[18.2] concentration-time graphs

Question 1

how can you use initial rate method to find the order wrt any species in the reaction mixture?

Answer 1

• do a series of experiments with different initial concentrations of reactants
• plot a concentration-time graph
• gradient of tangent is drawn at t = 0, which is the initial rate

Question 2

what is the importance of measuring initial rate?

Answer 2

concentration of all substances in the reaction are known exactly at this time

Question 3

describe the continuous method

Answer 3

1. reaction mixture is made up
2. sample of reacting mixture is taken at regular intervals
3. reaction is quenched if necessary
4. use titration or colorimetry to determine conc of reactant
5. plot graph of reactant conc vs. time
6. establish half-life of reaction at different concs

Question 4

describe the concentration-time graph for zero order

Answer 4

• straight line graph
• negative gradient
• no change in reaction rate
• value of gradient = k

Question 5

describe the concentration-time graph for first order

Answer 5

• downward curve
• decreasing gradient
• T ½ is constant

Question 6

describe the concentration-time graph for second order

Answer 6

• downward curve
• decreasing gradient
• steeper at start but gradually tailing off

Question 7

how do half lives change with decreasing concentration for zero order?

Answer 7

half life decreases with decreasing concentration

Question 8

how do half lives change with decreasing concentration for first order and how do you calculate k?

Answer 8

• length of half life is constnat
• t ½ = ln (2) / k so k = ln (2) / t½

Question 9

how do half lives change with decreasing concentration for second order?

Answer 9

length of half life increases with decreasing concentration