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[OBSELETE] S - Chem > topic 16 > Flashcards

topic 16 Flashcards

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1
Q

what is meant by rate of reaction

A

the change in the amount of reactants or products per unit time

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2
Q

recall the continuous monitoring methods that can be use to measure rate of reaction

A

measuring gas volume
change in mass
colour change
change in pH
titration
electrical conductivity

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3
Q

what is meant by a continuous monitoring method

A

measurements are taken over the duration of the reaction

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4
Q

how can measuring gas volume be used to measure rate of reaction

A
  • if gas is given off in the reaction→ can collect it in a gas syringe
  • record how much you’ve got at regular time intervals
  • this method can e.g be used for a reaction between an acid and a carbonate → CO2 given off
  • find conc. of reactant at each time point → use ideal gas equation to find moles and then use molar ratio to work out conc.
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5
Q

how can measuring change in mass be used to measure rate of reaction

A
  • if gas is given off → system will lose mass
  • you can measure the loss off mass at regular time intervals using a balance
  • work out moles to work out how much gas has been lost and how many moles of reactant is left
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6
Q

how can tracking colour changes be used to measure rate of reaction

A
  • track colour change of a reaction using colorimeter which measures the absorbance → how much light is being absorbed by the solution
  • the more concentrated the colour of the solution → higher absorbance
  • plot a calibration curve → known conc. of solution against absorbance
  • during the experiment take a small sample from reaction solution at regular intervals and read absorbance
  • use calibration curve to convert the absorbance into a conc at each time point
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7
Q

how can changes in pH be used to measure rate of reaction

A
  • if the reaction produces or uses H+ ions → pH of solution will change
  • if this is the case → you can measure the pH of the solution at regular intervals + calculate conc. of H+
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8
Q

how can a titration be used to measure rate of reaction

A
  • take small samples of a reaction at regular time intervals and titrate them using a standard solution
  • rate can be found from measuring the change in conc. of reactants/ products overtime
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9
Q

how can electrical conductivity be used to measure rate of reaction

A

if the number of ions changes so will electrical conductivity

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10
Q

how can a conc. time graph be drawn

A
  • by repeatedly taking measurements (continuous monitoring) → can draw a conc.-time graph
  • amount of reactant or product- y axis
  • time- x axis
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11
Q

how can the rate of reaction be determined from a conc. time graph

A
  • rate at any point in the reaction is given by the gradient at that point on the graph
  • if the graph is a curve → draw tangent to the curve and find gradient of that
    • draw tangent for the part of the curve that you want to find the rate of reaction from
  • gradient= change in y/change in x
    • or rise/run
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12
Q

what are the units for rate of reaction

A

conc. per unit time

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13
Q

why is there a negative gradient when measuring reactant concentration

A

this is because conc. of reactant falls to make product

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14
Q

what do orders tell you

A

tells you how a reactants conc. affects the rate

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15
Q

what does it mean if the order of reaction is 0 with respect to X

A

[X] changes and the rate stays the same
- so if [X] doubles → rate stays the same
- (so change in conc. of reactant doesnt affect the rate)

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16
Q

what does it mean if the order of a reaction is 1 with respect to X

A

rate is proportional to [X]
- if [X] doubles → rate will double
- whatever happens to the conc. of reactant, the same will happen to rate

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17
Q

what does it mean if the order of a reaction is 2 with respect to X

A

rate is proportional to[X]2
e.g - if [X] doubles → rate will be 2 to the power of 2 → 4 times faster
- if [X] triples → 3 to the power of 2→ rate is 9 times faster

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18
Q

how do you find the overall order of a reaction

A

overall order of reaction is the sum of the orders of all the reactants

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19
Q

how do you use a conc.-time graph to construct a rate-conc. graph

A
  • find gradient at various points on the graph
    • this will give the rate at that particular conc.
    • need to draw tangents and find their gradients
  • plot each point on a new graph with the axes rate and conc.
    • rate. - Y axis
    • conc.- X axis
  • draw a smooth line or curve through the points
  • the shape of the line will tell you the order of the reaction with respect to that reactant
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20
Q

what can a rate-conc. graph tell you

A

the order of a reaction

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21
Q

what does a horizontal line on rate-conc. graph mean

A
  • order is 0
    • changing the conc. doesnt change the rate
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22
Q

what does a rate-conc. graph with an order of 1 look like

A

if its a straight line through the origin → rate is proportional to [X]

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23
Q

what does a rate-conc. graph with an order of 2 look like

A

rate will be proportional to [X]2

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24
Q

what is half life + its symbol

A

the time it takes for half of the reactant to be used up

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25
Q

how can you work out half life

A
  • to work out half life →plot a conc. time graph
  • then draw lines across from the y-axis at points where the conc was halved
  • then read off the time taken from the x axis
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26
Q

how can half life be used to identify a first order reaction

A

half-lives are useful for identifying a first order reaction (without needed to draw a rate-conc. graph)
- half life is always constant for a first order reaction
- always takes the same amount of time to halve
- half life is independent of conc. → first order

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27
Q

what is half life with respect to volume

A
  • volume time graphs can also be used to identify first order reactions
  • e.g the half-life is the time it takes for the volume of reactant to half
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28
Q

what is initial rate of reaction

A

the initial rate of a reaction is the rate at the start of the reaction

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29
Q

how is the initial rates method carried out

A
  1. carry out separate experiments using different initial concentrations of one reactant
    - keep the rest f the conc. of reactants constant
  2. see how changing initial conc. affects initial rate and figure out the order for each reactant
    - can do this by carrying out experiments using continuous monitoring and drawing conc.-time graphs or volume-time graphs
    - calculate gradient of tangent at time=0 →gives initial rate
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30
Q

what is the clock reaction an example of

A

is an example of the initial rates method
the quicker the clock reaction finished the faster the rate of reaction

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31
Q

what do you do in a clock reaction

A

measure how the time taken for a set amount of product to form changes as you vary the conc. of one of the reactants

32
Q

why will there be a sudden inc in the conc. of a certain product

A

there will be a sudden increase in the conc. of a certain product as a limiting reactant is used up

33
Q

how do you know when the end point occurs in the clock reaction

A

usually an easily observable end point e.g colour change → tells you when the desired amount of product has formed

34
Q

what are the assumptions when carrying out a clock reaction

A
  • the conc. of each reactant doesnt change significantly over the time period of your clock reaction
  • temp stays constant
  • when end point is seen the reaction hasnt proceeded too far
35
Q

what is the equation for the iodine clock reaction and what is it also known as

A
  • is a clock reaction
    • also known as he Harcourt-Esson reaction
36
Q

how do you carry out the clock reaction

A

1) a small amount of sodium thiosulphate solution and starch are added to excess hydrogen peroxide and iodine ions in acid solution
- starch used as an indicator → turns blue-black in the presence of iodine

2) sodium thiosulphate that is added to the reaction mixture reacts instantaneously with any iodine that forms

3) all iodine that forms in the first reaction is used up straight away in the second reaction.
once all of the sodium thiosulphate is used up → any more iodine that forms will stay in solution
- so starch indicator will suddenly turn the solution blue-black → end of clock reaction

4)varying iodide or hydrogen peroxide conc. while keeping others constant → gives different times for the colour change

5) the time it takes for the reaction to occur along with the conc. of reactants allows you to calculate the initial rate with respect to iodide or hydrogen peroxide

37
Q

what is the equation for iodination of propanone and how can it be carried out

A

can be found by titrating

38
Q

how can the initial rate of iodination of propanone be found

A
  • monitor reaction by taking samples at regular intervals
  • first stop the reaction in each sample by adding sodium hydrogen carbonate → neutralise the acid
  • then titrate each sample against sodium thiosulphate and starch to work out the conc. of iodine
  • repeat experiment and change the conc. of just one reactant
39
Q

recall the general rate equation and its units

A

units moldm-3 s-1

40
Q

explain the rate equation

A
  • K- rate constant
  • the larger the rate constant, the faster the reaction
  • m and n are the orders of the reaction with respect to reactant A and B
  • m→ tells you how the conc. of reactant A affects the rate
  • n→ tells you how the conc. of reactant B affects the rate
  • overall order of the reaction is m+n
41
Q

how can you use the initial rates method to work out orders of reaction

A
  • by comparing initial rates of a reaction with varying conc. of reactants from e.g a table of data → can find the orders of reaction for reactants
  • once you know chemical equation for a reaction and orders of reaction → can write rate equation
  • so find the order for each reactant → look at the effect of increasing one reactant on the rate to find the order
    e.g see if its directly proportional, or no change
42
Q

how can you calculate the rate constant from orders and the rate of reaction

A
  • write rate equation
  • insert values that you have e.g rate and conc
  • rate goes on the left hand side and conc goes in the square brackets and the order outside the square brackets
  • if order of a reactant equals 0 → not included in equation
    • this is because [X] to the power of 0 is 1
  • rearrange the equation and calculate K
  • find units by inserting the units rather than values into the equation which is used to find K
43
Q

what is the effect of temperature on the rate constant

A
  • rate constant is always the same for a certain reaction at a particular temperature
  • if you increase temp, rate constant will inc aswell
44
Q

what is the rate determining step

A

rate determining step → slowest step in a multi-step reaction
in a reaction with a series of steps → each step can have a different rate

45
Q

what can the rate equation be used to work out

A
  • the rate equation can be used to work out the mechanism of a chemical reaction
46
Q

how to determine which reactants are involved in the rate determining step from the chemical equation

A
  • if a reactant appears in the rate equation→ it must affect the rate
    • so this reactant or something derived from it must be in the rate determining step
  • if reactant doesnt appear in rate equation → not involved in rate determining step
  • the rate determining step doesnt have to be the first step in the mechanism
  • the reaction mechanism cant usually be predicted from just the chemical equation
47
Q

how can you predict the rate equation from the rate determining step

A
  • the order of a reaction with respect to a reactant shows the number of molecules of that reactant which are involved in or before the rate determining step
  • e.g if the reaction is second order with respect to X → two molecules of X in the rate determining step
  • so look at the number of molecules of reactants in the rate determining step to determine the order of each reactant
  • then write the equation
48
Q

how can you predict the mechanism for a reaction from the rate equation

A
  • rate equation gives you the reactants that determine the rate of the reaction
    • gives reactants that are in the rate determining step
      if a reactant from the chemical equation isnt in the rate equation then this means it cant be involved in the reaction until after the rate determining step
49
Q

how can you find the mechanism for the reaction between propanone and iodine using the rate reaction- recall the rate equation and chemical equation as well

A
  • propanone and H+ are in the rate equation so they (or something derived from them) must be in the rate determining step
  • iodine → not in rate equation isnt involved until after the rate determining step
  • order of reaction for both propanone ad H+ is 1 → so one molecule of each
  • H+ is a catalyst - regenerated in another step
50
Q

recall the three different types of halogenoalkanes

A

primary
secondary
tertiary

51
Q

what is a primary halogenoalkane

A

halogen is joined to a carbon with just one alkyl group attached

52
Q

what is a secondary halogenoalkane

A

halogen is joined to a carbon with two alkyl groups attached

53
Q

what is a tertiary halogenoalkane

A

the halogen is attached to a carbon with 3 alkyl groups attached

54
Q

how can halogenoalkanes be hydrolysed

A

all of these halogenoalkanes can be hydrolysed/split by heating them with sodium hydroxide but they react using different mechanisms

55
Q

why are halogenoalkanes able to undergo nucleophilic substitution

A
  • nucleophile attacks and swaps with the halogen
  • C-halogen bond is polar as most halogens are more electronegative than carbon → so they draw e- to themselves
  • carbon is partially positive so is easily attacked by nucleophiles
    OH- is the nucleophile → provides pair of e- for the partially positive carbon
56
Q

what are the two types of mechanisms for nucleophilic substitution

A

SN1 and SN2

57
Q

what do SN1 reactions involve

A

SN1 reactions only involve 1 molecule/ion in the rate-determining step

58
Q

what do SN2 reactions involve

A

SN2 reactions involve 2 molecules, 1 molecule and 1 ion, or 2 ions in the rate-determining step

59
Q

what do primary halogenoalkanes only react by

A

only react by SN2 mechanism

60
Q

what can secondary halogenoalkanes react by

A

can react by both the SNS1 and SN2 mechanisms

61
Q

what can tertiary halogenoalkanes only react by

A

only react by the SN1 mechanism

62
Q

what are the characteristics of a halogenoalkane undergoing the SN2 mechanism

A

will include both reactants
- has a single rate determining step
- because of this a transition state is formed
- the rate equation shows that the rate is dependent on the conc. of both reactants
- order with respect to each reactant is 1
- one molecule of each reactant
- both reactants must be involved in or before the rate determining step
- fits the SN2 mechanism

63
Q

what are the characteristics of a halogenoalkane undergoing the SN1 mechanism

A
  • rate is only dependent on the conc. of the halogenoalkane
  • hydroxide ion is only involved after the rate determining step as its not in the rate equation
  • reaction happens in two steps
  • first step- halogen leaves the haloalkane
  • second step- nucleophile is then able to attack in the second step
64
Q

why does the SN1 mechanism occur the way it does

A

the reaction happens this way because → very little space around carbon as its surrounded by alkyl groups

65
Q

how can you calculate activation energy

A

you can use the Arrhenius equation to calculate the activation energy
links rate constant with activation energy and temp

66
Q

what is meant by activation energy

A

the minimum amount of kinetic energy particles need to react

67
Q

recall the Arrhenius equation and what each part of the equation is

A

A- arrhenius constant

68
Q

what is the link between activation energy and the rate constant(k)

A
  • as the activation energy increases, k gets smaller
    • if the activation energy increases this means that less particles have enough energy to react when they collide
    • a large activation energy will mean a slow rate
69
Q

what is the link between temp and the rate constant (k)

A
  • the equation also shows that as temp inc, k also inc
  • when temp inc→ particles have greater kinetic energy so are mere likely to collide
70
Q

how can you simplify the arrhenius equation to find Ea

A
  • can simplify the equation by getting rid of e part → take natural log (ln) of both sides. this gives:
    • lnK= lnA - Ea/RT
    • then rearrange: Ea/RT= lnA - lnK
    • multiply both sides by RT to get Ea on its own: Ea= (lnA - lnK) x RT
  • then substitute numbers in
71
Q

how can you use arrhenius plots to find out the activation energy

A
  • can use Arrhenius equation lnk= lnA - Ea/RT and plot a graph of lnk against 1/T (just add x10-3 to the end of temp)
  • then draw a line of best fit
  • this wil produce a graph with a gradient of -Ea/R
  • once you know this gradient → can find activation energy
    • change in y/change in x gives -Ea/R
  • rearrange this equation: to get Ea on one side of the equation
72
Q

how can you use an arrhenius plot to find the arrhenius constant

A
  • substitute value of the gradient and any point on the line into the equation lnk= -Ea/ RT + lnA
  • lnA = lnk + (-Ea/RT)
  • rearrange the equation to get lnA on one side of the equation
  • to work out A → press e button and calc and plug in the value of lnA
73
Q

what is the units for a reaction with an order of 0

A

mol dm-3 s-1

74
Q

what is the units for a reaction with an order of 1

A

s-1

75
Q

what is the units for a reaction with an order of 2

A

mol-1 dm3 s-1

76
Q

what is the units for a reaction with an order of 3

A

mol-2 dm-6 s-1

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