Describing Reactions

• Stoichiometry

• Thermodynamics

• Kinetics concerned with the speed or rates of chemical reactions

reacting ratios, limiting and excess reagents

reaction spontaneity

Reaction Rates

Br2 + HCOOH 2Br- + 2H+ + CO2

change in concentration change in time

Rate = D[Br2]Dt

Time (s) [Br2] (M)0.0 0.0120

50.0 0.0101

100.0 0.00846

150.0 0.00710

200.0 0.00596

250.0 0.00500

300.0 0.00420

350.0 0.00353

400.0 0.00296

Br2 + HCOOH 2Br- + 2H+ + CO2

3.80 x 10-5 M/s

1.14 x 10-5 M/s

Instantaneous Rate

Br2 + HCOOH 2Br- + 2H+ + CO2

Rate = D[Br2]Dt

= 2.96 x 10-5 M/s

Reaction Rate and Stoichiometry

Br2 + HCOOH 2Br- + 2H+ + CO2

Rate of change of Br- ?

Rate = D[Br2]Dt

= 2.96 x 10-5 M/s

Rate = D[Br2]Dt =

D[Br- ]Dt

12 =

D[H+ ]Dt

12

= D[HCOOH]

Dt

Reaction Rate and Stoichiometry

aA + bB cC + dD

Rate = D[A ]Dt

1a

= D[B ]Dt

1b =

D[C]Dt

1c =

D[D]Dt

1d

Rate Laws

• expresses relation of rate of reaction to concentrations of reactants

A + B C

rate = k [A]x [B]y

rate constant. temperature dependent

“order” of the reaction with respect to each reactantNOT based on coefficients!!!determined experimentally

A + B C

[A] (M) [B] (M) Rate (M/s)

0.10 0.10 0.20

0.20 0.10 0.40

0.30 0.10 0.60

0.30 0.20 2.40

0.30 0.30 5.40

rate = k [A]x [B]y

Reaction “Order”

rate = k [A]

rate = k [A]0

first order

zero order

Reaction Mechanism

(CH3)3CBr + OH- (CH3)3COH + Br-

rate = k [(CH3)3CBr]1[OH-]0

slow step

fast step

slow step

fast step

rate = k [(CH3)3CBr]1[OH-]0

Reaction “Order”

rate = k [A]

rate = k [A]0

rate = k [A]2[B]1

first order

zero order

second order with respect to Afirst order with respect to Bthird order overall

Unit of Rate Constant (k)• depends on overall order of the reaction• To determine the value and units of k, use

data from one trial and sub into rate law.

rate = k [A]1[B]2

0.20 = k [0.10]1[0.10]2 k=200