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CHEM1602 > Chapter 14: Chemical Kinetics > Flashcards

Chapter 14: Chemical Kinetics Flashcards

1
Q

What are the three factors that affect reaction rate in the collision model?

A
  1. Temperature
  2. Concentration of the reacting particles
  3. Structure and relative orientation of the reacting particles.
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2
Q

How does temperature affect reaction rate?

A

Larger velocities lead to more collisions and more energetic collisions. Therefore, rate increases with increasing temperature.

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3
Q

How does concentration affect reaction rate?

A

A higher concentration leads to a higher probability of collision. The rate of reaction depends on concentration.

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4
Q

How does “structure” affect reaction rate?

A

To react, molecules need to collide in the correct orientation.

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5
Q

How is rate defined for chemical reactions?

A

Rate is defined by how the concentration(s) of the species change per unit of time.

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6
Q

When writing the rate equation for a general reaction, why should you always use lowest integer coefficients?

A

Rate depends on how the reaction is balanced.

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7
Q

What factor affects the value of the rate constant (k)?

A

Temperature

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8
Q

What is n?

A

n is the reaction order, NOT the stoichiometric coefficient.

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9
Q

What does the rate equal for zeroth order reactions?

A

rate = k

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10
Q

What does the rate equal for first order reactions?

A

rate = k[A]^1

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11
Q

What does the rate equal for second order reactions?

A

rate = k[A]^2

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12
Q

For zeroth order reactions, doubling [A] does what to rate?

A

Nothing. Concentration does not change rate.

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13
Q

For first order reactions, doubling [A] does what to rate?

A

rate2 = 2 x rate1

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14
Q

For second order reactions, doubling [A] does what to rate?

A

rate2 = 4 x rate1

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15
Q

What is the integrated rate law?

A

rate = -∆[A]/∆t = k[A]^n
it solves the rate equations to write concentrations as a function of time.

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16
Q

What is the integrated rate law for zeroth order reactions?

A

[A]t = -kt + [A]˚

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17
Q

What is the integrated rate law for first order reactions?

A

ln([A]t/[A]˚) = -kt

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18
Q

What is the integrated rate law for second order reactions?

A

1/[A]t = kt + 1/[A]˚

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19
Q

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

A

t1/2 = [A]˚/2k

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20
Q

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

A

t1/2 = ln(2)/k

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21
Q

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

A

t1/2 = 1/k[A]˚

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22
Q

How does the length of the half-life change over time for zeroth order reactions?

A

The half-life decreases as the reaction proceeds.

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23
Q

How does the length of the half-life change over time for first order reactions?

A

The length of the half-life remains constant.

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24
Q

How does the length of the half-life change over time for second order reactions?

A

The half-life increases as the reaction proceeds.

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25
Q

Define half-life.

A

Half-life is the time required for the concentration of a reactant to fall to half of its initial value, that is [A]t = 1/2[A]˚.

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26
Q

For a graph that plots [A]t against time, what is the order of the reaction, and what is the slope?

A

Zeroth order with a slope of -k.

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27
Q

For a graph that plots ln[A]t against time, what is the order of the reaction, and what is the slope?

A

First order with a slope of -k.

28
Q

For a graph that plots 1/[A]t against time, what is the order of the reaction, and what is the slope?

A

Second order with a slope of k.

29
Q

For zeroth order reactions, what are the units of k?

A

M x s^-1 or M/s

30
Q

For first order reactions, what are the units of k?

A

s^-1 or 1/s

31
Q

For second order reactions, what are the units of k?

A

M^-1 x s^-1 or 1/M x s

32
Q

What is the y-intercept on the straight line plot of a zeroth order reaction?

A

[A]˚

33
Q

What is the y-intercept on the straight line plot of a first order reaction?

A

ln[A]˚

34
Q

What is the y-intercept on the straight line plot of a second order reaction?

A

1/[A]˚

35
Q

What is the Arrhenius equation?

A

k = Ae^(-EA/RT)

36
Q

What does A refer to in the Arrhenius equation?

A

This is the frequency factor or the number of attempted reactions.

37
Q

What does e^(-EA/RT) refer to in the Arrhenius equation?

A

This is the exponential factor. It refers to the fraction of attempts that have enough energy to react higher (higher than EA).

38
Q

In the Arrhenius equation, what does a large EA mean?

A

This means the exponential factor will be small, and few molecules have enough energy to react.

39
Q

In the Arrhenius equation, what does a small or zero EA mean?

A

This means the exponential factor will be large, and most collisions lead to a reaction.

40
Q

What is the linearized Arrhenius equation?

A

ln(k1/k2) = EA/R(1/T2 - 1/T1)

41
Q

What is the slope of the linearized Arrhenius equation?

A

slope = -EA/R

42
Q

What are the units for the R value that equals 8.314?

A

R = 8.314 J/mol x K

43
Q

How does an increase in temperature affect the rate constant?

A

As temperature increases, k should increase.

44
Q

How can the frequency factor (A) be further broken down?

A
  1. p = orientation factor
  2. z = collision frequency
    A = pz
45
Q

What is a reaction mechanism?

A

The series of chemical reactions that occur and add together to give the overall balanced chemical equation.

46
Q

What are elementary steps?

A

The individual steps of a reaction mechanism that describe a single molecular event.

47
Q

What is molecularity?

A

The number of reactant molecules in an elementary step.

48
Q

How is the rate equation for an elementary step defined?

A

Since it is a single event, the rate should be proportional to the concentration of reactants, and therefore the rate equation is defined by the reaction stoichiometry.

49
Q

What is the rate-determining step?

A

The slowest step in the reaction mechanism. The overall reaction rate and the rate of the slowest step should be nearly the same.

50
Q

How are intermediates considered in deriving the overall rate law for mechanisms?

A

Overall rate law cannot have intermediates. It must be in terms of reactants (and occasionally products).

51
Q

What is a catalyst?

A

A substance that increases the rate of a chemical reaction without being consumed by lowering the activation energy of the reaction.

52
Q

What are homogenous catalysts?

A

Catalysts in which the catalyst is in the same phase as reacting molecules.

53
Q

What are heterogeneous catalysts?

A

Catalysts in which the catalyst is in a different phase from the reacting molecules.

54
Q

How is the integrated rate law different from the rate law?

A

The rate law shows the relationship between the rate of a reaction and the concentration of a reactant. The integrated rate law shows the relationship between the concentration of a reactant and time.

55
Q

Where is the temperature dependence of the reaction rate found in the equation?

A

Temperature dependence is found in the rate constant (k). An increase in temperature generally results in an increase in k, which results in a faster rate.

56
Q

What is the relationship established in the Arrhenius equation?

A

This equation shows the relationship between the rate constant (k) and the temperature in kelvin (T).

57
Q

What is the relationship between the magnitude of the activation energy and reaction rate?

A

In general, the higher the activation energy, the slower the reaction rate (at a given temperature).

58
Q

What is the frequency factor?

A

The frequency factor is the number of approaches to the activation barrier per unit time. Represented as A in the Arrhenius equation.

59
Q

What is the exponential factor?

A

A number between 0 and 1 that represents the fraction of molecules that have enough energy to make it over the activation barrier on a given approach. The exponential factor is the fraction of approaches that are actually successful and result in the product. Represented by -Ea/RT in the Arrhenius equation.

60
Q

What does it mean when there is a low activation energy and a high temperature?

A

It makes the negative exponent small, so the exponential factor approaches one. For example, if the activation energy is zero, the exponent is zero, and the exponential factor is exactly one, meaning every approach to the activation barrier is successful.

61
Q

What does it mean when there is a large activation energy and a low temperature?

A

This makes the exponent a very large negative number, so the exponential factor becomes very small. For example, as the temperature approaches 0 K, the exponent approaches an infinitely large number, and the exponential factor approaches zero.

62
Q

How does an increase in temperature affect the thermal energy distribution of a reaction?

A

At any given temperature, a sample of molecules has a distribution of energies. As the temperature increases, the number of molecules having enough thermal energy to surmount the activation barrier increases dramatically.

63
Q

What is an Arrhenius plot?

A

A plot of the natural log of the rate constant (lnk) vs the inverse of the temperature in kelvins (1/T) yields a straight line with a slope of -Ea/R and a y-intercept of lnA.

64
Q

What is the collision frequency?

A

The number of collisions that occur per unit time, which we can calculate for a gas-phase reaction from the pressure of the gases and the temperature of the reaction mixture.

65
Q

What is the orientation factor?

A

The orientation factor is a number usually between 0 and 1 which represents the fraction of collisions with an orientation that allows the reaction to occur. A measure of how specific the orientation of the colliding molecules must be.

66
Q

What does a large orientation factor (near 1) mean?

A

This indicates that the colliding molecules can have virtually any orientation and the reaction will still occur.

67
Q

What does a small orientation factor (near 0) mean?

A

This indicates that the colliding molecules must have a highly specific orientation for the reaction to occur.

CHEM1602 (8 decks)

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