Module 3

Lesson 10 – Practical and Arrhenius

Objectives

MustDescribe qualitatively, using the Boltzmann distribution, the effect of temperature changes on the proportion of molecules possessing a certain energy.ShouldConduct an experiment to investigate the effect of temperature on rate of reaction.CouldManipulate the results of the experiment to calculate the activation energy using a graphical method and the Arrhenius equation.

Starter homework – peer assessAn experiment is conducted to measure the rate of hydrogen peroxide decomposition. Using collision theory and the Boltzmann distribution explain the following observations.

• When the temperature is raised by 20°C from 20°C to 40°C the rate increases to more than four times the original rate.

• A catalyst is tested at 40°C which increases the rate of reaction still further to double the previous rate

• Heating the mixture containing the catalyst still further to 50°C caused the reaction rate to fall.

[10 marks]

• In order to react molecules need to collide in the correct orientation and with sufficient energy [1 mark]

• The Boltzmann distribution shows the number of molecules with a certain energy. The number of molecules with the Ea is shown as the shaded area [2 marks]

• Diagram [2marks]

• When the temperature is higher the curve gets broader and the shaded area gets MUCH bigger. Many more molecules have enough energy to react when they collide. [1 mark]

• A catalyst reduces the activation energy (shown illustrated OK) [1 mark].

• This increases the proportion of molecules possessing the activation energy / area under curve increases [1 mark].

• The catalyst is an enzyme which becomes denatured at >40°C. [1 mark].

• The activation energy returns to the uncatalysed value and rate falls. [1 mark]

The Reaction

Dilute hydrochloric acid reacts with sodium thiosulphate solution to give a precipitate of sulphur

Na2S2O3 (aq) + 2HCl (aq) 2NaCl (aq) + H2O (l) + SO2 + S (s)

Practical

• Using fixed concentrations of 0.5M HCl and 0.1 M Na2S2O3, investigate the effect of temperature on the rate of reaction.

• You should investigate 5 different temperatures with three repeats across the whole group.

• Your temperatures should span approximately (but recorded accurately) 5°, 10°, 20°, 30°, 40°

• To do the repeats have all your boiling tubes containing your reactants in the water bath/trough at the same time.

Plenary

• Plot three graphs and bring to next lesson

• Time taken (t) against temperature (T)

• Rate (1/t) against temperature (T)

• ln(1/t) vs. 1/T

Extension – The rate equation

Rate of reaction is dependent on the concentrations of reacting species• Our reactants are Na2S2O3 and HCl

We can say that • RATE = k[Na2S2O3]m[HCl]n

• Where [Na2S2O3] is the concentration of thiosulphate in mol/l and

• [HCl] is the concentration of HCl in mol/l

Extension - Arrhenius

Rate = k[a]x[b]y

For constant [a] and [b] ie concentration kept constant

K = AexpEa/RT

Lnk = lnA+Ea/RT

So plot ln (1/t) vs 1/T has gradient Ea/R

Further extension

Orders of reaction – experiment varying concentration

RATE = k[Na2S2O3]m[HCl]n

• The small numbers ‘m’and ‘n’ are something we call the ORDER of the reaction.

• The ORDER could be 1, 2, 3 etc. We call this 1st, 2nd, 3rd order.

• We can ONLY determine these by actually doing the experiment.

• Plotting RATE (Y) against CONCENTRATION (X) tells us the order by looking at the shape of the graph.

RATE = k[Na2S2O3]m[HCl]n

• Rate = 1/time (Y AXIS)• Straight horizontal line = 0 order changing the concentration

of this has NO effect• Straight line upwards = 1st order doubling the concentration

doubles the rate.• Curved line upwards = 2nd order – doubling the concentration

more than doubles the rate.

RATE = k[Na2S2O3]m[HCl]n

• If you have looked at changing the concentration of thiosulphate you could tell me the ORDER with respect to this IE ‘m’

• If you have looked at changing the concentration of HCl you could tell me the ORDER with respect to this IE ‘n’

• If you have done both you can give me the whole RATE EQUATION for the reaction!!!!