AS - Group 7

Question 1

How do elements of Group 7 exist?

Answer 1

As diatomic molecules, F2, Cl2 etc

Question 2

What is another name for the group 7 elements?

Answer 2

The halogens

Question 3

How do fluorine, chlorine, bromine and iodine appear at room temperature?

Answer 3

Fluorine is a pale yellow gas.

Chlorine is a green gas.

Bromine is a red-brown liquid.

Iodine is a black/grey solid.

Question 4

Why does fluorine differ in its properties compared to the other halogens?

Answer 4

Because the F-F bond is unexpectedly weak, compared with that of the rest of the halogens.

The small size of the fluorine atom leads to repulsion between non-bonding electrons because they are so close together.

Question 5

Define electronegativity

Answer 5

A measure of the ability of an atom to attract a bonding pair of electrons towards itself within a covalent bond.

Question 6

What is the trend in electronegativity down group 7?

Answer 6

It decreases down the group.

Question 7

Why does electronegativity decrease down group 7?

Answer 7

The atomic radius increases down group 7 due to the extra energy level each element has compared to the one above it.

Larger atoms attract electrons less than smaller ones because the electrons are further from the nucleus and are shielded by more electrons. Therefore the elements are less electronegative going down the group.

Question 8

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

Answer 8

They increase down the group.

Question 9

Why do melting and boiling points increase down group 7?

Answer 9

As you go down the group, the size of the atom increases, as does the strength of the van der Waals forces between the molecules. These require more energy to break therefore making the melting and boiling points of the atoms greater.

Question 10

How is the trend in boiling points down group 7 shown?

Answer 10

In the changes of physical state from fluorine (gas) to iodine (solid).

Question 11

Melting and boiling points of the group 7 elements involve weakening of the van der Waals forces. But what remains intact?

Answer 11

The covalent bonds in the halogen molecules.

Question 12

How do halogens react, and what do they become?

Answer 12

They react by gaining electrons to become negative -1 ions.

Question 13

What is the trend in oxidising ability of the halogens down the group?

Answer 13

Decreases down the group.

Question 14

Why do halogens get less reactive down the group?

Answer 14

Because the atoms become larger. The outer shell is further from the nucleus, so electrons are less strongly attracted to it. This means it’s easier for the halogen to lose an electron, so is less oxidising but more reducing down the group.

Question 15

What is the general halogen displacement rule?

Answer 15

A halogen will displace a halide from solution if the halide is below it in the periodic table.

Question 16

What does chlorine displace?

Answer 16

Bromide and iodide.

Question 17

What does bromine displace?

Answer 17

Iodide.

Question 18

What does iodine displace?

Answer 18

Nothing, it has no reaction with fluoride, chloride or bromide.

Question 19

What colours are the following:

1. Chlorine water
2. Bromine water
3. Iodine solution?

Answer 19

1. Colourless
2. Orange
3. Brown

Question 20

What colours are the following:

1. Potassium chloride solution
2. Potassium bromide solution
3. Potassium iodide solution?

Answer 20

All colourless.

Question 21

What happens when chlorine water, bromine water or iodine solution reacts with potassium chloride solution?

Answer 21

No reaction.

Question 22

What happens when the following reacts with potassium bromide solution:

1. Chlorine water
2. Bromine water
3. Iodine solution?

Answer 22

1. Orange solution of bromine formed.
2. No reaction
3. No reaction

Question 23

What happens when the following reacts with potassium iodide solution:

1. Chlorine water
2. Bromine water
3. Iodine solution?

Answer 23

1. Brown solution of iodine formed
2. Brown solution of iodine formed
3. No reaction

Question 24

Why can you not investigate fluorine in an aqueous solution?

Answer 24

Because it reacts with water.

Question 25

What happens when halide ions lose electrons?

Answer 25

They become halogen molecules.

Question 26

What is the trend in the reducing ability of the halide ions down the group?

Answer 26

Reducing ability of halide ions increases down the group.

Question 27

Why does the reducing ability of the halide ions increase down the group?

Answer 27

The size of the ion increases down the group. The larger the ion, the more easily it loses an electron. This is because the electron is lost from the outer shell which is further from the nucleus as the ion gets larger so the attraction to the outer electron is less.

Question 28

What happens when sodium fluoride reacts with concentrated sulfuric acid?

Answer 28

Hydrogen fluoride gas and solid sodium hydrogensulfate are produced.

Misty fumes are seen as the extremely dangerous gas comes into contact with the moisture of the air.

The reaction stops here as the fluoride ion is too weak a reducing agent to reduce the sulpfur in sulfuric acid.

Question 29

What types of reactions occur between sodium fluoride or sodium chloride and concentrated sulfuric acid?

Why are they not redox reaction?

Answer 29

Acid-base reactions.

It’s not a redox reaction because the halide ion is too weak a reducing agent to reduce sulfur in sulfuric acid, it’s oxidation state remains +6.

Question 30

What happens when sodium chloride reacts with concentrated sulfuric acid?

Answer 30

Hydrogen chloride gas and solid sodium hydrogensulfate is produced.

Misty fumes arise as the gas comes into contact with the moisture of the air.

The reaction stops here as the chloride ion is too weak a reducing agent to reduce the sulfur in sulfuric acid.

Question 31

What happens when sodium bromide reacts with concentrated sulfuric acid?

Answer 31

Two reactions occur. First, sodium hydrogensulfate and hydrogen bromide are produced in an acid-base reaction. This produces misty fumes of HBr.

Bromide ions are strong enough reducing agents to reduce the sulfuric acid to sulfur dioxide (this gives off colourless, choking fumes).

Bromide ions are oxidised to bromine, which gives off brown fumes.

Question 32

What happens when sodium iodide reacts with concentrated sulfuric acid?

Answer 32

Several reactions occur. Hydrogen iodide is produced in an acid-base reaction (misty fumes).

Iodide ions are even better reducing agents than bromide ions and reduce the sulfur in sulfuric acid even further so that sulfur dioxide (colourless, choking has), sulfur (yellow solid), and hydrogen sulfide gas (toxic and smells like bad eggs) are produced.

Iodide ions are oxidised to grey/black solid iodine.

Question 33

What halide ion does not react with silver ions in aqueous solution to form a precipitate and why?

Answer 33

Silver fluoride as it is soluble in water.

Question 34

Describe and explain how you would carry out a method to test for halide ions using silver nitrate solution.

Answer 34

1. Add dilute nitric acid to removes ions which might interfere with the test (carbonate and hydroxide ions would form insoluble silver carbonate and insoluble silver hydroxide respectively)
2. Add a few drops of silver nitrate solution.
3. The halide precipitate forms.

Question 35

I’m the halide test, what colour precipitate will each halide produce?

Answer 35

Fluoride - no precipitate

Chloride - white precipitate

Bromide - cream precipitate

Fluoride - pale yellow precipitate

Question 36

How can you further test halide ions to confirm their identity after silver nitrate solution has been used?

Answer 36

Add ammonia solution

Question 37

What happens when dilute ammonia solution is added to the white precipitate, silver chloride?

Answer 37

It dissolves

Question 38

What happens when concentrated ammonia solution is added to the cream precipitate, silver bromide?

Answer 38

It dissolves

Question 39

What happens when concentrated ammonia solution is added to the pale yellow precipitate, silver iodide?

Answer 39

Nothing, silver iodide is insoluble even in concentrated ammonia solution.

Question 40

What happens when chlorine reacts with water?

Answer 40

Chloride ions and chlorite (I) ions are produced in a disproportionation reaction.

Question 41

What is chlorine used for?

Chlorine is toxic. How does this affect its used?

Answer 41

It’s used in water treatment for swimming pools and drinking water. It’s toxic, so society has to assess the advantages and disadvantages when deciding whether chemicals should be added to water supply. The benefits to health of water treatment by chlorine outweighs its toxic effects.

Question 42

What are the advantages of using chlorine in water treatment?

Answer 42

1. It kills disease causing microorganisms
2. Some chlorine remains in water and prevents reinfection further down the supply
3. Prevents growth of algae, eliminating bad tastes and smells
4. Removes discolouration caused by organic compounds

Question 43

What are the disadvantages of using chlorine in water treatment?

Answer 43

1. Chlorine is very harmful if breathed in as it irritates the respiratory system
2. Liquid chlorine on skin or eyes can cause severe burns
3. Water contains a variety of organic compounds which chlorine reacts with to form chlorinated hydrocarbons. These are carcinogenic. (The increased cancer risk is small compared to the risks of untreated water, a cholera epidemic for example, would kill thousands of people)

Question 44

What is produced when chlorine reacts with cold, dilute, aqueous sodium hydroxide to form?

What is the product used for?

Answer 44

Produces sodium chlorate (I) solution, NaClO, or household bleach as otherwise known.

This is used to kill bacteria. It’s used in water treatment, to bleach paper and textiles, and for cleaning.

Question 45

What is a disproportionation reaction?

Answer 45

A type of redox reaction, where atoms of the same element can have both an increase and decrease in oxidation state.

For example:
2NaOH + Cl2 —> NaClO + NaCl + H2O
0. +1. -1.