Measuring Reaction Rates Practical

Working and Solutions

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Technique 1: Collecting a Gas

4. Calculate the change in gas volume during the first, second and third thirds of the time.

Δ gas volume = difference in volume for each third.

5. Calculate the change in number of moles of gas during each third of the reaction, then divide this by the volume of the solution (in dm3) to give you a change in concentration.

n = v/Mv c= n/v (read values off your graph)

6. Use the equation in the introduction to calculate the rate of the reaction during each third of the experiment.

Rate (moldm-3s-1) = Δ[products]/Δtime

7. How do the three rates compare? Try you explain any difference?

Rate should be faster at the start because the concentration of the reactants is at the highest concentration. More reactant particles to collide, higher frequency of collisions, increased number of successful collisions produces higher rate of conversion to products.

(secs)

Analysis:1. Write a balanced equation for the reaction.

Na2(CO3) + 2HCl 2NaCl + CO2 + H2O

2. Draw a graph of volume of gas collected vs. time, with a line (curved) of best fit.

3 Use your line of best fit to determine the volume of gas collected at times one third, two thirds and all the way through the reaction.

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Technique 2: The Iodine Clock Technique

Analysis:

1. Calculate the number of moles of sodium thiosulphate involved in the reaction.

n = n/Mr = 9.5/248.2 = 0.0383 moles of Sodium thiosulphate

2. Calculate the concentration of sodium thiosulphate in solution A

c=n/v = 0.0383/0.02 = 1.92 moldm-3

3. Use the reaction equations to determine the number of moles of iodine produced at the point of colour change.

S2O32-:I2 2:1 ratio from the equation Number of moles of I2 = 0.0192

4. Determine the change in concentration of I2 (assuming that the clock wasn’t eating it).

When Iodine is formed at colour change c=n/v = 0.0192/0.04 = 0.479 moldm-3

5. Calculate the reaction rate.

Rate of reaction = Δ[I2]/ Δt = (0.479 – 0)/ Δt

(substitute how long it took for the colour change in seconds)

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Analysis:1.Write a balanced equation for this reaction…research it if you need

to.Na2S2O3 + 2HCl 2NaCl + SO2 + S + H2O

2.Determine the rate of reaction (Note: the ‘concentration’ of sulphur required to obscure the spot is about 7.5 x 10-3

mol dm-3).

Rate of Reaction = Δ[S]/Δt = (7.5 x 10-3 – 0)/Δt

3.Calculate the relative uncertainty in the rate you just determined. You will need to think in particular about the certainty of your timing.

Relative uncertainty of stopwatch = (0.1/ Δt) x 100Relative uncertainty of (2) measuring cylinder = (0.25/10) x 100 x 2 = 5%Add relative uncertainties together

Technique 3: The Obscured Mark Technique