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Aqa a level chemistry > Rate equations (y13) > Flashcards

Rate equations (y13) Flashcards

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1
Q

rate = k[A]^m [B]^n

A

m and n = orders of reaction with respect to reactants A and B

K = rate constant

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2
Q

rate K constant values

A

varies with temperature

rate equations show –> In rate equations, the mathematical relationship between rate of reaction and concentration gives
information about the mechanism of a reaction that may occur in several steps.

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3
Q

rate constant affected by temperature equation

A

k = Ae–Ea/RT

where A is a constant, known as the Arrhenius constant,
Ea is the activation energy and T is the temperature in K

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4
Q

key mark scheme points when making a graph

A

-sensible scales (cover at least half of the paper and not above squares)
-line through points must be smooth
-line of best fit ignores anomalies
-calculate gradient

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5
Q

expectations for a catalyst to be in rate equation

A

-It must have a measurable and quantifiable effect on the rate of reaction

-The catalyst must be homogeneous

-If a chemical appears in a rate equation but is not one of the reactants, then it is a catalyst

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6
Q

write the rate law for this equation

2NO (g) + O2 (g) –> 2NO2 (g)

A

K {NO]^2 {H2}^2

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7
Q

rate of reaction and units

A

Rate of reaction is the change in concentration of a product or reactant per unit time

Units of ROR = mol dm^-3 s^-1

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8
Q

Average

Instant

A

Average = averaged over the whole time of the experiment

Instant = rate at a particular time

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9
Q

rate expression

A

-rate expression tells us how the concentration of reactants and catalysts affect the reaction rate. Derived from experimental data

-rate = K{A}^a {B}^b

-rate of reaction depends on some or all the species in the reaction vessel

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10
Q

how can rate data be determined experimentally

A

-data of concentration and initial rate

-plotting a graph and look at shape

-plotting a graph with logarithmic scale of concentration and rate (will always be straight, gradient is the rate order)

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11
Q

changing temperature and pressure

A

If temperature increases or a catalyst is used the rate of reaction increases but concentrations remain constant. Rate constant (K) increases.

Rate constant (K) is unaffected by pressure change. When pressure increases, concentration increases and K remains constant

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12
Q

determining rate equation

A

K = rate / {R}

R = reacts with s to make products

Rate = K {R}

1) Find pair of experiments -> concentrations must be same

2) calculate how many times greater concentration of other reactant was

3) find rate reactant

4) calculate K and find units

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13
Q

order of a reaction

A

The order of a reactant shows how the concentration of a reactant affects the rate of reaction

It is the power to which the concentration of that reactant is raised in the rate equation

The order can be 0, 1 or 2

When the order of reaction of a reactant is 0, this means that it has no effect on the rate of the reaction and therefore is not included in the rate equation at all (unchanged)

When the order of reaction of a reactant is 1, the rate is directly proportional to the concentration of that reactant (x A)

When the order of reaction of a reactant is 2, the rate is directly proportional to the square of the concentration of that reactant (x A^2)

The overall order of reaction is the sum of the powers of the reactants in a rate equation

E.g Rate = k [NO]2 [H2]

Second-order with respect to NO

First-order with respect to H2

Third-order overall (2 + 1)

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14
Q

half life

A

The half-life (t1/2) is the time taken for the concentration of a limiting reactant to become half of its initial value

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15
Q

concentration vs time graph

A

-Graphs of concentration of reactant against time give some clues to the order or the reagent

-straight line = reaction is zero order –> constant with change if conc so conc doesn’t havent rate of reaction

-curve = first or second order

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16
Q

arrhenius equation

A

k = A e − E a R T

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17
Q

calculating gradient then change in concentration

A

1) Draw tangent then do change in y / change in X to get gradient

2) change in concentration x (A)2

3) Square root answer to get new {A}

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18
Q

A as pre-expoential factor

A

pre-exponential factor is related to collision frequency and the orientation of the particles (Same units as K)

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19
Q

rearranged arrhenius question (remove expoential)

A

Easier way of figuring out Arrhenius equation –> LnK = LnA – (Ea / RT)

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20
Q

rearranged to find activation energy

A

-Ea = RT (Ink – InA)

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21
Q

rearrnaged to find T

A

T = Ea / (InA – InK) x R

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22
Q

rearranged to find A

A

InA = InK + Ea / RT

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23
Q

arrhenius equation in relation to y=mx + c

A

Y= Ink = y-axis

M = Ea / R

X = 1/T = x axis

C = InA

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24
Q

rearrnged rate constant

A

K = rate / [C] [D]

units of K = mol-1dm3s-1

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25
Q

calculate Ea at 25 degrees in KJ
InA = 16.9
R = 8.31
K = 0.028
temp = 298

A

-Ea = RT (InK-InA) / 1000

(8.31) (In0.028 - 16.9) = 51

dont need In for A in equation as it is already provided in the equation

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26
Q

order reaction

A

relationship between concentration of the reactant and rate of reaction

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27
Q

order of reaction

A

-straight line –> if you change conc by x2 the rate does nothing

-straight line diagnoal –> if you change conc by 2 sometimes rate will be x2

-curve –> if you change conc by x2 sometimes the rate will be x4

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28
Q

zero order
A + B –> C

A

-increase A by a factor of X there is no change in rate

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29
Q

1st and 2nd order
A + B –> C

A

1st –> if we increase A by x and the rate increases by x we describe the order with respect to A as one

2nd –> if we increase A by x and the rate increases by X sqaured the rate is respective to A as two

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30
Q

K [NO]2 [H2]

A

NO = 2nd

H2 = 1st

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31
Q

which statement is correct

K = [X]2 [Y]

A

the rate of the reaction is halved if the concentraiton of X is halved and the concentration of Y is doubled

32
Q

units of rate constant

A

The units of k for a zero-order reaction are M/s,

the units of k for a first-order reaction are 1/s,

and the units of k for a second-order reaction are 1/(M·s)

33
Q

how to know if reaction is 2nd order

A

A reaction is second order overall when it is second order in one reagent, zero order in all others, or first order for two reagents (1 + 1 = 2). For example, the combination reaction A + B → C would be second-order overall if first-order in both A and B.

34
Q

1, 2, 3 calculating units

rate = moldm-3s-1
reactants = moldm-3

A

One:
rate = K [A]1 –> moldm-3s-1 /moldm-3 so units are s-1

Two:
rate = K [A]2 = mol-1dm3s-1

Three: mol-2 dm6 s-1

35
Q

half life

A

zero order = half life decreases over time + gradient is constant

1st order = half life is constant + gradient decreases exponentially

2nd order = half life increases over time + gradient decreases even faster

36
Q

explain why the student is able to deduce that iodine is zero order

A

-volume of sodium is directionally proportional

-propanone was in large excess = so only iodine is varied

37
Q

calculate the order of experiment with respect to B

A

B is doubled so rate double = 1st order

38
Q

determine Ea from table

A

subtract 2 temperatures close together or at ends of each table

In(1/time) - In(1/time) / 1/t - 1/ T

39
Q

if activation energy is high

A

rate constant is low

40
Q

if a reactant has a zero order, do you include it in the rate expression

A

no

41
Q

[A] =

A

rate / K

42
Q

calculate missing value in rate table

A

-find K first
-rearrange K = rate / [A] equation to find missing values

43
Q

method to measure change in volume of gas

A
  • Measure 50 cm3 of the 1.0 mol dm–3 hydrochloric acid
    and add to conical flask. * Set up the gas syringe in the stand
  • Weigh 0.20 g of magnesium. * Add the magnesium ribbon to the conical flask, place the
    bung firmly into the top of the flask and start the timer. * Record the volume of hydrogen gas collected every 15
    seconds for 3 minutes.
44
Q

what is the rate determining step in the 2 equations

1) (CH3)3CBR –> (CH3)3C+ + Br-
2) (CH3)3C+ + OH- –> (CH3)3COH

A

step 1 = longer to break strong bond in haloalkane

45
Q

zero order if ….

A

appears in mechanism after the slow step

46
Q

if NO2 apperars twice in rate determing step it is in

A

second order

47
Q

when drawing mechanisms refer to organic chem e.g if it a haloalkane and OH its

A

nucleophilic subsitution

48
Q

rate determing step

A

-rate determining step is the slowest part of the reaction that is key for the production of products

-reactions often occur in multiple steps as it is very unlikely that all the reactants collide at the same time

49
Q

equation for
H2O2 (Aq) + 2H+(aq) + 2I- (Aq) —> I2 (aq) + 2H2O

A

1) H2O2 + I- –> HOI + OH- (Slow = rate detemining step)

2) HOI + I- –> I2 + OH- (fast)

3) 2OH- + 2H3O+ –> 4H2O (l) (fast)

50
Q

A + B + C + D +E

A

in this equation C is the rate determining step as it limits the rate at which products are produced

51
Q

A + 2B –> C + D

A

Rate equation = K = [A][B]

Step 1 = A + B –> P (slow = RDS)

Step 2 = B + P –> C + D (Fast)

52
Q

A catalyst reacts with the reactants as is reproduced in a later step. It is not shown in the overall reaction, only the rate equation e.g K = [A] [H+] –> H+ is the catalyst in the overall equation A + 2B –> C + D

A

Step 1 = A + H+ –> AH (Slow RDS)

Step 2 = AH+ + B –> AB + H+ (fast)

Step 3 = AB + B –> D + E (fast)

53
Q

intermediate

A + B + C –> Y + Z

A

Step 1 = A + B –> D (D is an intermediate as it is used in 2nd reaction to make products and is not shown in the overall equation)

Step 2 = D + C –> Y + Z

54
Q

total order of an equation

A

Total order of K {CH3] [H+} = 2 as 1 + 1 = 2

55
Q

true or false –> any reactant involved after rate determing steps will be zero order

A

true

56
Q

when will a step no longer affect the rate of reaction

A

when the step occurs outside the rate detemining step

57
Q

rate equation for the reaction
slow = A + C –> X
fast = X + B –> D + E

A

K = [A]1 [C]1

58
Q

rate equation for the reaction
fast = A + C –> X
slow = X + B –> D + E

A

K = [A]1 [B]1 [C]1

59
Q

is step 1 or 2 the rate detemiing step

E + F –> EF = step 1
EF + F –> G = step 2

A

step 2

59
Q

step which requires catalyst =

step which creates intemediate =

A

rate determiining step

60
Q

y = mx + c

A

lnK = -Ea/RT + InA

61
Q

gradient (m) =

A

-Ea / R

62
Q

to get A from InA on calculator do

A

shift In to get exponential

63
Q

true or false -> if doubling the concentration of S has no effect on rate it is in the zero order

A

true

so rate equation doesnt include

64
Q

Show how these data can be used to deduce the rate expression for the
reaction between A and B

A

-B = experiment 1 and 2 is constant
-A x3 and rate = 3^2 so 2nd order in experiment 1 and 2
-A in experiment 2 and 3 only increases by 2 not 2^2 so B halves = respect to order 1
-rate = k [A]2 [B]

65
Q

why does doubling the temperature have a much greater effect than doubling conc of E
rate = K [E]

A

-reaction occurs when molecules have energy greater than or equal to activation energy
-doubling temperature increases kinetic energy
-doubling E only doubles number with E

66
Q

explain whether these haloalkanes would react with NaOH by the same mechanism

A

No – the mechanisms are different because with X the reaction is first order with respect to [NaOH] but with Y it
is zero order with respect to [NaOH]

67
Q

iodine clock

A

H2O2 + 2I- + 2H+ –> I2 + 2H2O

reactants = colourless
-products = black

react in the prescence of an acid

68
Q

initial rate -> iodine clock

A

-take gradient at t = O (initial rate)
-repeat after changing conc of a different reactant
-not time for colour change
-determine order

69
Q

large excess =

A

zero order

70
Q

how to calculate rate when [H+] = 0.35

A

draw tangent at 0.35
find gradient

71
Q

explain why the use of a large excess of H2O2 and I- means the rate of reaction at a fixed temperature depends only on the concentration of H+ (aq)

A

-H2O2 and I- concentrations are constant (negligble change)
-H2O2 and I- have no effect on rate as they are zero order

72
Q

state and explain what must be done to each sample before it is treated with an alkali

A

-stop the reaction
use ice

73
Q

explain how the graph shows the order with respect to H+ is zero

A

-contstant gradient
-H+ is decreasing but rate is not changing

74
Q

explain how you could use a series of experiments to determine the order of this reaction with respect to A

A + B + C –> D + E
-reagent X reacts with E

A

-measure known conc and volume of A and B
-add known conc and volume to container
-measure conc and vol of C in separate container
-add c and set timer
-record time for blue-black colour
-repeat with different conc of A but same for B and C
-keep constant temp using water bath
-plot graph of 1/time vs conc of A
-deduce order from graph

75
Q

units for 2nd order reaction

A

mol-1dm3s-1

Aqa a level chemistry (18 decks)

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