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Chemistry > 18 - Kinetics > Flashcards

18 - Kinetics Flashcards

1
Q

What is rate of reaction?

A

The change in concentration over time, at a constant temperature.

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2
Q

What is flash photolysis?

A

A method used to measure the rate of extremely fast reactions.

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3
Q

How does flash photolysis work?

A

The reaction is started by firing a pulse of light into the reaction mixture, which breaks chemical bonds and forms highly reactive free radicals which react rapidly.
Further probe flashed are shone a timed intervals, these are used to record the amount of light absorbed by one of the species in the reaction, therefore measuring its concentration.

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4
Q

What is one example where flash photolysis is used?

A

To investigate the mechanism of the depletion of the ozone layer.

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5
Q

What is colorimetry?

A

Monitoring the change in concentration of coloured compounds in solution, based on their absorption of specific wavelengths.
Optic filters are used to select the wavelength that the solute dissolves most.

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6
Q

If the pH changes, how could you measure the rate of reaction?

A

Use a pH meter.

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7
Q

If the number of ions changes, how could you measure the rate of reaction?

A

Test the conductivity.

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8
Q

If a gas is produced, how could you measure the rate of reaction?

A

Measure the amount of gas produced over time using a gas syringe.

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9
Q

What is the method for working out the rate of reaction using titrations?

A

Small samples of the reaction mixture are removed at specific time intervals and quenched to stop the reaction.
The samples are then analysed by titration.

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10
Q

What is the rate expression?

A

It describes the rate of reaction at a particular temperature. Depends on the concentration of species in the reaction.

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11
Q

What is the rate constant (k)?

A

A measure of the rate of reaction.

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12
Q

What is the order of reaction?

A

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

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13
Q

What is the overall order of reaction?

A

The sum of the orders of all species that appear in the rate equation.

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14
Q

What does it mean if a species has zero order?

A

The rate is not affected by the concentration of that species.

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15
Q

What does it mean if a species has first order?

A

The rate is directly proportional to the concentration of that species.
If the conc. doubles the rate doubles.

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16
Q

What does it mean if a species has second order?

A

The rate is proportional to the square of the concentration of that species.
If the conc. doubles the rate quadruples.

17
Q

What is a method for finding the order of reaction? (1)

A

Plot a graph of rate against concentration.
A horizontal line = zero order
A linear line = first order
A exponential line = 1st < order

18
Q

What is a method for finding the order of reaction? (2)

A

The initial rate method:
A series of experiments are carried out at a constant pressure with the concentration of one species varied.
For each experiment a concentration-time graph is plotted and a tangent is drawn at t=0 to calculate the initial rate.

19
Q

What is temperature?

A

A measure of the average kinetic energy of particles.

20
Q

Why is rate constant (k) temperature dependent?

A

As temperature increases more particles have the required activation energy, so the rate increases, therefore increasing the rate constant.

21
Q

What is the Arrhenius equation?

A

k = Ae^(Ea/RT)

22
Q

What is the A in the Arrhenius equation?

A

The pre-exponential factor that is the number of successful collisions between reactant molecules.

23
Q

How is the Arrhenius equation plotted as y=mx + c?

A

The graph is of lnk against 1/T, where lnA is the y-intercept and -Ea/RT is the gradient.

24
Q

What are the typical values of Ea?

A

40-400 kJ mol-1

25
Q

What is the rate determining step?

A

The slowest step in the mechanism, as it will have the biggest impact on the overall rate.

26
Q

What are SN1 reaction?

A

Nucleophilic substitution with an overall first order.
Tertiary halogenoalkanes.
Two-step reaction.

27
Q

What is the rate expression for SN1 reactions?

A

rate = k[halogenoalkane]

28
Q

What are SN2 reactions?

A

Nucleophilic substitution with an overall second order.
Primary halogenoalkanes.
An intermediate is formed.

29
Q

What is the rate expression for SN2 reactions?

A

rate = k[halogenoalkane][nucleophile]

Chemistry (24 decks)

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