Exothermic release energy into the surroundings temp of surroundings increases products have less energy - ΔE R P

Chapter 13

Endothermic absorb energy from the surroundings temp of surroundings decreases products have more energy +Δ E P R

• A + B ----> C + D + 30 joules• A + B ----> C + D ΔE = -30 joules• Forward reaction is exothermic

• Reverse reaction is endothermic• 30 joules + C + D ----> A + B• C + D ----> A + B ΔE = + 30 joules

Reactions involve

• Bond breaking –requires energy +ΔE

• Bond formation –releases energy -ΔE

• If more energy is needed to break bonds than is released +ΔE ( endothermic)

• If more energy is released in bond formation than is absorbed -ΔE (exothermic)

• Reaction mechanism- steps by which a reaction occurs

• Each step of a reaction mechanism involves the collision of two molecules.

Colliding molecules need

• 1. enough energy to break the bonds• Activation Energy ( A.E) or threshold energy• 2. the proper geometry or orientation (correct

angle)

Energy (P.E.) reaction coordinatec

Factors affecting rate of reaction

• Nature of reactants (number of bonds)• Surface area (solids and liquids)• Temperature• Concentration of reactants (solutions and

gases)• Catalyst

• Rate =(# of coll/time) (fract with A.E.) (fract with orientation)

• If orientation factor is 1 (orientation does not matter)

• TEMPERATURE• affects fraction with A.E.

Temp - measure of the average K.E.At a given temp all molecules do not have the same K.E.

# of molecules

Kinetic energy threshold energy (A.E)

• Rate =(# of coll/time) (fract with A.E.) (fract with orientation)

• Concentration• affects the # of collisions At a given temp

Rate = k (# of collisions /time)

Rate = k (# of collisions /time)

• k large - fast rxn• k small - slow rxn• Concentration of reactants raised to some

power (order)

• Rate = (k ) [A]x [B]y

• X and Y (orders)• Orders found only by experiment

Rate = (k ) [A]x [B]y

• If concentration of a reactant is doubled and the rate doubles the order is 1

• If concentration of a reactant is doubled and the rate quadruples the order is 2

• If changing concentration of a reactant has no effect on the rate of reaction it is not included in the rate law

• Sum of all orders is the order of the reaction

Reaction coordinate

Potential energy

Catalysts and rate

A catalyst does not change the K.E of the molecules

# of molecules

Kinetic energy

catalyst no catalyst

• Catalyst -changes the pathway (steps) Steps require less energy

• Changes the orientation requirement- more molecules have required orientation

• Not consumed in the reaction

Reaction mechanism and Rate Law

• Each step involves the collision of two molecules• –adding steps gives net reaction• Each step has its own rate law.• In the steps the coefficients are the orders of the

rate law• Slowest step determines the overall rate -rate

determining step• The rate law for the slowest step is the rate law

for the reaction.

Reaction Mechanism

• A + B- C + 2D• D+ B DB• DB + D F

• A + 2B - C + F

Determining Reaction Mechanism

• 1. Do an experiment to determine the rate law. Experimental rate law

• 2. Postulate possible steps by which the reaction could take place. Reaction Mechanism

A2+ B2 2AB

• rate = k [A2]2 (from experiment)• Possible mechanism 1• A2+ B2 2AB

• rate = k [ A2] [ B2]• Predicted does not match experimental• Not the reaction mechanism

A2+ B2 -> 2AB• rate = k [A2]2 (from experiment)• Possible mechanism 2• Step 1 A 2 + A2 2A + A2

• Step 2 A + B2 AB2

• Step 3 AB2 + A 2AB

• Step 1 rate = k [A2][A2] or rate =k [A2]2

• Step 2 rate = k [A][B2]

• Step 3 rate = k [AB2] [A]

• Do any rate laws match the experiment rate law? rate = k [ A2]2

2NO + O2 2NO2

Experiment Initial concentration [NO]

Initial concentration [O2]

Rate of formation of NO2 (M/s)

1 0 .015 M 0 .015 M 0.048

2 0 .030 M 0 .015 M 0.192

3 0 .015 M 0 .030 M 0.096

4 0 .030 M 0 .030 M 0.384