Reaction rates

Question 1

What are rates of reaction?

Answer 1

Rate = change in concentration / time
Usually measured in mol dm^3 s^-1

Question 2

What are orders of reaction?

Answer 2

Rate ∝ [A]^n
How changing the concentration changes the rate of a reaction.

Question 3

What is 0 order?

Answer 3

When the concentration of a reactant has no effect on the rate.

Question 4

What is 1st order?

Answer 4

rate ∝ [A]
If the concentration of the reactant is doubled, the reaction rate increases by a factor of 2.
If the concentration is tripled, the reaction rate increases by a factor of 3.

Question 5

What is 2nd order?

Answer 5

rate ∝ [A]^2
If the concentration of the reactant is doubled, the reaction rate increases by a factor of 2^2 = 4.
If the concentration is tripled, the reaction rate increases by a factor of 3^2 = 9.

Question 6

What is the rate equation?

Answer 6

The mathematical relationship between the concentrations of the reactants and the reaction rate.
rate = k [A]^m [B]^n
k is the rate constant

Question 7

What is the overall order?

Answer 7

The sum of orders with respect to each reactant.

Question 8

How do you determine orders from experimental results?

Answer 8

If continuously monitoring, the concentrations of any reactants not being investigated must be in excess, so they won’t change much during the reaction, and are 0 order.

Question 9

What are the methods for continuous monitoring?

Answer 9

For gases:
Measuring gas collection (gas syringe)
Measuring mass loss (using a balance)
For others:
Colorimetry
Change in pH (if hydrogen or hydroxide ions are produced or used up)

Question 10

How is rate measured with a colorimeter?

Answer 10

The wavelength of the light passing through a coloured solution is controlled using a filter.
The amount of light absorbed is measured.

Question 11

What is the method for using a colorimeter?

Answer 11

Set the colorimeter to measure the wavelength of light interested in measuring.
Calibrate the colorimeter by placing a sample of distilled water in a cuvette and place in the colorimeter and set to 0.
Carry out the reaction and at regular intervals take samples and measure the absorbance.

Question 12

What are the shapes of concentration time graphs?

Answer 12

0 order is a straight line with a negative gradient. The value of the gradient is equal to k.
1st order produces a downward curve with a decreasing gradient over time. The half life is constant.
2nd order is a downward curve, steeper at the start but tails off slower.

Question 13

How is the gradient of a curved graph determined?

Answer 13

Draw a tangent at the point you want to measure the rate - the line should have an equal gap either side of the point to the curve.
Find the gradient of the straight line by doing change in y / change in x

Question 14

What is half life?

Answer 14

The time taken for half of the reactant to be used up.
1st order reactions have a constant half-life.
*Draw it clearly on the graph with double headed arrows.

Question 15

How is the rate constant determined using half life?

Answer 15

k = ln2 / t1/2

Question 16

What are the shapes of rate concentration graphs?

Answer 16

0 order is a horizontal straight line with 0 gradient. The y-intercept gives the rate constant k.
1st order is a straight line graph through the origin. The gradient gives the rate constant k.
2nd order is an upwards curve with increasing gradient.

Question 17

What is the initial rates method?

Answer 17

Initial rate is the rate right at the start of the reaction when time = 0. It can be found by measuring the gradient of a tangent drawn at t=0 on a concentration time graph.

Question 18

What is a clock reaction?

Answer 18

This measures the time from the start of an experiment for a visual change to be observed - colour or precipitate.
Provided there is no significiant change in rate during this time, it is assumed the average rate of reaction is the same as the initial rate.
So initial rate is proportional to 1/t.

Question 19

What are iodine clocks?

Answer 19

Measures the time from the start to the appearance of the iodine colour of brown to appear. Starch is usually added which forms a blue black complex.
Measures the reaction: H2O2(aq) + 2I-(aq) + 2H+(aq) –> I2 (aq) + 2H2O (l)
A graph is plotted with 1/t on y-axis against iodine concentration, or concentration against time.

Question 20

What is the accuracy of clock reactions?

Answer 20

The longer the period of time over which the average rate is measured, the more rate changes over the time period.
So, it is less close to the initial rate and therefore less accurate over longer times.

Question 21

Why are there multi-step reactions?

Answer 21

A reaction only takes place when particles collide.
In reactions with multiple molecules, it is unlikely they would collide simultaneously so are more likely to have several steps.
The reaction mechanism is the series of steps that make up an overall reaction.
If the stoichiometry of the rate equation doesn’t match overall equation, then multi-step.

Question 22

What is the rate determining step?

Answer 22

The slowest step in a multi-step reaction.

Question 23

How are reaction mechanisms predicted?

Answer 23

It uses the rate equation to show which reactants are in the rate determining step.
It also shows the orders of the reactants, and so how many moles are reacting.
Usually, you react the rate equation reactants together to form a final product, and then an intermediate that then reacts in the next step to form another product.

Question 24

What is the effect of temperature on the rate constant?

Answer 24

As temperature increases, the rate increases and the rate constant k also increases.
Increasing the temperaure shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy.
As temperature increases, particles move faster and collide more frequently.

Question 25

What is the Arrhenius equation?

Answer 25

k = A e^ -Ea / RT (written in data sheet)
A is the frequency factor, everthing else the exponential factor - the proportion of molecules that exceed Ea and have sufficient energy for a reaction to take place.
R is the gas constant, T is temperature in kelvin.

Question 26

What is the logarithmic form of the Arrhenius equation?

Answer 26

ln k = - Ea/RT + inA
A plot of lnk against 1/T gives a straight line graph (y=mx +c)
The gradient is -Ea/R (so multiply gradient by R)
The intercept is lnA

Question 27

What are important things to remember for the Arrhenius calculations?

Answer 27

Temperature is in kelvin.
Activation energy needs to be in Jmol^-1.
Units of k are same as A.
To find lnk you need to e the value.