(3.2.3) IC - Group 7, the Halogens

What is the colour and physical state of the first four halogens?

F2 - pale yellow - gas

Cl2 - green - gas

Br2 - red-brown - liquid

I2 - grey - solid

What is the trend with atomic radius going down group 7?

They increase going down the group due to additional electron shells.

What is the trend with reactivity going down group 7?

1. They need to gain an electron to get a full outer shell.

2. As atomic radius increases this becomes harder as the positive attraction of the nucleus is weakened by additional shielding.

3. Therefore, it is harder to attract an electron so reactivity decreases down the group.

What is the trend with melting and boiling points going down group 7?

1. They increase as the atoms get larger going down group 7.

2. Larger atoms have more electrons, making the van der Waals’ forces between the molecules stronger.

3. This means they require a lot of energy to overcome.

What is the trend with electronegativity going down group 7?

1. It decreases going down the group.

2. This is because larger atoms attract less electrons than smaller ones, as their outer electrons are further from the nucleus and are more shielded.

3. The atoms get larger going down group 7.

Why do halogens become less oxidising going down group 7?

1. Halogens act as good oxidising agents as they accept electrons from the species being oxidised and are reduced.

2. This oxidising power decreases down the group as their ability to attract electrons decreases due to shielding and a greater atomic radius.

3. The relative oxidising strengths mean a halogen will displace any halide beneath it in the periodic table.

How can displacement reactions be used to identify the halide ions the solution?

Halide ions are colourless in a solution, but when a halogen is displaced, it shows a distinctive colour.

Why are halides good reducing agents?

They donate electrons to the species being reduced and are themselves oxidised.

What is the trend with reducing power of halides, going down the group?

1. It increases going down the group.

2. This is because electrons are easier to lose from larger ions due to shielding and a larger atomic radius.

Is the reaction between sodium chloride and concentrated sulfuric acid a redox reaction?

1. No as no oxidation state has changed

2. The chloride ion is too weak a reducing agent to reduce the sulfur in sulfuric acid.

What are the reactions between fluoride and chloride ions with H2SO4?

NaF + H2SO4 → NaHSO4 + HF

NaCl + H2SO4 → NaHSO4 + HCl

What is the redox reaction between bromide and H2SO4?

1. NaBr + H2SO4 → NaHSO4 + HBr

2. 2HBr + H2SO4 → Br2 + SO2 + 2H2O

3. There are two parts of this reaction due to the high reducing power.

What is occurs during the reaction between bromide and H2SO4?

Brown/orange fumes

What is the redox reaction between bromide and H2SO4?

1. NaI + H2SO4 → NaHSO4 + HI

2. 2HI + H2SO4 → I2 + SO2 + 2H2O

3. 6HI + SO2 → H2S + 3I2 + 2H2O

4. There are three parts of this reaction due to the high reducing power.

What is occurs during the reaction between iodide and H2SO4?

1. Purple fumes.

2. Bad egg smell.

Why can’t you investigate fluorine in an aqueous solution?

It reacts with water.

What are the results of Cl2, Br2 and I2 water reacting with the potassium halide version of themselves?

	KCl(aq) colourless	KBr(aq) colourless	KI(aq) colourless
Cl2(aq) colourless	no reaction	orange solution (Br2)	brown solution (I2)
Br2(aq) orange	no reaction	no reaction	brown solution (I2)
I2(aq) brown	no reaction	no reaction	no reaction

How do you test for halide ions?

Silver nitrate test

1. Add dilute nitric acid to remove ions that may interfere with the test.

2. Add a few drops of silver nitrate solution (AgNO3(aq)).

3. This forms a precipitate of silver halide.

4. The colour of the precipitate identifies the halide.

	Cl-	Br-	I-
AgNO3	White precipitate (AgCl)	Cream precipitate (AgBr)	Yellow Precipitate (AgI)
Dilute NH3	Precipitate dissolves	No Change	No Change
Concentrated NH3	Precipitate dissolves	Precipitate dissolves	No Change

What is bleach and how is it used?

1. Sodium chlorate (I) solution (NaClO(aq))

2. It is used to kill bacteria, when cleaning toilets, water treatment, etc.

What is the equation to form bleach and why is it a disproportionation reaction?

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

Chlorine is both oxidised and reduced, it starts with an oxidation state of 0, and has a +1 oxidation state in NaClO and a -1 oxidation state in NaCl.

What is the reaction between chlorine and cold water?

Cl2 + H2O → ClO- + Cl- + 2H+

Why is the reaction between chlorine and water a disproportionation reaction?

One of the chlorine atoms increase from 0 to +1 in the chlorate(I) ion and another from 0 to -1 in the chloride ion.

What is the reaction between chlorine and water in UV light?

2Cl2 + 2H2O →(UV)→ 4HCl + O2

What are the positive impacts of chlorine?

1. It kills bacteria, so adding chlorine or a compound containing chlorate ions to water can make it safe to drink or swim in.

2. It removes discolouration caused by organic compounds.

3. It prevents algae growth.

4. It eliminates bad tastes and smells.

What are the negative impacts of chlorine?

1. If breathed in it can irritate the respiratory system and damage lungs.

2. it can evaporate quickly forming chlorine gas which can produce an acid on skin contact.

3. It can cause burns to the skin.

4. Could be fatal.

If there are so many risks why do we use chlorine?

The benefits outweigh the risks.

How is bleach produced?

Chlorine reacts with cold, aqueous sodium hydroxide.

What is the equation for when chlorine reacts with cold, aqueous sodium hydroxide?

2NaOH + Cl2 → NaClO + NaCl + H2O

How do you test for sulfate ions (SO4 2-)?

1. Add some dilute HCl

2. Add some barium chloride

3. If a white precipitate of barium sulfate forms, it means the original compound contained a sulfate.

4. Barium sulfate forms as it is insoluble.

How do you test for hydroxide ions (OH-)?

1. These ions indicate that a substance is alkaline.

2. Therefore, they can identified with red litmus, which turns blue, or using universal indicator, which turns blue-purple.

How do you test for carbonate ions (CO3 2-)?

1. When an acid like HCl is added, the substance containing carbonate ions with produce effervescence and CO2 gas is given off.

2. This gas can be collected and bubbled through limewater, which will turn cloudy, confirming it contains carbon dioxide.

How do you test for group 2 ions?

Flame tests

Calcium (Ca2+)	Brick-red
Strontium (Sr2+)	Red
Barium (Ba2+(	Pale green

How do you test for ammonium ions (NH4+)?

1. If ammonium ions are present, ammonia gas is given off, which is a base.

2. Therefore, the presence of ammonium ions can be tested by holding red litmus over a petri dish of the substance being tested.

3. It will turn blue if ammonium ions are present.

4. Alternatively, they can be tested for in the same way, but by adding NaOH to produce ammonia gas faster.

How do you test the different halides using solubility in ammonia?

1. Chloride ion precipitate dissolves in dilute ammonia solution, forming a colourless solution.

2. Bromide ion precipitate dissolves in concentrated ammonia solution.

3. Iodide ion precipitate is insoluble in concentrated ammonia solution.

Cl- is most soluble and I- is least soluble.

How can you use dilute sodium hydroxide solution to identify group 2 ions?

1. Add NaOH dropwise into a test tube containing the metal ion solution, and observe the precipitate formation.

2. Keep adding NaOH in excess.

Metal Ion	With OH-	With excess OH-
Magnesium, Mg2+	Slight white precipitate	White precipitate
Calcium, Ca2+	Slight white precipitate	Slight white precipitate
Strontium, Sr2+	Slight white precipitate	Slight white precipitate
Barium, Ba2+	No change	No change

Why do precipitates form?

They are insoluble.