Unit 4 Rates of reaction

Question 1

define rate of reaction?

Answer 1

the change in concentration of a substance in unit time

Its usual unit is mol/dm cubed

Question 2

When graph of concentration against time is drawn, what does the gradient mean?

Answer 2

The rate of the reaction.

Question 3

When does measuring a change in mass work best?

Answer 3

This works if there is a gas produced which is allowed to

escape. Works better with heavy gases such as CO2

Question 4

What does quenching mean?

Answer 4

To stop a reaction. Sometimes it entails cooling it down.

Question 5

What is Colorimetry ?

Answer 5

If one of the reactants or products is coloured
then colorimetry can be used to measure the
change in colour of the reacting mixtures

Question 6

When can Measuring change in electrical conductivity be used to gauge a change in reaction rates?

Answer 6

if there is a change in the number

of ions in the reaction mixture

Question 7

What is used to measure the change in optical activity?

Answer 7

A polarimeter.

Question 8

What is the rate equation?

Answer 8

For the following reaction, mA + nB →
r = k[A]ᴹ[B]ᴺ
r is the rate of reaction.  k is the rate constant.
the [] mean the concentration of A or B.
m and n are reaction orders.

Question 9

What are reaction orders?

Answer 9

usually integers 0,1,2
0 means the reaction is zero order with respect to that reactant
1 means first order
2 means second order

Question 10

What is the total order for a reaction ?

Answer 10

The sum of the reaction orders

Question 11

What does a reaction order of 0 mean?

Answer 11

the concentration of A has no effect on the
rate of reaction
r = k[A] ⁰= k

Question 12

What does a reaction order of 1 mean?

Answer 12

the rate of reaction is directly proportional

to the concentration of A r = k[A]¹

Question 13

What does a reaction order of 2 mean?

Answer 13

the rate of reaction is proportional to the
concentration of A squared
r = k[A]²

Question 14

What is Continuous rate data?

Answer 14

This is data from one experiment where the concentration

of one substance is followed throughout the experiment.

Question 15

When continuously measuring the concentration of a reactant, why must the amounts of reactants not being measured by highly excessive?

Answer 15

This will insure the concentration will remain virtually the same and not affect the rate of the reaction.

Question 16

How is the data from continuous raw data used?

Answer 16

There is a graph drawn of [A] against time. Then the time for the value of [A] to halve is found.

Question 17

How can half lives be used to determine whether the reaction order?

Answer 17

The half-life of a first-order reaction
is independent of the concentration and is constant
If half-lives rapidly increase then the order is 2nd.

Question 18

What are the features of a concentration time graph with a reaction order of 0?

Answer 18

If zero order the rate stays constant as the reactant is used up. This means the concentration has no effect on rate. It will be line of constant negative gradient.

Question 19

How does temperature affect the rate constant?

Will it be affected by concentration?

Answer 19

The value of k refers to a specific temperature and
it increases if we increase temperature.
It won’t be affected by concentration.

Question 20

What are the different units for K?

Answer 20

For a 1st order overall reaction the unit of k is
s⁻¹
For a 2nd order overall reaction the unit of k is
mol⁻¹ dm³s⁻¹
For a 3rd order overall reaction the unit of k is
mold ⁻² dm⁶s⁻¹

Question 21

What is the overall reaction order?

Answer 21

The sum of the reaction orders for the two A and B.

Question 22

r = k [A] [B]2
If the concentration of a is doubled and the rate of b is tripled , what will happen to the rate of the overall reaction?

Answer 22

If the [A] is x2 that rate would x2
If the [B] is x3 that rate would x3²= x9
If both happened then the rate of reaction would be 9 X 2 which is 18 times more.

Question 23

How would the value be K be calculated with the equation r = k [X] [Y]² and if you knew that the initial concentration for x and y was 0.2 and the rate of reaction at this time was 2.4 * 10⁻⁶.

Answer 23

Rearrange the equation to get k = r / [X] [Y]²
put in the values.
2.4 * 10⁻⁶/ 0.2*(0.2²)=3.0 x 10 ⁻⁴mol⁻² dm⁶s⁻¹

Question 24

What affect does increasing the temperature have on the value of k?
How is the equation modelled?

Answer 24

Increasing temperature increases the rate constant k
given by the Arrhenius equation k = Ae⁻ᴱᵃ/ᴿᵀ
where A is a constant R is gas constant and Ea is activation energy.

Question 25

How can the Arrhenius equation be re arranged and used to find an activation energy?

Answer 25

ln k = constant – Ea/(RT)
k is proportional to the rate of reaction so ln k can be
replaced by ln(rate)
From plotting a graph of ln(rate) or ln k against 1/T the
activation energy can be calculated from measuring
the gradient of the line

Question 26

What is a mechanism?

Answer 26

A mechanism is a series of steps through which the reaction
progresses, often forming intermediate compounds. If all the
steps are added together they will add up to the overall
equation for the reaction

Question 27

How will each step in a reaction affect the overall rate of reaction?

Answer 27

Each step can have a different rate of
reaction. The slowest step will control the
overall rate of reaction. The slowest step
is called the rate-determining step.

Question 28

Hoe can the rate determining step be used to determine the order of reactions of substances?

Answer 28

If 0 moles of A is in the rate determining step then the reaction is 0 order with respect to A. If there is 1 mole of B it is first order with respect to B.

Question 29

If the reaction has an equation of A + 2B + C =D + E and the rate determining step is A + B=X + D, what is the rate equation?

Answer 29

r = k [A]¹ [B] ¹[C]⁰

Question 30

If the rate determining step contains a reactant that isn’t part of the reactants of the overall reaction, what do you do?

Answer 30

Include the reactants that form the reactant and they have the same order.

Question 31

What is Sn2 reaction?

Answer 31

A single step reaction. 2 molecules are involved and it happens with primary halogenoalkanes.

Question 32

Why don’t primary halogenoalkanes use the SN1 reaction/

Answer 32

because they would only form an unstable primary

carbocation.

Question 33

Why do tertiary halgonoalkanes use the Sn1 mechanism?

Answer 33

Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is made stabilised by the electron
releasing methyl groups around it. (see alkenes topic for another example of this). Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as in Sn2.

Question 34

What the Sn1 mechanism mean for the rate of reaction?

Answer 34

The first step of the Sn1 reaction will be the slower and the rate determining step.

Question 35

What are the different ways to measure the rate of reaction?

Answer 35

Volume of gas given off, loss of mass, colour change(with a colorimeter), electrical conductivity or pH if the no. of ions change.

Question 36

What is the concentration time graph for a 1st order reaction?

Answer 36

A sloped line with a constant half life.

Question 37

What is the concentration time graph for a 2nd order reaction?

Answer 37

A slopped line with a half life that increases.

Question 38

What are the rate concentration graphs for 0, 1st and 2nd order graphs?

Answer 38

A line of gradient 0. a line taking the form y=+x and a line taking the form y=x²

Question 39

how do you find the half life of a 0 order reaction with K?

Answer 39

If it was the decomposition of x to y it would be the concentration of x/2k

Question 40

how do you find the half life of a 1st order reaction with K?

Answer 40

0.69 / k

Question 41

how do you find the half life of a 2nd order reaction with K?

Answer 41

1/ k[x]

Question 42

What information about the rate determining step can be determined by the rate equation?

Answer 42

If a reactant is in the rate equation, it will appear in the rate determing step, the same goes for catalysts.

Question 43

What do the parts of the Arrhenius equation mean?

Answer 43

k=rate constant
Ea=activation engery
T=temperture (k)
R=gas constant (8.31)
A= another constant.

Question 44

what is the Arrhenius equation in log form?

Answer 44

Lnk=Ln A -(Ea/RT)

Question 45

What is a arrhenius plot?

Answer 45

ln k by 1/T. The gradient is -(Ea/R)

Question 46

What is a catalyst?

Answer 46

they increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy. They are chemically unchanged by the end of the reaction.

Question 47

Give an example of a homogeneous catalyst?

Answer 47

Enzymes in the body. Everything is aqueous in that instance.

Question 48

How can a heterogeneous catalyst get poisoned?

Answer 48

A substance can cling to the catalyst more strongley than the reactant does, preventing the catalyst getting involved in the reaction.

Question 49

How would a graph show that a reaction with respect to a product is first?

Answer 49

A straight line going through the origin.

Directly proportional.

Question 50

How does OH- react with a bromoalkane?

Answer 50

For tertiary the bromine leaves and the OH- joins

For primary it happens in one step.