Factors that affect reaction rateFactors that affect reaction rate

Concentration of REACTANTSConcentration of REACTANTS

Temperature (kinetic energy of molecules)Temperature (kinetic energy of molecules)

Presence of a catalystPresence of a catalyst

Physical state of reactants (surface area)Physical state of reactants (surface area)

Higher concentration = more Higher concentration = more collisionscollisions

Ways over (or around) the barrierWays over (or around) the barrier

Temperature increases reaction rate by Temperature increases reaction rate by increasing the fraction of molecules which increasing the fraction of molecules which have sufficient energy to jump the barrierhave sufficient energy to jump the barrier

A catalyst is a way to remove, or at least A catalyst is a way to remove, or at least lower the barrier. A catalyst acts to increase lower the barrier. A catalyst acts to increase the chemical reaction, but is not consumed the chemical reaction, but is not consumed itself during the reactionitself during the reaction

Catalysts modify the pathwayCatalysts modify the pathway

Addition of a chlorine Addition of a chlorine catalyst increases the rate catalyst increases the rate of decomposition of ozone of decomposition of ozone into Ointo O22 – the reason for the – the reason for the destructive influence of destructive influence of these compounds in the these compounds in the atmosphereatmosphere

Although two barriers are Although two barriers are present, both are smaller present, both are smaller than the one without the than the one without the catalyst, and the reaction catalyst, and the reaction proceeds more rapidlyproceeds more rapidly

Without catalysts, there would be no Without catalysts, there would be no life at all, from microbes to humans life at all, from microbes to humans

ENZYMES are ENZYMES are biological catalystsbiological catalysts

Most enzymes are Most enzymes are proteins – large proteins – large moleculesmolecules

Have correct shape to Have correct shape to bring reactant bring reactant molecules together in molecules together in correct orientationcorrect orientation

Natural selection produces Natural selection produces prodigious catalystsprodigious catalysts

Green lines are the Green lines are the amino acid backbone amino acid backbone of the enzymeof the enzyme

The white molecule is The white molecule is the fatty acidthe fatty acid

Red are iron atomsRed are iron atoms

Catalysts can be solids, liquids or Catalysts can be solids, liquids or gasesgases

The catalyst shape can direct certain The catalyst shape can direct certain products: zeolite molecular sieveproducts: zeolite molecular sieve

Reactant molecules

Product molecules

Mathematical treatment of reaction Mathematical treatment of reaction raterate

In the reactionIn the reaction

2N2N22OO55 = 4NO = 4NO22 + O + O22

We describe the rate according to the formWe describe the rate according to the form

Rate = Rate = kk[N[N22OO55]]nn

kk is the rate constant is the rate constant nn is the order of reaction with respect to N is the order of reaction with respect to N22OO55

NB: n NB: n is not the same as the coefficient in the equation!is not the same as the coefficient in the equation! Rate is normally measured by monitoring growth in Rate is normally measured by monitoring growth in

concentration of a product, or decline in concentration of a product, or decline in concentration of a reactant concentration of a reactant

More than one reactantMore than one reactant

A + B = products A + B = products

Rate = Rate = kk[A][A]mm[B][B]nn

m and n are individual orders for the reactants A m and n are individual orders for the reactants A and B. The overall order of reaction = m + nand B. The overall order of reaction = m + n

Strategies for determining orders of Strategies for determining orders of reactionreaction

Method of initial ratesMethod of initial rates At At tt = 0, product concentration is zero = 0, product concentration is zero Vary concentration of one reactant, keeping the other(s) constantVary concentration of one reactant, keeping the other(s) constant

Reaction is second order in NO and first order in OReaction is second order in NO and first order in O22

][][ 2

22 ONOkt

NORate

Effect of temperature: activation Effect of temperature: activation energyenergy

As temperature increases rate increasesAs temperature increases rate increases

Arrhenius expression: EArrhenius expression: EA A is activation energyis activation energy Plot of ln k vs 1/T is constant (T in Kelvin)Plot of ln k vs 1/T is constant (T in Kelvin)

AERTk Ae

Activation energy using two Activation energy using two temperaturestemperatures

Measure ratio of rates at two temperatures Measure ratio of rates at two temperatures (Rate is proportional to k)(Rate is proportional to k)

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