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Module 5 Chemistry > Chapter 18 - Rates of reaction > Flashcards

Chapter 18 - Rates of reaction Flashcards

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1
Q

what is the rate of a reaction

A

rate of reaction is the change in concentration of products or reactants per unit time

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2
Q

what is the generalised equation for rate of reaction

A

rate of reaction = Change in Conc./time

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3
Q

what are the units for rate of reaction

what is the shorthand for concentration

A

moldm^-3s^-1

[A]

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4
Q

what are the 4 factors on the rate of a reaction

what is the important proportionality regarding one of these

A
  • temperature
  • surface area
  • catalyst
  • concentration

Rate is directly proportional to [A] ^ n
where n is the order of the reaction

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5
Q

what is meant by a zero order reaction

A

rate direct prop. [A]^0

  • concentration has no effect on rate
  • n = 0
  • concentration doubles = rate stays the same
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6
Q

what is meant by a first order reaction

A

rate direct prop. [A]^1

  • concentration is directly proportional to rate
  • n = a
  • concentration doubles, rate doubles
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7
Q

what is a second order reaction

A

rate direct prop. [A]^2

  • concentration^2 is directly proportional to rate
  • n = 2
  • concentration doubles, rate quadruples
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8
Q

what is the rate equation for a reaction and how can you calculate the overall order

A

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

overall order = n+m

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9
Q

how can we calculate the units for the rate constant

A

rearrange for k
write out units

moldm^-3s^-1/(moldm^-3)^n+m

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10
Q

how can you find the order of reaction from experimental data

A

1) observe how the rate changes when the conc. of one reactant increases (whilst keeping all other conc.’s constant)
2) repeat for other reactants

quote data in answers

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11
Q

what is a continuous monitoring method and what does it measure

A
  • gives information on how the rate of reaction changes throughout the reaction
  • measures a property which is constantly changing
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12
Q

what is an initial rates method and what do we need to be careful of

A
  • gives information on how the initial rate varies as the starting concentration of a reactant changes
  • it can be inaccurate because the ROR at the start is not always the same as all the way through
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13
Q

what are the 4 main ways to measure rate of reaction and how can we do them, what types of measuring are they

A
  • measure volume or mass of gas produced, use an upturned measuring cylinder or measure loss of mass over time using a mass balance, continuous method
  • track the formation of a precipitate, time how long it takes for a precipitate to obscure a mark - initial
  • measure changes in pH using a pH metre - continuous
  • use a colorimeter - measures the absorbance of a particular wavelength of light by a solution, continuous
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14
Q

what does the gradient of a concentration-time graph represent

A

rate of reaction

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15
Q

what does a concentration-time graph look like for a zero order reaction/its features

A
  • straight negative gradient line
  • ROR never changes with concentration so grad doesn’t change
  • gradient = k
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16
Q

what does a concentration-time graph look like for a first order reaction/its features

A
  • downwards curve with flattening curve over time
  • ROR slows with time so gradient becomes less -ve
  • constant half life, time for conc. of reactant to half is constant
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17
Q

what does a concentration-time graph look like for a second order reaction/its features

A
  • same as shape for first order but steeper and tails off slower
18
Q

what is a half life

A

“the half life, t1/2, is the time taken for the concentration of a reactant to halve”

19
Q

how can we find half-life from a first order reaction

A
  • first order reactions have a constant half-life
  • measure successive periods where the concentration of the reactant has halved
  • if they are the same then the time period value is the half life and the reaction must be first order
20
Q

what are the two ways of determining k from a first order reaction

A
  • drawing a tangent

- using half life

21
Q

how can we use the tangent to calculate k from a first order concentration time graph

A
  • draw a tangent at a point
  • measure its gradient, this is rate
  • determine the value for conc. at the point the tangent touches the line
  • substitute to rate = k[A]^n
22
Q

how can we use the half life of a first order reaction to find k

A

k = ln(2)/(t1/2)

23
Q

what do the lines for 0th, 1st and 2nd order reaction look like on a concentration-rate graph

A

0th = flat line, rate does not change with concentration

1st = straight line up, rate does not change with concentration

2nd = quadratic style curve

24
Q

what are the features of 0th order line on a concentration-rate graph

A 
Grad = 0
rate = k
Y-intercept = k
25
Q

what are the features of 1st order line on a concentration-rate graph

A 
rate = k[A]
y = mx
k = gradient
26
Q

what are the features of 2nd order line on a concentration-rate graph

A

rate = k[A]^2
so if you plot rate-concentration^2
gradient = k

27
Q

what is the alternative way of drawing rate-concentration graphs to avoid any of the issues

A

log-log graphs
rate = k[A]^n
log(rate) = nlog[A] + log(k)
y = mx+c

28
Q

what is the initial rates method and what is a clock reaction

A
  • the initial rate is the instantaneous rate when t = 0
  • this can be found through the gradient of the tangent on a concentration-time graph when t = 0
  • clock reactions measure the time from the start of the experiment to the point where a visual change is observed
  • we assume there’s no significant change in rate and model
    Rate direct prop 1/T
  • this is repeated for different concs
29
Q

what is the iodine clock reaction

A
  • clock reactions use iodine’s reaction with starch to provide a clear colour change
  • make a reaction that produces iodine
  • add sodium thiosulphate and starch
  • as the iodine is formed it immediately reacts with the thiosulphate
  • once all the thiosulphate is used up, the starch/iodine solution goes blue-black and this is the end point
30
Q

how are reactions with more than 2 reactants likely to occur

A

with more than one step/stage of reaction

31
Q

define the rate determining step

A

“The rate determining step (the one which decides the overall rate) is the slowest step”

32
Q

what are the 3 main consequences of the rate determining step

A
  • increasing the concentration of any reactant that is not in the rate determining step has no effect - they have an order of 0
  • only the reactants in the rate determining step are in the rate equation
  • the molar quantities of the rate determining step correspond to their orders in the rate equation
33
Q

how can we form a mechanism from the rate equation

A

1) identify reactants of the rate determining step using the rate equation
2) identify stoichiometry
3) predict products
4) predict other step

  • combining the equations must reform the initial equation
  • equations must balance
  • species and molar ratios must agree with the rate equation
34
Q

what occurs in many cases where there is a 10 degree rise in temperature for a reaction

A

rate doubles

35
Q

what are the factors that affect the rate constant and how do they affect it

A
  • temperature, higher temperatures mean more particles have greater than (or equal to) the activation energy, particles are also more likely to collide
  • Ea, a lower Ea gives more particles able to react

the dominant factor is Ea and proportion of particles greater than Ea

36
Q

what is the arrhenius equation and units

A

k = Ae^(-Ea/RT)
activation energy = J
temperature = K

37
Q

what do the pre-exponential and exponential factor determine in the arrhenius equation

A

Pre-exponential factor = takes into account orientation

exponential factor = proportion of molecules greater than Ea

38
Q

how can we also write the arrhenius equation which is a more useful way

A 
K = Ae^(-Ea/RT)
ln(k) = (-Ea/R)(1/T) + ln(A)
y = mx+c
39
Q

why might a reactant have an order of effectively 0 even if it is in the equation

A

it is in excess

40
Q

what should we remember when rearranging a rate equation to calculate concentrations

A
  • watch out for squares/cubes in the powers

- easiest way to realise is to rearrange fully and not take short cuts

Module 5 Chemistry (6 decks)

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