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LC Chemistry > Rate Of Reaction > Flashcards

Rate Of Reaction Flashcards

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1
Q

Rate of reaction

A

The change in concentration per unit time of any one reactant or product

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2
Q

Factors that affect the rate of a reaction

A

Nature of the reactants
Surface area
Concentration
Temperature
Catalyst

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3
Q

how do we draw the rate of reaction diagram

A

product line starts on 0
reactants start high
x axis = time
y axis = concentration

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4
Q

when is the rate of reaction at its fastest?

A

start

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5
Q

why is the rate of reaction fastest at the start

A

there is the greatest concentration of reactants thus more effective collisions occur

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6
Q

why does the rate of reaction decrease as the reaction progresses

A

conc of reactants is reducing, less effective collisions

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7
Q

why do both the reactants and products level out at the same time on the graph?

A

when all of the reactants are used up, no more product is produced so the conc of both remains constant

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8
Q

effective collision

A

a collision that results in the formation of products.
the rate of a reaction depends on the number of effective collisions per second

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9
Q

what is required for a reaction to occur

A

particles to collide

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10
Q

when will collisions result in the formation of products

A

if a certain minimum energy is exceeded in the collision (activation energy). this is an effective collision1

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11
Q

what happens in an effective collision

A

bonds break (endothermic) and new bonds form (exothermic)

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12
Q

activation energy

A

the minimum energy that colliding particles must have for a reaction to occur

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13
Q

what does the size of the AE depend on

A

the nature of the reactants ie ionic or covalent

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14
Q

what happens to the rate of reaction if you decrease activation energy

A

increases

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15
Q

how can the activation energy be changed

A

catalysts

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16
Q

what does the max Boltzmann distribution curve show

A

the relationship between the number of particles and activation energy

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17
Q

explain how the max boltzmann distribution curve works

A

the peak of the graph is the average kinetic energy of gaseous particles
from the graph, the activation energy is higher than the average kinetic energy therefore, one a small portion of the particles have enough energy for successful collisions

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18
Q

what affect does heat energy have on activation energy

A

does not change

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19
Q

what affect does increasing temp have on rate of reaction

A

increases

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20
Q

why is the rate of reaction increased as the temp increases

A

at higher temperatures, molecules have more kinetic energy and are moving faster this leads to an increased number of collisions
there are more effective collisions at higher temperatures- more molecules have the activation energy so when they collide, they collide more effectively and product is formed

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21
Q

a reaction profile diagram

A

a graph which shows the change in energy of a chemical reaction over time as the reaction progresses

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22
Q

draw the diagram for exothermic reactions

A

y axis= energy
x axis = time

reactants start higher than products
show activation energy (from reactants to peak)
show change in activation energy from reactants to products
label reactants and products

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23
Q

draw the diagram for the endothermic rate of reaction

A

y axis= energy
x axis = time

reactants start lower than products
show activation energy (from reactants to peak)
show change in activation energy from reactants to products
label reactants and products

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24
Q

what is drawn if they ask for the reaction profile diagram of a catalyst

A

smaller peak

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25
Q

why does a catalyst lower activation energy

A

less energy is needed for effective collections thus more collisions are effective and more product is formed

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26
Q

why does the energy of reactants products and delta H the same with or without a catalyst

A

the same reactants are used and the same products are formed ie the nature is the same bonds are the same so energy will be the same
the only difference is the size of the activation energy

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27
Q

how do you monitor the rate of reaction

A

monitoring the removal or decrease in concentration of a reactant ie the change in mass of the reactant with time

monitoring the formation ie the increase in concentration

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28
Q

how can the volume of a gas be determined

A

downward displacement of water or using a gas syringe

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29
Q

average rate

A

calculated by dividing the total volume by the total time taken to go to completion

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30
Q

instantaneous rate

A

the rate of reaction at any one particular time during the reaction. draw a tangent to the curve at that particular time + find slope of that line

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31
Q

which nature of reactants results in a faster rate of reactants

A

ionic

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32
Q

why are ionic reactions faster

A

when 2 ionic substances react, the oppositely charged ions combine to form the precipitate

there are no bonds broken when ionic substances react as when in solution, the crystal lattice has already broken up

when 2 ionic substances react, the product forms immediately and a colour change is seen

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33
Q

what happens when acidified sodium dichromate reacts with ammonium iron 2 sulphate

A

when these 2 ionic substances react, the product forms immediately and a colour change from orange (Cr+6) to green (Cr+3) is seen

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34
Q

what happens when acidified sodium dichromate is added to ethanal

A

ethanoic acid is formed. it takes longer for the solution to change colour from orange to green because ethanal has covalent intramolecular bonds. these covalent bonds in the reactant must be broken first before new bonds can form

35
Q

what effect do powdered substances have on the rate of reaction

A

they have a larger surface area thus more surface is exposed and available for reaction. this means more sucsessful collisions occur between reactants

36
Q

write a balanced equation for the decomposition of hydrogen peroxide

A

MnO2
H2O2 —-> H2O + 1/2 O2

37
Q

what are the risks of using or storing hydrogen peroxide and how can it be managed

A

it is corrosive
dilute it

38
Q

if you have the same mass of a substance but some is powdered and some is solid how does the quantity of product formed differ

A

the same amount of product is formed at the end of both reactions because the same mass of marble is used in each reaction

the only difference is the rate of reaction

39
Q

Give the balanced equation of a the experiment which demonstrates the effect of particle size/ surface area on rate of reaction

A

CaCO3 + 2HCl ——> CO2 + H2O + CaCl2

40
Q

What is a the purpose of the cotton wool in the experiment which demonstrates the effect of particle size/ surface area on rate of reaction

A

Cotton wool allows carbon dioxide to escape but stops acid spray from escaping so that the decrease in mass is from carbon dioxide formation only

41
Q

Sketch the graph showing how surface area affects the reaction rate

A

X axis is time
Y axis is change in mass (g)

Reaction rate is faster with powdered calcium carbonate (steeper slope).
• The same mass of calcium carbonate used for both reactions, so same amount of product is produced - this is why they both finish with the same change in mass.

42
Q

How do you show the affect of surface area on rate of reaction

A

measure the volume of carbon dioxide produced every 30 seconds using powdered calcium carbonate and marble chips by collecting carbon dioxide in a gas syringe or by downwards displacement of water. Record the change in volume over time.

43
Q

What affect does increasing concentration have on rate? Explain

A

Increases- there are more particles in a fixed volume
There are a greater number of collisions in the container
There are more effective collisions and more product is produced

44
Q

What effect does increasing temperature have on rate of reaction

A

As temp increases, the kinetic energy of the colliding particles increases.
They collide more often with each other and the sides of the container.
Therefore there is a greater number of effective collisions per unit time therefore the rate of reaction increases

45
Q

What is a catalyst

A

A substance that alters the rate of a chemical reaction but is not consumed in the reaction

46
Q

Properties of catalysts

A

They are recovered unchanged at the end of an experiment + can be reused

Catalysts are specific

Catalysts only need small amounts to work

They can be poisioned

47
Q

What is an inhibitor

A

A catalyst that slows down a chemical reaction

48
Q

Give an example of an inhibitor

A

Glycerine

49
Q

Give an example of a reaction involving a catalyst

A

MnO2 used in the decomposition of hydrogen peroxide to form oxygen gas

50
Q

What is an enzyme

A

A substance that is produced by a living cell and acts as a biological catalyst

51
Q

How do catalysts effect the rate of reaction

A

It has no effect on the rate of reaction
It Provides an alternative pathway or route for the reaction
Lowers the activation energy needed for the reaction to occur
More reactant particles have enough energy to collide effectively and produce product

52
Q

Homogeneous catalysis

A

Both the reactants and the catalyst are in the same phase ie there is no boundary between the reactants and the catalyst

53
Q

What is an example of homogeneous catalysis

A

Iodine snake

54
Q

Explain the iodine snake experiment

A

Add concentrated potassium iodide (liquid catalyst) to washing up liquid and hydrogen peroxide.
Observe foaming due to O2 gas produced by the decomposition of H2O2. Oxygen gas is trapped in bubbles and forms a foam

55
Q

Heterogeneous catalysis

A

Reactants are in a different phase to the catalyst ie there is a boundary between the catalyst and the reactants

56
Q

Give an example of heterogeneous catalysis

A

Formation of ethene
Catalyst Al2O3 - white solid
Ethanol- liquid/vapour/gas

57
Q

Autocatalysis

A

Catalysis in which one of the products of the reaction acts as a catalyst for the reaction

58
Q

Give an example of autocatalysis

A

Reaction of acidified potassium permanganate and ethanedioc acid

59
Q

Explain the Reaction of acidified potassium permanganate and ethanedioc acid

A

When the first few drops were added, the pink potassium permanganate decolourises slowly
After this is decolourises more rapidly because the Mn+2 formed in the reaction acts as a catalyst

60
Q

What is the immediate formation theory

A

The catalyst works by forming an intermediate compound, which then reacts with a second reactant to form the product

61
Q

What are the problems with the intermediate formation theory

A

The intermediate only forms for a short period of time and may be difficult to detect

62
Q

What is evidence for the intermediate formation theory

A

Sodium tartrate + water + pink cobalt

Colour change is observed from green back to pink. The green substance is the intermediate and the Co+2 ions are not used up in the reaction so the pink catalyst is regenerated

63
Q

Write the mechanism for the intermediate formation theory

A

Step 1. Reactant + catalyst —-> intermediate
Step 2: intermediate + reactant —-> products + catalyst

Overall reaction :
Catalyst
Reactant + reactant —-> products

64
Q

Write the mechanism for the iodine snake reaction

A

H2O2 + I- —-> IO- + H2O

IO- + H2O2 —-> H2O + O2 + I -
I-
Overall : 2H2O2 —> 2H2O + O2

Catalyst= I-

65
Q

adsorption

A

means something accumulates on the surface of another substance

66
Q

what are the 3 steps of the surface adsorption theory

A

adsorption
reaction on surface
desorbtion

67
Q

describe the mechanism for the production of water using a catalyst

A

adsorption: h2 and o2 adsorb onto the surface of the Pt catalyst forming temporary bonds with the surface of the metal- this weakens the covalent bonds in the H2 and O2 molecules.

reaction on surface: there is a greater concentration of H2 and O2 on the surface of the Pt catalyst. they are more likely to collide effectively on the surface and form H2O

desorption: products leave the surface of the catalyst and diffuse away. the surface of the catalyst is now free for more reactant molecules to adsorb and the cycle begins again

68
Q

catalytic converter

A

a device in which the exhaust of a motor vehicle that contains catalysts which converts pollutants in the exhaust gases to less harmful substances

69
Q

how do catalytic converters work?

A

by lowering the activation energy

70
Q

what do exhaust fumes consist of?

A

contain carbon monoxide, nitrogen dioxide, and unburned hydrocarbons which are harmful to human health

71
Q

source of CO

A

incomplete combustion of hydrocarbons due to insufficient oxygen

72
Q

danger of CO

A

highly poisonous- can lead to death

73
Q

removal of CO

A

reacts with O2 to form CO2

74
Q

source of NOx

A

during combustion , heat is released ( combustion is exothermic) this causes N2 in the air to react with O2 to form NO and then NO2

75
Q

danger of NOx

A

poisonous and causes acid rain

76
Q

removal NOx

A

converted to N2 and O2

77
Q

source of smog

A

unburned hydrocarbons leads to smog

78
Q

danger of smog

A

hazardous if inhaled
causes a range of respiratory conditions

79
Q

removal of smog

A

reacts with NOx to form CO2, N2 and H2O on the surface of the catalyst

80
Q

what is the catalyst in the catalytic converter

A

thin layer of metal on a ceramic honeycomb layer structure such as platinum palladium or rhodium

81
Q

what is a catalytic poison

A

lead

82
Q

how does lead poison a catalyst

A

it makes the catalyst inactive. lead binds irreversibly to the catalyst and forms a permanent bond. the lead cant be removed the catalyst is now poisoned

83
Q

explain how a catalytic converter works

A

harmful pollutants in exhaust fumes pass through the catalytic converter
thin layer of metal (Pt, Pd or Rh) is coated on a ceramic honeycomb structure
less harmful gases emerge from the other side and are emitted to the atmosphere. all pollutants are converted into CO2 N2 and H2O which occur naturally in the atmosphere

84
Q

give the balanced equation of how a catalytic converter works

A

2CO + 2NO—->2CO2 + N2

LC Chemistry (22 decks)

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