Chapter 18

Question 1

What are the units for rate of reaction?

Answer 1

mol dm-3 s-1

Question 2

How can you measure rate of reaction?

Answer 2

Measuring the decrease in the concentration of a reactant over time
or Measuring the increase in the concentration of a product over time

Question 3

How is rate of reaction written?

Answer 3

rate =k [A] [B]

Question 4

What does the order of a reactant show?

Answer 4

shows how the concentration of a chemical, typically a reactant, affects the rate of reaction

Question 5

What does it mean when the order is 0?

Answer 5

Changing the concentration of the chemical has no effect on the rate of the reaction

Therefore, it is not included in the rate equation

Question 6

What does it mean when the order is 1?

Answer 6

The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction

The chemical is included in the rate equation

Question 7

What does it mean when the order is 2?

Answer 7

The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four

The chemical is included in the rate equation (appearing as a squared term)

Question 8

What is the overall order?

Answer 8

the sum of the powers of the reactants in a rate equation

Question 9

How do you calculate orders from data?

Answer 9

Identify two experiments where the concentration of the reactant you are looking at first changes, but the concentrations of all other reactants remain constant

Repeat this for all of the reactants, one at a time, until you have determined the order with respect to all reactants

Question 10

How can the rate constant (K) be calculated?

Answer 10

using the initial rates and the rate equation - You rearrange the rate equation so rate over the initial rates = k

Question 11

How does a 0 order Concentration time graph look?

Answer 11

Straight diagonal line from top left to bottom right.

Question 12

What does the gradient mean in a 0 order Concentration-Time graph?

Answer 12

The gradient = rate of reaction

since its 0 order, the reactant has no effect on the rate of the reaction, so the rate = the rate constant (K)

Question 13

How does a 1 order Concentration time graph look?

Answer 13

The graph is a curve going downwards and eventually plateaus.

Question 14

How does a 2 order Concentration time graph look?

Answer 14

The graph is a steeper curve going downwards

because the concentration of the reactant decreases more steeply with time.

Question 15

How can the order of reaction be deduced from a reactions half life?
(explain how 0,1 and 2 would look)

Answer 15

For a zero-order reaction the successive half-lives decrease with time (less time for concentration of reactant to halve)

The half-life of a first-order reaction remains constant throughout the reaction (same time for concentration to halve)

For a second-order reaction, the half-life increases with time (takes more time for concentration to halve)

Question 16

How does a 0 order rate-concentration graph look?

Answer 16

The rate doesn’t depend on the concentration of the reactant- The rate of the reaction, therefore, remains constant throughout the reaction.

The graph is a horizontal line

rate = K

Question 17

How does a 1 order rate-concentration graph look? How do you determine the rate of reaction?

Answer 17

Rate of the reaction increases as the concentration of the reactant increases.

Straight diagonal line.

rate = k[A]

gradient = rate of reaction

Question 18

How does a 2 order rate-concentration graph look?

Answer 18

The rate is directly proportional to the square of concentration of a reactant.

Curved line going up.

rate = k[A]2

Question 19

What does initial rate determine?

Answer 19

The rate at the very start of a reaction when t=0

Question 20

How do you find initial rate?

Answer 20

Drawing a concentration-time graph
Adding a tangent at t = 0
Calculating the gradient of the tangent

Question 21

What are clock reactions used for? How do they work?

Answer 21

Measuring the initial rate of reaction using a single measurement.

The time taken, t, for a specific visual change in the reaction to occur is measured

These changes could be a colour change or formation of a precipitate

Question 22

What is the assumption that a clock reaction depends on?

Answer 22

There is no significant change in the rate of reaction between the start of the reaction and the time when the measurement is taken.

Question 23

What is initial rate proportional to?

Answer 23

1/t

Question 24

How do you calculate rate constant from the half life?

Answer 24

k = ln2 / t1/2

rate constant = ln(2) over t half life.

Question 25

How does the iodine clock reaction work?

Answer 25

In each experiment, the solution is colourless at the start and the time t
is measured for the blue-black colour of the starch-iodine to appear. (starch is added to make the blue/black iodine is normally orange-brown)

The initial rate is proportional to 1/t
A graph of 1/t against concentration is then plotted.

Separate experiments are carried out using different concentrations of one of
the reactants and all other concentrations are kept constant. The colour change
is delayed by including a small amount of another chemical which actually removes iodine as it forms.
As soon as this chemical is all used up, the blue-black colour appears.

Question 26

What is the accuracy of a clock reaction dependant on?

Answer 26

The actual time measured, the longer it takes the less accurate the average rate of change will be. The rate will have changed over that period (so it changes less over a shorter period.)

Question 27

How does continuous monitoring work? (to investigate reaction rates)

Answer 27

collecting experimental data throughout the course of a reaction to plot a concentration-time graph .

Measuring the volume / amount of gas evolved over time

Measuring the mass of reactants lost over time

Question 28

How is colorimetry used to investigate reaction rates?

Answer 28

It measures colour absorbance which is proportional to the concentration of the coloured species.

Firstly you need to measure the absorbance of a set of standard solutions of iodine and obtain a calibration curve.

By calibrating the colorimeter with solutions of known concentration we can use the readings from this device to measure the concentration at a particular time. (draw tangents and work out the rate of reaction).

Question 29

What is the rate determining step?

Answer 29

The slowest step of a reaction.

(A chemical reaction can only go as fast as the slowest part of the reaction)

Question 30

Do the reactants of the slow or fast step appear in the rate equation?

Answer 30

If a reactant appears in the rate-determining step (slow step), then the concentration of that reactant will also appear in the rate equation

Question 31

What happens to the rate of reaction if temperature increases?

Answer 31

Increasing the temperature of a reaction increases the rate of a chemical reaction.

Question 31

What happens to the rate of reaction if temperature increases?

Answer 31

Increasing the temperature of a reaction increases the rate of a chemical reaction.

Question 32

What happens to the rate constant as the rate of reaction increases?

Answer 32

As the rate of reaction increases the rate constant will increase.

Question 33

What happens to K as temperature increases?

Answer 33

K increases as long as the concentration of the reactants remains unchanged.

Question 34

How many degrees increases does it take for rate of reaction to double?

Answer 34

approx 10 degrees. (The number of degrees needed to double the rate also changes gradually as temperature increases)

Question 35

What causes the increase of temperature to result in a higher rate/ rate constant.

Answer 35

Increasing temperature causes the particles to move around faster resulting in more frequent collisions

Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction.

Question 36

Does k change when the concentration of the reactants is changed?

Answer 36

The rate constant, k only remains constant if the concentration of the reactants is the only factor which is changed

Question 37

What equation shows the relationship between k, temperature and activation energy?

Answer 37

the Arrhenius equation

Question 38

What is the Arrhenius equation?

Answer 38

K= Ae ^ -Ea/RT

K= rate constant. A= Arrhenius Constant
R = Gas constant (8.31 JK-1mol-1)
T= Temperature (K)
e= mathematical constant

Question 39

What’s the easier way to use the Arrhenius equation?

Answer 39

Ln k = ln A - Ea/RT

Question 40

How can the Arrhenius equation be used in a graph?

Answer 40

Plot ln K against 1/t. You can use this to calculate Ea.

The gradient = -Ea/R (so if you times it by 8.31 you will get the activation energy)

The y intercept = ln A

if you write it in the form of y = mx + c
ln K = -Ea/R 1/t + ln A