Describe chloride reactions with concentrated sulfuric acid
Distinguish the non-redox acid–base reaction of chloride from the redox reactions of heavier halides.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State conditions for halogen reactions, distinguish halogen oxidising strength from halide-ion reducing strength, and balance redox equations. Chlorine, bromine, iodine and concentrated acids are hazardous; examples teach chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use molecular halogen formulas X₂, oxidation numbers, ionic equations and electron-transfer ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What gas is formed when chloride reacts with concentrated sulfuric acid?
What you will learn
- Write the chloride–concentrated sulfuric acid equation
- Identify HCl as the gaseous product
- Contrast acid–base behavior with redox
Chloride ions do not reduce sulfuric acid in the standard test
With solid sodium chloride and concentrated sulfuric acid, hydrogen chloride is produced: NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g). This is an acid–base reaction, not redox; oxidation numbers do not change.
Chloride is a weak reducing agent relative to bromide and iodide, so it does not reduce concentrated sulfuric acid under the standard syllabus comparison. Distinguish hydrogen chloride fumes from chlorine gas.
Conditions and hazards matter
The reagent is concentrated sulfuric acid; do not substitute dilute acid in the described test. HCl gas is corrosive and hazardous. This lesson explains the expected products only and does not provide a procedure for carrying out the reaction.
When a question compares halide salts, first identify whether the observed product follows acid–base behavior only or whether the halide ion is oxidised and sulfur is reduced.
Pause and explain
Is the standard NaCl plus concentrated H₂SO₄ reaction redox?
Worked example
Write the equation for sodium chloride with concentrated sulfuric acid and state whether it is redox.
Show the worked solution
- Na⁺ pairs with HSO₄⁻ to make sodium hydrogensulfate.
- The displaced product is HCl gas.
- Oxidation numbers remain unchanged, so this is acid–base rather than redox.
Answer NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g); this is not a redox reaction.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the equation for solid sodium chloride with concentrated sulfuric acid.
Give me a hint
Form sodium hydrogensulfate and hydrogen chloride.
Compare my reasoning
- Na⁺ pairs with HSO₄⁻.
- The chloride ion accepts H⁺ to form HCl.
- Check Na, Cl, H, S and O atoms.
NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g).
Look for these in your work
- Salt formula correct
- HCl product correct
- Equation balanced
A learner claims a pale acidic gas from NaCl and concentrated H₂SO₄ is Cl₂. Correct the claim.
Give me a hint
Use the acid–base equation and identify the anion's oxidation state.
Compare my reasoning
- The reaction forms HCl gas.
- Chloride stays at oxidation number −1.
- No Cl₂ is produced in this non-redox reaction.
The gas is HCl, not Cl₂; chloride remains at −1 and the reaction is acid–base.
Look for these in your work
- Gas identified
- Oxidation state checked
- Reaction class stated
Compare the chloride reaction with concentrated H₂SO₄ to the bromide reaction at the level of electron transfer.
Give me a hint
Chloride gives acid–base behavior; bromide can reduce sulfuric acid.
Compare my reasoning
- Cl⁻ forms HCl without changing oxidation number.
- Br⁻ can be oxidised to Br₂.
- Sulfuric acid can be reduced to SO₂ in the bromide reaction.
Chloride undergoes acid–base reaction without redox; bromide reduces concentrated sulfuric acid and is oxidised to Br₂.
Look for these in your work
- Chloride behavior correct
- Bromide electron loss stated
- Sulfur reduction distinguished
Common mistakes
- Calling HCl gas chlorine gas
- Writing a redox equation for chloride
- Using dilute sulfuric acid instead of concentrated acid
- Ignoring the condition specified in the question
What can you explain now?
What gas is formed when chloride reacts with concentrated sulfuric acid?
Compare with the explanation
Hydrogen chloride
The chloride ion is protonated to give HCl.
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Make it stick
What is the chloride reaction with concentrated H₂SO₄, and how do you know it is not redox?
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