Core Practical 13a – Rate of the Iodine–Propanone Reaction

Note – Core Pracitcal 13b is further down page

Reaction and Rate Expression

I₂ + CH₃COCH₃ + H⁺ → CH₃COCH₂I + 2H⁺ + I⁻

Rate = k[CH₃COCH₃][H⁺] (zero order in iodine).

Safety

• Iodopropanone is an eye irritant – pour mixtures away with running water in fume cupboard.
• Propanone is an irritant and highly flammable.
• Sulfuric acid is an irritant.
• Sodium thiosulfate produces SO₂ (toxic).
• Wear eye protection, ensure good ventilation, max run time = 12 mins.

Equipment and Reagents

• 1 mol dm⁻³ propanone and sulfuric acid
• 0.02 mol dm⁻³ iodine in potassium iodide
• 0.01 mol dm⁻³ sodium thiosulfate
• 1% starch solution
• Sodium hydrogencarbonate
• Pipettes, conical flasks, beakers, stop clock

Procedure

1. Mix 25 cm³ propanone with 25 cm³ sulfuric acid in a beaker.
2. Add 50 cm³ iodine solution and start the timer.
3. Every ~3 mins withdraw 10 cm³ reaction mixture.
4. Quench with sodium hydrogencarbonate, recording exact time.
5. Titrate iodine present with sodium thiosulfate using starch indicator.

6. Repeat 4–5 times without stopping the timer.

Data Processing

• Plot a graph of titre vs. time
• The titre ∝ [I₂]
• If zero-order with respect to iodine: plot will be a straight line
• Determine rate from gradient of plit or half-life if nonlinear

Sample Data

Core Practical 13b – Clock Reaction and Rate Equation

Clock Reaction

S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂

I₂ detected with starch → blue-black colour.

Safety

• Wear lab coat and goggles.
• Follow CLEAPSS Hazcards 47B, 95B, 95C.
• Use freshly prepared starch solution.
• Control temperature carefully (reaction is sensitive).

Method

1. Mix KI, Na₂S₂O₃ and starch in a beaker.
2. Add Na₂S₂O₈ to start the reaction, and start the clock.
3. Stop clock when blue-black colour appears.
4. Repeat with different [I⁻] or [S₂O₈²⁻].
5. Keep volume constant at 25.0 cm³ using water (see below table).
6. Record time, t and then use 1/t as rate (1/t is proportional to rate).

Sample Dilutions and Volumes

Data Analysis

• Use the time for blue colour to appear to calculate rate ≈ 1/t
• Plot rate vs. [I⁻] and rate vs. [S₂O₈²⁻]
• If straight line through origin, it’s first order

Rate expression: rate = k[S₂O₈²⁻][I⁻]

Mechanism

Suggested mechanism (based on balanced reaction equation - see top of page):
Step 1: I⁻ + S₂O₈²⁻ → S₂O₈I³⁻ (slow)
Step 2: S₂O₈I³⁻ + I⁻ → 2SO₄²⁻ + I₂ (fast)

Therefore, step 1 is the rate-determining step – consistent with rate = k[S₂O₈²⁻][I⁻]

Sources of Error

• Timing inaccuracy when colour appears.
• Misjudging end point intensity.
• Volume measurement uncertainty – reduce using burettes/pipettes

Matt’s exam tip

When iodine doesn’t affect rate, it proves reaction is zero order with respect to iodine. For clock reactions, initial rate is proportional to 1/t – always check units and axes carefully in graphs.