Halide Ions with Concentrated Sulfuric Acid

Learning Objectives (OCR Spec 3.1.3)

By the end of this lesson you should be able to:

• Describe the reactions of solid sodium halides (NaF, NaCl, NaBr, NaI) with concentrated sulfuric acid
• Classify each observation as either acid-base or redox (or both)
• Identify the reduced sulfur products (SO2, S, H2S) and their smells/colours
• Explain the observations in terms of the reducing ability of halide ions
• Appreciate that this is OCR-specific content covering spec 3.1.3(e)

Why This Reaction Matters

Concentrated sulfuric acid is a powerful oxidising agent as well as a strong acid. When added to a solid sodium halide (NaX), two things can happen:

1. Acid-base: H2SO4 protonates X- to form the gaseous hydrogen halide HX
2. Redox: If X- is a strong enough reducing agent, it will then reduce H2SO4 to a lower oxidation state of sulfur

Whether step 2 happens depends on the reducing ability of the halide, which increases down the group. This gives a clear demonstration of the reducing ability trend from the previous lesson.

Summary Table

Halide	Acid-base?	Redox?	Sulfur product	Overall observations
F- (NaF)	Yes	No	H2SO4 unreacted	HF(g) misty fumes only
Cl- (NaCl)	Yes	No	H2SO4 unreacted	HCl(g) misty fumes only
Br- (NaBr)	Yes	Partial	SO2	HBr(g) fumes + choking SO2 + orange vapour (Br2)
I- (NaI)	Yes	Extensive	SO2, S, H2S	HI(g) briefly + black/purple I2 + H2S (bad egg smell) + yellow S + SO2

Reducing ability increases F- → I-, so the reaction with H2SO4 becomes progressively more redox-dominated as you go down the group.

1. NaF with Concentrated H2SO4 (Acid-Base Only)

F- is a very weak reducing agent (F is the most electronegative element and holds onto its lone pair very tightly). The reaction is therefore acid-base only:

NaF(s) + H2SO4(l) → NaHSO4(s) + HF(g)

Observations:

• Steamy/misty white fumes of HF gas
• No colour change in the solid
• No SO2 smell, no sulfur precipitate

Oxidation numbers: all unchanged. F- stays at -1, S stays at +6. This is a non-redox reaction.

2. NaCl with Concentrated H2SO4 (Acid-Base Only)

Cl- is a slightly stronger reducing agent than F- but still too weak to reduce H2SO4. Again the reaction is acid-base only:

NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)

Observations:

• Steamy/misty white fumes of HCl gas
• No colour change
• No SO2, no sulfur, no H2S

Industrially, this is how HCl gas was once produced for the chemical industry. Oxidation numbers: unchanged (Cl stays -1, S stays +6).

3. NaBr with Concentrated H2SO4 (Acid-Base THEN Redox)

Br- is a strong enough reducing agent to partially reduce H2SO4. Two reactions occur:

Step 1 (acid-base):

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

Step 2 (redox): HBr (or Br-) then reduces H2SO4:

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

Or in ionic form:

2Br-(aq) + H2SO4(l) + 2H+ → Br2(l) + SO2(g) + 2H2O(l)

Oxidation number changes:

• Br: -1 → 0 (oxidised, in Br2)
• S: +6 → +4 (reduced, in SO2)

Observations:

• Initial steamy fumes of HBr
• Orange/brown vapour (Br2)
• Choking, acidic gas (SO2) - turns damp blue litmus red
• Dark red-brown liquid may form

4. NaI with Concentrated H2SO4 (Vigorous Redox)

I- is the strongest halide reducing agent, so H2SO4 is reduced even further - down to S and H2S:

Step 1 (acid-base, briefly):

NaI(s) + H2SO4(l) → NaHSO4(s) + HI(g)

Step 2 (redox to SO2):

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

Step 3 (further redox to S):

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

Step 4 (further redox to H2S):

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