(2) Rate Equations

Question 1

Define rate of reaction.

Answer 1

the change in concentration of a reactant or product per unit time

Question 2

Define Activation energy.

Answer 2

the minimum amount of energy needed to start a chemical reaction.

Question 3

Why does high temperature increase rate of reaction?

Answer 3

• More particles have energy greater of equal than activation energy
• There is higher frequency of successful collisions

Question 4

Why does a higher conc/pressure increase the rate of reaction?

Answer 4

• There are more particles within the same amount of space

- so more frequent and successful collisions

Question 5

Why does breaking a solid into smaller particles increase rate?

Answer 5

Increased surface area so more frequent, successful collisions

• more surface exposed for collisions

Question 6

What is a catalyst?

Answer 6

• A substance which increases the rate of reaction without being used up

Question 7

How does a catalyst work?

Answer 7

• Provides an alternate reaction route with a lower activation energy

Question 8

Maxwell-Boltzmann distribution key points?

Answer 8

• Peak is the most probable energy
• total amount of particles = area under graph
• area to the right of Ea represents the particles which have enough energy to react

Question 9

Majority of reactions start off with a fast rate of reaction and then slow down before the reaction stops. Why is this?

Answer 9

• Initially there’s a high concentration of reactants so large frequency of successful collisions
• as reactants start to get used up, successful collisions become less frequent
• once one or more reactants are used the can be zero successful collisions

Question 10

Define order.

Answer 10

sum of the order with respect to each reactant in the rate equation

(how changing the concentration affects the rate).

Question 11

Define 0 order.

Answer 11

rate is independent of concentration.

Question 12

Define 1st order.

Answer 12

rate is directly proportional to concentration.

Question 13

Define 2nd order.

Answer 13

rate is proportional to the square of the concentration.

Question 14

How do you show the three orders in an equation? Use [X] for reference.

Answer 14

Zero order = Not written in equation

First order= [X]

Second order = [X]2

Question 15

What’s the overall order of:

R=k[X][Y]2

Answer 15

3rd

Question 16

What’s the overall order of:

R=k[X][Y]2

Answer 16

3rd order

Question 17

What is the rate constant?

Answer 17

‘k’

• Specific for different reactions
• Only affected by temperature
• k=rate / reactants

Question 18

How do you determine order of reactions?

Answer 18

• Experimentally

Question 19

How can you determine the units for k?

Answer 19

Powers of rate minus Powers of reactants in rate equation

so for : Rate= k[X]2[Y]

1-3 = -2

mol dm-3 x -2 = mol-2 dm6 s-1

Question 20

What are the 2 types of experiment which can be done to determine the order with respect to different reactants?

Answer 20

• Continuous monitoring

- comparing initial rates

Question 21

How does the continuous monitoring method work?

Answer 21

There are two ways to measure the progress of a reaction:

1. Take samples at regular intervals
2. Using a visible indicator

Question 22

How would you solve the problem of the reaction mixture still reacting?

Answer 22

• use COLD DISTILLED water and COOL and DILUTE the reaction to stop it reacting further.

(QUENCHING the reaction)

Question 23

What is a problem when finding order by monitoring a physical property, such as volume of gas produced at regular intervals?

Answer 23

• Orders of reaction are for reactants
• We measure production of a product
• Volume would need to be converted to moles to calculate rate

Question 24

Where is concentration and time plotted on a CTT graph?

Answer 24

Concentration always on Y axis

Time on X axis

Question 25

Where do you draw tangent when asked to measure the initial rate and rate at a specific time?

Answer 25

Initial rate = point where the time is at 0 but and conc is at highest

Specific time= tangent just touches the specific time

Rate = gradient of tangent

Question 26

What is the order when conc change from 0.1 to 0.03 and rate changed from 2x10^-5 to 6x10^-4?

Answer 26

0.1 –> 0.03 was x3.3 (0.1/0.03)

2x10^-5 –> 6x10^-4 was x3.3 (2x10^-5/ 6x10^-4)

change in rate is directly proportional to change in conc so its FIRST ORDER

Question 27

What is the order when conc change from 0.1 to 0.03 and rate changed from 2x10^-5 to 2x10^-5?

Answer 27

0.1 –> 0.03 was x3.3 (0.1/0.03)

2x10^-5 –> 2x10^-5 had no change

• change in conc has no change in rate so its ZERO ORDER

Question 28

What is the order when conc change from 0.1 to 0.2 and rate changed from 2x10^-5 to 8x10^-5?

Answer 28

0.1 –> 0.2 was x2

2x10^-5 –> 8x10^-5 was x4

(4 is 2^2)

rate is proportional to the square of the concentration so its SECOND ORDER

Question 29

What do you have to make sure of when working out order for a specific reactant on a table with multiple reactants?

Answer 29

• Working out order from experiments where other reactants have CONSTANT CONCENTRATION

Question 30

What’s the scale factor when concentration of a reactant is halved?

Answer 30

x0.5

Question 31

Outline the iodine clock reaction.

Answer 31

• Measure known volumes of reactants. Keep them separate so the reaction doesn’t start.
• Measure a known volume of sodium thiosulphate.
• Start the stop clock when the last of the reactants is added to a conical flask.
• Do not add sodium thiosulphate last.
• Stop the timer when the mixture turns blue/black and record.
• Repeat the experiment using the same total volume altering the volume of KI

Rate = 1/time

Plot a graph of 1/t against vol of KI

Question 32

What is the equation for Arrhenius and what does each symbol stand for?

Answer 32

K= Ae ^-Ea/RT

K= rate constant

A= Arrhenius constant

e= exponential

Ea= activation energy

R= gas constant

T= Temp (K)

Question 33

What is the opposite of exponential?

Answer 33

natural log

Question 34

What is the simplified equation of Arrhenius?

Answer 34

lnK = lnA - Ea/RT

Question 35

If Ea increases what happens to K?

Answer 35

K will decrease

• less particles with sufficient energy for a successful collision to occur

Question 36

What happens to K when Temperature increases?

Answer 36

K will increase

• particles have more KE so higher chance of successful collisions

Question 37

What is the rearranged formula to find Ea from Arrhenius?

Answer 37

Ea= (lnA - lnK) x RT

always work them out separately then multiply for more accurate answer

Question 38

Compare Arrhenius equation to the straight line equation..

Answer 38

y= mx + c AND lnk= lnA- Ea/RT

y= lnK

m= -Ea/R

X= 1/T

c= LnA

Question 39

What is the equation to calculate Ea using gradient?

Answer 39

> work out gradient using diif y/diff x

> gradient = -Ea/R

> Ea = -(gradient x R)

Question 40

What is the equation to calculate A (from lnA) on a graph?

Answer 40

Pick two point from graph eg (-1.1, 3.2x10^-2)

lnk= - (Gradient x X axis value) + lnA

> add gradient on lnk to get lnA

> use e button and input lnA to get A on its own

Question 41

Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction at other times during the experiments.

Answer 41

The concentrations are known at the start.

Question 42

Key practical notes?

Answer 42

1) The concentration does not itself need to be known; It is assumed that the volume is proportional to the concentration
2) In initial rates experiments the rate of reaction is proportional to 1/time

Rate = 1/t

Question 43

Define ‘Rate determining step’.

Answer 43

THE SPECIES IN THE RATE DETERMINING STEP MUST MATCH THE SPECIES IN THE RATE EQUATION.