

Question 21

how to write half equations for the reducing power

Answer 21

add water and h+ to balance

Question 22

what solution can be used to test for halide ions

Answer 22

Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding silver nitrate solution dropwise

Question 23

why is nitric acid used before adding silver nitrate

Answer 23

Remove carbonate ions
The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions

Question 24

what is the general equation for halide test

Answer 24

Ag+ (aq) + X- (aq) → AgX (s)

Question 25

what are the results of halide test

Answer 25

Silver chloride (AgCl) is a white precipitate
Silver bromide (AgBr) is a cream precipitate
Silver iodide (AgI) is a yellow precipitate

Question 26

why do you add dilute and conc ammonia after silver nitrate test

Answer 26

Because the white, cream and yellow precipitates could look very similar in colour, ammonia is often used as a follow up test to determine which halide ion is present
Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion
If the precipitate dissolves in dilute ammonia the unknown halide is chloride
If the precipitate does not dissolve in dilute, but does dissolve in concentrated ammonia the unknown halide is bromide
If the precipitate does not dissolve in dilute or concentrated ammonia, then the unknown halide is iodide

Question 27

what are the reactions with H2SO4 + general equation

Answer 27

Chloride, bromide and iodide ions react with concentrated sulfuric acid to produce toxic gases
These reactions should therefore be carried out in a fume cupboard
The general reaction of the halide ions with concentrated sulfuric acid is:
H2SO4(l) + X-(aq) → HX(g) + HSO4-(aq)

Question 28

reaction of bromine with sulphuric

Answer 28

The reaction of sodium bromide and concentrated sulfuric acid is:
H2SO4 (l) + NaBr (s) → HBr (g) + NaHSO4 (s)

The concentrated sulfuric acid oxidises HBr which decomposes into bromine and hydrogen gas and sulfuric acid itself is reduced to sulfur dioxide gas:
2HBr (g) + H2SO4 (l) → Br2 (g) + SO2 (g) + 2H2O (l)

The bromine is seen as a reddish-brown gas

Question 29

reaction of iodine with sulphuric

Answer 29

The reaction of sodium iodide and concentrated sulfuric acid is:
H2SO4 (l) + NaI (s) → HI (g) + NaHSO4 (s)

Hydrogen iodide decomposes readily
Sulfuric acid oxidises the hydrogen iodide to form several products:
The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur dioxide gas:
2HI (g) + H2SO4 (l) → I2 (g) + SO2 (g) + 2H2O (l)

Iodine is seen as a violet/purple vapour also as a black solid
The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur:
6HI (g) + H2SO4 (l) → 3I2 (g) + S (s) + 4H2O (l)

Sulfur is seen as a yellow solid
The concentrated sulfuric acid oxidises HI and is itself reduced to hydrogen sulfide:
8HI (g) + H2SO4 (l) → 4I2 (g) + H2S (s) + 4H2O (l)

Hydrogen sulfide has a strong smell of bad eggs

Question 30

what is a disproportionation reaction

Answer 30

is a reaction in which the same species is both oxidised and reduced
The reaction of chlorine with dilute alkali is an example of a disproportionation reaction

Question 30

how is chlorine useful

Answer 30

Chlorine can be used to clean water and make it drinkable
Chlorine is used in water treatment to kill bacteria. It has been used to treat drinking water and the water in
swimming pools. The benefits to health of water treatment by chlorine outweigh its toxic effects.

The reaction of chlorine in water is a disproportionation reaction in which the chlorine gets both oxidised and reduced
Chloric(I) acid (HClO) sterilises water by killing bacteria
Chloric acid can further dissociate in water to form ClO-(aq):
HClO(aq) → H+(aq) + ClO-(aq)

ClO-(aq) also acts as a sterilising agent cleaning the water

Question 31

what is the reaction of chlorine in water

Answer 31

Chlorine gets oxidised as there is an increase in ox. no. from 0 to +1 in ClO-(aq)

Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl-(aq)

Question 32

what is the reaction of chlorine with sunlight

Answer 32

2Cl2 (g) + 2H2O(l) = 4HCl (aq) + O2 (g)
Pale green to colourless

Question 33

chlorine with water

Answer 33

Cl2(g) + H2O (l) ⇌ HClO (aq) + HCl (aq)

Question 34

chlorine with water and sunlight

Answer 34

occurs.
2Cl2 + 2H2O = 4HCl + O2

Question 35

Reaction of chlorine with cold dilute NaOH solution

Answer 35

Cl2 (and Br2, I2) in aqueous solutions will react with cold sodium hydroxide. The colour of the halogen
solution will fade to colourless.
Cl2 (aq) + 2 NaOH (aq)  NaCl (aq) + NaClO (aq) + H2O (l)