11Stuff about Reaction RatesStuff about Reaction Rates

The raterate of a chemical reaction describes the speedspeed or how fast how fast the reaction occurs.

22A rate can be expressed as:

* how much reactant is used * how much reactant is used up in up in a a unit of time, unit of time, oror

* how much product forms in a unit * how much product forms in a unit of of time.time.Unit of time usually = seconds or hours; “how much” usually = concentration units.

33“Concentration” means quantity of mass/volume of solution.

# of moles/Liter is standard concentration unit.

44So raterate is given as

concentration/concentration/timetime,where “change in value” = final value - initial value.

ExampleExample: 1 hour reactionFinal concentration 1 mole/LInitial concen. = 3 mole/Lrate = (3-1)/1= 2 mole/L/hr

55The study of reaction rates is called kinetics.

Why do kinetics experiments?

* kinetic data gives info about the most critical moment in a

reaction, the point of point of highest highest energy energy ==> the transition transition statestate.

66Reactions involve breaking and making bonds.

Each type of bond has its own particular strength or energy.

When reactions occur, energy changes occur as well.

77Collision Model for Reactions* molecules are in constant * molecules are in constant motion in solution.motion in solution.

*sometimes when molecules *sometimes when molecules collide with each other, collide with each other,

reaction occurs. reaction occurs.

*an effective collision leads to *an effective collision leads to formation of a product.formation of a product.

88*to have an effective *to have an effective collision, the molecules collision, the molecules

must must collide with enough collide with enough energy and energy and the proper the proper geometry.geometry.

* the minimum energy needed * the minimum energy needed to to have an effective collision is have an effective collision is

called the activation energy.called the activation energy.

99The activation energy is a barrier the reactants must overcome in order to react.

The higherhigher the activation energyactivation energy, the slowerslower the reactionreaction.

Low activation energy Low activation energy = faster faster reactionreaction.

1010Analogy: driving through high country over mountains.

sea level

roadroad

actual actual elevationelevation

height height barrierbarrier

mountainmountain

1111More formally:

EEnneerrggyy

Reaction progressReaction progress

productreactant

transition state

activation activation energyenergy

1212In order to make a small net change in elevation, a larger increase in height must be accomplished first.

The activation energy = energy increase from reactants to transition state.

1313At the transition state:* old bonds are partially * old bonds are partially brokenbroken, and

* new bonds are partially formed* new bonds are partially formed.

Duration of transition state: Duration of transition state: about 10about 10-14 -14 seconds (time of seconds (time of vibration of one bond).vibration of one bond).

1414Net energy change in reaction = energy of products - energy reactants.

OR

EEreaction reaction = E= Eproductsproducts - E - Ereactantsreactants..

1515

EEnneerrggyy

Reaction progressReaction progress

productreactant

transition state

EEactact

EEreactionreaction

1616WhenEEreaction reaction < 0< 0, reaction is

exothermicexothermic (reaction produces heat)

EEreaction reaction > 0> 0, reaction is endothermicendothermic

(reaction absorbs heat)

1717exothermic endothermic

Energy

EErxnrxn < 0 < 0 EErxnrxn > 0 > 0

1818What affects reaction rate?

* increase concentration * increase concentration of reactants = increase collisions.

* increase temperature * increase temperature = = increase # of molecules that have the needed activation

energy

1919** add a catalyst add a catalyst = lowers

activation energy by changing path by which reaction occurs.

analogy: cut a road along side of mountain, dig a tunnel.

2020

Other descriptions of energy changes in reactions...

* entropy, symbol = S* entropy, symbol = S

* enthalpy, * enthalpy, symbol = H symbol = H

* free energy, symbol = G* free energy, symbol = G

2121EntropyEntropy is a measure of the disorder or randomness in a system.

Entropy increases (Entropy increases (S > 0) S > 0) if: • #products> #reactants;• products are in a less ordered

physical state than reactant (ex: solid --> liquid; liquid -->

gas)

2222

The entropy or messiness of the universe tends to increase over time.

2323EnthalpyEnthalpy is the change in heat change in heat energy energy associated with a reaction at constant pressure.

Enthalpy changes come from making and breaking chemical bonds.

Make bonds = release heat

Break bonds = absorb heat

2424Each type of bond has a different strength or energy value.

If product bonds are stronger If product bonds are stronger than reactant bonds, heat is than reactant bonds, heat is released: released: Enthalpy = Enthalpy = H < 0.H < 0.

If product bonds are weaker than If product bonds are weaker than reactant bonds, heat is reactant bonds, heat is absorbed: absorbed: Enthalpy = Enthalpy = H > 0.H > 0.

2525Free energy change in a reaction is a combination of enthalpy and entropy change at a particular temperature:

G = G = HH - - TT**SS

(T=temperature in kelvins)

H usually larger in value than H usually larger in value than S alone.S alone.

2626

Free Energy refers to the excess or usable energy available from a reaction. In other words,

heat energy released – energy needed to order the system = energy left over for use in doing work.

2727G < 0G < 0, reaction is exergonicexergonic, likely to occur.

G> 0G> 0,, reaction is endergonicendergonic, not likely to occur.

If G < 0, reaction may still not occur if it is very slow

= if Eactivation is too large.

2828

G tells us if reaction is energetically possible or likely, but not how fast.

Eactivation determines speed and if reaction is likely to take place in an observable time frame.

2929

In previous slides, Ereaction most closely approximates Enthalpy of reaction.

Enthalpy changes are larger than entropy changes alone.

Free energy changes involve comparing H and T*S.