Kinetics(II)

Question 1

define continuous experiments

Answer 1

one experiment is conducted and the change in concentration of reactant/product is measured at regular time intervals.

Question 2

state the conditions needed for continuous experiments

Answer 2

1. temperature should be kept constant throughout the experiment.
2. A low concentration of the reactant to be investigated is used while the other reactants are in large excess.

Question 3

state how sampling and titration can be carried out to perform continuous experiments.

Answer 3

1. preparation of the reaction mixture
2. withdrawal of samples from the mixture at regular time inteverals.
3. quenching of reaction mixture to stop reaction at specific time.

different ways to quench:
- addition of base to remove acid catalyst present
- sudden introduction of a large volume of water
- submerge reaction sample into an ice bath to achieve rapid cooling.

4. the quenched sample is then titrated against a suitable reagent of known concentration to detemrine the amount of reactant that remains at the time of quenching.

Question 4

example of how it can be carried out

Answer 4

reaction of propanone with iodine using H+ catalyst:
CH3COCH3(aq) + I2(aq) (H+)= CH3COCH2I(aq) + HI(aq)

1. Preparation of reaction mixture:
   reactants anc acid catalyst of known volumes and concentrations are mixed, and stopwatch is started at the point of mixing.
2. Withdrawal of samples for analysis:
   At regular time intervals, samples are withdrawn from the reaction mixture.
3. Quenching of reaction sample:
   Excess quenching reagent NaHCO3(aq)/Na2CO3(aq) is added to sample, removing H+ catalyst from the reaction. As quenching stops the reaction in each sample, the concentrations of reactant and products in these samples remain constant. dilution can also be used to slow down the reaction tremendously so that the concentrations remain relatively constant.
4. Analysis of sample via titration:
   2S2O32-(aq) + I2(aq) = S4O62-(aq) + 2I-(aq).
   for each sample, we only get one attempt at titration.
5. repeat steps 2 to 4:
   Determine the amount of unreacted iodine in the reaction in the raction mixture at different time intervals and plot an appropriate graph.

Question 5

state how observing the change in gas volume can be used to find the rate of reaction

Answer 5

this method is used only if gas is formed.

if aqueous reactants are used, the gas should be insoluble in water.

1. measure the volume of gas formed at regular time intervals during reaction.
2. plot a graph of volume of gas against time.
3. the order of reaction with respect to the reactant can be determined using the half-life method
4. the rate of reaction at any time t is given by the gradient of the graph at time t.