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CHEM160X Simona > SECTION E > Flashcards

SECTION E Flashcards

1
Q

What is the rate of a rection?

A

“The rate at which reagents are used up and products are formed”

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2
Q

How is the rate of a reaction defined in terms of the concentrations of both reagents and products?

A
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3
Q

What is the rate law?

A

The rate law expresses the relationship between the rate of reaction and the concentrations of reactants.

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4
Q

What is the order of reaction?

A

The exponent to which [reactants] are raised in the rate law. It is determined experimentally.

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5
Q

What is k?

A

K is the rate constant for the reaction. It depends onlyontemperature(andthenatureofthereactants).

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6
Q

What do the integrated rate laws allow us to calculate?

A

Concentrations of each component as a function of time.

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7
Q

What is the integrated rate law for zero order reactions?

A
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8
Q

What are the units of k for zero, first and second order reactions?

A
  1. mole L-1 sec-1
  2. sec-1
  3. L mole-1 sec-1
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9
Q

How can we best describe the rate of a zero order reaction?

A
  • It remains constant with time;
  • the concentration of [A] decreases linearly with time
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10
Q

What is the integrated rate law for a first order reaction?

A
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11
Q

For a first order reaction, which of the two following graphs is linear?

a) [A] vs. t
b) ln[A] vs. t

A

B

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12
Q

What is the half life?

A

1/2 is the time required for the concentration of the reagent to drop to half its original value

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13
Q

What is the half life for first order reactions?

A

For first order reactions, it is independent of concentration.

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14
Q

What is the significance of k?

A

It is the rate consant. Also called the proportionality constant, it depends on t (Arrhenius relation) but is independent of concentrations, and tells us how fast the reaction goes.

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15
Q

Radioactive decay follows ____ order kinetics.

A

First order

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16
Q

Which equation(s) best describea radioactive decay?

A
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17
Q

Give an equation for radioactive decay that includes the half life

A
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18
Q

What is the integrated rate law for a second order reaction?

A
19
Q

What is the defining feature of the half life for a second order reaction?

A

It depends on [A]o

20
Q

If a plot of 1/[A] vs t is linear, what is the order of the reaction?

A

Second order.

21
Q

What are pseudo first order conditions and what are they used for?

A

They are used for experimental determination of the order of reaction:

A + B → products

Rate = k[A]a[B]b

 Need to isolate the variation in [A] so Keep [B] the same but let [B] >>[A]
[B] is effectively constant (pseudo-order conditions) (isolation)

 Run the reaction at several different [A]0. Plot [A] vs. time

 Determine initial rate for each curve (draw tangent).

 Note that if we take logs: log(Rate) = log(kobs) + alog[A]

Get straight line graph with slope equal to the order of the reaction.

 The ratio of any two rates will be in proportion to the ratio of initial concentrations to the power of a.

+ log(Rate) = log(kobs) + alog[A] —> as it the gradient of a graph of log(rate) vs. log[A]

22
Q

In general, is the order associated with the stoichiometry of the reaction?

A

Nope

23
Q

What does the order of a reaction reflect?

A

The underlying mechanism of the reaction

24
Q

The order of elementary (unimolecular, bimolecular) reactions is equal to the ____________.

A

Molecularity (number of molecules that come together to react in an elementary reaction)

25
Q

What is the rate determining step?

A

If a reaction proceeds by a series of steps then the rate constant, k, of the slowest step governs the rate of the reaction.

26
Q

The ______ ________ _________ can be used to infer reaction mechanisms.

A

Steady state approximation.

27
Q

What is the Arrhenius relation?

A
28
Q

What is the Arrhenius plot?

A

lnK vs. 1/T

Gradient: -Ea/R

Intercept: lnA

29
Q

What is the significance of A in the Arrhenius relation?

A

It is the pre exponential factor. It takes into account the total number of collisions occurring per second in the reaction + the orientation of the molecules in the collision, which is also important.

30
Q

Why do reactions depend on T?

A

Most reactions depend on collisions in order to occur. Molecules require a collision of a threshold (minimum) energy in order to occur. – Activation Energy, EA.

31
Q

What is catalysis?

A

Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst. With a catalyst, reactions occur faster and with less energy. Because catalysts are not consumed, they are recycled. Often only tiny amounts are required.

32
Q

What is a catalyst?

A

A substance which takes part in a reaction and changes its rate but which can be recovered unchanged at the end of the reaction (at least in theory).

A catalyst provides the reaction with a separate mechanism (pathway) which has a lower activation energy than the unassisted route.

33
Q

What is a homogeneous catalyst?

A

Catalyst is in the same phase as the reactants.

Example: any enzyme

34
Q

What is a heterogeneous catalyst?

A

Catalyst is in a different phase as the reactants e.g. reactants are gases, catalyst is solid. They are materials with the capability of adsorbing molecules of gases or liquids onto their surfaces.

Example: Pt catalyses CO + 1/2*O2 —–> CO2

35
Q

What is the M-M equation?

A

V: rate of reaction

Vmax: limiting rate of reaction

Km: [S] at which the enzyme is 50% saturated

36
Q

In M-M kinetics, how does the order of the reaction change as a function of [S]?

A

[S] << Km: first order

[S] >> Km: zero order

37
Q

What is the turnover number of an enzyme?

A

Kcat: it is defined as the maximum number of molecules of substrate that an enzyme can convert to product per catalytic site per unit of time.

It informs about the efficiency of the catalyst, how many substrate molecules can be processed per mol of enzyme per unit of time.

38
Q

What is the catalytic efficiency of an enzyme?

A

Kcat/Km

It gives an indication of how efficient the enzyme is. It cannot be greater than the limit imposed by diffusion control (diffusion-limited enzymes).

39
Q

What is the equation of the LB plot?

A
40
Q

What does a LB plot look like?

A
41
Q

What is the equation for the Eadie-Hofstee plot?

A

Tip: take M-M equation, cross-multiply and divide by [S]

42
Q

What does the EH plot look like?

A
43
Q

What type of inhibition is this?

A

Competitive

44
Q

What type of inhibition is this?

A

Non-competitive

(Km unchanged, Vmax is decreased)

CHEM160X Simona (8 decks)

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