10.2 - Tests for Halides

Question 1

Reaction of iodide ions with concentrated sulphuric acid:

• general equation with NaI
• equation of HI decomposing to form iodine and sulphur/SO2 as well
• equation of forming H2S as well

Answer 1

H2SO4(l) + NaI (s) → HI (g) + NaHSO4(s)

• hydrogen iodide decomposes rapidly
• misty fumes of HI
• sulphuric acid oxidises the hydrogen iodide to form different products
  concentrated H2SO4 oxidises HI and is itself reduced to SO2 gas

2HI (g) + H2SO4(l) → I2(g) + SO2(g) + 2H2O (l)

• iodine is a purple vapour
• choking gas of SO2

6HI (g) + H2SO4(l) → 3I2(g) + S (s) + 4H2O (l)

• concentrated H2SO4 oxidises HI and is reduced to sulphur as well
• sulphur is a yellow solid

8HI (g) + H2SO4(l) → 4I2(g) + H2S (s) + 4H2O (l)

• HI is oxidised by concentrated H2SO4 and is reduced to H2S
• H2S has a strong smell of bad eggs

Question 2

Reaction of bromide ions with concentrated sulphuric acid, chemical equation of this

Answer 2

H2SO4(l) + NaBr (s) → HBr (g) + NaHSO4(s)

• oxidises HBr and it decomposes into bromine and hydrogen gas and H2SO4 is reduced into SO2 gas

2HBr (g) + H2SO4(l) → Br2(g) + SO2(g) + 2H2O (l)

• bromine is a reddish brown gas
• misty fumes of HBr gas
• choking gas of SO2

Question 3

Reaction of chloride ions with concentraterd sulphuric acid, what is the equation for this reaction

Answer 3

• drops of concentrated H2SO4 is added to NaCl crystals to produce HCl gas
• H2SO4(l) + NaCl (s) → HCl (g) + NaHSO4(s)
• the gas produced are white fumes
• misty fumes of HCl

Question 4

Reactions of halides with concentrated sulphuric acid - what does it produce and where is it normally taken place

what’s the general equation?

Answer 4

chloride, bromide and iodide ions react with concentrated H2SO4 to produce toxic gases
* therefore is carried out within a fume cupboard
* H2SO4(l) + X-(aq) → HX(g) + HSO4-(aq)

Question 5

Identifying halides through using ammonia

Answer 5

• dilute ammonia dissolves the chloride precipitate
• concentrated dissolves the bromide precipitate
• if it doesn’t dissolve the iodide precipitate is present

Question 6

Silver halide precipitate colours

Answer 6

Silver chloride (AgCl) is a white precipitate

Silver bromide (AgBr) is a cream precipitate

Silver iodide (AgI) is a yellow precipitate

Question 7

Process of identifying halide ions

Answer 7