Elements from the sea

Question 1

What are the potential products of electrolysis in an aqeous solution?

Answer 1

Cathode
If the metal is less reactive than hydrogen (e.g sliver or copper) then the metal will be formed
If the metal is more reactive than hydrogen (e.g all group 1 and 2 metals and aluminium) hydrogen gas will be formed

Anode
If a halide is present the halogen will be formed (only if the solution is concentrated)
If the solution doesn’t contain a halide (or a dilute conc of solution containing halide) oxygen will be formed

Question 2

What is the half equation for the formation of oxygen at the anode?

Answer 2

4OH- (aq) –> O2 (g) + 2H2O (l) + 4e-

Question 3

How do you get chlorine/ iodine/ bromine from brine?

Answer 3

chlorine - electrolysis
bromine - bubble chlorine gas through brine (the chlorine will displace the bromine
2Br- + Cl2(g) –> 2Cl- + Br2(g)
iodine - the same

Question 4

What do oxidation number represent?

Answer 4

How many electrons an atom has

Question 5

How do you carry out a iodine - sodium thiosulfate reaction to find the concentraation of a oxidising agent?

Answer 5

1) Measure out 25cm2 of potassium iodate(V)
2) Add to excess acidic potassium iodide
The iodate(V) ions will oxidise the iodide ions to iodine
IO3-(aq) + 5I-(aq) + 6H+ —> 3I2(aq) + 3H2O(l)
3) Titrate this solution with sodium thiosulfate
The iodine reacts with thiosulfate ions
I2(aq) + 2S2O3 2- (aq) —> 2I- (aq) + S4O6 2- (aq)
do this by…
1) From the burette add sodium thiosulfate solution to the solution containing iodine in the flask
2) Its hard to see the end point so when the iodine colour fades to pale yellow add starch indicatior
3) Add sodium thiosulfate drop wise until the blue colour dissapears
4) This means all the iodine has reacted now you can calculate the moles of iodine in the solution

Question 6

Describe the trend in volatility down group 7

Answer 6

Decreases down the group
Increasing strength of istantaneous dipole- induced dipole bonds (IDIDs) - these increase as the size of atoms increase

Question 7

What are the colours of the halogens at RTP, in water and in hexane?

Answer 7

Halogen RTP Water Hexane
Florine pale yellow gas X X
Chlorine green gas colourless colourless
Bromine red- brown liquid orange orange/ red
Iodine grey solid brown purple

Question 8

What is the trend in reactivity down group 7?

Answer 8

Halogens react by gaining 1 electron
Down the group the atoms become larger - the outer electrons become futher from the nucleus and are more sheilded making it harder to gain an electron so reactivity decreases down the group

Question 9

How does displacment work in halgoens and halides?

Answer 9

A more reactive halogen will displace a less reactive halide
Chlorine will displace bromine and iodine
Bromine will displace iodine
Iodine won’t displace
Chlorine is more reactive and is a better oxidising agent so is better at getting that extra electron to become Cl-

Question 10

How do you make hydrogen halides?

Answer 10

Add an ionic halide (NaCl) and add it to a conc acid (phosphoric acid)
Sulfuric acid can be used to make HCl but not HBr or HI

Question 11

What happens when sulfuric acid reacts with the hydrogen halides?

Answer 11

HF and HCl don’t react
HBr reduces H2SO4 to SO2
2HBr + H2SO4 —> Br2 + SO2 + 2H2O
HI reduces H2SO4 to H2S
8HI + H2SO4 –> 4I2 + H2S + 4H2O