B.Tech./MSc/BSc Chemistry Lab Experiment: Decomposition of Diacetone alcohol – Dilatometry method

Decomposition of Diacetone Alcohol – Dilatometry Method

Aim

To study the kinetics of the decomposition of diacetone alcohol at 25 °C in the presence of alkali using the dilatometric method.

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Principle

The decomposition of diacetone alcohol in an alkaline medium follows the reaction:

(CH₃)₂C(OH)CH₂COCH₃ ⟶ 2 CH₃COCH₃

In the presence of excess alkali, the reaction rate is independent of the alkali concentration and depends only on the concentration of diacetone alcohol. This behavior makes it a pseudo-first order reaction.

Since the products occupy a larger volume than the reactants, the change in volume during the reaction can be used to monitor the reaction progress. Thus, the dilatometric method is especially suitable for studying this reaction.

The rate constant k for a first-order reaction is given by:

k = (2.303 / t) × log(V∞-V₀) / (V∞ - Vt))

Where:

• V₀ = initial volume

• Vt = volume at time t

• V∞ = final volume

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Apparatus and Chemicals Required

Sl. No.	Apparatus	Chemicals
1	Dilatometer	Diacetone alcohol
2	Graduated pipette	Sodium hydroxide (0.5N & 0.25N)
3	Stop watch
4	Standard flask
5	Burette stand
6	Thermostatic bath

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Procedure

1. Clean and dry the dilatometer. Clamp it on a stand and immerse it in a thermostatic water bath maintained at 25 °C.

2. Pour the required amount of 0.5N NaOH into the bulb of the dilatometer.

3. Add 2 ml of diacetone alcohol, simultaneously starting the stopwatch.

4. Record the scale reading at intervals of 30 seconds.

5. Continue taking readings until after t½, which satisfies:
   t½ × [OH⁻] = 5

6. After this point, record at least 10 readings at equal intervals.

Repeat the procedure using 0.25N NaOH, and calculate the rate constants using the following relation:

k = (2.303 / tn) × log((r' - rn) / (r' - r))

Where:

• r and r' are scale readings at times tn and t½ + tn, respectively.

Plotting log(r' – r) against tn yields a straight line. The slope of this line gives the rate constant k.

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Result

• Rate constant (k) for 0.5 N NaOH = [to be filled after experiment]

• Rate constant (k) for 0.25 N NaOH = [to be filled after experiment]

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