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CHEM 110 > Block 3 > Flashcards

Block 3 Flashcards

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1
Q

Define rate of reaction

A

the change in concentration of a reactant (measured in MolL-1) per unit of time.

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2
Q

What is the instantaneous rate equation and what dose it tell us about reaction rate

A

For a reaction aA +bB -> cC + dD,
rate = 1/a (change in [A] over time) = 1/c (change in [C] over time) …etc for B and D.
This tells us that reaction rate is relative to both the rate of consumption and the rate of formation and that these two can be different depending on the moles being made.

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3
Q

In basic terms what is the reaction rate and how can it be measured

A

The number of times a reaction happens every second. It can be measured by how fast the reactant decreases or how fast the product is formed

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4
Q

In a plot of concentration vs time, the instantaneous rate of reaction is given by what?

A

the tangent to the slope at that point in time

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5
Q

What is the difference between the instantaneous rate and the average rate ?

A

Average rate is the change in rate over a time interval reaction, whereas Instantaneous rate is the rate at a specific point in time during the course of the reaction. As a result they can be very different.

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6
Q

What does “ change in” or delta mean

A

Final - initial

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7
Q

What are the factors affecting rate of reaction

A

Nature of reaction, Temperature, The action of light (dependent on wavelength), State (greater SA), Concentration and Catalysts (SA of catalysts matter too).

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8
Q

Where do rate laws come from

A

they are experimentally derived

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9
Q

What does second order in A mean

A

The rate is proportional to [A]^2, if A doubles then the rate quadruples.

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10
Q

The rate of reaction “with respect to” [B] means what

A

Its how the rate of the reaction changes entirely based on the changing of just that reactant

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11
Q

What is the unit of k for first order reaction

A

s-1 (per second)

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12
Q

What is the unit of k for second order reaction

A

L mol-1 s-1 (Litre per mole per second)

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13
Q

What is the unit of k for third order reaction

A

L^2 mol-2 s-1 (Litres squared per moles squared per second)

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14
Q

What is the unit of k for a zero order reaction

A

mol L-1 s-1 (mole per litre per second)

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15
Q

What is the first order integrated rate law

A

[R] = [R initial] e^-kt

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16
Q

How to check if a reaction is really first order (also what equation does this involve?)

A

Plot a graph of ln [R] against time as x.
using the ln [R] = ln [R initial] - kt which emulates y= c + mx. A first order reaction will have a straight line, slope -k

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17
Q

For some second order reactions why do we treat them like pseudo first order reactions and what does this mean

A

2 reactants contributing to the rate law may be changing it at different rates. This can be mitigated by making the conc of one of the reactants much larger than the other so that changes in its conc are negligible therefore treated as if the rate = k’ [R1]

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18
Q

What unit of k does a pseudo first reaction have

A

s-1

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19
Q

What is the intergrated form of second order reaction equation

A

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

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20
Q

How to check if its a second order reaction?

A

The graph of 1/[R] over time is a straight line graph y= mx + c if its a second order reaction

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21
Q

What is the half life of a reaction in a chemical reaction? What is half life denoted by

A

The time (usually in seconds) for the concentration of the reactant to fall to half of its initial value. It is denoted by t(small 1/2)

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22
Q

For first order reaction half life only depends on

A

the rate constant (k)

23
Q

For second order half life what does it depend on

A

rate constant k and the initial concentration of reactants [R]

24
Q

Define the transition state of a reaction (step 3)

A

this is the middle step of an elementary reaction where the molecule has the most potential energy between the reactants and products for a short time but requires the most energy, thus at the highest point of the graph.

25
Q

What is the step 2 of the reaction (step after the resting state of the molecule)

A

The potential energy is increasing because the molecules are re-orientating in order for the reaction to take place.

26
Q

What is activation energy

A

This is the difference in energy between the transition state and the initial resting state of the molecule ; it is the energy required to proceed with the reaction and determines whether a reaction will happen or not.

27
Q

What physical process does the activation energy of a unimolecular reaction represent?

A

Could be radioactive decay or a molecule falling apart. Collisions help it to have enough energy to fall apart- its vibrating

28
Q

A collision is effective if

A

the molecule has enough energy to surpass the activation energy and the molecules are correctly aligned

29
Q

If the activation energy is high how fast is the reaction

A

it is slow

30
Q

What is this Arrhenius equation used to calculate and what are the units… k= Ae^(-Ea/RT) .
What is the key relationship

A

Used to calculate the activation energy for a reaction. k= rate constant, A = constant about the orientation, R= gas constant , T= temperature in Kelvin, Ea= activation energy. In this equation temperature is relative to Ea.

31
Q

Describe the two ways to solve for activation energy

A
  1. taking the ln of the arhenius equation to give an equivalent y= mx +c graph of rate constant vs time experimentally.
  2. using the simultaneous equation form of arhenius that has two rate constants known at two different temperatures. k1T1 is a pair
32
Q

What is the Boltzmann distribution and what is the principle relationship of temperature to energy that sits behind this

A

The distribution of different molecule energies at a given temperature. Temperature is proportional to average energy if temperature scale used is absolute. (Kelvin).

33
Q

What is a rate limiting step

A

The slow step of the reaction mechanism, the reaction step that has the biggest activation energy

34
Q

How can you identify the reactive intermediates in the reaction mechanism?

A

They appear on both sides of the arrow; produced in one step to be used up in another.

35
Q

Compared to reactants or products how is the energy and stability of reactive intermediates?

A

They are extremely reactive with a higher energy than either reactants or products and they cannot be detected or isolated in the reaction.

36
Q

Define catalysts, what is their characteristic feature

A

They are substances that speed up the reaction without being overall being consumed in the overall process -> usually react in early step and then regenerated in a later step.

37
Q

What are the two types of catalysts, which is better and why and whether they appear in the rate law

A

Homogenous catalysts are catalysts that are in the same state/phase - can mix. Appears in the rate law.
Heterogenous catalysts are catalysts in a different phase/ state (eg immiscible liquids).

38
Q

What is the main message of catalysts and competing reactions

A

When two reactions are possible for the same reactants, the one with the lower Ea is favoured. However the use of the catalyst can lower the Ea of one reaction so that becomes the favoured product.

39
Q

What are the two things that catalysts do for a reaction and how do these relate to the Arhenius equation and speed of reaction

A

Catalysts provide an alternate pathway for reaction that has a lowered Ea or they help to align the molecules-> increases the probability of a successful reaction (change in A). AS a result, the k (rate constant) of catalysed reaction is always greater than uncatalysed, so it is a faster reaction

40
Q

From what step can the rate law for the reaction be written directly from

A

the rate limiting/ slow step.

41
Q

What is more likely ? collision with 2 things or collision with 3 things

A

collision with 2 things

42
Q

What are the key principles of reaction mechanisms and rate laws

A

Mechanisms must be made up of elementary steps
The slow step is the rate determining step from which the rate law of the reaction can be directly written from
Intermediates cannot appear in the final rate law

43
Q

What do we do if our rate law contains some reactive intermediates

A

You can use the equlibrium constant big K to find a way to substitute for the reactive intermediate. Then by substituting this reaction in with the original rate equation you will get a rate constant k’ = original k * K

44
Q

How do you write equilibrium constant

A

K= [products]/[reactants]

45
Q

The rate law can tell us about the

A

mechanism of the reaction

46
Q

How do you find the rate of formation of a product

A

You look at the number of moles and then times that by ‘the rate of reaction’

47
Q

How to go from °C to K

A

°C + 273.15

48
Q

What is the unit for Ea

A

KJ mol-1

49
Q

An elementary reaction has how many transition states and reactant steps

A

1

50
Q

What is a rate constant

A

Proportionality constant in rate law.

51
Q

How do you define any “# order reaction”

A

The rate is proportional to […]. Overall reaction order in rate law is #. Then give all examples of the equations you could get.

52
Q

Define rate law

A

the relationship between rate of reaction and concentration. eg [A] proportional to rate

53
Q

What is the rule for remembering the units for rate constant

A

starting from 0 where it is molL-1 s-1, each next one is timesed by mol-1 and L

CHEM 110 (8 decks)

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