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CHEM260 Kinetics > Rate Laws And Arrhenius > Flashcards

Rate Laws And Arrhenius Flashcards

1
Q

How do thermodynamics and kinetics differ?

A

Thermo compares free energy of reactants and products, calculating the driving force IF a transformation occurs. Timescale not considered

Kinetics provide information on how a transformation occurs and on what time scale

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2
Q

How do you work out the rate equation?

A

Has to be found experimentally, NOT given by the stoichiometric coefficients - unless elementary.

Cannot be found by saying the number of molecules and working it out from that

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3
Q

What is the Isolation method?

A

If a reaction goes A + B —> P

And B is in vast excess, it can be said that [B]t is constant. ([B]o = [B]t)

This rate equation is then said to be pseudo-first order

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4
Q

What is the initial rate method?

A

Where the initial rate is measured of a reaction, changing the initial concentration of one reactant to see what effect that has on the initial rate.

Complication; the concentrations and rate of reaction all change with time

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5
Q

What is the integrate rate law for a 0th order reaction?

A

[R] = [R]o - kt

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6
Q

What is the integrate rate law for a 1st order reaction?

A

[R] = [R]o exp(-kt)

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7
Q

What is the integrated rate law for a 2nd order reaction?

A

1/[R] - 1/[R]o = kt

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8
Q

How do you establish if a reaction is elementary?

A

By experimentally determining the reaction mechanism

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9
Q

How do you find the order of an elementary reaction?

A

By the stoichiometry, this is not contradictory as one needs to do experiments to see if a reaction is elementary or not

Even if the order does match the stoichiometry, doesn’t mean the reaction is necessarily elementary

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10
Q

If the initial rate of a reaction is v = k’[A]o^n, what is the linearised form?

A

Log v = log k’ + nlog [A]o

Plot log v against log [A]o

The gradient = n

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11
Q

What is the Arrhenius equation?

A
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12
Q

What does the Ea/RT of the Arrhenius equation signify?

A

It is the fraction of encounters (A+B) that have sufficient energy to react

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13
Q

How does temperature affect a small Ea?

A

A small Ea is HIGHLY temperature dependent

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14
Q

What does the pre-exponential factor A consider?

A

Collision frequency and orientation term

Individual for each reaction

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15
Q

What is the half life of a zeroth order reaction?

A

t = [R]o / 2k

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16
Q

What is the half life of a first order reaction?

A

t = ln2 / k

17
Q

What is the half life of a second order reaction?

A

t = 1 / k[R]o

18
Q

What is the x axis when plotting the Arrhenius equation?

A

x = 1 / T

TIME DEPENDENT

19
Q

How do you obtain A graphically?

A

If on computer, it is the intercept.

If on paper, cannot extrapolate to y axis so obtain A from one data point - best to find from multiple points and give the mean value, with standard error, etc.

20
Q

What is the gradient when the Arrhenius equation is plotted graphically?

A

m = -Ea/R

21
Q

What is the rate of removal of R for a zeroth order reaction?

A

v = - d[R]/dt = k[R]^0 = k

22
Q

How do you find the integrated rate equation for a zeroth order reaction?

A

Rewrite v = - d[R]/dt = k[R]^0 = k as d[R] = -kdt

Integrate d[R] = -kdt to get:

[R]t = [R]o - kt

23
Q

What is the gradient of an integrated zeroth order reaction?

A

Gradient = -k

As [R]t = [R]o - kt

24
Q

What is the rate of removal of R for a first order reaction?

A

v = -d[R]/dt = k[R]^1

25
Q

How do you find the integrated rate equation for a first order reaction?

A

Rearrange v = -d[R]/dt = k[R]^1

To get: d[R]/[R] = -kdt

Integrate from t=0 to time t.

26
Q

What is the intercept and gradient for an integrated first order rate equation?

A

Plot of ln[R] vs t is linear with gradient -k and intercept ln[R]o

27
Q

Is the shape of a 1st order graph dependent of the initial concentration of R?

A

NO, it is independent of the initial concentration

28
Q

What is the rate of removal of A in a second order reaction?

A

v = -d[A]/dt = k[A]^2

29
Q

How do you find the integrated rate equation of a second order reaction?

A
30
Q

What is the intercept and gradient for an integrated second order rate equation?

A

The plot of 1/[A]t vs t is linear with gradient k and intercept 1/[A]o

31
Q

Is the shape of a 2nd order graph dependent of the initial concentration?

A

Yes, it does depend on the initial concentration

32
Q

What is the equation for the half-life of a zeroth order reaction?

A
33
Q

What is the equation for the half-life of a first order reaction?

A
34
Q

Is the half-life f 0th, 1st and 2nd order reactions dependent or independent of the initial concentration?

A

0th = dependent on [R]o

1st = independent of [R]o

2nd = dependent on [R]o

35
Q

What is the equation for ‘Lifetime’ and when is it applicable?

A
36
Q

What is the equation for the half-life of a second order reaction?

A

CHEM260 Kinetics (5 decks)

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