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Chem 110 > Block 3: Kinetics > Flashcards

Block 3: Kinetics Flashcards

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1
Q

What is chemical kinetics?

A

The study of the rates of chemical reactions and the factors that affect them

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2
Q

What is the rate of a chemical reaction?

A

The change in concentration per unit time

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3
Q

What are the units of a rate?

A

molL-1/s-1

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4
Q

What is the formula for the rate of reaction:

aA + bB –> cC + dD

A

Rate = -(1/a) x (change in [A])/(change in t)

This is equal to +(1/c) x (change in [C])/(change in t)

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5
Q

What is the rate proportional to, and give an example to illustrate this.

A

Rate is proportional to the moles of the chemical.

Eg. in 3O2 –> 2O3, the rate is 1/3 the disappearance of O2, or 1/2 the appearance of O3.

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6
Q

What does the rate measure?

A

The initial rate of reaction, or average rate of the overall reaction. The initial rate is faster than the average rate.

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7
Q

How do we make the average rate as close as possible to the initial rate?

A

Use a shorter timespan

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8
Q

What factors affect the rate of reaction?

A
  • Nature of the reaction itself
  • Concentrations of the reactants
  • Temperature
  • State of the reactants
  • Presence of a catalyst
  • Action of light
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9
Q

What is a rate law?

A

The relationship between the reaction rate and the reagents’ concentrations.

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10
Q

What are the 4 different orders, and what do they mean?

A

1st order: The rate id directly proportional to the conc.
2nd order: The rate is proportional to the conc. squared
3rd order: The rate is proportional to the conc. cubed
Zero order: The rate is not related to the conc.

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11
Q

What happens if a reaction has more than one reactant affecting rate, and what is the resultant order (ie not a reactive intermediate)? +Example

A

The rate is proportional to the concentrations of all reactants.
Eg. CH3Br + OH- –> CH3OH + Br-.
The rate is proportional to [CH3Br] and [OH-].
Rate is proportional to [CH3Br][OH-]
It is 1st order for each reactant, but 2nd order overall.

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12
Q

What is k?

A

The rate constant for these reactions. It differs depending on the reaction, and only changes within a single reaction with temperature.
Now we can phrase rate constants as
rate = k[A][B].

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13
Q

What are the units of k?

A

They depend on the order of the reactions.
As k = rate/[A][B], the units are also molL-1s-1/[units for A][units for B].
For example, if both A and B were first order, molL-1 x molL-1 = mol2L-1, and the equation simplifies to mol-1Ls-1.
(the way to work this out is to subtract the bottom line from the top line: minus 2 = -1, -1 minus -2 = 1 etc)

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14
Q

How do we determine the order of a reaction experimentally?

A

If the reaction rate doubles when one reactant is doubled (and other(s) constant), that reagent is 1st order. If reaction rate quadruples/octuples/no change, it is 2nd/3rd/0 order.
The overall rate is determined by simply adding the reactants’ orders.

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15
Q

How do we determine the order of a reaction using graphs of reaction rate?

A

When we graph concentration vs. time:
We want a straight line, where the gradient = k. This is because k, the gradient, must be constant throughout the reaction.
However, we need to know what form of graph to plot, necessitating the use of the integrated rate law.

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16
Q

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

A

[R] = [R]o e^-kt. This simplifies to ln[R] = ln[R]o - kt

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17
Q

What is the function of the integrated rate law?

A

This allows us to work out k, if we know the concentration of R at 2 points in time.

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18
Q

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

A

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

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19
Q

What is the integrated rate law for a 3rd order reaction?

A

1/[R]^2 = 1/[R]o^2 + kt

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20
Q

What is the integrated rate law for a 0 order reaction?

A

[R]/t

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21
Q

How does the k value tell us the speed of the reaction?

A

A larger k value denotes a faster reaction.

22
Q

What is a half life?

A

The time in seconds for the reactant’s concentration to halve.

23
Q

What is the symbol of a half life?

A

t1/2

24
Q

What is the formula for half life in a 1st order reaction, and what does the formula tell us?

A

t1/2 = ln2/k.
It is not dependant on the concentration of the reactant, so its half life will be constant regardless of the concentration.

25
Q

What is the formula for half life in a 2nd order reaction, and what does the formula tell us?

A

t1/2 = 1/ ([R]o x k)
The rate depends on the initial concentration of the reactant, so its half life will increase exponentially as concentration falls.

26
Q

What does the Arrhenius equation describe?

A

The relationship between k and temperature

27
Q

What is the Arrhenius equation?

A

k = Ae ^ (Ea/RT)
Where: R = gas constant
A= pre exponential factor. Both Ea and A are different in different reactions. The units of A are the same as for k.

28
Q

What does the equation rearrange to to find Ea?

A

Rearrange so that lnk = lnA - Ea/RT.

Now, the slope = -Ea/R, and Ea = -slope x R

29
Q

What is collision theory?

A

In order for two molecules to react, they must first collide at the correct energy, in the correct orientation.

30
Q

What is an elementary reaction?

A

One step in a series of steps of an overall reaction

31
Q

What is a bimolecular reaction?

A

It is the most common form of a reaction where two molecules collide and react

32
Q

What is a unimolecular reaction?

A

A less common reaction in which a single molecule reacts with itself

33
Q

What is a termolecular reaction?

A

A less common reaction where three molecules collide and react

34
Q

What happens to reaction rate as T increases, and why?

A

The molecules gain Ek and move more rapidly, allowing them to reach Ea and overcome it. More will collide with sufficient energy and react, increasing the reaction rate.

35
Q

What is a reaction mechanism?

A

An overall reaction from reactants to products written as a sequence of 2 or more steps (one slow, others fast).

36
Q

What are reactive intermediates?

A

Species which feature on both the product and reactant sides of the equation. They have a very reactive high energy state, and can’t be detected or isolated in a reaction. They don’t feature in the overall calculation of K.

37
Q

What is the RDS, and why does it form?

A

Different steps in the reaction have different Ea and k values. The one with the larges Ea takes the most energy to overcome, so the rate of the overall reaction is limited to how long it takes to overcome that single step. This is called the Rate Determining Step (RDS)

38
Q

What does the rate of reaction depend on in terms of the RDS?

A

The rate is equal to the rate of the RDS, so is dependant on only the reactants in this step, and nothing else. The overall rate law also equals the RDS rate law

39
Q

What happens when the 1st step is not the RDS, and the rate depends on a product from the 1st step?

A

We assume step 1 is in equilibrium, and that by using the concs of the reactants used to produce the intermediate, we can approximate the conc of the actual intermediate, to find the rate constant.

40
Q

What equation do we use for finding the conc. of intermediate C using concs of A and B? How can we then use this to find the rate of reaction C + D –> P ?

A

We know that K = [C] / [A][B]
Therefore [C] = K[A][B].
Rate = kK[A][B][D]

41
Q

How do we use reaction mechanisms to determine if something is Sn1 or Sn2?

A

In Sn1, the slow step is R-X –> R+ + X-, while the fast step is R+ + Nu- –> RNu. Therefore the rate is 1st order in RX but 0 order in X. We can prove this experimentally.

In Sn2, it all happens as one step, so the rate is proportional to both RX and Nu. Therefore we can experiment to determine which type of substitution is occurring.

42
Q

What is a catalyst?

A

It speeds up a reaction by providing an alternate reaction pathway at a lower activation energy. They may give rise to the same or different products if the reaction were uncatalysed.

43
Q

What is a heterogeneous catalyst?

A

The catalyst is in a different state to the reagents

44
Q

What is a homogeneous catalyst?

A

The catalyst is in the same state as the reactants.

45
Q

Does the catalyst appear in the rate expression?

A

Yes, if it is present. It may not be in the overall equation, but it features in the rate law for the slow step, as catalysts are primarily needed to speed up the slow step

46
Q

How do catalysts speed up a reaction?

A
  • They may provide an alternative reaction pathway at a lower Ea, or in the case of heterogeneous catalysts, they may hold the/a reagent in the correct orientation to react.
47
Q

What gives the enzyme its function?

A

The active site, made up of specific folding of proteins

48
Q

What is a competing reaction?

A

One reactant may react to yield one of two reactants determined by different rate constants. As temperature affects rate constant, altering the temperature alters the ratio of the k values and so how much of each product is produced. The reaction with the higher Ea value will increase faster than the lower Ea.

49
Q

What is an explosion?

A

Characterised by reactions whose rates dramatically increase as the reaction proceeds. Many reactive intermediates are formed and re-react with each other and original reagents, forming more and more reaction as time goes on.

50
Q

What are oscillating reactions?

A

Products are generatd in bursts. This is due to the fact that the some of the elementary reactions are catalysed by their own products - they are autocatylitic as both reactant and product produce the same compound.

Chem 110 (6 decks)

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