Chemical Kinetics

Rates of chemical reactions

Every chemical reaction has its own signature rate

• Diamonds are made by converting Carbon from its graphite state.

• takes millions years to consume the graphite (the reactant) to form a diamond (the product).

• this is why we place such a high value on diamonds

• the chemical reaction of photosynthesis (a plants ability to make its own food, which is the reaction's product along with Oxygen) takes place on a time scale as short as... ...10^-12 seconds (0.000000000001 s)

Chemical Kinetics

• We can think of reaction rate in terms of the change of concentration with time.

• IF A→B, then...

rate = - ΔA/Δt or rate = ΔB/Δt

...wherein ΔA and ΔB are changes in the concentration (the molarity) over a time period.

• The rate of reaction A→B, represented as the decrease of A molecules with time and as the increase of B molecules with time.

B molecules

A molecules

Deriving rates of reactions: you must first determine the concentration of the reactant at different intervals

of time

• Average rate – the mean rate of chan

ge in the concentration of the reactant over the whole duration of the reaction

- ΔA/Δt

• Instantaneous rate– a specific rate at a part

icular moment in time– If temperature is the s

ame and the initial concentration of the reactant used in the reaction is the same, the rate will always be the same at that moment in time every time you perform the reaction

Reactant concentration is proportional to the reaction rate...

Instanteous Rate

rate is directly proportional to Br2 concentration...

)(2)()()()(2 22 gaqaqaqaq COHBrHCOOHBr

2

2

k rate

constant a provide

must we),(equation an

intoality proportion make to

α rate

Br

Br

rate constant (k)• a constant of proportiona

lity between the reaction rate and the concentration of reactant.

• plot the data in a graph:rate vs. Br2 concentration• Is it a straight line or

a curved line?The fact that the graph is a straight line shows that the rate is directly

proportional to concentration.

time (s) Br2 M reaction rate (M/s)

0.0 0.0120 4.2 x 10^-5

50.0 0.0101 3.52 x 10^-5

100.0 0.00846 2.96 x 10^-5

150.0 0.00710 2.49 x 10^-5

units for rate constant

• Because reaction rate has the units M/s, and Br2 is in M, the unit of k is...

1/s or s^-1• k is not affected by the concentration of Br

2. • To be sure, the rate is greater at a higher c

oncentration and smaller at a lower concentration of Br2, but the ratio of rate/Br2 remains the same provided the temp. remains constant.

Find the rate constant for the following reaction

time (s) Br2 M reaction rate (M/s)

0.0 0.0120 4.2 x 10^-5

50.0 0.0101 3.52 x 10^-5

100.0 0.00846 2.96 x 10^-5

150.0 0.00710 2.49 x 10^-5

)(2)()()()(2 22 gaqaqaqaq COHBrHCOOHBr

k = rate/Br2

Use the chart to determine the rate con

stant

3.49 x 10^-3 s^-1

Reaction Rates and Stoichiometry

• STOICHIOMETRY - The quantitative study of reactants and products in a chemical reaction – Balancing equations and using molar masses

to figure the amount of each substance being used as reactants to determine how much product is being converted during a reaction

– Unbeknownst to you, we have been operating this concept, but we have yet to define it...

Reaction Rates and Stoichiometry• I have introduced to you simple situations...A→B• what if...2A→B• Uh-ohhhh... What to do? What to do?• Well, this expression is stating that 2 moles of A

disappear for each mole of B that forms.• The rate of disappearance of A is twice as fast

as the rate of appearance of B. • How could we express this rate?

Reaction Rates and Stoichiometry

trate

trate

B

or A

2

1How about like this...

THE RULE:take the reciprocal of every product's coefficient and eve

ry negative reciprocal of every reactant's coefficient

FOR EXAMPLE

t

O

t

Orate

OO gg

32

)(3)(2

2

1

3

1

23

YOUR TURN...

)(22(g)2(g)4(g)

(g)2(g)2(g)3(g)

OH2COO2CH

OH6NO4O54NH

.products.. of appearance

and reactants of ncedisappeara

of in terms reactions following

for these sexpression rate theWrite

g

The next step is to find the actual rate

)(22)(2)(2 22 aqgl OHOOH Molecular oxygen is reacting at the rate of 0.02

4 M/s. At what rate is H2O2?

sMt

OH

sMt

OHt

OH

t

O

sMt

Ot

OH

t

O

t

OHrate

/0048.0

)/024.0(2)(

2

1

/024.0

2

1

2

1

22

22

222

2

2222

HOMEWORK CHECK!

• 13.7• 2NO + O2 → 2NO2

• Suppose that at a particular moment during the reaction nitric oxide (NO) is reacting at the rate of 0.066 M/s.

a) At what rate is NO2 being formed?

b)At what rate is molecular oxygen reacting?

2NO + O2 → 2NO2

NO is reacting at the rate of 0.066 M/s• To express the rate of reaction we need to do three th

ings...– Use proper sign (- or +)– Determine the reciprocal of the stoichiometric coeffi

cient for each molecule in the reaction– Make all molecular rates equal to one another

t

NO

t

O

t

NO

22

2

1

1

1

2

1

2NO + O2 → 2NO2

NO is reacting at the rate of 0.066 M/s

sM

t

NO066.0

t

NO

t

NO

t

NO

sM

2

2

2

1066.0

2

1

2

1

2

1

t

NOsM

2066.0

• At what rate is NO2 being formed?

t

NO

t

O

t

NO

22

2

1

1

1

2

1GIVEN

-

_

2NO + O2 → 2NO2

NO is reacting at the rate of 0.066 M/s

sM

t

NO066.0

• At what rate is molecular oxygen reacting?

t

NO

t

O

t

NO

22

2

1

1

1

2

1GIVEN

t

O

t

O

t

NO

sM

2

2

1

1066.0

2

1

1

1

2

1

t

O

t

O

sM

sM

2

2

033.0

066.02

1

-

_

_

_

N2 + 3H2 → 2NH3Molecular hydrogen reacts at a rate of 0.074 M/s

• 13.8• Suppose that at a particular moment during the

reaction molecular hydrogen is reacting at the rate of 0.074 M/s.

a) At what rate is ammonia being formed?b) At what rate is molecular nitrogen reacting?

t

NH

t

H

t

N

322

2

1

3

1

1

1

N2 + 3H2 → 2NH3Molecular hydrogen reacts at a rate of 0.074 M/s

• At what rate is ammonia being formed? s

M

t

H074.02

t

NH

t

H

t

N

322

2

1

3

1

1

1

t

NH

t

NH

t

H

sM

3

32

2

1074.0

3

1

2

1

3

1t

NHsM

3074.0

3

2

_

-

_

t

NHsM

3049.0

GIVEN

N2 + 3H2 → 2NH3Molecular hydrogen reacts at a rate of 0.074 M/s

• At what rate is molecular nitrogen reacting? s

M

t

H074.02

t

NH

t

H

t

N

322

2

1

3

1

1

1

-

GIVEN

t

N

t

N

t

H

sM

2

22

1

1074.0

3

1

1

1

3

1

t

NsM

2074.0)1(

3

1

t

NsM

2025.0

-

HERE ARE YOUR TESTS