Chemistry - 3.2.3: Group 7 (17), the Halogens

Halogens

The elements in group 7 (17)

Appearance of fluorine?

Pale yellow gas

Appearance of chlorine?

Green gas

Appearance of bromine?

Red-brown liquid

Appearance of iodine?

Black solid

What happens of the physical properties of the halogens as you go down the group?

They become darker and denser

Why does fluorine have many untypical properties?

Fluorine atoms are very small, so the lone electrons in F2 repel each other - this means the F-F bond is (surprisingly) weak

What is the trend in electronegativity for the halogens and why?

It decreases as you go down the group - increased atomic radius and shielding, so weaker attraction between nucleus and bonding pair of electrons in covalent bond

What is the trend in boiling point for the halogens and why?

It increases as you go down the group - atoms become larger, so have more electrons, making induced dipole-dipole forces stronger

What is the trend in atomic radius for the halogens and why?

It increases as you go down the group - each element has one extra filled main level of electrons compared with the one above it

What is the relationship between volatility and boiling point?

The lower the boiling point, the more volatile the element

What happens to halogens during chemical reactions?

They are reduced and gain an electron to form negative -1 ions.

What is the trend in how easily the halogens are oxidised and why?

It increases going down the group (i.e. they lose electrons more easily as you go down the group)

What type of reaction occurs when halogens react with aqueous metal halides

A displacement reaction - the halogen that is more reactive (above it in the periodic table) will displace the halogen that is less reactive

Displacement reaction

A chemical reaction in which one atom or group of atoms replaces another in a compound.

Why can you not investigate fluorine in an aqueous solution?

It reacts with water

What is the trend in the strength of the reducing agents of the halide ions and why?

Halide ions become better reducing agents as you go down the group - size of ion becomes larger, so less attraction between outer electrons and nucleus, so easier to lose an electron

What forms when sodium chloride reacts with sulfuric acid?

Steamy fumes of HCl
Solid sodium hydrogensulfate

Symbol equation for sodium chloride and sulfuric acid

NaCl (s) + H2SO4 (l) -> NaHSO4 (s) + HCl (g)

Is the reaction between NaCl and H2SO4 a redox reaction and why?

No - no oxidation state has changed

What type of reaction occurs between NaCl and H2SO4?

An acid-base reaction

What forms when sodium fluoride reacts with sulfuric acid?

Steamy fumes of HF
Solid sodium hydrogensulfate

Symbol equation for sodium fluoride and sulfuric acid

NaF (s) + H2SO4 (l) -> NaHSO4 (s) + HF (g)

Is the reaction between NaF and H2SO4 a redox reaction and why?

No - no oxidation state has changed

What type of reaction occurs between NaF and H2SO4?

An acid-base reaction

What forms when sodium bromide reacts with sulfuric acid?

Steamy fumes of HBr
Brown fumes of Br2
Colourless So2

Symbol equation for sodium bromide and sulfuric acid

1. NaBr (s) + H2SO4 (l) -> NaHSO42 (s) + HBr (g)

2. 2H+ + 2Br- + H2SO4 (l) -> SO2 (g) + 2H2O (l) + Br2 (l)

OR
3. 2NaBr + 2H2SO4 -> Na2SO4 + Br2 + SO2 + 2H2O

Why are there two reactions between sodium bromide and sulfuric acid?

Bromide ions are larger than other halogens (except iodine) so are stronger reducing agents, so reduce the H2SO4 to SO2 (oxidation state of sulfur changes from +6 to +4, for bromine it changes from -1 to 0)

What type of reaction occurs between NaBr and H2SO4?

First reaction: acid-base reaction
Second reaction: redox reaction (also exothermic)

What forms when sodium iodide reacts with sulfuric acid?

Steamy fumes of HI
Black solid of iodine
Bad egg smell of H2S
Potentially: Yellow solid sulfur + colourless So2

Symbol equation for sodium iodide and sulfuric acid

1. NaI (s) + H2SO4 (l) -> NaHSO4 (s) + HI (l)

2. 8H+ + 8I- + H2SO4 (l) -> H2S (g) + 4H2O (l) + 4I2 (s)

OR
3. 2NaI + 2H2SO4 -> Na2SO4 + I2 + H2S + 2H+ + 2e-

Why are there two reactions between sodium iodide and sulfuric acid?

Iodide ions are larger than other halogens so are stronger reducing agents (better than bromine) so reduce sulfur even more (from +6 to 0 and then -2)

When reacting sodium iodide with sulfuric acid, why is some yellow solid sulfur sometimes seen?

Sulfur passes through oxidation state 0 (oxidation state of uncombined sulfur)

What type of reaction occurs between NaI and H2SO4?

First reaction: acid-base reaction
Second reaction: redox reaction

How do you use silver nitrate ions to identify halide ions?

1. Use acidified silver nitrate solution (acidified with dilute nitric acid)
2. Add a few drops of the AgNO3 solution
3. Observe precipitate that forms:
Fluorine: colourless
Chlorine: white
Bromine: cream
Iodine: yellow

Why do you need to acidify the silver nitrate solution in order to test for halide ions?

To get rid of any soluble carbonate or hydroxide impurities, which would interfere with the test by forming insoluble silver carbonate/hydroxide precipitate

Why can you not use H2SO4 or HCl to acidify silver nitrate and test for fluoride ions?

They would each provide precipitates of silver, which would invalidate the test

Why does silver fluoride form a colourless solution?

It is soluble in water so does not form a precipitate

How can you further test solutions of halide ions in order to identify them?

Add ammonia:
Chloride: dissolves in dilute ammonia
Bromide: dissolves in concentrated ammonia
Iodide: insoluble in concentrated ammonia

Symbol equation of chlorine and water

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

What type of reaction occurs when chlorine reacts with water?

A reversible, disproportionation (redox) reaction

What happens to the oxidation number of chlorine when it reacts with water?

Changes from 0 to -1 and +1

What forms when chlorine reacts with water?

Chloric (I) acid [HClO]

What is the reaction between chlorine and water used for?

To kill harmful microorganisms and purify water for drinking and swimming pools to prevent life-threatening diseases

What is chloric (I) acid used for?

Bleach + it is an oxidising agent that kills bacteria by oxidation

Symbol equation of chlorine and water in sunlight

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

What is an alternative to using chlorine directly in swimming pools + the symbol equation?

Adding solid sodium (or calcium) chlorate (I):
NaClO (s) + H2O <-> Na+ (aq) + OH- + HClO (aq)

When using sodium/calcium chlorate in swimming pools, why must the pool be kept slightly acidic?

In an alkaline solution, the position of equilibrium shifts to the left so the HClO is removed as ClO- ions - must be kept acidic to stop this from happening
(why?? OH- is basic/alkaline, so equilibrium shifts to lower pH)

What happens if swimming pool water is kept too acidic when trying to maintain optimum conditions for chlorinating the pools with NaClO?

Metal components may corrode and swimmers may be affected

What forms when chlorine reacts with cold, dilute sodium hydroxide and give its properties?

Sodium chlorate (I) - an oxidising agent, used in bleach

Symbol equation for chlorine and sodium hydroxide

Cl2 (g) + 2NaOH (aq) -> NaClO (aq) + NaCl (aq) + H2O (l)

What type of reaction occurs when chlorine reacts with sodium hydroxide (an alkali)?

A disproportionation reaction

Why is chlorine used to kill bacteria in swimming pools even though it is toxic?

It is only used in small amounts - the health benefits outweigh the risks

Why can chlorine oxidise bromide ions, but iodine cannot? (1)

Chlorine is a better/stronger oxidising agent (as smaller, less shielding, stronger attraction with outer electron so easier to gain an electron)

Why is using sodium chlorate (I) more efficient than using chlorine to treat swimming pools (4)?

- Chlorine forms HClO and HCl by a disproportionation reaction
- In sunlight, these products decompose to form HCl and O2
- In sunlight, for NaClO, there is only 1 reaction, which forms HClO and NaOH
- So, NaClO is better as a greater proportion of Cl is converted to ClO-

What are 3 disadvantages of using chlorine to treat public drinking water supplies?

- Chlorine gas is very harmful if breathed in, so is difficult to store/transport safely
- Liquid chlorine causes severe chemical burns, so is difficult to store/transport safely
- Chlorine reacts with organic compounds in water to form chlorinated hydrocarbons, which can be carcinogenic

Symbol equation for when silver halides dissolve in ammonia?

AgX (s) + 2NH3 (aq) -> Ag(NH3)2 +(aq) + X- (aq)

What colour solution does aqueous bromine form?

Orange

What colour solution does aqueous iodine form?

Brown

What's a better oxidising agent and why - Cl2 or Cl-?

Cl2 - Cl- already has full outer shell of electrons so cannot gain/accept more from another species

Colour of bromine solution vs colour of iodine solution

Bromine solution: orange, iodine solution: dark brown