3.2.3.1 Trends in properties (group 7)

Question 1

What is the formula and name for all halide ions?

Answer 1

1-, -ide

Question 2

What is the state and appearance of fluorine at room temperature and its characteristics?

Answer 2

Very pale yellow gas
Highly reactive

Question 3

What is the state and appearance of chlorine at room temperature and its characteristics?

Answer 3

Greenish and reactive gas
Poisonous at high concentrations

Question 4

What is the colour of fluorine in a solution?

Answer 4

No colour, doesn’t really form solutions

Question 5

What is the colour of chlorine in solutions?

Answer 5

Often colourless, other times very pale green

Question 6

What is the state and appearance of bromine at room temperature and its characteristics?

Answer 6

Red liquid
Gives off dense brown poisonous fumes

Question 7

What is the colour of bromine in solutions?

Answer 7

Yellow solutions

Question 8

What is the state and appearance of iodine at room temperature and its characteristics?

Answer 8

Shiny grey/black solid, sublimes to purple gas
Solid sublimes to purple gas

Question 9

What is the colour of iodine in solutions?

Answer 9

Brown solution (sometimes black solid present)

Question 10

Describe the trend of electronegativity down group 7

Answer 10

Decreases, number of shells increases down the group, shielding effect increases, attraction between nucleus and shared electron pair decreaes

Question 11

Describe the trend of boiling points down group 7

Answer 11

Increases from F to I, molecules have more electrons, Van der Waal’s forces become stronger, so more energy needed to overcome forces

Question 12

What are the chemical properties of halogens?

Answer 12

Gain electrons in chemical reactions
Reactivity of halogens decreases down group 7
They are good oxidising agents

Question 13

What is the trend in oxidising abilities of halogens?

Answer 13

F2>CL2>Br2>I2

Question 14

Describe the trends in reducing ability of the halide ions

Answer 14

Going down the group, reducing ability increases
because halide ion becomes larger as they have more electron shells. Halide ions have greater shielding effect, so it is easier to lose outer electrons as they are less strongly held by nucleus.

Question 15

Describe the reaction of NaF with conc. H2SO4

Answer 15

Acid-base step: NaF(s) +H2SO4 (l) -> NaHSO4(s) +HF(g)
F- ions are not strong enough reducing agents to reduce the S in H2SO4, so no redox reaction occurs
Observation: White steamy fumes

Question 16

Describe the reaction of NaCl with conc. H2SO4

Answer 16

Acid-base step: NaCl(s) + H2SO4(l) -> NaHSO4(s) +HCl(g)
Cl- ions are not strong enough reducing agents to reduce the S in H2SO4, so no redox reaction occurs.
Observation: White steamy fumes

Question 17

Describe the reaction of NaBr with conc. H2SO4

Answer 17

Acid-base step: NaBr(s) + H2SO4(s) -> NaHSO4 (s) + HBr(g)
Br- ions are stronger reducing agents, so after acid-base reaction they reduce the sulfur in H2SO4 from +6 to +4 in SO2
Redox step: 2HBr + H2SO4 -> SO2 (g) + Br2 (g) + 2H2O(l)
role of H2SO4 - oxidising agent
Observation:
Orange fumes of bromine evolved and a colourless acidic gas (SO2)

Question 18

Describe the reaction of NaI with conc. H2SO4

Answer 18

Acid-base step: NaI (s) +H2SO4(l) -> NaHSO4(s) + HI(g)
I- ions are the strongest halide reducing agents and can reduce sulfur from +6, to +4, to 0, to -2
Redox steps:
2HI + H2SO4 -> SO2(g) + I2(s) + 2H2O(l)
6HI + H2SO4 -> S(s) + 3I2(s) + 4H2O(l)
8HI + H2SO4 -> H2S(g) + 4I2(s) + 4H2O(l)
role of H2SO4: oxidising agent
Observations: White steamy fumes, black solid and purple fumes of iodine evolved, colourless acidic gas (SO2), H2S has with foul egg smell

Question 19

What are the redox equations for the reaction of NaI with conc. H2SO4?

Answer 19

Oxidation: 2I- -> 2e- + I2
Reduction:
H2SO4 + 2H+ + 2e- -> SO2 + 2H2O
H2SO4 + 6H+ + 6e- -> S + 4H2O
H2SO4 + 8H+ + 8e- -> H2S + 4H2O

Question 20

What is the reaction when chlorine reacts with water?

Answer 20

Redox reaction:
Cl2(g) + H2O(l) <-> HClO(aq) +HCl(aq)
Oxidation half-equation:
H2O + Cl2 -> 2HClO + 2H+ +2e-
Reduction half-equation:
Cl2 + e- -> 2Cl-

Question 21

What is used to test for halide ions?

Answer 21

Dilute nitric acid then silver nitrate solution

Question 22

What are the observations when halide ions react with acidified silver nitrate?

Answer 22

F- = colourless solution
Cl- = white ppt
Br- = cream ppt/ pale yellow
I- = yellow ppt

Question 23

What is the ionic equation for halide ions reacting with acidified silver nitrate?

Answer 23

Ag+ (aq) + X-(aq) -> AgX(s)
- if the halide is given as a solid, make sure it is dissolved in water before doing the test.

Question 24

Why is nitric acid added?

Answer 24

To remove carbonate ions that will interfere with the test

Question 25

Why is HCl not used?

Answer 25

It interferes with the test (forms a white precipitate)

Question 26

How can you distinguish between ions in a precipitate?

Answer 26

Add dilute ammonia solution or concentrated ammonia solution

Question 27

What is the effect of adding dilute ammonia solution to precipitate?

Answer 27

Cl- = ppt dissolves
Br- = ppt remains insoluble
I- = ppt remains insoluble

Question 28

What is the effect of adding concentrated ammonia solution to precipitate?

Answer 28

Cl- = ppt dissolves
Br- = ppt dissolves
I- = ppt remains insoluble

Question 29

What is the equation when halide precipitate dissolves in ammonia solution?

Answer 29

AgCl(s) + 2NH3(q) -> Ag(NH3)2+(aq) + Cl-(aq)