Rate Constants and Temperature

Question 1

What 2 factors affect the rate constant when increasing the temperature?

Answer 1

1. Increasing the temperature shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy, Ea.
2. As the temperature increases, particles move faster and collide more frequently with the correct orientation.

Question 2

Why is the change in rate when temperature is increased mainly determined by Ea?

Answer 2

Because the increased frequency of collisions is comparatively small compared with the increase in the proportion of molecules that exceed the Ea from the shift in the Boltzmann distribution.

Question 3

What does the rate constant k with temperature graph look like?

Answer 3

An upward curve with an increasing gradient.

Question 4

What is the Arrhenius equation in words?

Answer 4

An exponential relationship between the rate constant k and temperature T.

Question 5

What is the Arrhenius equation?

Answer 5

k = A e-^Ea/RT

k = rate constant.
A = frequency factor/pre-exponential factor.
Ea = activation energy.
R = gas constant (8.314).
T = temperature in kelvin.

Question 6

What does the exponential factor represent?

Answer 6

The proportion of molecules that exceed Ea and that have sufficient energy for a reaction to take place.

Question 7

What is the frequency factors/pre-exponential factor? Increase?

Answer 7

Takes into account the frequency of collisions with the correct orientation. It can increase with the increase of temperature but still remains fairly constant.

Question 8

What is the Arrhenius equation when expressed as a logarithmic relationship?

Answer 8

ln k = - Ea/RT + In A.

Question 9

How does the Arrhenius equation be related graphically (y=mx+c)?

Answer 9

ln k = -Ea/R 1/T + In A

 y =    m    x    x    +     c

Question 10

Overall, what is the Arrhenius graph plot with In k?

Answer 10

• In k is on the y-axis.
• 1/T / K-1 is on the x-axis.
• In A is the intercept c.
• Gradient is - Ea/R

Question 11

What are the steps in using the Arrhenius equation to calculate Ea and A?

Answer 11

1. Calculate the values of ln K and 1/T from the values of T/K and k/s-1:
   - 1/T / k-1 = 1 divided by the temperature in kelvin.
   - In k = ln (k) on your calculator.
2. Plot a graph of In k against 1/T:
   - In k on the y-axis.
   - 1/T / K-1 on the x-axis.
3. Calculate the activation energy from the gradient of the straight line = change in y/change in x multiplied by 8.314.
4. Calculate In A from the intercept:
   - The intercept is where the line meets the y-axis.
   - A = e^intercept number = In A.

Question 12

How do you plot the axis on rate constant and temperature graphs?

Answer 12

• If your In k values are negative, plot 1/T / K-1 on the x-axis at the top.
• If your In k values are positive, plot 1/T / K-1 on the x-axis at the bottom like the usual.