3.5 - Chemical Kinetics

Question 1

What factors effect the rate of a chemical process?

Answer 1

• temperature
• concentration
• surface area of solid
• presence of a catalyst

Question 2

What is the reaction rate?

Answer 2

Change in concentration of a reactant or a product per unit time
- how quickly reactants are used up
- how quickly products are formed

Question 3

What is the average rate?

Answer 3

Total change in the concentration of reactants/products divided by the time taken for the reaction to take place

Question 4

What is the instantaneous rate?

Answer 4

Rate of change occuring at a specific time
(find the gradient of a tangent y/x)

Question 5

What is the rate equation?

Answer 5

Mathmatical expression in which the rate is given in terms of concentrations of the reactants and the rate constant, k
- each term is raised to a power which relates to the effect the reactant has on the rate of reaction
rate = k(A)^x(B)^y
The sum of the powers gives overall order, can be anything but usually 0, 1 or 2

Question 6

What is zero order?

Answer 6

The concentration of the reactant does not affect the rate of the reaction

Question 7

What is first order?

Answer 7

If the concentration doubles, the rate will double

Question 8

What is second order?

Answer 8

If the concentration of the reactants doubles, the rate of the reaction will be 4x faster

Question 9

What are rate graphs?

Answer 9

• rate vs concentration/ concentration vs time
• the shape of these graphs can tell us the order of the reaction with respect to the reactant
• the gradient of these graphs tells us the value of the rate constant or the rate respectively

Question 10

What does 1st order graph look like for RVC

Answer 10

Straight diagonal line
The rate is proportional to the conc of A
If the concentration of A doubles, so does the rate

Question 11

What does zero order graph look like for RVC?

Answer 11

Straight line horizontal
The rate is independent of the concentration of A in a reaction which is zero order in respect to A
If A changes, the rate stays the same

Question 12

What does 2nd order graph look like for RVC?

Answer 12

Curved upwards
The rate is proportional to the second power of A
If conc of A triples, the rate will be 9x faster
If plotted (A)2, line will look like 1st order

Question 13

What does zero order graph look like for CVT?

Answer 13

As the reaction proceeds A is used up and the concentration drops
A does not affect the rate
Diagonal line downwards

Question 14

What does a 1st order graph look like for CVT?

Answer 14

A is used up and the concentration drops
A does not effect the rate and so its dropping concentration causes rate to decrease and hence the speed in which its being consumed reduces
Curved line downwards

Question 15

What does a 2nd order graph look like for CVT?

Answer 15

A is used up and conc drops
A affects rate more significantly and causes rate to decrease sharply, steeper than first order
Steep curved line downwards

Question 16

What is half life and what is the equation for it?

Answer 16

The half life of a reaction, t1/2, is the time taken for the concentration of the reactant to reach half its initial value
(A)t1/2 = 1/2(A)t0

Question 17

How can we tell apart 1st and 2nd order for CVT graphs?

Answer 17

The half life of a first order reaction is independant of concentration, so the half life stays the same.

Question 18

What is the relationship/equation for the rate constant (k) and the half life of a reaction?

Answer 18

t1/2 = 0.693/k

Question 19

What is a rate determining step?

Answer 19

The slowest elementary step and as such limits the overall rate of a multistep mechanism
As a general rule, if a reactant does not appear in the rate equation, it must come after the rate determining step

Question 20

What is the link between rate and temperature?

Answer 20

As you increase the temperature, a larger fraction of particles have sufficient energy to react
They react to form intermediates of the products having overcome the activation energy and collide in the correct orientation

Question 21

What is the Arrhenius equation?

Answer 21

k = Ae ^ -Ea/RT
k = rate constant
T = temperature (in kelvin)
A = pre-exponential factor, a constant for each chemical reaction
Ea = activation energy
R = gas constant

Question 22

What does the Arrhenius equation tell us about the factors affecting rate of reaction?

Answer 22

If we raise the value of T, the power to which Ae is raised would get smaller
As the power is negative, the smaller the power the larger Ae becomes (and thus k larger too)
Decreasing the activation energy also has the same effect and increases k (seen in adding a catalyst)
Raising the temperature increases the value of the rate constant and hence the rate