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Chemistry - Module 5 > 18 - Rates > Flashcards

18 - Rates Flashcards

1
Q

Two ways of determining rate

A

• Measuring the decrease in the concentration of a reactant over time

• Measuring the increase in the concentration of a product over time

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2
Q

How can rate equations be determines

A

can only be determined experimentally not by mole ratios

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3
Q

Rate-concentration = 0 order

A

o Changing the concentration of the chemical has no effect on the rate of the reaction

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4
Q

Is the 0 order included in rate equation

A

No

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5
Q

Rate from 0 order rate-concentration graph

A

o Rate = K // y-intercept

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6
Q

Rate - concentration 1st order

A

o The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction

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7
Q

Rate from a first order rate-concentration graph

A

o Rate = gradient

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8
Q

Rate - concentration 2nd order

A

o The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four

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9
Q

Rate from 2nd order rate-concentration graph

A

o Upward curve = plot rate against conc squared = straight line = k = gradient

  • gradient of straight line
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10
Q

Overall order of a rate equation

A

sum of the powers of the reactants in a rate equation

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11
Q

Order with respect to [(CH3)3CBr]

A

• From the above table, that is experiments 1 and 2
o The [(CH3)3CBr] has doubled, but the [OH-] has remained the same
o The rate of the reaction has also doubled
o Therefore, the order with respect to [(CH3)3CBr] is 1 (first order)

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12
Q

Order with respect to [OH-]

A

• From the above table, that is experiments 1 and 3
o The [OH-] has doubled, but the [(CH3)3CBr] has remained the same
o The rate of reaction has increased by a factor of 4 (i.e. increased by 22)
o Therefore, the order with respect to [OH-] is 2 (second order)

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13
Q

Rate equation of this

A

Rate = k [(CH3)3CBr] [OH-]2

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14
Q

Draw 0 order rate concentration graph

A
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15
Q

Draw second order rate-concentration graph

A
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16
Q

Draw third order rate-concentration graph

A
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17
Q

How to find k

A

Use one row from table

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18
Q

Units of k

A
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19
Q

Monitoring rate with a colorimeter

A

• In colorimeter – wavelength of light passing through a coloured solution is controlled using a filter

• Amount of light absorbed is measures

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20
Q

How to use a colorimeter - 6 marks

A
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21
Q

Concentration - time graph 0 order

A

• In a zero-order reaction, the concentration of the reactant is inversely proportional to time

• When the order with respect to a reactant is 0, a change in the concentration of the reactant has no effect on the rate of the reaction

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22
Q

Rate constant of concentration time graph

A

gradient= rate constant k

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23
Q

Draw 0 order concentration time graph

A
24
Q

1st order concentration-time graph

A

• In a first-order reaction, the concentration of the reactant decreases with time

o The graph is a curve going downwards and eventually plateaus:

25
Q

Draw a first order concentration time graph

A
26
Q

How to deduce order of reaction from graph

A

Half life

27
Q

Half life is only for…

A

Concentration - time graph

28
Q

Zero order half life

A

successive half-lives decrease with time

29
Q

First order half life

A

half-life of a first-order reaction remains constant throughout the reaction

30
Q

Second order half life

A

half-life increases with time

31
Q

Define half life

A

the amount of time taken for the amount (or concentration) of the limiting reactant in a reaction to decrease to half its initial value

32
Q

Determination of K for a first order reaction - two ways - concentration-time

A
  • rate
    -half life
33
Q

1st order reaction has… (concentration-time)

A

Constant half life

34
Q

Calculating the rate constant from the rate - first order concentration - time

A
35
Q

Calculating the rate constant from half-life - first order concentration - time

A
36
Q

Clock reaction

A

Idk

37
Q

What is the rate determining step

A

Reactant in rate-determining step is in the rate equation

38
Q

Feature of rate determining step

A

Slowest step of

39
Q

Example of rate determining step

A
40
Q

Increasing temp on rate

A

• Increasing temp increases rate of reaction (may not impact yield due to la chateliers principle)

• Therefore increasing temp increases the value of the rate constant, k, assuming that the concentration of the reactants remains unchanged

41
Q

relationship between the rate of reaction and temperature

A

exponential

42
Q

When temp of a reaction mixture increases,

A

particles to move around faster resulting in more frequent collisions

o proportion of successful collisions increases, meaning a higher proportion of the particles possess the activation energy to cause a chemical reaction

43
Q

Boltzmann distribution

A
44
Q

Arrhenius equation

A
45
Q

Calculate the activation energy of a reaction which takes place at 400 K, where the rate constant of the reaction is 6.25 x 10-4 s-1.
A = 4.6 x 1013 and R = 8.31 J mol-1 K-1.

A

Take natural logs of both sides

46
Q

Arrhenius plot:

A

• A graph of ln k against 1/T can be plotted, and then used to calculate Ea

47
Q

How to draw Arrhenius plot - remember equation (look at it in form y=mx+c)

A
48
Q

How to find rate constant from concentration time graph - method 2

A
49
Q

what is the rate constant

A

a constant of proportionality in the rate equation

50
Q

how would you prove that the gradient of the graph gives you the rate constant

A

For a first-order reaction of a reactant A:

rate = k[A]

Therefore, the gradient of a rate–concentration graph gives the value of the rate constant, k.

51
Q

if you equate the rate to be 1/t what assumption is made + how valid is this

A

The assumption is that the reaction proceeds at constant rate to the measured
point.

The assumption is reasonable so long as the point is soon after the start of the reaction.

52
Q

how could you follow the progress of a reaction between gaseous hydrogen and gaseous bromine to make gaseous hydrogen bromide

A

A colorimeter

could be used to detect the loss of colour of the bromine as it formed hydrogen bromide

53
Q

when predicting equations what should you focus on

A

the number of moles in the rate equation ( the power) and the number of moles in the overall equation

54
Q

does the order of the reactant in the rate equation match up to the number of moles in the rate determining step

A

?

55
Q

what is the pre-exponential factor

A

A

56
Q

units of the parts of the Arrhenius equation ???

A

R - Jmol−1K−1
A -
K -
Ea - Jmol-1
T - K

Chemistry - Module 5 (7 decks)

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