topic 6/16- kinetics

Question 1

give two equations for the rate of a reaction

Answer 1

ror= amount of reactant used/time
ror= amount of product formed/time

Question 2

how could you measure the rate of reaction at a particular point on a rate of reaction graph?

Answer 2

draw a tangent and calculate the gradient

Question 3

state an example of an experimental method for following ROR and give its equation

Answer 3

Mg (s) + 2HCl -> MgCl2 (aq) + H2 (g)

Question 4

define rate of reaction

Answer 4

the change in concentration of reactants or products per unit time (=chang in conc/time)

Question 5

state the 2 most common units for ROR

Answer 5

mol/dm3/s or mol/dm3/min

Question 6

collision theory states that for a reaction between two particles to occur:

Answer 6

1. the particles must collide with each other with sufficient energy (Ea)
2. the particles must collide in the correct orientation

Question 7

define activation energy

Answer 7

the minimum energy needed for a reaction to occur on colliding of particles

Question 8

state 5 factors affecting the rate of reaction

Answer 8

1. concentration
2. pressure
3. surface area
4. temperature
5. enzymes

Question 9

explain how an increase in concentration/pressure affects the rate of reaction

Answer 9

• no of particles per given volume increases
• increased number of frequent (successful collisions)
• rate increases

Question 10

explain how an increase in surface area affects the rate of reaction

Answer 10

• exposes more reactant particles
• increased number of collisions and successful collisions per second
• increased rate of reaction

Question 11

the average kinetic energy of particles is proportional to…

Answer 11

their temperature in kelvin

Question 12

explain the effect of an increase in temperature on the rate of reaction

Answer 12

• increased energy, so KE of particles increases
• a greater proportion of the particles have the required activation energy
• more collisions per second
• increased number of collisions result in a reaction
• ROR increases

Question 13

draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases.

Answer 13

elsewhere

Question 14

draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases, which shows how increasing the temperature affects the ROR

Answer 14

elsewhere

Question 15

define a catalyst

Answer 15

a substance that increases the rate of a chemical reaction but is chemically unchanged at the end of the reaction

Question 16

describe the effect of a catalyst on the ROR

Answer 16

a catalyst speeds up the rate of reaction by providing an alternative pathway with a lower activation energy
(= increase frequency of successful collisions)

Question 17

draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases, which shows how a catalyst lowers the activation energy

Answer 17

elsewhere

Question 18

draw an energy profile diagram showing the same reaction with and without a catalyst

Answer 18

elsewhere

Question 19

give the two major economic advantages of the use of catalysts in industry

Answer 19

• they increase the rate of a chemical reaction meaning that more of the desired product can be made in a given time period
• reactions can take place at lower temperatures resulting in a decrease in the energy costs to the manufacturer

Question 20

define a heterogenous catalyst

Answer 20

one that is in a different phrase to that of the reactants

Question 21

investigating the iodine-propanone reaction
- 3 steps
- how can we stop the reaction in the sample?
- what do we titrate the iodine against and how?

Answer 21

• the course of the reaction is followed by taking samples from the iodine-propanone mixture at regular intervals, stopping the reaction and then determining the amount of iodine present by titration.
• by adding an excess of sodium hydrogen carbonate to the mixture, which removes the acid (sulphuric acid) catalysing the reaction
• titrate against sodium thiosulphate- wait for the iodine to go from orange to straw coloured/colourless and then add starch, which is blue/black in the presence of iodine

Question 22

give 3 ways of monitoring concentration changes in a reaction

Answer 22

• mass (loss of mass as gas released)
• volume (of gas produced)
• colour ( production of solid and cross)

Question 23

give the equation and units for rate of reaction

Answer 23

change in concentration (mol/dm3)/time (s)

Units: mol/dm3/s

Question 24

for a reaction A+B-> products, give two scenarios regarding the rate

Answer 24

scenario 1: the rate is proportional to the concentration of A (if we double the concentration of A, the rate also doubles)

scenario 2: the rate is proportional to the square of the concentration of A (if we double the concentration of A, the rate quadruples)

Question 25

state the rate equation and describe each of the terms

Answer 25

Rate = k[A]^a[B]^b - k is the rate constant; it is constant for each reaction as long as neither temperature is changed nor catalyst is added - [A] and [B] are the concentrations in mol/dm3 of reactants A and B respectively - a and b are the orders of reaction with respect to reactant A and B

Question 26

zero order with respect to reactant A

Answer 26

- reaction rate not affected by [A] - if A were doubled, the reaction rate would not change - rate = k - zero order reactants do not appear in the equation

Question 27

first order with respect to reactant A

Answer 27

- reaction rate is proportional to [A] - if A were multiplied by 2, reaction rate would double, if multiplied by 4, reaction rate would quadruple - rate = k[A]

Question 28

second order with respect to reactant A

Answer 28

- reaction rate is proportional to [A]^2 - if [A] were multiplied by 2, reaction rate would quadruple, if multiplied by 4, it would x8 - rate = k[A]^2

Question 29

how do we work out the units of the rate constant, k, and why?

Answer 29

the units of k will depend on the form of the rate equation, so need to be derived separately from each reaction equation

Question 30

effect of an increased concentration of a reactant on - rate of reaction - value of rate constant

Answer 30

- increases unless the order is 0 - no change

Question 31

effect of an increased temperature of a reactant on - rate of reaction - value of rate constant

Answer 31

- increases - increases

Question 32

effect of the addition of a catalyst on - rate of reaction - value of rate constant

Answer 32

- increases - increases

Question 33

give two strategies that can be employed to find reaction orders from experiments

Answer 33

- continuous monitoring - using initial rates

Question 34

define continuous monitoring

Answer 34

the quantity (ie concentration) of a reactant is measured at intervals (eg every 30 seconds) during the course of a reaction

Question 35

describe 3 ways in which continuous monitoring can be used

Answer 35

1. concentration-time graphs 2. finding half-lives 3. rate-concentration graphs

Question 36

1. concentration-time graphs

Answer 36

concentration on y axis, time on x axis, where the shape indicates the order of reactant with respect to that reactant

Question 37

draw the graph shapes for zero order, first order, and second order concentration-time

Question 38

2. finding half-lives

Answer 38

the half-life of a reactant is the time taken for its concentration to reduce by half - first order- half life is constant - second order- half life not constant

Question 39

3. rate-concentration graphs

Answer 39

- to determine how the rate changes with concentration it is necessary to take tangents of the concentration-time graph - these tangents give the rate of the reaction at that particular time - concentration on x axis, rate on y axis

Question 40

draw the graph for zero, first, and second order rate-concentration graphs

Question 41

in a 'clock method', rate =

Answer 41

1/t

Question 42

strategy B; using initial rates

Answer 42

- several experiments undertaken, changing concentration of only one reactant at a time and keeping all others the same

Question 43

how is initial rate determined?

Answer 43

- finding the gradient of a reactant concentration against time graph at time=0 - timing how long it takes for a certain amount of product to be made and calculating 1/time as initial rate

Question 44

why is the reaction between iodine and propane in acid solution known as an auto-catalysis reaction?

Answer 44

because the catalyst (H+) is made as a product

Question 45

describe the reaction order of an SN1 reaction mechanism for halogenoalkanes

Answer 45

overall order=1 check booklet for more info

Question 46

describe the reaction order of an SN2 reaction mechanism for halogenoalkanes

Answer 46

overall order=2 check booklet for more info

Question 47

the effect of temperature on the rate is determined by the

Answer 47

rate constant

Question 48

altering the temperature or adding a catalyst will change the

Answer 48

rate constantt

Question 49

Arrhenius equation

Answer 49

k=Aexp(-Ea/RT) A- Arrhenius constant/pre-exponential factor/frequency factor/steric factor - incorporates effect of collision frequency and orientation Ea- minimum energy for reaction to occur (J/mol) R- gas constant (8.31 J/K/mol) absolute temperature (K)

Question 50

taking the natural logarithm of the Arrhenius equation converts it to a more useful form

Answer 50

ln(k) = ln(A) - (Ea/RT) - this equation is in the form y=mx + c - ln(k) is y axis - ln(A) is c - (-Ea/R) is gradient, m - 1/T is x axis

Question 51

describe how you would use the Arrhenius equation to find activation energy

Answer 51

1. measure the rate of reaction at a range of different temperatures 2. calculate the nature logarithm of each rate to give ln(rate) 3. convert each 'C temp to Kelvin by adding 273 and find 1/T (units K^-1) 4. plot a graph of ln(rate) on the y axis against 1/T/K^-1 and draw a line of best fit 5. find the gradient by using m=y2-y1/x2-x1 6. activation energy (j/mol) = -R x Gradient 7. convert answer to units kJ/mol

Question 52

remember

Answer 52

1/T values will often be of the order !0^-3 so don't forget to write x 10^-3 on your temperature scale

Question 53

reactions may occur by more than one step; what determines the rate of reaction?

Answer 53

the slowest step (rate determining step)

Question 54

look at how to form the rate equation from reactions with more than one step

Question 55

the molecularity of an elementary step is

Answer 55

the number of reactant particles taking part in that step

Question 56

the order of a reaction can either be ----- or ------ in nature, and can describe

Answer 56

integer, fractional; the number of a specific reactant's particles taking part in the rate-determining step.

Question 57

rate equations can only be determined

Answer 57

experimentally

Question 58

how do catalysts alter a reaction mechanism?

Answer 58

they introduce a step with lower activation energy

Question 59

which step of the reaction will have a higher Ea

Answer 59

the rate-determining step