8 Reactivity trends

Question 1

Why are Group 2 elements called reducing agents?

Answer 1

1) When group 2 elements are oxidised, they lose 2 electrons from their outer shell.
2) Another species will gain these two electrons and be reduced.
3) The group 2 element is called a reducing agent because it has reduced another species.

Question 2

What is the trend of hydroxides in water down the group?

Answer 2

• Solubility increases
• pH increases
• Alkalinity increases

Question 3

What is the solubility of Mg(OH)2 in water?

Answer 3

Only slightly soluble, as it has a low OH- concentration and pH of 10

Question 4

What is the solubility of Ba(OH)2 in water?

Answer 4

Much more soluble, as it has a greater OH- concentration and a pH of 13.

Question 5

What are some uses of Group 2 compounds in agriculture?

Answer 5

Calcium hydroxide is added to fields as lime by farmers to increase pH in acidic soils. (CaOH2)

Question 6

What are some uses of Group 2 compounds in medicine?

Answer 6

Group 2 bases are often used as antacids for treating acid indigestion. Mg(OH)2

Question 7

Why does boiling point increase down group 7 (halogen group)?

Answer 7

• There are more electrons as you go down and so stronger London forces.
• More energy is required to break the intermolecular forces.
• Therefore boiling point increases.

Question 8

Why are group 7 elements called oxidising agents?

Answer 8

1) Each halogen atom is reduced, gaining one electron to form a 1- halide ion.
2) Another species loses electrons to halogen atoms.
3) The halogen is called an oxidising agent because it has oxidised another species.

Question 9

What colour is fluorine at RTP?

Answer 9

Pale yellow gas

Question 10

What colour is chlorine at RTP?

Answer 10

Pale green gas

Question 11

What colour is bromine at RTP?

Answer 11

Red-brown liquid

Question 12

What colour is iodine at RTP?

Answer 12

Shiny grey-black solid

Question 13

Reactivity of halogens…

Answer 13

decreases down the group.

Question 14

What happens in a halogen-halide displacement reaction?

Answer 14

A solution of each halogen is added to aqueous solutions of the other halides.
E.g a solution of chlorine is added to two aqueous solutions containing bromine and iodine ions

Question 15

What are the results of the displacement reactions?

Answer 15

• Chlorine displaces bromine (orange colour appears from Br2 formation)
• Chlorine displaces iodine (violet colour appears from I2 formation)
• Bromine displaces I- (violet colour from I2 formation)
• Iodine has not reacted at all.

Question 16

Why is cyclohexane added?

Answer 16

• An organic non-polar solvent such as cyclohexane is added and the mixture is shaken.
• The non-polar halogens dissolve more readily in cyclohexane than in water.
• In cyclohexane the colours are easier to tell apart.
  CHLORINE: PALE GREEN
  BROMINE: ORANGE
  IODINE: VIOLET

Question 17

Why does reactivity decrease down group 7?

Answer 17

• Atomic radius increases
• More inner shells so shielding increases
• Less nuclear attracton to capture an electron from another species
• Reactivity decreases

Question 18

What is a disproportionation reaction?

Answer 18

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

Question 19

What are the benefits and risks of chlorine use?

Answer 19

BENEFITS: Ensures water is fit to drink and that bacteria is killed
RISKS: Extremely toxic gas and is a respiratory irritant in small concentrations

Question 20

What is qualitative analysis?

Answer 20

It relies on simple observations rather than measurements.

Question 21

What is the carbonate test?

Answer 21

1) In a test tube, add dilute nitric acid to the solid and solution to be tested.
2) If you see bubbles, the unknown compound could be a carbonate.
3) To prove that the gas is carbon dioxide, bubble the gas through lime water (calcium hydroxide). Carbon dioxide should react to form a fine white precipitate of calcium hydroxide which turns lime water cloudy.

Question 22

What is the sulfate test?

Answer 22

• Aqueous barium ions are added to a solution of an unknown compound.
• The formation of a white precipitate of barium sulfate is the basis for the test as it is very insoluble.

Question 23

What is the halide test?

Answer 23

1) Add aqueous silver nitrate, AgNO3, to an aqueous solution of a halide.
2) The silver halide precipitates are different colours:
CHLORIDE: WHITE
BROMIDE: CREAM
FLUORINE: YELLOW
3) Add aqueous ammonia to test solubility of precipitates.
CHLORIDE: SOLUBLE IN DILUTE NH3
BROMIDE: SOLUBLE IN CONC NH3
IODIDE: INSOLUBLE IN CONC NH3