2.3 Group 7

Question 1

What is the trend in boiling point down group 7 ?

Answer 1

•Increases
•Larger atom sizes leads to larger molecules with stronger van der waals, meaning more energy is required.

Question 2

What is the trend in electronegativity down group 7 ?

Answer 2

•Decreases
•This is because the attractive force is weaker in the larger molecules, therefore they are less likely to attract a pair of electron in a covalent bond.

Question 3

What do you use to test for halide ions ?

Answer 3

Acidified AgNO3.

Question 4

Why do you acidify Ag with NO3 not HCl when testing for halide ions ?

Answer 4

•Prevents false positive as Cl- ions will give false positives.

Question 5

Write the result and equation for testing for Cl- .

Answer 5

•Ag+ + Cl- -> AgCl
•White ppt forms.

Question 6

Write the result and equation for testing for Br- .

Answer 6

•Ag+ + Br- -> AgBr
•Cream ppt forms.

Question 7

Write the result and equation for testing for I- .

Answer 7

•Ag+ + I- -> AgI
•Yellow ppt forms.

Question 8

What happens to each of the silver halides when dilute NH3 is added.

Answer 8

AgCl dissolves in both dilute and concentrated ammonia
AgBr only dissolves in concentrated
AgI will not dissolve.

Question 9

What is the trend in oxidising ability down group 7 ? Why ?

Answer 9

It decreases down the group
This is because Cl has the fewest occupied electron shells, the greatest force of attraction between outer electrons and nucleus, therefore it gains electrons and is reduced the easiest.

Question 10

Write the equation for Cl2 oxidising 2I- .

Answer 10

Cl2 + 2I- -> 2Cl- + I2.

Question 11

What is the trend in reducing ability of the halides down the group ? Why ?

Answer 11

It increases down the group
This is because I- has the most occupied electron shells, so outer electrons are further from the nucleus, it has the weakest force of attraction between positive charge of nucleus, easiest to be oxidised and lose electrons .

Question 12

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Question 13

What are the products of Br- + H2SO4 ?

Answer 13

2HBr + 2SO2.

Question 14

Does Cl- reduce H2SO4 ?

Answer 14

No, it is not a powerful enough reducing agent. Only HCl is formed.

Question 15

Why is chlorine added to drinking water ? Why is it safe ?

Answer 15

Forms ClO- ions which oxidise (killing) all microorganisms in water
Once it has done its job, little remains and the health benefits outweigh the risks.

Question 16

What are the potential risks of adding chlorine to drinking water ?

Answer 16

Chlorine is toxic and damages the respiratory system in large quantities
It can form carcinogenic hydrocarbons.

Question 17

Why is the ozone not used to purify water in the UK?

Answer 17

It is more expensive than chlorine and evaporates from water quicker.

Question 18

What is the equation of the reaction of Cl2 with water ?

Answer 18

Cl2 (g) + H2O (l) -> HClO (aq) + HCl (aq)

Question 19

What type of reaction is the reaction of chlorine with water ?

Answer 19

Disproportionation; chlorine is both oxidised and reduced.

Question 20

What are the two forms of chlorate ion ?

Answer 20

ClO- is chlorate (1)
ClO3 - is chlorate(V).

Question 21

What is the equation for making bleach ?

Answer 21

Cl2 + 2NaOH -> NaCl + NaClO + H2O
NaClO is bleach.

Question 22

Give the equation for the reaction of chlorine and water in the presence of sunlight.

Answer 22

2Cl2 + 2H2O -> 4HCl + O2 (g).

Question 23

What is desalinisation?

Answer 23

Converts saltwater into clean potable water
This happens by reverse osmosis or by vacuum distillation at low pressure and temperatures.

Question 24

What are the advantages and disadvantages of desalinisation ?

Answer 24

Advantages - safe, clean, potable water produced anywhere
Disadvantages - uses lots of energy, reverse osmosis has low efficiency, can disturb ecosystems.