Rate Equations

Question 1

How can rate be found from a graph?

Answer 1

Gradient of a concentration time graph

If is curved then a tangent can be used to find the rate.

Question 2

Define rate of reaction

Answer 2

Change in concentration of a substance in unit time

Question 3

Rate =

Answer 3

k [A]^a[B]^b

k = rate constant
[x] = concnetration of a substance
^x are the orders of reaction

Question 3

What are orders of reaction?

Answer 3

They say the effect the concentration of substances have on the rate of a reaction.

Question 4

What are the different orders of reaction?

Answer 4

Zero order - change in concentration has no effect on rate

First order - Changes in concentration are proportional to change in rate

Second order - Changes in concentration has a squared proportional change on rate. eg. if [A] doubles then the rate quadruples.

Question 5

How can orders be worked out?

Answer 5

Only experimentally not by looking at equations

Question 6

How does the rate constant link to rate of reaction?

Answer 6

K is dependent on particular temperatures.

The larger the value of K the faster the rate of reaction.

Question 7

How do you calculate the initial rate from a graph?

Answer 7

Tangent at 0 minutes

Question 8

How do you calculate the rate equation use the initial rates?

Answer 8

Repeat the experiment several times but each time alter the concentration of one of the reactants.

Calculate the initial rate of reaction and see how it changes with each concentration change. Eg. does it double (1st order)

Allows you to figure out the orders of reaction

Question 9

How can rate concentration graphs help us identify order?

Answer 9

The shape:

Horizontal = 0 order
Changing concentration doesn’t affect rate

Straight diagonal line = 1st order
Changing concentration changes the rate equally

Steep curve = 2nd order
Changing concentration changes the rate squared

Question 10

What is the rate determining step?

Answer 10

Slowest step in a multi step reaction.

Question 11

If a reactant appears in the rate equation it must …

Answer 11

Appear in the rate determining step

Question 12

How to calculate the rate equation from a multi step reaction?

Answer 12

Identify the rate determining step

These reactants from the must appear in the rate equation.
The number of moles of these reactants represents the order of reaction

If a reactant in the rate determining step is formed in a prior reaction than those reactants must also feature in the rate equation

Question 13

Arrhenius equation

Relationship between activation energy and rate constant?

Answer 13

As activation energy gets smaller the rate constant gets bigger.

As when activation energy decreases rate of reaction increases as more particles have sufficient energy to collide and react.

Question 14

Arrhenius equation

Relationship between temperature and rate constant?

Answer 14

As temperature increases so does the rate constant.

Question 15

How do you calculate the activation energy from an arrhenius plot?

Answer 15

plot graph of lnk against 1/T and then draw a line of best fit.

The gradient represents -Ea/R

Question 16

How do you calculate the arrhenius constant from an arrhenius plot?

Answer 16

Plot graph of lnk against 1/T

Substitute in formula into y=mx+C

m = (-Ea/R)
c = (lnA)