Kinetics

Question 1

Define thermodynamics

Answer 1

it tells us whether a reaction can proceed spontaneously but doesn’t inform us about the rate at which the reaction proceeds

Question 2

Whats kinetics?

Answer 2

it looks at how the enrgy is distributed as the reaction proceeds

Question 3

Information for kinetics can’t be gained theretically, why is this the case? how is information gained instead?

Answer 3

It has to be gained experimentally, as plugging information into equations doesn’t tell you anything about the kinetics of something

Question 4

whats speed/ velocity?

Answer 4

the distance something has travelled in a given time

Question 5

Whats the rate of reaction?

Answer 5

the amount of material produced or consumed in a given time

Question 6

Label the key aspects on this energy profile

Answer 6

Question 7

Whats the difference between the reactants energy and products energy?

Answer 7

∆G

Question 8

Why are transition states so unstable?

Answer 8

due to their high energies

Question 9

Whats one of the main problems that arises when studying reaction rates?

Answer 9

the transition states and intermediates cannot usually be probed directly as they are very short lived

Question 10

Whats the thing we can only measure to calculate reaction rates?

Answer 10

we can only measure the concentration of the reactants and/or the products with time, and use this information to calculate the reaction rate

Question 11

What are the 2 key enzymes reaquired in this reaction?

What cofactor is useful for this reaction to occur also?

Answer 11

the cofactor, zinc, is needed for the enzyme to work as it helps to hold the enzyme, alcohol and NAD+ together in the correct position

Question 12

why must acetaldehye be degraded quickly?

Answer 12

its toxic

Question 13

whats is rate defined as?

Answer 13

the change of the concentration of a species as a function of time

Question 14

On a concentration-time graph, what does the gradient tell you?

d[reactant/product]/ dt

Answer 14

the rate at the particular point during the reaction

Question 15

Many concentration-time relationships are non-linear what does this mean for the rate?

Answer 15

the rate will change as a function of time (the rate is not the same throughout the reaction)

Question 16

What are the two generalisations of the rate constant?

Write any equations that support these generalisations

Answer 16

1. The rate usually decreases if the concentration of the reactants are decreased

d[products] / dt = K[A]^X[B]^Y

^{K= the rate constant}

^{X and Y= the orders of the reaction}

2. The rate always increases with increasing temperature

K= A exp (-E_a / RT)

_{A and Ea can be treated as constants over small temperature ranges}

Question 17

What are the 2 main reaction rate theories?

Answer 17

1. collision theory
2. transition state theory

Question 18

Whats meant by a simple reaction?

Answer 18

A reaction that occurs in a single step with no intermediates

Question 19

Whats the collision theory?

What it focusses on/ assumptions

Answer 19

• this theory focusses on the molecular nature of the reaction and provides more insight into the behaviour of individual molecules
• assumption of theory; the reactants and products must collide before the reaction occurs
• they must collide with enought energy to overcome the activation energy barrier for the reaction

Question 20

do spontaneous reactions have activation energy?

Answer 20

yes

Question 21

What is the rate of collision detemined by and how does this effect the rate of reaction?

Answer 21

1. the concentration of products and reactants (more molecules increase the chance of collision)
2. molecule size (bigger molecules have more chance of collision)
3. speed (molecules moving faster are more likely to collide )

Question 22

Whats the equation for rate of collision?

Answer 22

rate of collision= πr²V_R[A][B]

Question 23

Whats the formula for rate and for K in the Collision theory law ?

Answer 23

Rate= k [A][B]

K= πr²V_R exp (-E_a /RT)

Question 24

What is rate related to?

Answer 24

The reactants

Question 25

What are the units of rate?

Answer 25

moldm^-3s^-1

Question 26

Is the reaction rate constant K, a constant value? if not then what makes the value change?

Answer 26

It' s not constant, it can vary depending on temperature, adding catalyst or changing the catalyst for example

Question 27

What does the order of a reaction tell us?

Answer 27

What happens in the rate determining step of a particular mechainsm

Question 28

Tell me the rate equation and units of K for the following types of reactions... 1. **A --\> B order :0** 2. **A --\> B order: 1** 3. **A --\> B order: 2** 4. **A + B --\> C order: 2**

Answer 28

**A --\> B order: 0** rate equation: K units for K: mol dm^-3 S^-1 **A --\> B order: 1** rate equation: K[A] units for K: S^-1 **A --\> B order: 2** rate equation: K[A]² units for K: mol^-1 dm³ S^-1 **A + B --\> C order: 2** rate equation: K [A][B] units for K: mol^-1 dm³ S^-1

Question 29

what two methods can be used to determine the reaction order and rate constant? How do you know/ decide what method to use?

Answer 29

1. differential or direct method 2. integration method To decide what method to use, look at the system with several measurements and look at the concentration as a function of time

Question 30

What occurs in the direct method?

Answer 30

1. A rate equation is produced for the reaction **A --\> Products** 2. logs are taken from each side of the reaction **Log (rate) = log (k [A]^a)** **Log (rate)= alog[A] + log (k) (this is in form y= mx +c)** 3. this then puts the rate equation in the form y= mx + C **Y= log(rate)** **M= a** **X= log[A]** **C= log(k)** 4. this can then be plotted on a graph and then extrapolated

Question 31

Draw rate-concentration graphs for zero, first and second order reactions

Answer 31

Question 32

Draw the conc-time graphs for zero, first and second order reactions

Answer 32

Question 33

Draw the rate of reaction- concentration graph for zero, first and second order reactions

Answer 33

Question 34

What is a first order reaction?

Answer 34

if doubling the concentration of a reactant has a double effect on the rate

Question 35

What is a second order reaction?

Answer 35

If doubling the concentration has a quadrupling effect on the rate

Question 36

Whats a third order reaction?

Answer 36

If doubling the concentration results in an eight-fold increase in rate

Question 37

whats K' ?

Answer 37

Pseudo which is a first order rate constant

Question 38

Whats the integrated method?

Answer 38

the rate equation is made form the components of the reaction where they are then integrated and converted to a linear format in order to plot as a straight line of a graph

Question 39

What time of graph does the integration method lead to being able to produce?

Answer 39

A concentration- time plot

Question 40

For the following reaction write the rate equation with rate being written as a differential? and then the integrated rate equaiton your would get from this? **A --\> products**

Answer 40

Question 41

Write the following integrated rate equation in the form y=mx + c

Answer 41

Question 42

Write the following secord order reaction in linear form from the integrated rate equation... **A --\> product**

Answer 42

Rate = -d[A]/dt rate= k[A]² put these two equations together and solve by integration -d[A]/dt = k[A]² 1/[A]_t – 1/[A]₀ = kt (then put in form y=mx +c) Y= 1/ [A]_t M= K X= t C= 1/[A]₀

Question 43

Whats more accurate, the integrative method or the direct method and why?

Answer 43

The integrative method is more accurate than the direct method as the kinetic parameters are determined directly from a relationship between time and a function of the concentration

Question 44

The characteristic kinetic plot is just what you need to plot in order to get a straight line Units of rate constant is variable Rate is always the same

Answer 44

Learn this table off by heart

Question 45

Whats a chemical half life? How is it denoted?

Answer 45

The time taken for a chemical species to fall to half its initial concentration [A] --\> [A] / 2 A half life is denoted as: t_1/2

Question 46

What order reaction has a constant half life?

Answer 46

first order reactions have a constant half life

Question 47

When a reaction is faster, is there a shorter or longer half-life?

Answer 47

shorter

Question 48

What is the rate of reaction proportional to?

Answer 48

The rate constant, thus the larger the rate constant the shorter the half life

Question 49

How does the half life change with second order cases?

Answer 49

it gets longer and longer as time goes on as the reaction is slowing down

Question 50

Does this graph show a zero, first or second order reaction?

Answer 50

You can't tell which one it is apart from the fact its not first as the half life isn't constant

Question 51

Whats the half life eqaution for a second order reaction? What does the equation suggest?

Answer 51

t_1/2 = 1 / k[A]₀ This inverse relationship suggests that as the initial concentration of reactant is increased, there is a higher probability of the two reactant molecules interacting to form a product Also implies that since the half-life is longer when the concentration are low, species decaying according to second order kinetics may exist for longer amount of time if their initial concentration are small

Question 52

What are the steps in order to work out the kinetics of an unknown mechanism?

Answer 52

1. start with the overall reaction 2. guess some mechanism(s) 3. derive corresponding rate laws (don't need to derive as they're given) 4. compare with experimental data 5. and repeat from step 3 as needed if you dont get a straight line then something has gone wrong in the experiment or the mechanism is too complex (not zero, first or second)

Question 53

Whats the RDS?

Answer 53

The rate-determining step (RDS) is a single step in the mechanism which is much slower that there other steps it can be thought of like a 'bottle neck' (nothing can happen until its happened, it influences the overall reaction)

Question 54

Look at S_N1 and S_N2 reactions and facts about their RDS

Question 55

Observations are made from experimental observations of what...

Answer 55

The rate laws intermediates of the reaction

Question 56

Whats the collision theory formula and what does it tell us?

Answer 56

**Rate= T [concentration]ⁿ** * Rate is increasing dramatically * T: if rate is increasing dramatically, something on the other side of the equation must be increasing too. Must be the temperature * [Concentration]ⁿ: concentration is not changing much

Question 57

What are the two forms of **Arrhenius equation** in 1889 and what does each part stant for/ units

Answer 57

Form one: **K = Ae^(-Ea'/KBT)** Form two: **K= Ae^(-Ea/RT)** K= rate constant of the reaciton A= pre-exponential factor (normalising factor) E'_a= the activation energy for the reaction (J) E_a= the activation energy for the reaction (J mol^-1) T= temperature (K) K_B= Boltzmann's constant R= gas constant

Question 58

In Arrhenius' equaiton there is an exponential factor, what does this mean for the results/effects?

Answer 58

its not linear and the effects are dramatic

Question 59

For the Arrhenius equation, complete these relationships... if K increase T \_\_\_\_\_ If K decreases T \_\_\_\_\_ if K decreases the E_a \_\_\_\_\_ If K increases the E_a \_\_\_\_\_

Answer 59

if K increase T **increases** If K decreases T **decreases** if K decreases the E_a **increases** If K increases the E_a **decreases**

Question 60

convert the following equation into a linear format... **K= Ae^-Ea/RT** Write out in form y= mx+c

Answer 60

**lnK = lnA - E_a / RT** y= ln K m= -E_a/R x= 1/ T c= ln A (used to work out pre-exponential factor)

Question 61

How do you work out the E_a from a graph?

Answer 61

E_a= - gradient x R

Question 62

If T was raised by 10˚c, then the rate would double What two ways can the arrhenius equation be written in order to form a general temperature between temperature and rate and write out the general relationship formula...

Answer 62

**K₁₌ Ae^(-Ea/RT1) ** **K₂= Ae^(-Ea/RT2)** General relationship: ln [K₂/K₁] = E_a/R {1/T₂ - 1/T₁} NOTE; units of A are identical to units of K Temperature is always in kelvin Take the natural log of the K2/K1 ratio