18.2 Concentration-time Graphs

Question 1

What methods of continuous monitoring can be used to measure the rate of a reaction?

Answer 1

1. gas collection
2. mass loss
3. colorimetry

Question 2

Describe a method using colorimetry, which can be used to determine the initial rate of the reaction below:

I₂ + CH₃COCH₃ ⟶ CH₃COCH₂I + I^- + H⁺

Answer 2

1. prepare standard solutions of I₂, (each of different concentration)
2. calibrate the colorimeter using water
3. place a filter which has a complementary colour to the solution. e.g blue/green, since iodine solutions are orange-brown
4. measure the absorbance readings of each I₂ solution
5. plot a calibration curve, (indicates absorbance readings of each concentration of I₂₎
6. carry out the reaction, taking absorbance readings at regular time intervals.
7. compare the absorbance readings to the calibration curve to determine the concentration of I₂ at each time.
8. plot a graph of I₂ concentration against time.
9. find the gradient of the tangent at t = 0, which is the rate.

Question 3

What is half-life?

Answer 3

time taken

for concentration of a reactant

to half

Question 4

How can you differentiate between a concentration-time graph of a 1st & 2nd order reaction?

Answer 4

1st order has constant half life

2nd order has increasing half life

Question 5

How can you find the value of k from a 1st order concentration-time graph?

Answer 5

USING RATE: draw tangent @ specific concentration & find gradient. gradient = rate. sub in rate & concentration into rate equation to find k.

USING HALF-LIFE: find half life. k = ln(2)/half life

Question 6

What is the equation for finding k using the half life?

Answer 6

k = ln(2)/half life