CI - Initial rates experiments

Question 1

What can be investigated using the initial rates method?

Answer 1

How a reaction depends on the concentration of a reactant.

Question 2

What does the initial rates method tell you?

Answer 2

The rate at the beginning of a reaction.

Question 3

What is the initial rate of a reaction?

Answer 3

The rate right at the start of the reaction.

Question 4

What happens in the initial rates method?

Answer 4

You time how long it takes for a set amount of product to form at the beginning of the reaction and then use this data to calculate the initial rate of the reaction.

You repeat the experiment several times, each time changing the initial concentration of the of the reactants.

You can use the data you collect to calculate the initial rate of the reaction using the formula:

initial rate = amount of reactant used or product formed / time.

You can also estimate the initial rate of a reaction from a concentration-time graph by drawing a tangent to the curve at t = 0 and measure its gradient.

Question 5

What is changed each time in the initial rate method?

Answer 5

The initial concentration of one of the reactants.

Question 6

When you plan an initial rates experiment, what assumptions do you make?

Answer 6

• The concentration of the other reactants isn’t changing significantly - you usually do this by having all of the other reactants present in excess.
• The temperature stays constant.
• The reaction has not proceeded too far when you take your measurement.

Question 7

What can you assume as long as all your assumptions regarding the set up of your initial rate experiment are reasonable for your experiment?

Answer 7

That the rate of reaction will be constant during the time period of your measurement. So the rate that you calculate will be a good estimate of the initial rate of your reaction.

Question 8

What is the equation for working out initial rate?

Answer 8

initial rate = amount of reactant used or product formed / time.

Question 9

Give a type of initial rate experiment

Answer 9

Clock reactions.

Question 10

What happens in a clock reaction?

Answer 10

You measure how the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants.

Question 11

What are clock reaction an example of?

Answer 11

Initial rate experiment.

Question 12

Why are clock reactions good initial rate experiments?

Answer 12

Because they have an easily observable end point, usually a colour change, that tells you when the desired amount of product has formed.

Question 13

What does the speed of a clock reaction tell you?

Answer 13

The quicker the clock reactions finishes, the faster the initial rate of the reaction.

Question 14

What is the most famous clock reaction?

Answer 14

The iodine clock reaction.

Question 15

What is the equation for the iodine clock reaction?

Answer 15

H2O2 (aq) + 2I- (aq) + 2H+ (aq) –> 2H2O (l) + I2 (aq)

Question 16

What happens in the iodine clock reaction?

Answer 16

• A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in acid solution. (Starch is used as an indicator - it turns blue-black in the presence of iodine.)
• The sodium thiosulfate that is added to the reaction mixture reacts instantaneously with any iodine that forms: 2S2O3^2- (aq) + I2 (aq) –> 2I- (aq) + S4O6^2- (aq).
• To begin with, all the iodine that forms in the first reaction is used up straight away in the second reaction. But once all the sodium thiosulfate is used up, any more iodine that forms will stay in solution, so the starch indicator will suddenly turn the solution blue-black. This is the end point of the clock reaction.
• Varying the concentration of iodine ions or hydrogen peroxide will give different times for the colour change.

Question 17

What happens at the end of a clock reaction?

Answer 17

There is an easily observable end point - usually a colour change, that tells you when the desired amount of product has formed.