Rate experiments + rate determining step

Question 1

how do you find the initial rate of reaction from a graph?

Answer 1

Draw a tangent at zero

and calculate the gradient

Question 2

intitial rates method:

Answer 2

• Repeat an experiment several times using different initial concentrations of the reactants ( just change one of the concentrations of the reactants at a time)
• Observe how initial concentrations affect initial rates, and you can find the “order” for each reactant.

Question 3

iodine clock reaction (initial rate method)

Answer 3

• small amount of sodium thiosulfate and starch added to the reaction mixture, sodium thiosulfate instantly reacts with the iodine that’s being formed
• However once all the sodium thiosulfate has been used up, any more iodine that is formed stays in the solution.
• turning the starch indicator blue-black.

Question 4

during iodine clock reaction how do you get different times for colour change?

Answer 4

• vary the concentration of iodine or hydrogen peroxide whilst keeping everything else constant.

Question 5

what is the continuous monitoring method

Answer 5

Recording the amount of product/reactant at regular time intervals.

Question 6

the step with the slowest rate is known as the…..

Answer 6

Rate determining step

Question 7

what does order of reaction show regarding the rate determining step

Answer 7

how many molecules of reactant that are involved with the rate determining step

Question 8

what does the rate determing step help to work out

Answer 8

Reaction mechanisms
- 1st mechanism: with C-Br bond it is a strong bond which is hard to break- so as this is a slow rate of reaction ( rate determing step)

• 2nd mechanism: If OH- concentration is high , positive ions are more likely to collide with the OH- ions- fast rate. OH- ions are not in the rate equation, so can not be involved in the rate determing step