3.3.3 group 7 , the halogens

Question 1

explain the trend in the boiling points of the elements

Answer 1

• down the group boiling point increases
• number of electrons increases so strength of van der waals forces increases
• more energy needed to overcome these bonds

Question 2

explain the trend in electronegativity of the elements

Answer 2

• down the group electronegativity decreases
• atomic radius increases due to more energy levels, shielding effect increases
• weaker attraction between nucleus and shared pair of electrons in a covalent bond

Question 3

what is the trend in strength of oxidising agent?

Answer 3

• down the group elements decrease in oxidising strength

Question 4

what makes something a good oxidising agent?

Answer 4

the better a species is at gaining electrons

Question 5

outline what happens in a halogen displacement reaction

Answer 5

• they are redox reactions
• the halogen acts as an oxidising agent gaining electrons from the halide ion and becoming reduced

Question 6

explain the trend in reducing ability of the halide ions?

Answer 6

• as you go down the group the reducing ability increases
• as you go down the group the ionic radius increases increasing the ability of the halide ion to lose an electron

Question 7

how can the reducing abilities of the halides be compared?

Answer 7

• react with concentrated sulphuric acid
• takes part in 2 stages: an acid base displacement reaction and redox reaction

Question 8

what sodium halides take part in the acid base displacement reaction?

Answer 8

all sodium halides

Question 9

what sodium halides take part in the redox reaction?

Answer 9

NaBr and NaI

Question 10

in the displacement reaction, what are the roles of the solid sodium halides and the sulphuric acid?

Answer 10

sodium halides: base (proton acceptor)
sulphuric acid: acid (proton donor)

Question 11

why is it only NaBr and NaI that take part in the redox reaction?

Answer 11

• fluoride and chloride ions are not strong enough reducing agents to reduce sulphuric acid

Question 12

what is a product of a redox reaction between sulphuric acid and bromide ions?

Answer 12

SO2 (choking gas)

Question 13

what is the oxidation product from a redox reaction between sulphuric acid and iodide?

Answer 13

I2 - observed as a black solid / purple vapour

Question 14

what is the reduction product observed in a redox reaction between sulphuric acid and iodide?

Answer 14

S - observed as a yellow solid

Question 15

what is reduction product in a redox reaction between sulphuric acid and iodide?

Answer 15

H2S - observed as a gas with a bad egg smell

Question 16

write a half equation to show the oxidation of bromide ions to bromine

Answer 16

2Br- — Br2 + 2e-

Question 17

write a half equation for the reduction of sulphuric acid to sulphur dioxide

Answer 17

H2SO4 + 2H+ + 2e- — SO2 + 2H2O

Question 18

write a half equation to show the oxidation of iodide ions to iodine

Answer 18

2I- — I2 + 2e-

Question 19

write a half equation for the reduction of sulphuric acid to sulphur

Answer 19

H2 SO4 + 6H+ + 6e- — S + 4H2O

Question 20

Write a half equation for the reduction of sulphuric acid to hydrogen sulphide

Answer 20

H2SO4 + 8H+ + 8e- — H2S + 4H2O

Question 21

what products does iodide reduce to?

Answer 21

S
H2S
SO2

Question 22

what products does bromide reduce to?

Answer 22

SO2 only

Question 23

what can you use to identify and distinguish between halide ions?

Answer 23

acidified silver nitrate

Question 24

explain how you would distinguish chloride ions

Answer 24

• acidify with nitric acid
• add silver nitrate
• add dilute then conc ammonia
• white precipitate forms that dissolves in NH3 forms

Question 25

write out an ionic equation between chloride ions and silver ions

Answer 25

Cl- + Ag+ — AgCl

Question 26

explain how you would distinguish bromide ions

Answer 26

- acidify with nitric acid - add silver nitrate - add dilute then concentrated ammonia - cream precipitate forms that dissolves in concentrated ammonia

Question 27

explain how you would distinguish iodide ions

Answer 27

- acidify with nitric acid - add silver nitrate - add dilute then concentrated ammonia - yellow precipitate forms that doesn’t dissolve in either dilute or concentrated ammonia

Question 28

explain how you would test for ammonium ions (method, observation and ionic equ)

Answer 28

- add NaOH and gently heat - hold damp red litmus paper at the mouth of a boiling tube - observation: fumes turn litmus paper from red to blue - NH4+ (aq) + OH-(aq) — NH3(g) + H2O(l)

Question 29

explain how you would test carbonate ions (method,observation and ionic equ)

Answer 29

- add dilute HCl - colllect gas and bubble through limewater - observation: fizzing, bubbling effervescence limewater turns cloudy due to white precipitate forming - CO3 2- +2H+ — CO2 +H2O

Question 30

explain how you would test for sulphate ions (method, observation and ionic equ)

Answer 30

- acidify with HCl - add BaCl2 - observation: white precipitate forms - Ba2+ + SO42- — BaSO4

Question 31

write an equation to show what occurs when chlorine reacts with cold water

Answer 31

Cl2 + H2O — 2H+ + Cl- + ClO-

Question 32

write an equation to show the reaction of chlorine and water in direct sunlight

Answer 32

2Cl2 +2H2O — 4HCl + O2

Question 33

what is a benefit of using chlorine in water treatment?

Answer 33

- chlorine is toxic to bacteria which can cause disease so it protects health if population

Question 34

what is a risk of using chlorine in water treatment?

Answer 34

- chlorine is also toxic to humans so there are risks associated with gas leaks during chlorination - risk of formation of chlorinated hydrocarbons

Question 35

write an equation to show the reaction between chlorine and cold aqueous sodium hydroxide

Answer 35

2NaOH + Cl2 — NaClO + NaCl + H2O

Question 36

what is an observation for the displacement reaction for all sodium halides?

Answer 36

misty white fumes

Question 37

what is an observation for the displacement reaction for all sodium halides?

Answer 37

misty white fumes