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A level Chemistry > Chapter 10 - Rates of Reaction > Flashcards

Chapter 10 - Rates of Reaction Flashcards

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1
Q

rate of reaction is?

A

the change in the conc of r or p in a given time

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2
Q

how is rate calculated?

A

Change in conc/ time

- rate in moldm(^-3)s(^-1)

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3
Q

collision theory?

A

in order for a reaction to occur, 2 molecules must collide:

  • with sufficient energy to overcome Ea
  • w the correct orientation
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4
Q

factors affecting the ROR?

A
  • temp
  • pressure
  • conc
  • SA
  • Presence of catalyst
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5
Q

factor: temp?

A

⬆temp =⬆KE of molecules

as KE ⬆, no. of successful collisions ⬆, R.O.R ⬆

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6
Q

factor: pressure?

A
  • ⬆ P means molecules are closer together, more likely to collide and react w energy in excess of Ea, rate of collisions ⬆, ROR V
    INCREASING THE PRESSURE ONLY INCREASES THE RATE OF GASEOUS REACTIONS
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7
Q

what does collisions are more freq mean?

A

more coliisons occur in a given period of time

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8
Q

factor: conc?

A
  • ⬆ conc of a r ⬆ no. of moleculs of that paticular r
  • ⬆ conc means molecules are closer together and there is a ⬆ chance of them colldiing w sufficient energy to overcome Ea
  • ⬆ in the no. of successful collisons in a given period of time
  • ROR ⬆
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9
Q

factor: SA?

A

⬆ sa of the r ⬆s the area over which a reaction can occur

ROR ⬆

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10
Q

factor: presence of a catalyst?

A

adding a catalyst to a rection provides an an alternate pathway w a lower Ea

  • more particles have energy > or equal to the Ea of the catalysed reaction
  • ⬆ successful collisons take place per unit time, so adding a catalyst ⬆ ROR
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11
Q

industrially, the yield of a reaction must be balanced against?

A

the economic feasibility

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12
Q

Haber Process equation?

A

N2(g) + 3H2 (g) (reversible reaction arrow) 2NH3 (g)

ΔH = -92

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13
Q

Haber Process optimum conditions?

A
  • high pressure : L: 4 moles R: 2 moles so equil. moves to tight
  • low temp: forwards reaction is exothermic so ⬇ temp will cause equi to shift to right
    BUT THERE ARE SOME ISSUES WITH USING THESE CONDITIONS IN INDUSTRY
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14
Q

prblms with using high pressure?

A
  • increasing pressure increases the conc of gases which increases ROR
  • But, lots of energy is required to maintain these high pressures - expensive
  • storing gases at such high pressure is unsafe
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15
Q

issues w low temp?

A

Decreases the ROR

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16
Q

what does the use of the Iron catalyst mean for the Haer Process?

A

Equil is established quickly at lower temp

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17
Q

what allows almost a 100% conversion rate in Haber Process?

A

unreacted reactants are recycled

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18
Q

At equil, the concs of r+ p are?

A

constant, but that does not mean they are of equal conc

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19
Q

what does the Equil. constant indicate?

A
  • where the equil lies

- Ratio of reactants and products

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20
Q

what does the magnitude of Kc indicate?

A

the extent of reaction

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21
Q

in order to monitor the ROR we can monitor?

A
  • conc of r/p -e.g. titration
  • gas volume of product e.g. using gas syringe
  • change in the mass of substances formed - e.g. using a balance to change in mass
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22
Q

ROR graph:?

A

X axis: time

y: change in (conc of e.g.)

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23
Q

calc the IROR from a graph?

A
  • IROR shown at t=0

- draw tangent that is straight line to curve

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24
Q

when the graph plateaus of?

A

rate = 0

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25
Q

gradient = ?

A

rate

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26
Q

what does a reaction profile diagram show?

A

the progress of reaction plotted against energy

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27
Q

homogeneous catalyst e.g.?

A
  • enzymes within saliva
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28
Q

heterogeneous catalysts e.g.?

A

catalytic converter in the exhaust pipes of cars

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29
Q

the importance of catalysts in industry?

A
  • by lowering Ea, lower energy demands of the process so:
  • process requires less energy ➡reduces cost
  • lower demands for fossil fuels ➡ lower greenhouse gas emissions ➡ better for envir
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30
Q

Boltzmann distribution curve shows?

A

the distribution of molecular energies in a gas at constant temp

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31
Q

BDC - molecules?

A
  • some mols move fast & have high energy
  • some move slow and have a low energy
  • but majority have an average energy
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32
Q

BDC: area under curve =?

A

the total no. of molecules in sample

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33
Q

why does the BDC never touch the x axis?

A

no molecules have 0 energy

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34
Q

the BDC starts?

A

at origin

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35
Q

effect of temp on BDC?

A
  • curve moves to right, lower peak

- AREA UNDER CURVE REMAINS SAME AS THE NUMBER OF MOLECULES IN THE SYSTEM REMAINS CONSTANT

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36
Q

effect of catalyst on BDC?

A

addition of a catalyst does not CHANGE THE DISTRIBUTION OF MOLECULAR ENERGIES (CURVE DOES NOT MOVE, ONLY Ea LINE DOES)

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37
Q

list the factors altering the ROR?

A
  • Conc
  • temp
  • catalyst
  • SA
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38
Q

describe how conc alters the ROR?

A

Increasing the conc increases the ROR as it inc the amount of particles per dm3 thfr collision freq is the highest - increases the no. of successful coliisons

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39
Q

describe how temp inc ROR?

A

An increase in temp inc the proportion of particles with E> or equal to Ea which means more successful collisions thus a higher rate

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40
Q

describe how a catalyst increases ROR?

A

Catalysts provide a new mechanism for the reaction with lower Ea so that the proportion of particles w E greater than or equal to Ea is inc thus more successful collisions so a higher ROR

41
Q

Describe how SA increases ROR?

A

A larger SA from small particles means more reactant particles can come into contact, thfr more successful collisions so ROR is inc.

42
Q

pressure is abt? but conc is abt?

A

pressure is gas, conc is solution

43
Q

why use a gas syringe when CO2 is produced?

A
  • to measure V of CO2 produced using gas syringe

- CO2 is denser than air, so will get into syringe + don’t need to bubble thru H20 - would dissolve

44
Q

when drawing a gradient triangle, make it as ?

A

large as possible - tangent

45
Q

⭐at higher conc, there is the largest amount of?

A

particles PER DM3

46
Q

Steps to calculating the ROR from gradients of graphs?

A

1) draw tangent using SHARP PENCIL
2) determine change in y and change in x
3) gradient = change in y/ change in x (rise/run!!)
4) use the axis units to determine units for rate
5) make gradient triangle as large as possible

47
Q

what is a catalyst?

A

a substance that inc the ROR w/o getting used up in the reaction. A catalyst provides an alt mechanism w a lower Ea

48
Q

examples of heterogeneous catalysts?

A
  • iron
  • aluminium oxide
  • nickel
  • Rhodium, Platinium, Paladium
49
Q

examples of homogeneous catalysts?

A

acid

chlorine radicals

50
Q

what is activation energy?

A

the minimum energy required for a particle to be involved in a successful collision

51
Q

homogeneous catalysts are?

A

catalysts and reactants are in the same state

52
Q

heterogeneous catalysts are?

A

catalyst and reactants are in different states (heterosexual like diff sex remember )

53
Q

catalytic convertors?

A
  • are heterogeneous catalysts
  • hydrocarbon fuels are burnt in the internal combustion engines of many vehicles
  • in limited O2, CO - a toxic gas can form, high temps can also cause oxides of N to form from N in air: N2(g) + O2 -> NO(g)
54
Q

catalytic convertors allow?

A
  • the NO and CO to react. Harmless N gas and non-toxic CO2 is formed
  • 2NO + CO -> CO2 + N2
    catalyst = platinum
55
Q

catalytic convertors - what happens to the unburnt hydrocarbons?

A

they’re oxidised to water and CO2

56
Q

heterogeneous catalysis - how exactly does it work?

A
  • step 1: ADSORPTION: weak bonds form between r + catalyst, weakens bonds in reactants
  • 2: the reaction - bonds break and form
    3: DESORPTION: weak bonds to catalyst break + products leave surface
57
Q

what does a Boltzmann distribution show?

A

the range of energies of all of the molecules in a gas at a constant temp. We can use em to explain how changes like adding catalysts or changing the temp affect the ROR

58
Q

When drawing a boltzmann distribution for T higher make sure?

A

SAME AREA UNDER BOTH CURVES

59
Q

What does a higher temp Boltzmann distrubution curve look like?

A

further right & lower peak

60
Q

make sure BDC does not touch?

A

x axis at end

start at origin.

61
Q

why does temp inc rate?

A

at a higher temp, the proportion of particles with E> or = to Ea has inc - more sucessful collisions -> higher rate

62
Q

the effect of catalyst on a BDC?

A
  • NO EFFECT ON ACTUAL CURVE

- Ea line shifts to left ( becomes lower)

63
Q

which carbonates are soluble ?

A

only aqueous carbonates are G1, rest are insoluble

64
Q

how does the use of catalysts help chemical companies make their processes more sustainable ?

A
  • reduced Ea so lower temp so less fossil fuels -> lower demand for fossil fuels -> less global warming
  • catalysts aren’t used up so can be used over and over
  • cataylsed reactions can be caused to produce other products/ more products (e.g. catalytic convertors)
  • bio-enzymes - biodegradable
65
Q

what is dynamic equilibrium?

A
  • EQUILibrium is when the rate of forwards and reverse reactions are EQUAL
  • dynamic means the f+b reactions are still taking place at a particle level
  • the amounts of the reactants and products don’t change (but the actual atoms, molecules or ions do) but THE AMOUNTS DO NOT NEED TO BE EQUAL
66
Q

for dynamic equilibrium to occur u must have a closed system. what is meant by this?

A

no chemicals can be taken out or added in in a closed system

67
Q

which factors affect the position of equilibrium (yield of product)?

A
  • pressure
  • temp
  • adding/ removing a chemical (not conc)
68
Q

state ‘Le Chatelier’s principle’

A

if the conditions of an equilibrium are changed, the position of the equilibrium moves to minimise (readjust) the change

69
Q

what effect does a catalyst have on the position of equilibrium?

A

NONE- it speeds up the forwards and reverse reaction equally

70
Q

how could the position of equilibrium be monitored in an equilibrium reaction?

A

colour changes

71
Q

inc the temp shifts the equilibrium in?

A

the endothermic direction

72
Q

inc the pressure shifts the equilibrium to?

A

the side w the fewer moles of gas

73
Q

Dynamic definition?

A

the reaction is taking place at a molecular level

74
Q

equilibrium definition?

A

when the forwards and reverse reactions are taking place at the same rate

75
Q

what does it mean if equilibrium lies to the left?

A

there are a higher conc of r than p

76
Q

what does it mean if equil lies to the right?

A

there are a higher conc of product than reactant at equil, higher yield of product

77
Q

if an acid/ alkali is being added?

A

be careful, bc the acid will react w the alkali or vice versa using it up, so the equil will shift to make more of the acid/alkali

78
Q

y is graphite more thermodynamically (heat) stable than diamond?

A

it has less energy/ enthalpy

79
Q

Equil- sometimes a ? has to be made

A

compromise between yield and ROR

80
Q

Equilibrium constant (Kc) can be calculated w the equation?

A

[products] / [reactants]

81
Q

Kc - [] means?

A

the conc of (in mol dm-3)

82
Q

Kc- [X] means

A

the concs of r and products in moldm-3

83
Q

Kc - []^y means

A

the molar amounts in the equation - (big numbers)

84
Q

to calc Kc what do we need to know?

A

the concs of reactants and products at equilibrium

85
Q

Kc - working out the units?

A

write them out and do the cancelling to see what’s left

86
Q

what can Kc values be used to do?

A

estimate the position of equilibrium from the value/ size of Kc

87
Q

if Kc = 1 then?

A

equal amounts of reactants and products

88
Q

if Kc > 1 then?

A

high yield of product, equil is shifted forwards (to right)

89
Q

if Kc < 1 then?

A

low yield of product, equil is shifted backwards (to left)

90
Q

what is meant by ROR?

A

THE rate of a chemical reaction measures how fast a reactant is being used up or how fast a product is being formed

91
Q

units of rate ?

A

moldm-3s-1

92
Q

the progress of a chemical reaction can be followed by?

A
  • monitoring the removal (⬇ in conc) of a r

- following the formation ( ⬆ in conc) of a p

93
Q

apart from conc, what else may change as the reaction proceeds?

A

gas V, mass of r or p and colour

94
Q

if the reaction produces a gas, 2 methods that can be used to determine the ROR?

A
  • Monitoring the V of a gas produced at regular time intervals using gas collection
  • monitoring the loss of mass of r using a balance
95
Q

IROR is when?

A

t=0

96
Q

how does a homogeneous catalyst work?

A
  • the catalyst reacts w the r to form an intermediate

- the intermediate then breaks down to give the product and regenerates the catalyst

97
Q

features of the boltzmann distribution?

A
  • no molecules have 0 energy- the curve starts at the origin
  • the area under the curve is equal to the total no. of mols
  • there is no max E for a mol- the curve does not meet the x axis at high energy. The curve would need to reach infinite energy to meet the x axis
98
Q

why do the concs of reactants and products not change in an equilibrium?

A
  • as fast as the r are becoming ps, the ps are reacting to become rs
  • thr, the concs remain unchanged even tho the f+b reactions are still taking place

A level Chemistry (39 decks)

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