Halide Salt Reactions and Disproportionation

We have already had a look at the oxidising power of halogens, so we are now going to look at the reducing power of halides and how it can influence reactions.

A reducing agent is a substance that reduces another substance while it itself is oxidised (loses electrons). The reducing power of a substance refers to its ability to reduce other substances. The reducing power of halides increases down group 7. This is because, down group 7, the ions get bigger so it is easier for outer electrons to be taken by other substances since the attraction between the nucleus and outer electrons is smaller.

We can carry out and observe reactions with halides and concentrated sulfuric acid to demonstrate the reducing power of halides.

When fluoride and chloride-containing halide salts react with concentrated sulfuric acid, an acid-base reaction occurs rather than a redox reaction. This happens because fluoride and chloride ions do not have a lot of reducing power, so cannot reduce sulfuric acid.

\text{NaF}_{\text{(s)}} + \text{H}_2\text{SO}_{4\text{(l)}} \rarr\text{NaHSO}_{4\text{(s)}} + \text{HF}_{\text{(g)}}

\text{NaCl}_{\text{(s)}} + \text{H}_2\text{SO}_{4\text{(l)}} \rarr\text{NaHSO}_{4\text{(s)}} + \text{HCl}_{\text{(g)}}

Observation

• White, misty fumes are produced by \text{HF} and \text{HCl} in each reaction.

Bromide ions are stronger reducing agents than chloride and fluoride ions. While bromide-containing halide salts also undergo an acid-base reaction, the strong reducing power of bromide ions also allows for a redox reaction to take place.

The following equations show the reactions that take place when sodium bromide (NaBr) initially reacts with concentrated sulfuric acid. In the first equation, an acid-base reaction takes place and in the second reaction, a redox reaction takes place.

\text{NaBr}_{\text{(s)}} + \text{H}_2\text{SO}_{4\text{(l)}}   \rarr   \text{NaHSO}_{4\text{(s)}} + \text{HBr}_{\text{(g)}}

2\text{HBr}_{\text{(aq)}} + \text{H}_2\text{SO}_{4\text{(l)}}  \rarr  \text{Br}_{2\text{(g)}} + \text{SO}_{2\text{(g)}} + 2\text{H}_2\text{O}_{\text{(l)}}

Observations

• In the acid-base reaction, white misty fumes of \text{HBr} are released.
• In the redox reaction, orange fumes from \text{Br}_2 and a colourless acidic gas, \text{SO}_2, are evolved.

You will need to be able to carry out simple test-tube reactions to identify cations such as group 2 elements and ammonium ions \left(\text{NH}^{4+}\right) and anions such as halide ions, hydroxide ions, carbonate ions and sulfate ions.

Cations

Group 2: Group 2 metal cations can be identified by reacting them with an ammonium solution, dilute sodium hydroxide or dilute sulfuric acid.

Ammonium Solution
Only magnesium will form a white precipitate when it reacts in an ammonium solution.

Dilute \textbf{NaOH}

1. Mix an equal amount of the test solution containing the group 2 metal cation and dilute sodium hydroxide.
2. Then continue adding sodium hydroxide dropwise until it is in excess.

Group 2 metal cations usually form a colourless solution. In excess \text{NaOH}, some of them will form precipitates.

	Group 2 Metal Cation
	Magnesium	Calcium	Barium	Strontium
Equal Volume of \text{NaOH}	Off White Precipitate	Off White Precipitate	Colourless Solution	Off White Precipitate
Excess Volume of \text{NaOH}	White Precipitate	Off White Precipitate	Colourless Solution	Off White Precipitate

 

Dilute \textbf{H}_2\textbf{SO}_4

1. In a test tube, mix an equal amount of the test solution containing the group 2 metal cation and dilute sulfuric acid.
2. Then continue adding sulfuric acid by drops until it is in excess.

	Group 2 Metal Cation
	Magnesium	Calcium	Barium	Strontium
Equal Volume of \text{H}_2\text{SO}_4	White Precipitate	Off White Precipitate	Off White Precipitate	White Precipitate
Excess Volume of \text{H}_2\text{SO}_4	White Precipitate	Off White Precipitate	Colourless Solution	White Precipitate

 

Ammonium ions: Ammonium ions can be identified through a reaction with sodium hydroxide solution.

1. In a test tube, mix an equal amount of the solution containing ammonium ions and sodium hydroxide solution.
2. Warm the mixture gently. If ammonium ions are present, fumes of ammonia gas should evolve.
3. Place damp red litmus paper at the mouth of the test tube. If the litmus paper changes colour to blue, the gas is ammonia and ammonium ions must be present.

 

Anions

Hydroxide Ions: Damp red litmus paper will turn blue when placed in an aqueous solution containing hydroxide ions.

Carbonate ions: Carbonate ions can be identified through a reaction with hydrochloric acid.

1. In a test tube, mix an equal amount of the test solution containing carbonate ions and dilute hydrochloric acid.
2. Bubble the gas produced through limewater. If the lime water goes cloudy, carbonate ions are present.

Sulfate ions: Sulfate ions can be identified through a reaction with barium chloride.

1. In a test tube, mix an equal amount of the substance containing sulfate ions, dilute hydrochloric acid and barium chloride.
2. If a white precipitate is formed, sulfate ions are present

Halide Ions (Aqueous): Halide ions in an aqueous solution can be identified through a reaction with silver nitrate. For different halides, different coloured precipitates are formed.

Halide Ions (Solid): Halide ions in solids can be identified through a reaction with concentrated sulfuric acid.

1. Using a spatula, place a small amount of the solid containing halide ions into a test tube and then add a few drops of concentrated sulfuric acid.
2. Record the observation.
3. Fumes from the above reaction should always turn damp blue litmus paper red if halide ions are present

	Halide Ion
	Chloride	Bromide	Iodide
Concentrated \text{H}_2\text{SO}_4	White Misty Fumes	Orange Fumes	Purple Fumes and Black Precipitate