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Chemistry > 1.8 rate equations > Flashcards

1.8 rate equations Flashcards

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1
Q

Practicals in this topic

A

7a- iodine clock
7b- measuring the progress of a reaction

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2
Q

what is continuous monitioring?

A

following a reaction all the way through by recording the amount of product or reactant that you have at regular time intervals

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3
Q

examples of continuous monitoring methods

A

gas volume
changes in pH
colour change
loss of mass
clock reactions (eg iodine clock)

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4
Q

how to find rate at a specific point on a curve

A

draw a tangent

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5
Q

what is the rate equation?
units?

A

rate= K [A]^m [B]^n
[ ] means concentration of
m and n are the orders of reactions
k is the rate constant
units are usually moldm-3s-1

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6
Q

what does the rate equation show?

A

how the rate is affected by the concentrations of reactants
if a reactant is in the rate eq, then it must affect the rate

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7
Q

what is 0 order?

A

if conc of reactant changes and rate stays the same, the order of reaction with respect to that reactant is 0

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8
Q

what is 1st order?

A

if rate is proportional to conc of a reactant then it is 1st order w.r.t that reactant
eg [A] doubles, rate doubles
[A] triples, rate triples

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9
Q

what is 2nd order?

A

if rate is proportional to conc of reactant ^2, then it is 2nd order w.r.t that reactant
eg [A] doubles, rate quadruples
[A] triples, rates x9

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10
Q

what is the overall order of reaction?

A

the orders of each reactant added up
(m+n)

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11
Q

what is k?

A

the rate constant- a constant that links conc of reactants to ror
the bigger it is, the faster the rate

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12
Q

when does k changes?
why?

A

increase temp, k rises because increased temp means increased ror
conc of reactants hasnt changed so increased ror must be due to increased k

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13
Q

how do you calculate k?

A

using the rate equation (tells you orders) and experimental data

also need to calculate units (cancel out)

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14
Q

what is initial rate?
how do you find it?

A

rate at start of a reaction
using a conc-time graph made by using continuous monitoring method and plotting results
draw tangent at time=0 and find gradient

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15
Q

how can you use initial rate method to find order of reaction for each reactant?

A
  • carry out reaction and monitor progress (dont have to monitor to completion as only working out initial rate)
  • draw conc-time graph
  • repeat using diff initial conc of one of reactants but keep other concs and conditions same
  • draw another conc-time graph
  • calculate initial rate
  • look at how initial conc effects initial rate and use this to work out order
  • repeat for each reactant
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16
Q

how to work out order when more than one concentration changes

A

example on page 56
and practice

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17
Q

how do you work out reaction orders from graphs?

A

make a rate-conc graph using a conc-time graph:
find grad at various points- plot this on new graph and join points
0 order= horizontal line
1st order= straight line through origin
2nd order= curved line

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18
Q

what is the rate determining step?

A

the step of a mechanism that decides the overall rate

19
Q

how to work out which is the rate determining step

A

write out what the rate eq would be for each step (m and n are the number of molecules)
the one that matches up with the actual rate eq is the rate determining step

20
Q

how can you work out the reaction mechanism?

A

by knowing which reactants are in the rate determining step (see page 63)

21
Q

what is the arrhenius equation? (describe)

A

shows how the rate constant varies with temperature and activation energy

22
Q

what is the arrhenius equation? (equation)

A

K= Ae ^ -Ea/RT

k= rate constant
Ea= activation energy (Jmol-1)
T= temperature
R= gas constant (8.31 JK-1mol-1)
A= arrhenius constant
e= button on calc

23
Q

relationship between k and Ea

A

Ea gets bigger, k gets smaller

24
Q

relationship between k and temp

A

temp rises, k increases

25
Q

what form of arrhenius eq do you use to find K or A?

A

K= Ae ^ -Ea/RT

26
Q

what form of arrhenius eq do you use to find T or Ea?

A

Ln(K) = -Ea/RT + Ln(A)

27
Q

what is an arrhenius plot?

A

A graph of K against 1/T

28
Q

logarithmic form of arrhenius eq in form of y=mx+c

A

ln(k) = -Ea/R 1/T + ln(A)
ln(k)= y axis
-Ea/R= gradient
1/T= x axis
ln(A)= y intercept

29
Q

how to gather data for an arrhenius plot

A

carry out same experiment a few times at diff temps and work out rate at same point for each one

30
Q

what is partial pressure?

A

in a mixture of gases, each individual gas exerts its own pressure

31
Q

what is the total pressure of a gas mixture?

A

sum of all partial pressures (all diff unless present in equal quantities)

32
Q

what is a mole fraction?

A

the proportion of a gas mixture that is made up of a particular gas

33
Q

what are the 2 formula involving mole fractions?

A

mole fraction of a gas in a mixture= no of moles of gas / total moles of gas in mixture

partial pressure of a gas in a mixture = mole fraction of gas x total pressure of the mixture

34
Q

what is Kp?

A

equilibrium constant for a reversible reaction where some or all of the reactnat and products are gases

35
Q

what is the expression for Kp?
(for aA(g) + bB(g) -> dD(g) + eE(g))

A

Kp= (PD)d (PE)e / (PA)a (PB)b

36
Q

what is le chatalier’s principle?

A

if a reversible reaction at equilibrium is subject to change, the equilibrium shifts to counteract the change

37
Q

effect of temp on Kp

A

if shift in eq to counteract temp change causes more product to be formed, Kp increases
vice versa

37
Q

effect of pressure on Kp

A

doesnt affect Kp
partial pressures of reactants and products at new eq position keep Kp constant

38
Q

effect of catalyst on Kp

A

no effect
increases ror equally in both directions

39
Q

why are samples of a reaction mixture added to NaHCO3 before they are titrated?

A

To stop the reaction
(NaHCO3 neutralises the acid)

40
Q

why does doubling temp have a greater effect on rate that doubling conc?

A

Reaction occurs when molecules have E>Ea

Doubling T by 10 °C causes many more molecules to have this E

Whereas doubling [E] only doubles the number with this E

41
Q

what is a reaction order

A

power to which a concentration is raised by in the rate equation

42
Q

why does an excess of a reactant mean that it has 0 order/ can be ignored?

A

their concentration remains essentially constant/ any change is negligible so rate only depends on other reactant

43
Q

how can a graph of vol against time show that a reactant has 0 order?

A

constant gradient
so ror doesnt change as conc changes

Chemistry (18 decks)

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