3.1.9/10 : Group 2 and 7

Question 1

Write the equation for the reaction of magnesium with oxygen

Answer 1

2Mg(s) + O2(g) -> 2MgO(s)

Question 2

Describe what you would expect to see when magnesium ribbon is added to an excess of dilute hydrochloric acid

Answer 2

Slight fizzing as gas (hydrogen) is released, magnesium ribbon will dissolve, leaving behind magnesium chloride solution

Question 3

Describe one difference you would see if strontium, instead of magnesium, was reacted with dilute hydrochloric acid

Answer 3

Strontium would dissolve and produce gas bubbles more quickly

Question 4

Explain why barium is more reactive than calcium

Answer 4

Barium has…
- a larger atomic radius
- more electron shielding
This decreases the electrostatic attraction from the nucleus to the outer electrons (this outweighs the increase in nuclear charge), so outer electrons are lost more easily

Question 5

What change in oxidation stage of the group 2 metals usually occurs in redox reactions? Is this an example of oxidation or reduction and why?

Answer 5

Oxidation state changes from 0 to +2

This is because when group 2 metals become metal ions, they lose 2 e- to form ions with 2+ charges

It is oxidation as the metal loses electrons and the oxidation state increases

Question 6

Explain the trend in first and second ionisation energy for the metals in group 2

Answer 6

I.E decreases going down the group because…
- the number of shells increases
- atomic radius increases
- distance of the outer e- from the nucleus increases
- shielding effect increases
This causes a weaker electrostatic attraction between the nucleus and outer electrons, so less energy is needed to remove an electron
- the nuclear charge also increases however the resulting increased attraction is outweighed by the increases in distance and shielding

Question 7

Explain the trend in reactivity of the group 2 metals

Answer 7

• Reactivity increases going down the group as…
• number of shells increases
• atomic radius increases
• distance between nucleus and outer electrons increases
• shielding effect increases
• so the electrostatic attraction between the nucleus and outer electrons decreases
• so less energy is needed to remove the 2 electrons
• so they react more easily and quickly

Question 8

Explain the trend in solubility and alkalinity going down group 2

Answer 8

The solubility of the group 2 metals hydroxides in water increases down the group.

When a hydroxide is more soluble than another, it will release more OH- ions, so will make a more alkaline solution, with a higher pH of around 10-12

Question 9

What happens when group 2 compounds react with acids?

Answer 9

They form a salt and water

The solid oxide/hydroxide will dissolve

Question 10

What are the three main uses of group 2 compounds?

Answer 10

1. Neutralising acidic soils
2. Indigestion remedies
3. Building and construction uses

Question 11

Which metal hydroxide is commonly used by farmers and for what reason?

Answer 11

Calcium hydroxide is used as ‘lime’ to reduce the acidity levels of the soil

Question 12

Which group 2 hydroxide is commonly used to treat indigestion and why is it safe to swallow?

Answer 12

Magnesium hydroxide, in milk of magnesia, is used to neutralise the excess HCl in the stomach, producing a salt and water.

It is safe to swallow as the magnesium hydroxide is not very soluble in water so most of it remains a solid until it reacts with the acid in the stomach and neutralised jt

Question 13

What is a disadvantage of using group 2 carbonates as building materials?

Answer 13

They readily react with acids - most rainwater has an acidic pH, which leads to erosion of objects made using limestone or marble, such as buildings

Question 14

What are two properties of halogens?

Answer 14

• low melting and boiling points
• exist as diatomic molecules

Question 15

What is the trend in boiling points going down group 7?

Answer 15

• boiling point increases
• as each successive element has an extra shell of electrons (more electrons)
• which leads to a higher level of London forces between the molecules
• making them require more energy to overcome

Question 16

Why are halogens strong oxidising agents?

Answer 16

They are highly electronegative so very good at attracting electrons

Question 17

What happens to the reactivity and oxidising power of the halogens going down the group?

Answer 17

• reactivity and oxidising power decreases going down the group
  This is because:
• the atomic radius increases (so the nuclear pull is further away from the incoming electrons)
• the electron shielding increases
• so the electrostatic attraction of the nucleus on the incoming electrons decreases
• so the ability to gain an electron and form 1- ions decreases

Question 18

Why are halogens more soluble in organic solvents such as cyclohexane?

Answer 18

Both are non-polar so the only intermolecular forces to overcome are London forces, like dissolves like

Question 19

What is the colour of each of these halogens in 1. Water and 2. Cyclohexane?
- Chlorine
- Bromine
- Iodine

Answer 19

Chlorine = pale green in water, pale green in cyclohexane
Bromine = orange in water, orange in cyclohexane
Iodine = brown in water, violet in cyclohexane

Question 20

What is disproportionation?

Answer 20

Disproportionation is a reaction in which the same element is both reduced and oxidised.

Question 21

How many valence electrons do the halogens have?

Answer 21

7

Question 22

Which of the halogens can displace iodide ions?

Answer 22

Cl2, Br2

Question 23

Give the reaction for the addition of chlorine to water

Answer 23

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

Question 24

What reagents are used to produce the chlorate (I) ion used in bleach manufacturer?

Answer 24

Cl2 (aq) and NaOH (aq)

Question 25

What are the advantages and disadvantages of the use of chlorine in drinking water?

Answer 25

✅ chlorine kills bacteria making it safe to drink
❌ however it can form potentially carcinogenic chlorinated hydrocarbons, chlorine gas is toxic

Question 26

Describe a test that can be used to identify halide ions in solution

Answer 26

Add silver nitrate solution, then use the colour of the precipitate to identify the halide ions

Question 27

What further test may be required to identify halide ions if colours are hard to distinguish?

Answer 27

• addition of dilute NH3 will dissolve AgCl
• addition of concentrated NH3 will dissolve AgBr
• AgI will not dissolve in either concentrated or dilute NH3

Question 28

What is a displacement reaction and what type of reaction is it?

Answer 28

In a displacement reaction, a more reactive element will displace a less reactive element from its compounds. Halogen displacement reactions are redox reactions.