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chemistry > topic six > Flashcards

topic six Flashcards

1
Q

what requirements must be fulfilled for a chemical reaction to take palce

A

correct orientation
sufficent energy (reaching activation energy)

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2
Q

what is activation energy

A

the minimum amount of energy that colliding particles must have for a chemical reaction to occur.

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3
Q

what is the transition state

A

the highest energy state on a reaction coordinate; it indicates a point at which new bonds are being formed at the same time as old bonds are being broken.

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4
Q

in general, faster reactions tend to haev lower or higher activation energies

A

lower, whereas slower reactions tend to have higher activation energies

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5
Q

if activation enerfy is met will the reaction take place

A

if the correct orientation yes

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6
Q

what is the maxwell boltzmann distribution

A

a graph where the x axis is kinetic energy and the y axis fraction of particles

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7
Q

the area beneath the curve in the MB curve is directly proportional to

A

directly proportional to the number of molecules having a value of kinetic energy in that range.

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8
Q

as temp increases in the mb curve,

A

the particles gain ke and the curve flattens out

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9
Q

how is the maxwell boltzmann distribution helpful

A

in understanding how changes in temperature and the use of a catalyst will affect the rate of a reaction.

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10
Q

the rate of a chemical reaction is a measure of what of the reaction

A

the speed of the reaction

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11
Q

what can the reate of reaction be defined as

A

the hange in conc of a reactant or product per unit of time

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12
Q

rate of reaction =

A

change in reactant / change in time

increase in product conc / change in time

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13
Q

rate of reaction is not constnat over the course of reaction so you must consider it at

A

a particular time

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14
Q

how can instantaneous rate of reaction can be determined

A

graphically

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15
Q

what is avg rate of reaction

A

calculated as the avergae over a particualr time period or interval

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16
Q

what factors affect rate of reaction

A

temp (affects freq of collisions and energy they have)
concentration (affects freq of collisions)
pressure (affects freq of collisions)
surface area (increase as more molecules available)

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17
Q

when does particle size affect rate of reaction

A

when the solid is being reacted directly, not when it is dissolved in solution.

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18
Q

how do catalysts icnrease rate of reaction

A

providing an alternative reaction pathway so a greater proportion of reactant particles now have energy equal to, or greater than, the original activation energy for the reaction. This results in an increase in the frequency of successful collisions between reactant particles and an increase in the rate of reaction.

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19
Q

catalysts at the end of a reaction

A

remains chemically unchanged and sometimes can be reused

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20
Q

The factor by which the concentration of a reactant affects the rate of a reaction is known as

A

its order of reaction

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21
Q

what is the only way the order of a reaction with respect to a particular reactance can be determined

A

through experimental data

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22
Q

order of reactions can be:

A

zero, first or second order

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23
Q

what does the rate expression give

A

the relationship between reactant concentrations and the rate of reaction.

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24
Q

what is k

A

the rate constant

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25
Q

what are the superscribts in hte rate expression

A

the orders of reaction with respect to the letter

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26
Q

what is the overall order of reaction

A

the sum of indiviual orders of reaction

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27
Q

zeroth order

A

Rate is independent of [A]; any change in the concentration of A does not affect the rate of reaction

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28
Q

first order

A

Rate is directly proportional to [A]; if the concentration of A is doubled, the rate also doubles

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29
Q

The rate constant k is a constant for

A

for a reaction at a particular temperature (it is temperature-dependent).

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29
Q

second order

A

Rate changes as the square of [A]; if the concentration of A is tripled (3x), the rate increases by a factor of 9 (32 = 9)

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30
Q

the rate expression gives the relationship between the conc of the reactants and the

A

overall rate of reaction

31
Q

k is only constant at

A

a particular temp
it is temp dependent

32
Q

zero order units

A

mol dm-3 s-1

33
Q

first order units

A

s-1

34
Q

second order units

A

mol-1 dm3 s-1

35
Q

third order units

A

mol-2 dm6 s-1

36
Q

what type of graph can be used to find the order of a reaction with respect to a reactant

A

a concentration (y) time (x) graph
or a rate (y) conc (x) graph

37
Q

for zero order, the ct graph is

A

straight line

38
Q

for first order ct graph, the line is

A

exponential curve

39
Q

for second order ct graph the line is a steeper

A

exponential curve

40
Q

for zero order rc graph

A

horizontal straight line

41
Q

for first order rc graph

A

straight line

42
Q

for second order rc graph

A

parabolic curve

43
Q

what is half life

A

the time it takes for the concentration of a reactant to decrease by half

44
Q

a reaction that takes place in a single step is known as a

A

elementary reaction

45
Q

Many reactions, however, do not take place in a single step and are called

A

non elementary reactions

46
Q

what are the steps in a non elementary reaction called

A

elementary steps

47
Q

the slowest elementary step in a reaction mechanism is the

A

rds

48
Q

the elementary steps must add together to give

A

the overall balanced equation for the reaction

49
Q

the rate expression deduced from the rate-determining step must be consistent with

A

the experimentally determined rate expression

50
Q

The NO3, which appears in both steps but not in the overall equation, is known as

A

a reaction intermediate.

51
Q

what does moelcularity tell us

A

the number of reactant particles in an elementary step

52
Q

If only one particle is involved in the elementary step, it is

A

unimolecular

53
Q

If two reactant particles are involved,

A

it is bimolecular

54
Q

if three reactant particles are involved,

A

termolecular

55
Q

if two reactant particles are involved

A

its bimolecular

56
Q

if three reactant particels are involved

A

termolecular

57
Q

what is an intermediate

A

a species that is formed from the reactants in a chemical reaction which then goes on to react further to form the products.

58
Q

what is the transition state

A

the highest energy state on a reaction coordinate; it indicates a point at which new bonds are being formed at the same time as old bonds are being broken.

59
Q

the rds has the highest

A

activation energy as its the slowest

60
Q

why do catalysts appear in the elementary steps of a reaction mechanism and in the rate-determining step but not in the overall balanced equation for the reaction

A

they are chemically unchanged and can be reused

61
Q

what value constant does a catalyst change

A

increasing the value of the rate constant k

62
Q

what maeks somehting bimolecuelar

A

elementary steps have two reactant particels

63
Q

rate constant k is temp dependent, this relatiponsh[ is shown by the arrhenius equation:

A

k=Ae ^−Ea/RT

64
Q

what is a in the arrhenius equation

A

the pre-exponential factor or the frequency factor.

65
Q

what is Ea in the arrhenius equation

A

activation energy (J/mol^-1)

66
Q

what is r in the arrhenius equation

A

universal gas constant (8.31 J K^1mol^-1)

67
Q

what is e in the arrhenius equation

A

eulers number

68
Q

what does the arrhenius constant take into account with a

A

the frequency of collisions and the probability that they have the correct orientation (or geometry)

69
Q

The expression (e−Ea /RT) (the exponential factor) is the fraction of molecules that have

A

sufficient kinetic energy to react at a certain temperature

70
Q

modified version of arrhenus equation (to find ativation energy)

A

lnk=−Ea/RT +lnA

71
Q

lnk =−Ea/RT +lnA is a form of y = mx+c

A

therefore, a graph of ln k against 1/T gives a straight line (Figure 1). The gradient of the line is equal to –Ea/R (where R is the universal gas constant with a value of 8.31 J K−1 mol−1). The intercept on the y-axis is equal to ln A. Once we have determined the activation energy for the reaction, we can calculate the Arrhenius constant by substituting the activation energy value into the Arrhenius equation.

72
Q

Gradient=

A

−Ea/R

73
Q

−Ea=

A

gradient×R

74
Q

the rate constant, k, for a reaction increases ???? with increasing temperature

A

exponentially

chemistry (13 decks)

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