Chapter 8: Reactivity trends (8.3)

Question 1

Test for Carbonate ions (CO3 2-)

Answer 1

dissolve in distilled water. Add nitric acid (HNO3). Bubble gas through limewater to test for CO2. If limewater turns cloudy then test is positive

Question 2

Test for sulphate ions (SO4 2-)

Answer 2

dissolve in distilled water. Add barium chloride (but if you are doing a halide test after use barium nitrate) . If white precipitate (barium sulphate) forms then test is positive

Question 3

Test for halide ions (Cl-, Br-, I-)

Answer 3

Dissolve in distilled water. Add silver nitrate. Since silver halides are insoluble a coloured precipitate shows a positive test. Cl- = white. Br- = cream. I- = yellow

Question 4

Test for ammonium ions (NH4+)

Answer 4

add aqueous sodium hydroxide. Gently heat mixture. Use damp red litmus paper to test the gas evolved. If litmus paper turns blue then ammonia gas is present and the test is positive

Question 5

Test for hydroxide ions (OH-)

Answer 5

add universal indicator and the solution will turn blue

Question 6

Why do you do the carbonate test first?

Answer 6

You do the carbonate test first as it does not interfere with the other tests as none of the other tests produce bubbles with dilute acids

Question 7

Why do you do the sulphate test second?

Answer 7

Silver carbonate and Barium Carbonate also form white precipitates therefore you have to remove carbonate impurities before hand

Question 8

Why do you do the halide test last?

Answer 8

You are adding solutions containing Ag+ (aq) ions but silver carbonate and sulphate are both insoluble in water and will form as precipitates so this test must be carried out last.

Question 9

What happens when magnesium is burnt in oxygen?

Answer 9

It burns with a bright white light and forms magnesium oxide

Question 10

Water happens when a group 2 metal is reacted with water?

Answer 10

It creates an alkaline hydroxide and hydrogen gas

Question 11

What happens when you react an acid with a metal?

Answer 11

You get a salt and hydrogen gas.

Question 12

Why does reactivity increase down group 2?

Answer 12

Increased shielding
Atomic radius increases
Less attraction for outer electrons
Less energy to remove outer electron (ionisation energy)

Question 13

What happens to the alkalinity of the group 2 hydroxides as you move down the group?

Answer 13

Solubility increases - so the solution contains more OH- ions and therefore the PH increases and the alkalinity also increases.

Question 14

How do you test the PH of all the group 2 hydroxides ?

Answer 14

Add a spatula of each group 2 hydroxides to water and shake - you will get a saturated solution with some white solid undissolved at the bottom. Measure the PH and the alkalinity will increase down the group.

Question 15

How are group 2 compounds used in agriculture?

Answer 15

Ca(OH)2 added to fields to neutralise acidic soils

Question 16

How are group 2 compounds used in medicine?

Answer 16

Mg(OH)2 and CaCO3 used as ‘antacids’ to treat indigestion (neutralise the acid in the stomach)

Question 17

How does boiling point change as you go down the halogens and why?

Answer 17

Boiling point increases because there are more electrons = stronger London forces (more energy required to break intermolecular forces so boiling point increases)

Question 18

How does the physical state of the halogens change as you go down the group?

Answer 18

F and Cl - gas
Br - red/brown liquid
I - shiny grey/black solid

Question 19

What happens to the halogens in a redox reaction?

Answer 19

gain one electron to form a 1- halide ion (oxidising agent)

Question 20

What happens when you add a more reactive halogen to an aqueous solution of a weaker halogen?

Answer 20

reaction takes place. More reactive halogen displaces the halide to form a solution (solution changes colour)

Question 21

What colour are solutions of iodine and bromine in water and what does this mean?

Answer 21

The are a similar orange/brown colour depending on the concentration so we have to add a non-polar solvent such as cyclohexane

Question 22

What colour is iodine in cyclohexane?

Answer 22

Violet

Question 23

What colour is bromine in cyclohexane?

Answer 23

Orange

Question 24

How does the reactivity change as you go down the group of halogens and why?

Answer 24

Atomic radius increases so there are more inner shells and therefore shielding increases. Furthermore there is less nuclear attraction to capture an electron from another species so the reactivity decreases.

Question 25

What is a disproportionation reaction?

Answer 25

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

Question 26

What products do you get when you react chlorine and water?

Answer 26

Cl2 + H2O –> HClO + HCl

chloric(I) acid (HClO) and hydrochloric acid (HCl)

Question 27

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

Answer 27

Cl2 + H2O –> HClO + HCl

Chlorine is oxidised as it goes from 0 to +1 in chloric (I) acid and from 0 to -1 in hydrochloric acid

Question 28

What products are produced when you react hot and cold aqueous sodium hydroxide and chlorine?

Answer 28

Cold : 2NaOH + Cl2 –> NaClO + NaCl + H2O

Question 29

What is sodium chlorate (NaClO) used in?

Answer 29

Household bleach

Question 30

What are the benefits of chlorine use?

Answer 30

It kills bacteria

Makes sure water is fit to drink (purifies it)

Get a safe water supply after a natural disaster

Question 31

What is a risk of chlorine use?

Answer 31

It can react with organic hydrocarbons such as methane to form chlorinated hydrocarbons which are suspected of causing cancer.

Toxic gas, large concentrations can be fatal.

(Could be other methods to purify water, risks vs benefits, people can choose)

Question 32

Why do you add aqueous ammonia to the halide solutions after silver nitrate?

Answer 32

White/cream/yellow hard to tell apart.

Adding ammonia tests the solubility of the precipitate, so you can tell the precipitate colours apart

Question 33

What is the solubility of silver chloride, silver bromide and silver iodide in ammonia?

Answer 33

Silver chloride - soluble in dilute NH3
Silver bromide - soluble in conc NH3
Silver iodide - insoluble