Concentration-Time Graphs

Question 1

How can a colorimeter be used to continuously monitor rate of reaction?

Answer 1

• wavelength of the light passing through a coloured solution is controlled using a filter
  —> filter is a complementary colour to what is being absorbed (opposite on colour wheel)
• amount of light being absorbed by the solution is measured
• plot calibration curve of absorbance against concentration
• carry out reaction and take absorbance reading at time intervals
• determine the concentration of iodine at each absorbance reading
• plot graph of concentration against time to determine order of reaction and rate.

Question 2

Describe the shape of a concentration-time graph with zero order.

Answer 2

• straight line with -ve gradient
• rate does not change
• gradient = k

Question 3

Describe the shape of a concentration-time graph with first order.

Answer 3

• downward curve with decreasing gradient over time
• the rate of reaction is slowing down
• time for concentration to half is constant (half-life is the same)

Question 4

Describe the shape of a concentration-time graph with second order.

Answer 4

• downwards curve that is initially steep but is tailing off more slowly

Question 5

What is half life and how can it be used to confirm the reaction is first order?

Answer 5

• time taken for the concentration of a reactant to decrease by half
• successive half life’s are the same

Question 6

Why is the non recorded reactant used in excess when recording concentration-time?

Answer 6

• the non recorded reactant has effectively zero order so does not affect the rate of reaction

Question 7

How can the rate constant be determined using the concentration-time graph for a first order reaction?

Answer 7

1. • Tangent drawn at specific concentration
     —> gradient = rate
   • use rate equation to calculate rate constant
2. Rate constant = ln2 / half life