Chapter 18 - Rates of reaction

Question 1

What is the ‘rate equation’?

Answer 1

Rate = Quantity reacted or produced / time
or
Rate = change in concentration / time

Question 2

What are the units for rate?

Answer 2

mol dm^-3 s^-1

Question 3

What are the 3 ‘orders’ of a reaction?

Answer 3

Zero order, first order and second order

Question 4

What is ‘zero order’?

Answer 4

When [A] is risen to the power of 0 and the concentration doesn’t effect the rate

Question 5

What is ‘first order’?

Answer 5

When [A] is risen to the power of 1 and any change to the concentration gives the same factor effect to the rate

Question 6

What is ‘second order’?

Answer 6

When [A] is risen to the power of 2 and any change to the concentration gives a squared factor of the value on the effect on the rate

Question 7

How do you work out the overall order?

Answer 7

Add all the order values together
(eg. rate=k[A]+[B]^2 , the over rate order = 2+1 = 3)

Question 8

How do you work out the units for the rate constant k?

Answer 8

Make k the subject of the rate equation then divide the rate (mol dm^-3 s^-1) by how the orders amount of concentration (eg second order = mol dm^-3 mol dm^-3)

Question 9

What are the rate constant k units for a first order equation?

Answer 9

s^-1

Question 10

What are the rate constant k units for a second order equation?

Answer 10

mol^-1 dm^3 s^-1

Question 11

What are the rate constant k units for a zero order equation?

Answer 11

mol dm^-3 s^-1

Question 12

What is ‘continuous monitoring’?

Answer 12

When you monitor the measurements throughout a reaction (usually to create a concentration-time graph)
This can be done by monitoring the mass lost, gas collection or by colour change

Question 13

What apparatus is used to measure colour change?

Answer 13

Colorimeter (measures the amount of light absorbed by a solution)

Question 14

What would the orders look like on a concentration-time graph?

Answer 14

Zero order - straight line with a negative gradient
First order - downward curved line with a decreasing gradient (have a constant concentration half life)
Second order - similar to first order except the line starts off steeper and levels off faster

Question 15

What is a ‘half-life’?

Answer 15

The time taken for half of the reactants to be used up (pattern is called exponential decay)

Question 16

How can you work out the rate from a first order graph?

Answer 16

By working out the gradient of a tangent at a given point (gradient = change in y/ change in x)
or by working out k from the natural log2 (ln2)/ half-life value

Question 17

What would the orders look like on a rate-concentration graph?

Answer 17

Zero order - horizontal line (concentration of zero order have no effect on the rate, rate = k)
First order - straight line graph (y=x, as rate is directly proportional to the concentration)
Second order - upwards curve with an increasing gradient

Question 18

How do you work out the rate constant k of a second order rate-concentration graph?

Answer 18

Create another graph of the values of rate against the concentration squared, then the gradient of the new line will be the rate constant k

Question 19

What is the ‘initial rate of reaction’?

Answer 19

The instantaneous rate of reaction at the start of a reaction (when t=0)

Question 20

How can you find the initial rate?

Answer 20

By finding the gradient of a tangent at t=0 on a concentration-time graph

Question 21

What is another way of finding the initial rate of reaction with one measurement?

Answer 21

A clock reaction (recording the time it takes for a colour change or precipitate to be formed in a reaction), therefore initial rate is 1/t

Question 22

What is the ‘reaction mechanism’?

Answer 22

The series of steps that make up an overall reaction.

Question 23

What is the ‘rate determining step’?

Answer 23

The slowest step in a multi-step reaction mechanism

Question 24

How can you predict reaction mechanisms?

Answer 24

• Rate equations only include reacting species involved in the rate determining step
• The orders in the rate equation match the number of species involved in the rate-determining step

Question 25

How can you determine the overall reaction from the multi-step mechanism?

Answer 25

Cross cancel any compounds that appear on the left and right hand side of the reaction until you are left with your overall reactants and products

Question 26

Describe what effect temperature has on the rate constant, making references to the Boltzmann distribution curve

Answer 26

Increasing the temperature causes the curve to shift to the right, increasing the proportional number of particles that are able to overcome the Ea (activation energy) as particles move faster under higher temperatures which leads to a higher frequency of collisions (at the right energy and orientation)

Question 27

What does the Arrhenius equation suggest?

Answer 27

The exponential relationship between the rate constant k and temperature T

Question 28

What is the Arrhenius equation and how can it be written as a logarithmic equation?

Answer 28

k = Ae ^(Ea/RT)
or ln k = ln A - (Ea/RT)

Question 29

What value does R, T, A and Ea represent?

Answer 29

R - 8.314 J mol-1 K-1
T - temperature (K)
A - frequency factor / pre-exponential factor
Ea - activation energy (exponential-factor)

Question 30

What is y-intercept in the logarithmic equation for Arrhenius?

Answer 30

ln A

Question 31

What is the gradient value in the logarithmic equation for Arrhenius?

Answer 31

-Ea/RT

Question 32

What is the x value in the logarithmic equation for Arrhenius?

Answer 32

1/T

Question 33

What is the label that goes on the y-axis of a graph with the logarithmic Arrhenius equation?

Answer 33

ln k