In simple terms
A friendly intro before the formal notes — no formulas yet.
Halide Identity Parade
We can identify different halide ions by their unique chemical fingerprints. Their reactions with silver nitrate and concentrated sulfuric acid reveal a clear pattern of reactivity down the group.
Imagine a line-up of suspects (the halide ions). To identify them, you give them two different challenges. The first is a reaction with silver nitrate, which makes them reveal a specific coloured 'tag' (precipitate). The second is a strength test against a powerful opponent (concentrated sulfuric acid), where the strongest suspect (iodide) creates the most chaos (produces the most reduction products).
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Test for halide ions using acidified silver nitrate. Observe the precipitate colours: AgCl is white, AgBr is cream, and AgI is yellow.
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Observe the reaction with concentrated sulfuric acid. Iodide ions are strong enough reducing agents to reduce H₂SO₄ to H₂S, S, and SO₂.
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Recognise that the reducing power of halide ions increases down the group (I⁻ > Br⁻ > Cl⁻) as the outermost electron is easier to remove.
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See how a more reactive halogen displaces a less reactive halide from solution, e.g., chlorine displaces iodide ions to form iodine.
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Full topic notes
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1. Reactions with Aqueous Silver Nitrate
A key test for identifying halide ions in solution involves adding aqueous silver nitrate. Before adding the silver nitrate, the solution must first be acidified with a few drops of dilute nitric acid. This is to prevent the precipitation of other silver salts, such as silver carbonate, which would interfere with the test. Upon adding silver nitrate, a precipitate of the corresponding silver halide is formed.
General ionic equation: (where X = Cl, Br, or I)
Chloride (Cl⁻): Forms a white precipitate of silver chloride (AgCl).
Bromide (Br⁻): Forms a cream precipitate of silver bromide (AgBr).
Iodide (I⁻): Forms a yellow precipitate of silver iodide (AgI).
Confirmatory Test: Solubility in Aqueous Ammonia
The colours of silver chloride and silver bromide can be difficult to distinguish. To confirm the identity of the halide, aqueous ammonia is added to the precipitate. The different solubilities of the silver halides in ammonia provide a conclusive result. The silver chloride dissolves by forming a soluble complex ion, the diamminesilver(I) ion.
Equation for dissolving AgCl:
AgCl(s): Dissolves in dilute aqueous ammonia.
AgBr(s): Insoluble in dilute aqueous ammonia, but dissolves in concentrated aqueous ammonia.
AgI(s): Insoluble in both dilute and concentrated aqueous ammonia.
Be precise with your descriptions. Use 'white', 'cream', and 'yellow' for the precipitates. When discussing ammonia, always specify whether it is 'dilute' or 'concentrated' as it is crucial for distinguishing AgCl from AgBr.
2. Reactions with Concentrated Sulfuric Acid
The reaction between a solid ionic halide (e.g., NaX) and concentrated sulfuric acid demonstrates the increasing reducing power of the halide ions as you go down the group. Concentrated sulfuric acid is a strong acid but also a powerful oxidising agent. For chloride and fluoride, it acts only as an acid. For bromide and iodide, it acts as both an acid and an oxidising agent.
With NaCl(s) or NaF(s): An acid-base reaction occurs. Misty fumes of the hydrogen halide (HCl or HF) are observed. No redox reaction takes place as Cl⁻ and F⁻ are not strong enough reducing agents to reduce H₂SO₄.
With NaBr(s): The Br⁻ ion is a stronger reducing agent. It first reacts in an acid-base reaction to produce HBr gas. However, some of the HBr is then oxidised by the H₂SO₄ to bromine, Br₂. The H₂SO₄ is reduced to sulfur dioxide, SO₂.
- Acid-base: (misty fumes)
- Redox: (brown fumes of Br₂, choking SO₂)
With NaI(s): The I⁻ ion is the strongest reducing agent of the common halides. It reduces the sulfuric acid to several different products, depending on the extent of reduction. You will observe a complex mixture of products.
- Acid-base: (misty fumes)
- Redox: The HI produced is readily oxidised. Multiple redox reactions occur: (purple vapour/black solid of I₂, choking SO₂) (yellow solid of sulfur) (bad egg smell of H₂S)
Trend in Reducing Power: .
Reason: As you descend the group, the ionic radius increases and the number of inner electron shells (shielding) increases. The attraction between the nucleus and the outermost electron weakens, making the electron easier to lose. Therefore, the ion is more easily oxidised and acts as a better reducing agent.
Oxidation States of Sulfur: In H₂SO₄, S is +6. In SO₂, S is +4. In S, S is 0. In H₂S, S is -2.
3. Halogen Displacement Reactions
These reactions illustrate the trend in oxidising power of the halogen elements. A more reactive halogen (a stronger oxidising agent) will displace a less reactive halide ion from its aqueous solution. The reactivity as an oxidising agent decreases down Group 17: .
Chlorine water + Potassium Bromide: . The colourless solution turns orange/brown.
Chlorine water + Potassium Iodide: . The colourless solution turns brown. If starch is present, it turns blue-black.
Bromine water + Potassium Iodide: . The orange solution turns a darker brown.
No Reaction: Bromine water added to potassium chloride, or iodine solution added to potassium chloride or bromide, will result in no reaction as the halogen is less reactive than the halide it is trying to displace.
Remember the two key trends. Oxidising power of the HALOGENS () decreases down the group. Reducing power of the HALIDE IONS () increases down the group. Don't mix them up!
Worked examples
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An unknown solution containing a sodium halide is tested. Addition of acidified silver nitrate solution produces a cream precipitate. This precipitate is found to be insoluble in dilute aqueous ammonia, but it dissolves when concentrated aqueous ammonia is added. Identify the halide ion and write the two ionic equations for the reactions that occurred.
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Identification: A cream precipitate suggests the presence of bromide ions, Br⁻. This is confirmed by its solubility in concentrated ammonia but not dilute ammonia.
Describe the expected observations when a small amount of concentrated sulfuric acid is added to solid potassium iodide. For any two different redox reactions occurring, write a balanced chemical equation.
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Observations:
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What is the colour of the precipitate formed when aqueous silver nitrate is added to a solution containing chloride ions, Cl⁻?
A white precipitate of silver chloride (AgCl).
Key takeaways
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Chloride (Cl⁻): Forms a white precipitate of silver chloride (AgCl).
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Bromide (Br⁻): Forms a cream precipitate of silver bromide (AgBr).
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Iodide (I⁻): Forms a yellow precipitate of silver iodide (AgI).
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Test Your Knowledge on Halide Reactions
Test Your Knowledge on Halide Reactions
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