PMT Group 7

Question 1

List 2 properties of halogens

Answer 1

● Low melting and boiling points

● Exist as diatomic molecules

Question 2

What is the trend in boiling

point down group 7? Why?

Answer 2

Increases down the group because:

• size of atom increases as more occupied electron shells
• therefore stronger London forces of attraction between molecules
• therefore take more energy to break

Question 3

What is the trend in reactivity down group 7? Why?

Answer 3

Reactivity decreases because:
● Atomic radius increases
● Electron shielding increases
● Ability to gain an electron and form 1- ions
decreases due to lower attraction between that electron and the nucleus

Question 4

Explain the trend of electronegativity down group 7

Answer 4

Down the group the electronegativity of the elements decreases.

This is because the atomic radii increases due to the increasing number of shells so
there is reduced nuclear attraction between the outermost electron and the nucleus.

Question 5

What is the trend in oxidising

ability down group 7? Why?

Answer 5

Decreases down group (Cl strongest, I weakest)
This is because Cl has the fewest occupied electron shells,
greatest force of attraction between outer electrons and
nucleus and thus is the easiest to gain electrons and be
reduced → best oxidising agent

Question 6

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

Answer 6

Increases down the group (Cl- weakest, I- strongest)

I- has the most occupied electron shell so outer electrons are further from the nucleus, and so it has the weakest force of attraction between outer electrons and positive charge of nucleus.

Therefore it is the easiest to be oxidised and lose electrons → best reducing agent

Question 7

Which species is oxidised in
this reaction:
Br2 (l) + 2Na (s) 2NaBr (s)

Answer 7

Na has been oxidised

Oxidation state of 0 to +1

Question 8

When a more reactive halogen displaces a less reactive halide, what is the reaction called?

Answer 8

Displacement reaction

Question 9

What is the colour of chlorine

in water?

Answer 9

Pale green

Question 10

What is the colour of bromine

in water?

Answer 10

Orange

Question 11

What is the colour of iodine in

water?

Answer 11

Brown

Question 12

What is the colour of chlorine

in cyclohexane?

Answer 12

Pale green

Question 13

What is the colour of bromine in cyclohexane?

Answer 13

Orange

Question 14

What is the colour of iodine in

cyclohexane?

Answer 14

Violet

Question 15

Out of the 3 halides Cl- , Br- & I- , which ones can be oxidised by chlorine?

Answer 15

Br- & I- ions

Question 16

Write the equation for chlorine
oxidising bromide ions in water
and associated colour change

Answer 16

Cl₂ (aq) + 2Br- (aq) → 2Cl- (aq) + Br₂ (aq)

Yellow solution

Question 17

Write the equation for Cl2
oxidising 2I- in cyclohexane
and associated colour change

Answer 17

Cl₂ (aq) + 2I- (aq) → 2Cl- (aq) + I₂ (aq)

Purple solution

Question 18

Out of the 3 halides Cl- , Br- & I- , which one of these can be oxidised by bromine?

Answer 18

I- ions

Question 19

Write the equation for bromine
oxidising iodide ions in water
and associated colour change

Answer 19

Br₂ (aq) + 2I- (aq) → 2Br- (aq) + I₂ (aq)

Brown solution

Question 20

Out of the 3 halides Cl- , Br- &
I- , which one of these can be
oxidised by iodine?

Answer 20

Does not oxidise Cl-

or Br-

Question 21

Define disproportionation

Answer 21

The oxidation and reduction of the same

element in a redox reaction

Question 22

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

Answer 22

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

Question 23

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

Answer 23

Disproportionation; chlorine is both

oxidised and reduced

Question 24

Why is chlorine added to

drinking water?

Answer 24

It kills the bacteria in the water and

makes it safer to drink

Question 25

What are the two forms of the | chlorate ion?

Answer 25

ClO- is chlorate (I) | ClO₃- is chlorate (V)

Question 26

What is the equation for forming bleach and conditions?

Answer 26

Cold dilute alkali Cl₂ (aq) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H₂O (l) NaClO is bleach

Question 27

Show that the reaction of chlorine with hot dilute NaOH is a disproportionation reaction

Answer 27

``` 3Cl2 (aq) + 6 NaOH(aq) 5 NaCl (aq) + NaClO3 (aq) + 3H2O (l) Chlorine is been reduced and oxidised. Oxidation state of chlorine has gone from: 0 to -1 in NaCl = reduction 0 to +1 in NaClO3 = oxidation ```

Question 28

What do you use to test for | halide ions?

Answer 28

Acidified AgNO3

Question 29

Why do you add HNO3 ? Why not HCl?

Answer 29

To remove CO₃2- Adding HCl would add Cl- ions, giving a false positive result

Question 30

Result and equation for Cl- test

Answer 30

White precipitate | Ag+ + Cl- --> AgCl(s)

Question 31

What is the result and | equation for the test for Br-?

Answer 31

Cream ppt | Ag+ + Br- → AgBr (s)

Question 32

What is the result and | equation for the test for I-?

Answer 32

Yellow ppt | Ag+ + I- → AgI (s)

Question 33

``` What happens (+ equations) to each of the silver halide precipitates when dilute/conc NH3 are added? ```

Answer 33

AgCl- dissolves in both dilute and conc AgCl (s) + 2NH₃ (aq) → [Ag(NH₃)₂]+ (aq) + ClAg Br- only dissolves in conc AgBr (s) + 2NH₃ (aq) → [Ag(NH₃)₂]+ (aq) + Br AgI- will not dissolve in either

Question 34

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

Answer 34

Decreases down group (Cl best, I worst) Because: Cl has fewest occupied electron shells, greatest force of attraction between outer electrons and nucleus, easiest to gain electrons and be reduced → best oxidising agent

Question 35

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

Answer 35

Increases down the group (Cl- worst, I- best) Because: I- has the most occupied electron shells, so outer electrons are further from the nucleus, weakest force of attraction between outer electrons and positive charge of nucleus → easiest to be oxidised and lose electrons → best reducing agent

Question 36

``` What products are formed when I- reduces H2SO4 ? Do equations for all 4. ```

Answer 36

H₂SO₄ + 2I- → SO₄2- + 2HI H₂SO₄ + 2H+ + 2I- → SO₂ + I₂ + 2H₂O (SO2 is a choking gas with a pungent odour) H₂SO₄ + 6H+ + 6I- → S + 3I₂ + 4H₂O (S is a yellow solid) H₂SO₄ + 8H+ + 8I- → H₂S + 4I₂ + 4H₂O (H2S smells of bad/rotten eggs)

Question 37

What are the products of Br- + H2SO4 ?

Answer 37

HBr and SO₂

Question 38

Does Cl- reduce H2SO4 ?

Answer 38

No, not a powerful enough reducing agent; only | HCl is formed

Question 39

How can you test for carbonate ions, CO3 2-?

Answer 39

Add strong acid to the sample Collect the gas produced Pass through lime water

Question 40

What are the observations for a positive test of carbonate ions, CO3 2-?

Answer 40

Fizzing | Limewater turns cloudy

Question 41

Write an equation for the | carbonate ion test

Answer 41

CO3 2- (aq) + 2H+ (aq) → H2O (aq) + CO2 (g)

Question 42

How can you test for sulfate ions, SO4 2-?

Answer 42

● Add dilute hydrochloric acid and | barium sulphate to the sample

Question 43

What are the observations for a positive test of sulfate ions, SO4 2-?

Answer 43

White precipitate of barium sulfate is | produced

Question 44

Write an equation for the | sulfate ion test

Answer 44

Ba2+ (aq) + SO4 2- (aq) → BaSO4 (s)

Question 45

What do you use to test for | halide ions?

Answer 45

Acidified AgNO3

Question 46

When testing for carbonate, sulfate and halide ions, in which order should the tests be carried out and why?

Answer 46

1. Carbonate test 2. Sulfate test 3. Halide test Because barium ions forms insoluble precipitate of BaCO3 and silver ions form insolube precipitate of Ag2SO4

Question 47

How can you test for ammonium ions, NH4 + ?

Answer 47

Add sodium hydroxide to the sample and warm it Test the gas produced with red litmus paper

Question 48

What are the observations for | positive ammonium ions test?

Answer 48

● Red litmus paper turns blue | ● Ammonia has a pungent smell

Question 49

Write the equation for | ammonium ions test

Answer 49

NH4+ (aq) + OH- (aq) → NH3 (aq) + H2O (aq)