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MBBCh 1- CHEM1048A > Chemical Kinetics > Flashcards

Chemical Kinetics Flashcards

1
Q

What is kinetics?

A

Kinetics is the study of how fast chemical reactions occur.

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2
Q

There are 4 important factors which affect rates of reactions:

A
  • Physical state
  • Concentration
  • Temperature
  • Catalyst
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3
Q

How does the physical state of the reactants affect the rate of a reaction?

A

o In order to react, molecules must come in contact with each other.
o The more homogeneous the mixture of reactants, the faster the molecules can react.

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4
Q

How does the concentration of reactants affect rate of reaction?

A

o Increase in reactant concentration is likely to increase the rate of reaction, due to the increase in the frequency of collisions between reactant molecules.

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5
Q

How does the temperature of reactants affect rate of reaction?

A

o At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy.

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6
Q

How does the presence of a catalyst affect rate of reaction?

A

o Catalysts speed up reactions by changing the mechanism of the reaction.
o Catalysts are not consumed during the course of the reaction.

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7
Q

The rate of a reaction can be determined by monitoring the change in _____ of either reactants or products with time.

A

concentration

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8
Q

Average rate =

A
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9
Q

For the reaction A —-> B there are two ways of measuring rate:

A

– the speed at which the products appear (i.e. change in moles of B per unit time), or
– the speed at which the reactants disappear (i.e. the change in moles of A per unit time).

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10
Q
  • A plot of concentration vs. time for this reaction yields a curve like this.
  • The slope of a line tangent to the curve at any point is the______ rate at that time.
A

instantaneous

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11
Q

Why does the average rate decrease as the reaction proceeds?

A

• This is because as the reaction goes forward, there are fewer collisions between reactant molecules.

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12
Q

All reactions slow down over time. Therefore, what is the best indicator of the rate of a reaction?

A
  • Therefore, the best indicator of the rate of a reaction is the instantaneous rate near the beginning.
  • At t=0 the instantaneous rate is called the initial rate of the reaction.
  • From this point the term rate shall mean instantaneous rate.
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13
Q

• In general for the reaction

aA + bB —–> cC + dD, what is the reaction rate?

A
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14
Q

What is this equation called?

A

• This equation is called the rate law, and k is the rate constant.

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15
Q

For a general reaction with rate law

Rate =

A

we say the reaction is mth order in reactant 1 and nth order in reactant 2.

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16
Q

What is the overall order of reaction?

A
  • The overall order of reaction is m + n + ….
  • A reaction can be zeroth order if m, n, …are zero.
  • Note the values of the exponents (orders) have to be determined experimentally. They are not simply related to stoichiometry.
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17
Q

What a zero order reaction?

A

A reaction is zero order in a reactant if the change in concentration of that reactant produces no effect.

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18
Q

What is a first order reaction?

A

A reaction is first order if doubling the concentration causes the rate to double.

A first-order reaction has a rate dependent on the concentration of a single reactant raised to the 1st power.

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19
Q

A reaction is th order if doubling the concentration causes an 2 increase in rate.

A

nth

2n

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20
Q

Thus for a first order reaction, the differential equation is?

A
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21
Q

What is the integrated law for a first order reaction?

A
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22
Q

ln [A]t = -kt + ln [A]0

What is the graph for this equation?

A

Therefore, if a reaction is first-order, a plot of ln [A] vs. t will yield a straight line, with a slope of -k and y-intercept of ln[A]0.

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23
Q

What is half life?

A
  • Half-life, t1/2 is the time required for one-half of a reactant to react.
  • Because [A] at t1/2 is one-half of the original [A], [A]t = 0.5 [A]0.
24
Q

Half Life

For a first-order process, this becomes:

A
25
Q

What is the relationship between temperature and rate?

A
  • Most reactions speed up as temperature increases, e.g. food spoils when not refrigerated.
  • Since the rate law has no temperature term in it, the rate constant must depend on temperature.
26
Q
  • In a chemical reaction, bonds are broken and new bonds are formed.
  • Molecules can only react if they ____ with each other.
A

collide

27
Q

Explain the relationship between temperature and rate in terms of collision theory.

A

The higher the temperature, the faster the molecules will move on average, thereby increasing both the rate of collisions and the rate of reaction.

28
Q

Complication: not all molecular collisions lead to products. In fact, only a small fraction of collisions lead to product. Why? Two reasons.

A

• In order for the reaction to occur the reactant molecules must collide in the correct orientation and with enough energy to form products.

29
Q

There is a minimum amount of energy required for the reaction: the ____ energy, Ea.

A

activation

30
Q

o Arrhenius: molecules must possess a minimum amount of energy to react. Why?

A
  • In order to form products, bonds must be broken in the reactants.
  • Bond breakage requires energy.
31
Q

Consider the rearrangement of methyl isonitrile:

A
32
Q
  • The reaction coordinate diagram shows the energy of the reactants and products (and, therefore, △E).
  • The high point on the diagram is the _____ state.
  • The species present at the transition state is called the activated _____.
  • The activation energy _____ is the energy gap between the reactants and the activated complex.
A

transition
complex
barrier

33
Q

We can show the fraction of molecules, f, with energy equal to or greater than Ea is:

A

where R is the gas constant (8.314 J/mol·K).

34
Q

What is temperature defined as?

A
  • Temperature is defined as a measure of the average kinetic energy of the molecules in a sample.
  • At any temperature there is a wide distribution of kinetic energies.

[ • As the temperature increases, the curve flattens and broadens.
• Thus at higher temperatures, a larger population of molecules
has higher energy.]

35
Q

Arrhenius discovered most reaction-rate data obeyed the equation:

A

– k is the rate constant, Ea is the activation energy, R is the gas constant (8.314 J/Kmol-1) and T is the temperature in K.
– A is called the frequency factor.
– A is a measure of the probability of a favourable collision.
– Both A and Ea are specific to a given reaction.

36
Q

Taking the natural logarithm of both sides, the equation becomes:

What is the curve?

A

Therefore, if k is determined experimentally at several temperatures, Ea can be calculated from the slope of a plot of ln k vs. 1/T.

37
Q

Suppose we have two values of the rate constant, k1 and k2 , determined at two different temperatures T1 and T2 respectively, then we apply the Arrhenius equation to both sets of conditions:

A
38
Q

What is the reaction mechanism?

A
  • The sequence of events that describes the actual process by which reactants become products is called the reaction mechanism.
  • The reaction mechanism gives the path of the reaction.
  • Mechanisms provide a very detailed picture of which bonds are broken and formed during the course of a reaction.
39
Q

What is an elementary reaction?

A

• An elementary reaction is any reaction that occurs as a result of a single molecular collision or in a single step.

40
Q

The balanced chemical equation provides information about the _____ and end of the reaction.

A

beginning

41
Q

What is molecularity?

A

Molecularity: the number of molecules involved in an elementary reaction.

42
Q

–Unimolecular:
–Bimolecular:
–Termolecular:

A

–Unimolecular: one molecule in the elementary reaction,
–Bimolecular: two molecules in the elementary reaction, and
–Termolecular: three molecules in the elementary reaction.

• It is not common to see termolecular processes (statistically improbable).

43
Q

• If a reaction proceeds via several ______ steps (a multistep mechanism), then the elementary steps must add to give the balanced chemical equation.

A

elementary

44
Q

What is an intermediate species?

A

• An Intermediate: a species which appears in an elementary step, but is not a reactant or product e.g.
NO3 in the example above.

45
Q

Rate Laws for Elementary Reactions

• The rate law of an elementary step is determined by its molecularity:

A
46
Q
  • In a multistep process, one of the steps will be slower than all others.
  • The overall reaction cannot occur faster than the slowest step.
  • ___-___ ____: is the slowest of the elementary steps. It governs the rate law for the overall reaction.
A

Rate-determining step

47
Q

What is a catalyst?

A

A catalyst is a substance that increases the rate of a reaction, without itself being consumed by the reaction.

48
Q

How do catalysts increase the rate of reaction? (2)

A
  • Catalysts increase the rate of a reaction by decreasing the activation energy of the reaction.
  • Catalysts change the mechanism by which the process occurs.
49
Q

• There are two types of catalysts:

A

– homogeneous, and

– heterogeneous

50
Q

How does Homogeneous Catalysis work?

A

• Generally, catalysts operate by lowering the activation energy for a reaction.

51
Q

How does a Heterogeneous Catalysis work? (3)

A

• Heterogeneous catalyst exists in a different phase from the reactant molecules.

  1. First step is adsorption (the binding of reactant molecules to the catalyst surface).
  2. Adsorbed species (atoms or ions) are very reactive.
  3. Molecules are adsorbed onto active sites on the catalyst surface.
52
Q

One way a catalyst can speed up a reaction is by holding the reactants together and helping bonds to ____.

A

break

53
Q

What are the applications of catalysts in automobiles?

A

Automobiles are equipped with catalytic converters, which are part of their exhaust systems. The exhaust gases contain CO, NO, NO2 and unburned hydrocarbons which pass over surfaces impregnated with catalysts. The catalysts promote the conversion of the exhaust gases into CO2, H2O, and N2.

54
Q

Are enzymes catalysts? (3)

A
  • Enzymes are catalysts in biological systems.
  • They are large molecules with specifically shaped active sites.
  • The substrate fits into the active site of the enzyme much like a key fits into a lock.
55
Q
  • Most enzymes _____ very specific reactions.
  • Substrates undergo reaction at the active site of an enzyme.
  • A _____ locks into an enzyme and a fast reaction occurs.
  • The products then move away from the enzyme.
A

catalyze

substrate

56
Q
  • Only substrates that fit into the enzyme lock can be involved in the reaction.
  • If a molecule binds tightly to an enzyme so that another substrate cannot displace it, then the active site is blocked and the catalyst is inhibited (____ _____).
  • The number of events (_____ number) catalyzed is large for enzymes (103 - 107 per second).
A

(enzyme inhibitors)

turnover

57
Q

What is the molecular model of an enzyme?

A

(a) A molecular model of the enzyme lysozyme. Note the characteristic cleft, which is the location of the active site.
(b) Lysozyme with a bound substrate molecule.

MBBCh 1- CHEM1048A (8 decks)

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