# Kinetics

Kinetics

• How quickly something happens (PharmD and the rate of a release of a drug from a capsule)

• Rate is how quickly something is performed.

Rate = 1/time = 1/sec

• In a chemical reaction, as it proceeds, initially reactants are turning into products

• The graph of these initial changes are:

• In words, the rates can be expressed as:

• the formation of products/time

• or disappearance of reactants/time (-ve sign)

• The change in reaction rate per time is most commonly expressed as:

• Change in conc/unit time in mole dm^-3s^-1

• This can be taken from a tangent to a curve off of a graph at several points with reference to the time.

• Sometimes the disappearance of reactants per time are at different rates which can be seen in graph form and is indicated by A and B and their coefficients in a chemical equation:

• 2A + B → C

• The shape of curve indicates that the rate of a reaction is not constant during the reaction but is fastest at the start and slows down as the reaction proceeds.

• The change in rate has to do with the effects of the change in concentration.

• We measure initial rates by drawing a tangent to the graph at t=0

• As a reaction proceeds, the slope of a concentration time graph decreases.

Ways to measure rates of reactions

• Change in volume

• Change in mass

• Change in light transmission (spectroscopy)

• Change in concentration using titration to stop it (quenching)

• Change in conductivity (graphed)

• Change in color (clock reactions)

Maxwell Boltzmann

Enthalpy

Example

Experiment

[NO(g)] mol dm^-3

[CO(g)] mol dm^-3

[O2(g)] mol dm^-3

Initial rate / mol dm^-3 s^-1

1

1.00x10^-3

1.00x10^-3

1.00x10^-1

4.40x10^-4

2

2.00x10^-3

1.00x10^-3

1.00x10^-1

1.76x10^-3

3

2.00x10^-3

2.00x10^-3

1.00x10^-1

1.76x10^-3

4

4.00x10^-3

1.00x10^-3

2.00x10^-1

7.04x10^-3

Rate = k[NO]^2[H2]

Experiment

Initial [C4H9Br] / mole dm^-3

Initial [OH-] / mol dm^-3

Initial rate of reaction / mol dm^-3 min^-1

1

0.010

0.010

2.0x10^-3

2

0.020

0.010

4.0x10^-3

3

0.020

0.020

4.0x10^-3

Determining the value of rate constant “k”

Rate = k[C4H9Br]

.002M/min = k(.01M)

.002/.01 = k

K = 0.200/minute

Elementary steps

H2 + NO → X fast

X + NO → Y + H2O slow

Y + H2 → N2 + H2OZ fast

Rate = k[NO]^2[H2]