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AEC > Kinetics II > Flashcards

Kinetics II Flashcards

1
Q

What is meant by the term rate of reaction ?

A

The rate of change of concentration of a reactant or product with time.

Units - mol dm^-3 s^-1

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2
Q

What is the rate equation?

A

r = k[A]m[B]n

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3
Q

What is meant by the term order with respect to substance in a rate equation?

A

The power to which the concentrations of that substance is raised to in the rate equation

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4
Q

Overall order of reaction?

A

The sum of the powers to which the concentrations of the reactants are raised in the experimentally determined rate equation

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5
Q

What is meant by the term rate constant?

A

k is the constant of proportionality that connects the rate of reaction with the concentration of the reactants.

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6
Q

How can the orders be worked out?

A

They are worked out experimentally , they have nothing to do with the stoichiometric coefficients in the balanced equations

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7
Q

A + 2B + 3C -> products.
It was found to be first order in the A and B and second in C.
Write the rate equation for this reaction.

A

Rate = k [A] [B] [C]^2

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8
Q

Name methods of measuring rate of reaction

A

Measuring the volume of gas evolved
Measuring the change in mass of a reaction mixture
Monitoring the change in intensity of colour of a reaction mixture (calorimetry)
Measuring the change in concentration of a reactant or product using titration
Measuring the change in pH of a solution
Measuring the change in electrical conductivity of a reaction mixture

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9
Q

How would the reaction between calcium carbonate and dilute hydrochloric acid to measure the ROR ?
CaCO3 (s) + 2HCl (aq) -> CaCl2 (aq) + H20 (l) + CO2 (g)

A

measuring the volume of gas given off at regular intervals

By measuring the change in mass of the reaction with time

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10
Q

How would the reaction between propanone and iodine in aqueous solution measure the ROR? CH3COCH3(aq) + I2 (aq)-> CH3COCH2I (aq) + H+ (aq) + I- (aq)

A

to follow the reaction by monitoring the decrease in intensity of colour of reaction mixture as I 2 is. the only coloured species

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11
Q

What are the two most common techniques for collecting and measuring the volume of gas evolved during a reaction?

A

Collection over water into a measuring cylinder
Collection using a gas syringe

Chosen one will depend partly on the level of precision required. Greater degree of precision in gas syringe but if a larger volume of being measured not as precise

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12
Q

Measuring the change in mass of a reaction mixture

A

reaction flask and contents are placed on a digital balance and the decrease in mass is measured as the reaction proceeds.

More precise when the gas given off has a relatively high density

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13
Q

calorimetry

A

monitoring the colour change if the reactant or product is coloured

The reactants are mixed and a clock is started. Change is measured in set intervals

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14
Q

How is Titration used to find the ROR

A

Measure out samples of the reactants with known concentrations
Mix them together start a clock and stir the mixture thoroughly
At regular time intervals withdraw samples using a pipette and quench the reaction. Note the time when half the contents of the pipette is placed
The quenched solution is then titrated against a suitable standard solution

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15
Q

When can titration be used to find the ROR?

A

When an acid, alkali or iodine is a reactant or product.
Acid titrated with standard alkali
Alkali titrated with standard acid
Iodine titrated with sodium thiosulfate

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16
Q

measuring the electrical conductivity

A

measuring change via conductivity meter

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17
Q

In a reaction A —> B+C, the value of [A] fell from 0.12 mol dm -3 to 0.012 mol dm -3 in the 60s. Explain why it is inaccurate to state that the rate of reaction is 0.0018 Moldms-1?

A

For this to be accurate the concentration must change by 10% or less. Here it changes by 90%

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18
Q

What is the unit for zero order?

A

mol dm-3 s-1

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19
Q

What is the unit for first order?

A

s-1

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20
Q

What is the unit for second order?

A

dm^3 mol-1 s-1

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21
Q

What is the unit for third order?

A

dm^6 mol-2 s-1

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22
Q

The slowest step in a reaction determines

A

the overall ROR

It is known as the rate determining step

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23
Q

How can we determine the order of reaction with respect to each reactant?

A

continuous method

initial rate method

24
Q

What is a continuous method?

A

This is data from one experiment where the concentration of one substance is followed throughout the experiment.

25
Q

In order for the continuous method to be used,

A

the concentrations of the reactants not being followed must be in large excess in the experiment so their concentrations stay virtually constant and do not affect rate

26
Q

How is the data processed in the continuous method?

A

By drawing a concentration-time graph then calculating the successive half lives

27
Q

If the half lives are constant …

A

it is 1st order

28
Q

If the half lives rapidly increase

A

it is 2nd order

29
Q

If the rate stays constant as the reactants is used up

A

it is zero order

Concentration has no effect on rate

30
Q

What is meant by the term half life?

A

Time taken for the concentration of the reactants to halve

31
Q

From a conc-time graph, if it is a horizontal line

A

the reaction is not taking place = 0

32
Q

From a conc-time graph, if a straight line downwards is obtained

A

the rate is constant - zero order

33
Q

From a conc-time graph, if a downward curve is obtained

A

the rate is decreasing as concentration falls hence first order or greater

34
Q

How is the rate calculated at any particular concentration?

A

by drawing a tangent at the wanted concentration then using gradient

35
Q

Graphs of initial rate against concentration show what?

A

different orders

36
Q

How is the initial rate calculated using a graph?

A

The initial rate may have been calculated from taking gradients from concentration /time graphs

37
Q

In an initial rate graph , zero order means

A

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

38
Q

In an initial rate graph first order

A

For first order: the rate of reaction is directly proportional to the concentration of

A r = k[A]1

39
Q

In an initial rate graph second order is

A

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

40
Q

When using data from experiments within table format to figure out the order,

A

at least one of the concentrations must be the same, when identifying a pattern ethene the concentration of the other reactant and initial rate.
Often experimental values need to be rounded up to the whole number when getting a pattern.

41
Q

Determining zero order from rate-conc graphs

A

horizontal line 1/t = k[A]0

42
Q

Determining first order from rate-conc graphs

A

straight line 1/t = k[A]1

43
Q

Determining second order from rate-conc graphs

A

curved line. 1/t = k[A]2

However not able to eternise directly hence draw another graph 1/t against [A]^2. F straight its 2nd

44
Q

For a rate concentration graph to show the order of a particular reactant….

A

For a rate concentration graph to show the order of a particular reactant the concentration of that reactant must be varied whilst the concentrations of the other reactants should be kept constant.

45
Q

Propanone reacts with iodine in acidic solution (the acid is a catalyst) . Give the chemical equation

A

CH3COCH3(aq) + I2(aq) →

CH3COCH2I(aq) + H+(aq) + I–(aq)

46
Q

Investigating the rate of reaction between iodine and propanone

A

This reaction can be followed by removing small samples from the reaction mixture with a volumetric pipette. The sample is then quenched by adding excess sodium hydrogencarbonate to neutralize acid catalyst which stops the reaction. Then the sample can be titrated with sodium thiosulfate using a starch catalyst

47
Q

Give the orders for rate of reaction between iodine and propanone

A

This reaction is zero order with respect to I2 but 1st order with respect to the propanone and acid catalyst

48
Q

Give the rate equation for rate of reaction between iodine and propanone

A

The rate equation for the reaction is.

Rate = k[CH3COCH3(aq)][H+(aq)]

49
Q

RDS for the reaction between iodine and propanone

A

The rate determining step of this reaction must therefore contain one propanone molecule and one H+ ion forming an intermediate. The iodine will be involved in a subsequent faster step.

50
Q

What is meant by the term activation energy?

A

the minimum amount of energy required to initiate a reaction

51
Q

What is meant by the term heterogeneous catalyst?

A

Is in a different phase to that of the reactants

52
Q

What is meant by the term homogeneous catalyst?

A

Is in the same phase as the reactants

53
Q

Why does an increase in temperature increase the rate of reaction?

A

an increase in the fraction of molecules with energy equal or greater to the Ea for the reaction

Overall increase in the frequency of collisions between the reactions molecules

54
Q

State the Arrhenius equation

A

k = Ae^ (-EA/RT)
Increasing temperature increases the rate constant k.
A is the Arrhenius constant, R is the gas constant, and EA is activation energy.

55
Q

Units in Arrhenius Equation

A

Temperature uses the unit K
R = 8.31 J mol-1K-1 ActivationenergywillneedtobeinJmol-1 tomatchtheunits of R
The unit of the Arrhenius constant A will be the same as the unit of the rate constant k

56
Q

Taking the natural log of an Arrhenius Equation

A

lnk = (-Ea/R . 1/T) + lnA

57
Q

If a graph of lnk is plotted against 1/T

A

A straight line is obtained
The gradient = -Ea/R
The intercept at y axis is lnA0

AEC (19 decks)

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