Kinetics 2

Question 1

What are the two different ways in which kinetics and thermodynamics are related?

Answer 1

Kinetics= RATE 
Thermodynamics= ENERGETICS 

1) Equilibrium constants can be calculated from free energy (thermodynamics) which can then be expressed in terms of rate constants (kinetics)
2) Activation energy: Rates of reaction and rate constants can be expressed in terms of a free energy change taking place during the reaction

Question 2

What happens to the rate when the system is at equilibrium?

What is the equation for Keq?

Answer 2

Forward rate= Reverse rate
v = k1 [A] – k–1 [B]

Keq = [B] / [A]
Therefore: Keq = k1 / k–1

Question 3

What is the equation for Kd at equilibrium for a dissociation of a receptor-ligand complex RL?

Answer 3

RL R + L

Rate of forward reaction: vf = k1 [RL]
Rate of backward reaction: vr = k–1 [R] [L]

At equilibrium vf = vr
k1 [RL] = k–1 [R] [L] 
Keq  = [R] [L] / [RL] =  k1 / k–1
In this case, Keq = Kd, the dissociation constant
Kd has units: M

Question 4

What are the different stages of a chemical reaction?

What is activation energy? What is the sign for activation energy?

Answer 4

Reactants —> Transition State —-> Products

Reactants have more energy than products but transition state has the most energy (energy maximum)= is a curve

Activation energy: This is the minimum energy needed for the reaction to occur- the free energy needed to reach the transition state and it depends on the route of reaction
ΔG°‡

Question 5

Where doe the energy for the molecules to reach the transition state come from?

Answer 5

For a first order process: Internal molecular motion

For a second order process: Comes from collision energy when molecules collide= External molecular motion

Activation energy: Supplied by internal and external kinetic energy of molecules

Question 6

How does increasing temperature increase the rate of reaction?

Answer 6

Energy in movement of molecules is predicted by Boltzmann distribution= At higher temperatures, the faster the molecules= Energy is used for transition state

At higher temperatures, there are more molecules which have energy > Ea ( ΔG°‡ )= Proportion of molecules able to reach the transition state increases= Rate of reaction increases

At higher temperatures, the curve shifts to the right

Question 7

What is the equation which relates the rate constant for the reaction (k) to a free energy change (–ΔG°‡)?

Answer 7

Arrhenius equation:
k = A e^(–ΔG°‡ /RT)
OR taking natural logs:

ln (k) = ln (A) – ΔG°‡ /RT

Question 8

What do you use the Arrhenius equation for?

Answer 8

To show the effect of a change of temperature on the rate constant - and therefore on the rate of the reaction.

Predicts an exponential relationship between the rate constant (k) and the temperature (T)

Activation energy depends on temperature

Question 9

How do you plot a graph to estimate the activation energy?

Answer 9

1) Measure the rate of reaction at different temperatures
2) Work out the reaction constant, k
3) Plot ln (k) against 1/T
Gives a straight line, and the slope –ΔG°‡/R from which ΔG°‡ can be deduced (not entirely sure how…)

Question 10

How can reactions be made to go faster?

Answer 10

1) Increase temperature

2) Decrease the energy of activation= Catalyst= Lower “hump” on the reaction coordinate diagram

Question 11

Does a catalyst affect Keq?

Answer 11

NO!!!

Catalyst does NOT change the overall ΔG or alter equilibrium constants or equilibria