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IB chemistry > topic 6/16- kinetics > Flashcards

topic 6/16- kinetics Flashcards

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1
Q

give two equations for the rate of a reaction

A

ror= amount of reactant used/time
ror= amount of product formed/time

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2
Q

how could you measure the rate of reaction at a particular point on a rate of reaction graph?

A

draw a tangent and calculate the gradient

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3
Q

state an example of an experimental method for following ROR and give its equation

A

Mg (s) + 2HCl -> MgCl2 (aq) + H2 (g)

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4
Q

define rate of reaction

A

the change in concentration of reactants or products per unit time (=chang in conc/time)

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5
Q

state the 2 most common units for ROR

A

mol/dm3/s or mol/dm3/min

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6
Q

collision theory states that for a reaction between two particles to occur:

A
  1. the particles must collide with each other with sufficient energy (Ea)
  2. the particles must collide in the correct orientation
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7
Q

define activation energy

A

the minimum energy needed for a reaction to occur on colliding of particles

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8
Q

state 5 factors affecting the rate of reaction

A
  1. concentration
  2. pressure
  3. surface area
  4. temperature
  5. enzymes
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9
Q

explain how an increase in concentration/pressure affects the rate of reaction

A
  • no of particles per given volume increases
  • increased number of frequent (successful collisions)
  • rate increases
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10
Q

explain how an increase in surface area affects the rate of reaction

A
  • exposes more reactant particles
  • increased number of collisions and successful collisions per second
  • increased rate of reaction
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11
Q

the average kinetic energy of particles is proportional to…

A

their temperature in kelvin

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12
Q

explain the effect of an increase in temperature on the rate of reaction

A
  • increased energy, so KE of particles increases
  • a greater proportion of the particles have the required activation energy
  • more collisions per second
  • increased number of collisions result in a reaction
  • ROR increases
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13
Q

draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases.

A

elsewhere

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14
Q

draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases, which shows how increasing the temperature affects the ROR

A

elsewhere

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15
Q

define a catalyst

A

a substance that increases the rate of a chemical reaction but is chemically unchanged at the end of the reaction

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16
Q

describe the effect of a catalyst on the ROR

A

a catalyst speeds up the rate of reaction by providing an alternative pathway with a lower activation energy
(= increase frequency of successful collisions)

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17
Q

draw the Maxwell-Boltzmann distribution plot showing the distribution of energies among the molecules in gases, which shows how a catalyst lowers the activation energy

A

elsewhere

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18
Q

draw an energy profile diagram showing the same reaction with and without a catalyst

A

elsewhere

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19
Q

give the two major economic advantages of the use of catalysts in industry

A
  • they increase the rate of a chemical reaction meaning that more of the desired product can be made in a given time period
  • reactions can take place at lower temperatures resulting in a decrease in the energy costs to the manufacturer
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20
Q

define a heterogenous catalyst

A

one that is in a different phrase to that of the reactants

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21
Q

investigating the iodine-propanone reaction
- 3 steps
- how can we stop the reaction in the sample?
- what do we titrate the iodine against and how?

A
  • the course of the reaction is followed by taking samples from the iodine-propanone mixture at regular intervals, stopping the reaction and then determining the amount of iodine present by titration.
  • by adding an excess of sodium hydrogen carbonate to the mixture, which removes the acid (sulphuric acid) catalysing the reaction
  • titrate against sodium thiosulphate- wait for the iodine to go from orange to straw coloured/colourless and then add starch, which is blue/black in the presence of iodine
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22
Q

give 3 ways of monitoring concentration changes in a reaction

A
  • mass (loss of mass as gas released)
  • volume (of gas produced)
  • colour ( production of solid and cross)
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23
Q

give the equation and units for rate of reaction

A

change in concentration (mol/dm3)/time (s)

Units: mol/dm3/s

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24
Q

for a reaction A+B-> products, give two scenarios regarding the rate

A

scenario 1: the rate is proportional to the concentration of A (if we double the concentration of A, the rate also doubles)

scenario 2: the rate is proportional to the square of the concentration of A (if we double the concentration of A, the rate quadruples)

25
Q

state the rate equation and describe each of the terms

A

Rate = k[A]^a[B]^b

  • k is the rate constant; it is constant for each reaction as long as neither temperature is changed nor catalyst is added
  • [A] and [B] are the concentrations in mol/dm3 of reactants A and B respectively
  • a and b are the orders of reaction with respect to reactant A and B
26
Q

zero order with respect to reactant A

A
  • reaction rate not affected by [A]
  • if A were doubled, the reaction rate would not change
  • rate = k
  • zero order reactants do not appear in the equation
27
Q

first order with respect to reactant A

A
  • reaction rate is proportional to [A]
  • if A were multiplied by 2, reaction rate would double, if multiplied by 4, reaction rate would quadruple
  • rate = k[A]
28
Q

second order with respect to reactant A

A
  • reaction rate is proportional to [A]^2
  • if [A] were multiplied by 2, reaction rate would quadruple, if multiplied by 4, it would x8
  • rate = k[A]^2
29
Q

how do we work out the units of the rate constant, k, and why?

A

the units of k will depend on the form of the rate equation, so need to be derived separately from each reaction equation

30
Q

effect of an increased concentration of a reactant on
- rate of reaction
- value of rate constant

A
  • increases unless the order is 0
  • no change
31
Q

effect of an increased temperature of a reactant on
- rate of reaction
- value of rate constant

A
  • increases
  • increases
32
Q

effect of the addition of a catalyst on
- rate of reaction
- value of rate constant

A
  • increases
  • increases
33
Q

give two strategies that can be employed to find reaction orders from experiments

A
  • continuous monitoring
  • using initial rates
34
Q

define continuous monitoring

A

the quantity (ie concentration) of a reactant is measured at intervals (eg every 30 seconds) during the course of a reaction

35
Q

describe 3 ways in which continuous monitoring can be used

A
  1. concentration-time graphs
  2. finding half-lives
  3. rate-concentration graphs
36
Q
  1. concentration-time graphs
A

concentration on y axis, time on x axis, where the shape indicates the order of reactant with respect to that reactant

37
Q

draw the graph shapes for zero order, first order, and second order concentration-time

A
38
Q
  1. finding half-lives
A

the half-life of a reactant is the time taken for its concentration to reduce by half
- first order- half life is constant
- second order- half life not constant

39
Q
  1. rate-concentration graphs
A
  • to determine how the rate changes with concentration it is necessary to take tangents of the concentration-time graph
  • these tangents give the rate of the reaction at that particular time
  • concentration on x axis, rate on y axis
40
Q

draw the graph for zero, first, and second order rate-concentration graphs

A
41
Q

in a ‘clock method’, rate =

A

1/t

42
Q

strategy B; using initial rates

A
  • several experiments undertaken, changing concentration of only one reactant at a time and keeping all others the same
43
Q

how is initial rate determined?

A
  • finding the gradient of a reactant concentration against time graph at time=0
  • timing how long it takes for a certain amount of product to be made and calculating 1/time as initial rate
44
Q

why is the reaction between iodine and propane in acid solution known as an auto-catalysis reaction?

A

because the catalyst (H+) is made as a product

45
Q

describe the reaction order of an SN1 reaction mechanism for halogenoalkanes

A

overall order=1 check booklet for more info

46
Q

describe the reaction order of an SN2 reaction mechanism for halogenoalkanes

A

overall order=2 check booklet for more info

47
Q

the effect of temperature on the rate is determined by the

A

rate constant

48
Q

altering the temperature or adding a catalyst will change the

A

rate constantt

49
Q

Arrhenius equation

A

k=Aexp(-Ea/RT)

A- Arrhenius constant/pre-exponential factor/frequency factor/steric factor - incorporates effect of collision frequency and orientation
Ea- minimum energy for reaction to occur (J/mol)
R- gas constant (8.31 J/K/mol)
absolute temperature (K)

50
Q

taking the natural logarithm of the Arrhenius equation converts it to a more useful form

A

ln(k) = ln(A) - (Ea/RT)
- this equation is in the form y=mx + c
- ln(k) is y axis
- ln(A) is c
- (-Ea/R) is gradient, m
- 1/T is x axis

51
Q

describe how you would use the Arrhenius equation to find activation energy

A
  1. measure the rate of reaction at a range of different temperatures
  2. calculate the nature logarithm of each rate to give ln(rate)
  3. convert each ‘C temp to Kelvin by adding 273 and find 1/T (units K^-1)
  4. plot a graph of ln(rate) on the y axis against 1/T/K^-1 and draw a line of best fit
  5. find the gradient by using m=y2-y1/x2-x1
  6. activation energy (j/mol) = -R x Gradient
  7. convert answer to units kJ/mol
52
Q

remember

A

1/T values will often be of the order !0^-3 so don’t forget to write x 10^-3 on your temperature scale

53
Q

reactions may occur by more than one step; what determines the rate of reaction?

A

the slowest step (rate determining step)

54
Q

look at how to form the rate equation from reactions with more than one step

A
55
Q

the molecularity of an elementary step is

A

the number of reactant particles taking part in that step

56
Q

the order of a reaction can either be —– or —— in nature, and can describe

A

integer, fractional; the number of a specific reactant’s particles taking part in the rate-determining step.

57
Q

rate equations can only be determined

A

experimentally

58
Q

how do catalysts alter a reaction mechanism?

A

they introduce a step with lower activation energy

59
Q

which step of the reaction will have a higher Ea

A

the rate-determining step

IB chemistry (29 decks)

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