Reaction Rates and Temperature

k(T) = A e–Ea/RT

R is the gas constant: RT has energy units.Ea is the activation energyA is the pre-exponential factor(or Arrhenius A factor)it has the same units as k

Reaction Rates and Temperature: The Arrhenius Equation

Reaction can only occur if collision takes place.

Colliding molecules must have correct orientation and energy.

Collision rate is directing proportional to the concentration of colliding particles.

Collision theory

Transition State Theory Explains the reaction resulting from the collision of

molecules to form an activated complex. Activated complex is unstable and can break to form

product.

Exothermic Reaction Endothermic Reaction

Enzyme kinetics: what do enzymes actually do?

Proteins with catalytic properties due to its power of specific activation

Enzymes lower the activation energy barrier, thus make the reaction more likely. This is observed as as an increased reaction rate. Enzymes DO NOT change ΔG0’, i.e., whether or not the reaction is favourable.

Enzyme may be used again

Enzyme-substrate complex

E

S

P

E

E

P

Reaction coordinate

Temperature coefficient (Q10).

21

a

1

2T

1

T

1

R

E

k

klnFrom Arrhenius

Equation:

Rate constant increases when T2>T1 The Rate of Increase is proportional to Ea

Typical standard free energies of activation (15 - 70 kJ M-1) give rise to increases in rate by factors between 1.2 and 2.5 for every 10 K rise in temperature.

Denaturing reactions have standard free energies of activation of about 200 - 300 kJ mole-1 (Q10 in the range 6 - 36) which means that, above a critical temperature, there is a rapid rate of loss of activity

Temperature / °C

Enzyme activity

0 10 20 30 40 50

Q10 Denaturation

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