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Chemistry > Topic 16 Kinetics II > Flashcards

Topic 16 Kinetics II Flashcards

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1
Q

Overall rate of reaction

A

The change in concentration of a species divided by the time it takes for the change to occur. All reaction rates are positive.

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2
Q

Equation for overall rate of reaction:

A

Rate= change in concentration of a product/time= -change in concentration of a reactant/time.

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3
Q

Units for rate (most commonly)

A

mol dm^-3 s^-1

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4
Q

Methods of Measuring Rate of Reaction (6)

A
  • Measuring the volume of a gas evolved.
  • Measuring the change in mass of a reaction mixture.
  • Monitoring the change in intensity of colour of a reaction mixture by colourimetry.
  • 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 mixture.
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5
Q

Measuring the volume of a gas evolved

A

2 possible methods:
1. Collection over water into a measuring cylinder.
2. Collection using a gas syringe.

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6
Q

Measuring the change in mass of a reaction mixture

A

Used for reactions in which a gas is evolved.
The reaction flask & contents are placed on a digital balance, and the decrease in mass is measured as the reaction proceeds.

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7
Q

When is measuring the change in mass of a reaction mixture most suitable for measuring rate of reaction?

A

When the gas evolved in high-density (e.g., CO2) as the gas given off has a relatively high density, so it’s more precise as measurement uncertainties are less significant.

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8
Q

Measuring the change in concentration of a reactant or product using titration:

A

Use a pipette to remove aliquots from the reaction mixture at regular intervals. The reaction in the aliquot is then quenched and titrated.

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9
Q

How is the reaction in the aliquot quenched?

A

The reaction is slowed to almost zero by immersing it in an ice bath, or stopped by adding another species.

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10
Q

Measuring the change in concentration of a reactant or product using titration: iodine + propanone.

A

This reaction is catalysed by an acid. NaHCO3 is added to the aliquot to stop the reaction by removing the acid catalyst. The remaining iodine is titrated against sodium thiosulfate.
CH3COCH3 + I2 –> CH3COCH2I + H+ + I-
I2 + 2S2O3^2- –> 2I- + S4O6^2-
All species are aqueous.

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11
Q

Any other property that shows a significant change can be measured to determine reaction rate, for example…

A

Dilatometry/volume of a liquid.
Chirality.
Refractive index.

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12
Q

Measuring the change in electrical conductivity of a mixture: how?

A

If the number/type of ions changes, a conductivity meter can be used to measure changes in the conductivity of the solution.

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13
Q

What causes rate constants to change?

A

Temperature.

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14
Q

Units for the rate constant

A

Vary. They must be determined in each rate equation. E.g., 0 order is mol dm^-3 s^-1 & 1st order is s^-1.

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15
Q

Reactions involving simultaneous collision of more than 2 particles.

A

Very rare.

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16
Q

Rate-determining step

A

The slowest step in the mechanism for a reaction.

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17
Q

Overall order

A

Sum of all the individual orders.

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18
Q

Order of a reactant species

A

The power to which the concentration of the species is raised in the rate equation.

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19
Q

Rate equation

A

An equation expressing the mathematical relationship between the rate of reaction and the concentrations of the reactants.

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20
Q

Which species appear in the rate equation, and so affect the rate of reaction?

A

All reactant species either in or before the rate-determining step.

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21
Q

Half-life of a reaction

A

The time taken for the concentration of a reactant to fall to half its initial value.

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22
Q

Instantaneous reaction rate

A

The gradient of a tangent drawn to the line of the graph of concentration against time. Except for a zero order reaction, the instantaneous rate varies as the reaction proceeds.

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23
Q

What does a concentration-time graph look like for a zero order reaction?

A

A straight line with a negative gradient.

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24
Q

Determining the rate equation by the continuous method:

A

Withdraw samples of the reaction mixture at regular time intervals. the reaction in the sample is stopped by quenching. The concentration of a reactant is determined by titration.
Draw a concentration-time graph.
Determine the half-life for the reaction at different concentrations.

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25
Q

If the half-lives on a concentration-time graph are constant…

A

…it’s first order w.r.t. a reactant.

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26
Q

If the half-lives on a concentration-time graph double as the reaction proceeds…

A

…the reaction is 2nd order w.r.t a particular reactant.

27
Q

Rate=

A

1/t, this is an approximation used to find the orders. It is a worse approximation for larger values of t. It is used as the initial rate, but is technically the mean rate.

28
Q

Initial rates method

A

Several reaction mixtures are set up, and the initial rate (time taken for a fixed amount of reactant to be used up or a fixed amount of product to be formed) is measured.

29
Q

What does a rate-concentration graph look like for a 0 order reaction?

A

A straight horizontal line. (Rate is 1/t on the y-axis.)

30
Q

What does a rate-concentration graph look like for a 1st-order reaction?

A

Direct proportionality. A straight line with a positive gradient through the origin. (Rate is 1/t on the y-axis.)

31
Q

What does a rate-concentration graph look like for a 2nd-order reaction?

A

An upward-sloping curve, but it is impossible to tell directly that a rate-concentration graph is 2nd-order. So you have to plot a rate-[concentration]^2 graph.

32
Q

What does a 2nd-order rate-[A]^2 graph look like?

A

A straight line with a positive gradient through the origin.

33
Q

Elementary

A

A single collision between 2 reactant particles (with correct orientation & energy). The rate law can only be found from the balanced equation, if the reaction is elementary.

34
Q

The sum of the steps in a mechanism=

A

The stoichiometric equation.

35
Q

How do we know what species (and their ratio) are in the rate-determining step?

A

Indicated by the rate equation.

36
Q

How can we experimentally test a proposed mechanism?

A

Use a wider range of concentrations.
Use instrumental analysis, e.g., NMR to detect the presence of intermediates.
Carry out the reaction with a deuterated reactant (that is included in the rate equation), as a C-D bond is harder to break than a C-H bond.

37
Q

Activation energy

A

The minimum energy that colliding particles must possess for the reaction to occur. (It represents the energy that colliding particles must obtain to reach the energy level of the transition state, before that reactants can react to form products.)

38
Q

Why, when the original route is still available, do most collisions resulting in reaction occur via the catalysed route?

A

The fraction of particles possessing this lower activation energy will be greater.

39
Q

Homogeneous catalyst: in the same phase as the reactants. Examples…

A

H+ in the iodination of propanone.
Fe2+ of Fe3+ in: S2O8^2- + 2I- –> 2SO4^2- + I2 (all aq).
S2O8^2- + 2Fe2+ –> 2SO4^2- + 2Fe3+
2Fe3+ + 2I- –> 2Fe2+ + I2
If Fe3+ is the catalyst, the steps occur in the reverse order.

40
Q

Heterogeneous catalyst: in a different phase to the reactants. Examples…

A

Iron in the Haber process. V2O5 (Vanadium (V) oxide) in the contact process.

41
Q

How does heterogeneous catalysis work?

A

Adsorption: the reactants are first adsorbed onto the surface of the catalyst.
Reaction: reactant molecules are held in positions that enable them to react together.
Desorption: the product molecules leave the surface.

42
Q

What controls rate of reaction in heterogeneous catalysis?

A

How fast the reactants adsorb and products desorb. Once the surface of the catalyst is covered, rate cannot increase further, even with increases in the pressure of reactants.

43
Q

Autocatalysis

A

The reaction is catalysed by one of its products.
5(COOH2) (aq) + 2MnO4^- (aq) + 6H+ (aq) –> 10CO2(g) + 2Mn^2+ (aq) + 8H2O) (l)
The reaction is slow at RTP, until Mn2+ is produced in a catalytic amount.

44
Q

Describe a concentration-time graph for an autocatalysed reaction.

A

The reaction starts off slowly, then speeds up. The reaction then slows down before eventually stopping. This is reflected in the gradient.

45
Q

What other factors might produce a graph similar to the concentration-time graph for autocatalysis?

A
  • If the reaction involves a solid reacting with a liquid, there may be a substance coating the solid that the liquid must penetrate before it can react.
  • If the reaction is strongly exothermic and the temperature is not controlled, the heat evolved during the reaction speeds up the rate.
46
Q

Why does an increase in temperature increase rate of reaction?

A

An increase in the fraction of molecules with energy equal to or greater than the activation energy for the reaction.
An overall increase in the frequency of collisions between reacting molecules.

47
Q

Arrhenius equation

A

k= A x e to the power of (-Ea/RT)

T is the absolute temperature. R is the gas constant. Ea is the activation energy.

48
Q

What does A stand for in the Arrhenius equation?

A

A constant termed the pre-exponential factor. It is a measure of the rate at which collisions occur irrespective of their energy. It includes the fact that reactions can only occur, if molecules are correctly orientated at the time of collision.

49
Q

What do we obtain, if we take loge of the Arrhenius equation?

A

ln k = -Ea/R x 1/T + ln A

50
Q

If a graph of ln k is plotted against 1/T, what can we tell from said graph?

A

A straight line with gradient -Ea/R is formed, so we can determine Ea experimentally. ln a is the y-intercept.

51
Q

What does e^(-Ea/RT) represent?

A

The fraction of collisions that have energy equal to or greater than the activation energy.

52
Q

How does Ea affect reaction rate?

A

Reactions with a large Ea are slow, but the rate increases rapidly with an increase in temperature.
Reactions with a small Ea are fast, but the rate does not increase as rapidly with an increase in temperature.
Catalysed reactions have small values of Ea.

53
Q

When drawing reaction profile diagrams, how do you distinguish between a transition state and an intermediate?

A

An intermediate has an energy medium, whereas a transition state occurs at the top of an energy curve and has an energy maximum.

54
Q

What is the difference between a transition state and an intermediate regarding lifetime?

A

An intermediate is a definite chemical species which exists for a finite length of time. A transition state has no significant, permanent lifetime of its own; it usually exists for ~ 10^-15 s.

55
Q

In the SN1 hydrolysis of a tertiary halogenoalkane, which step has the higher activation energy?

A

The first step as it’s the rate-determining step.

56
Q

Why is an SN2 reaction fastest with a primary halogenoalkane?

A

Alkyl groups are much larger than hydrogen atoms, so the more alkyl groups around the central carbon in the transition state, the higher the activation energy for its formation.

57
Q

Why is an SN1 reaction fastest with a tertiary halogenoalkane?

A

Alkyl groups donate electrons by the inductive effect. This means that the stability of the carbocations increases as the number of alkyl groups donating electrons towards the positive carbon increases. As the stability of the carbocation increases, the activation energy for the reaction leading to its formation decreases.

58
Q

How does using a catalyst increase the rate constant, k?

A

A catalyst provides an alternative pathway with a lower activation energy, so a greater proportion of particles have sufficient energy to react, so there’s a faster reaction.

59
Q

What does it mean when the gradient of a concentration/time graph doesn’t change?

A

The rate doesn’t change, so the reaction is 0 order with respect to a particular reactant.

60
Q

How can you tell which is the RDS?

A

It will have the proportions of reactants that match the rate equation.

61
Q

For first order, rate is directly proportional to…

A

… change in concentration.

62
Q

SN1 rate equation

A

Only one reactant in the RDS.

63
Q

How do you calculate the reaction rate in an iodine-clock reaction?

A

Time how long it takes for the blue-black complex to form with starch.

64
Q

Question structure: justify the order w.r.t. a reactant.

A

Using experiments 1 & 2, how did the concentration change, and how did the rate change?

Chemistry (23 decks)

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