pratical 7 measuring rates of reaction

Question 1

A(aq) + B(aq) + C(aq) → D(aq) + E(aq)

In aqueous solution, A, B, C and D are all colourless but E is dark blue.

A reagent (X) is available that reacts rapidly with E. This means that, if a small amount of X is included in the initial reaction mixture, it will react with any E produced until all of the X has been used up.

Explain, giving brief experimental details, how you could use a series of experiments to determine the order of this reaction with respect to A. In each experiment you should obtain a measure of the initial rate of reaction.

Answer 1

Stage 1 Preparation

1a Measure (suitable/known volumes of) A, B and C (ignore quoted values for volume)

1b Use of colorimeter

1c into separate container(s) – (allow up to two reagents measured together into one container) – ignore use of X

Stage 2 Procedure

2a Start clock/timer at the point of mixing

2b Take series of colorimeter readings at regular time intervals

2c Use of same concentration of B and C / same total volume / (same volume/amount of X)

2d Same temperature

2e Repeat with different concentrations of A (can be implied through different volumes of A and same total volume)

Stage 3 Use of Results

3a Plot absorbance vs time and measure/calculate gradient at time=0

3b plot of gradient against volumes/concentrations of A

3c description of interpreting order from shape of 1/time vs volume or concentration graph

Question 2

Continuous Monitoring

Potential 6 marker:

Describe how you can determine the order with respect to NaOH in the hydrolysis of methylethanoate ester [6]

CH3COOCH3 + NaOH → CH3COONa + CH3OH

Answer 2

Stage 1: Method

1a- Add 50cm3 of 1 moldm-3 CH3COOCH3 to a 250cm3 beaker + 50cm3 of 1 moldm-3 NaOH, start the timer.

1b – Every 30 seconds, transfer 5cm3 sample from mixture to a clean 250ml conical flask using a 10ml measuring cylinder and teat pipette

1c – Quench the reaction by adding 20cm3 of cold water

1d – Titrate sample with 1moldm-3 HCl in burette to determine the concentration of NaOH

Stage 2: Use of results

2a – Plot the concentration of NaOH /moldm-3 (y axis) vs time/secs (x axis)

2b – Draw tangents t=30 secs & t=60 secs and calculate the gradients

2c – Rate = gradient which will allow you to determine the rate at two different concentrations

Stage 3: Analysis

3a – Compare the change in concentration to change in rate.

3b – If the conc is x ½ and the rate x ½ = 1st order wrt NaOH as the concentration is directly proportional to rate.

3c – If the conc is x ½ and the rate decreases by x ¼ = 2nd order wrt NaOH as concentration2 is directly proportional to rate.

3d – If conc is x ½ and rate does not change, 0 order wrt to NaOH.