3 - Halide ions Flashcards
State the four halides
Fluoride, chloride, bromide, iodide
Are halides oxidising or reducing agents?
Reducing agents
Why are halides reducing agents?
They donate an electron
What is the trend in reducing ability of the halide ions down group 7?
Reducing ability increases
Why does reducing ability of halide ions down group 7 increase?
The electrostatic forces of attraction between outer electrons and the nucleus are weaker so they are easier to lose
Which halide is the strongest reducing agent?
Iodide
Give an equation for the reaction between solid sodium fluoride / chloride and concentrated sulfuric acid where HF / HCl is produced
State an observation of this reaction
NaF (s) + H2SO4 (aq) → NaHSO4 (aq) + HF (g)
NaCl (s) + H2SO4 (aq) → NaHSO4 (aq) + HCl (g)
- White misty fumes of HF or HCl are produced
What kind of reaction occurs between fluoride / chloride and concentrated sulfuric acid?
Acid-base
What is the role of the chloride ions when reacted with concentrated sulfuric acid?
They are a proton acceptor
Why is there only an acid-base reaction and no further redox reaction when fluoride and chloride react with concentrated sulfuric acid?
Fluoride and chloride both have low reducing abilities
Give an equation for the reaction between solid sodium bromide and concentrated sulfuric acid where HBr is produced
State an observation of this reaction
NaBr (s) + H2SO4 (aq) → NaHSO4 (aq) + HBr (g)
- Misty fumes of HBr gas are produced
Give an equation for the reaction between hydrogen bromide and concentrated sulfuric acid where SO2 is produced and state what occurs in terms of reduction
State the observations of this reaction
2HBr (g) + H2SO4 (aq) → Br2 g + SO2 (g) + 2H2O (l)
- Bromide ions then reduce the S in H2SO4 from +6 to +4
- Orange bromine vapour and choking SO2 gas are produced
Give the overall equation of the reaction between solid sodium bromide and concentrated sulfuric acid
2NaBr + 3H2SO4 → 2NaHSO4 + SO2 + 2H2O
Give an equation for the reaction between solid sodium iodide and concentrated sulfuric acid where HI is produced
State an observation of this reaction
NaI (s) + H2SO4 (aq) → NaHSO4 (aq) + HI (g)
- Misty fumes of HI gas are produced
Give an equation for the reaction between hydrogen iodide and concentrated sulfuric acid where SO2 is produced and state what occurs in terms of reduction
State the observations of this reaction
2HI (g) + H2SO4 (aq) → I2 g + SO2 (g) + 2H2O (l)
- Iodide ions reduce the S in H2SO4 from +6 to +4
- Violet iodine vapour and choking SO2 gas is produced
Give an equation for the reaction between hydrogen iodide and concentrated sulfuric acid where solid sulfur is produced and state what occurs in terms of reduction
State the observations of this reaction
6HI (g) + H2SO4 (aq) → 3I2 (s) + S (s) + 4H2O (l)
- Iodide ions reduce the S in H2SO4 from +6 to 0
- Solid yellow sulfur is produced
Give an equation for the reaction between hydrogen iodide and concentrated sulfuric acid where H2S gas is produced and state what occurs in terms of reduction
State the observations of this reaction
8HI (g) + H2SO4 (aq) → 4I2 (s) + H2S (g)+ 4H2O (l)
- Iodide ions reduce the S from H2SO4 from +6 to -2
- H2S gas is produced
- Rotten egg smell of H2S gas is observed
State the half equation for the reduction of H2SO4 to SO2
H2SO4 + 2H+ + 2e- → SO2 + 2H2O
State the half equation for the reduction of H2SO4 to S
H2SO4 + 6H+ + 6e- → S + 4H2O
State the half equation for the reduction of H2SO4 to H2S
H2SO4 + 8H+ + 8e- → H2S + 4H2O
State the general half equation of the oxidation of halide ions
2X- → X2 + 2e-
X = halogen
What solution can be used to test for the presence of halide ions?
Aqueous silver nitrate (AgNO3)
Why must the silver nitrate (AgNO3) be acidified?
To remove any ions like carbonates that may form precipitates
If impurities are not removed it can show misleading results
When testing for halide ions, what colour precipitate appears if chloride is present?
White
Ag+ (aq) + Cl- (aq) → AgCl (s)
When testing for halide ions, what colour precipitate appears if bromide is present?
Cream
Ag+ (aq) + Br- (aq) → AgBr (s)
When testing for halide ions, what colour precipitate appears if iodide is present?
Pale yellow
Ag+ (aq) + I- (aq) → AgI (s)
Silver bromide and silver iodide have similar colours and can be hard to distinguish between. To confirm the identity of the halide present we can add excess…
ammonia solution
What is the effect of adding dilute ammonia solution to silver chloride?
The silver chloride will dissolve, confirming that chloride ions are present
What is the effect of adding concentrated ammonia solution to silver bromide?
The silver bromide will dissolve, confirming that bromide ions are present
What is the effect of adding concentrated ammonia solution to silver iodide?
Silver iodide does not react with ammonia as it is too insoluble
