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Chemistry > Rates Of Reactions > Flashcards

Rates Of Reactions Flashcards

1
Q

Define rates of reaction

A

Rate of reaction is the change in the concentration per unit of time of any one reactant or product

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2
Q

How are rates of reaction measured?

A

the formation of a product per unit time can be measured
the loss of a reactant per unit time can be measured

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3
Q

How to calculate average rate of reaction?

A

Average rate of reaction = Amount of substance formed or lost/ time taken for that to happen

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4
Q

What is ment by instantaneous rate of reaction

A

the rate of a reaction at any one particular time during the reaction

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5
Q

how to find instantaneous rate of reaction from a graph

A

draw a tangent to the curve at time asked
choose two good points.
find slope using formula

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6
Q

What dertimes weather a reaction does or does not take place

A

Reactants will collide with each other when mixed.
These reactats have a certain amount of energy.
if the colliding reactants meet the required energy(acivation energy) it will be an effective collision

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7
Q

What is activation energy

A

The minium combined energy of colliding particles for effective collisions/ for an reaction to take place

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8
Q

What is mean by an effective collision

A

A collision that reached activation energy and results in the formation of products

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9
Q

Draw out an large activation energy reaction profile diagram

A

reactants
large activation energy = slow rate of reaction
activation energy (Eact)
products
delta H

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10
Q

Describe the energy of products vs reactants in an exothermic reaction

A

energy of products is less than energy of reactants
ΔH = Energy of products – Energy of reactants
(ΔH will be a negative value)

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11
Q

Describe the energy of products vs energy of reactants in an endothermic reaction

A

energy of products is greater than energy of reactants
ΔH = Energy of products – Energy of reactants
(ΔH will be a positive value

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12
Q

Name the 5 factsots affecting the rate of a reaction

A

1-Nature of reactants
2-Particle size
3-Concentration
4-Temperture
5-Catalyst

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13
Q

Describe the nature of the reactants (Type of bonding) that occours involving ionic compounds in solution

A

Reactions involving ionic compounds in solution are faster.
-In solution, ionic compounds are dissociated, preset as free ions.
No bond breaking is required, the activation energy is lower.

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14
Q

Describe the nature of the reactants (Type of bonding) that occours involving covalent compounds in solution

A

Reactions involving covalent compounds have slower reactions
-Covalent compounds are not dissociated.Their bonds need to be broken and new ones reformed, The activation energy is higher

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15
Q

How does particle size affect the rate of a reaction, and explain why

A

Smaller particle size increases rate of a reaction
Larger size decreases the rate of a reaction

Reason
Smaller particles have a greater surfance area, meaning greater frequency of collisions occour, means that there will be more effective collisions- more collisions reach activation energy

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16
Q

Dust explosions present a risk in industry, Give five conditions necessary for a dust explosion to occour

A

particles must be combustible
dust particles must be dry
a source of ignition
Oxygen must be present
must be in an enclosed space

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17
Q

How does concentration of reactants affect the rate of a reaction, explain the reason

A

Increasing concentration increases rate of reaction
Decreasing concentration decreases rate of a reaction

-More concentrated solution has a greater number of particles.
Greater number of particles means greater frequency of collisions will occur, meaning more effective collisions will occour. More collision reach the required activated energy

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18
Q

What is the relationship between Concentration and the rate of reaction

A

directly proportional

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19
Q

Why does the rate of a chemical reaction generally decrease with time

A

as reaction proceeds, more reactants are used up, concentration decreases. Less frequecy of collisions, less effective collisions, less collisions reach activation energy

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20
Q

In the expeirment showing the effect of particle size of calcium carbonate in HCl, describe the difference between whole marble chips vs ground marble

A

smaller particle size - had a greater rate of reaction, steeper slope. The smaller particle size levels off first( reaction finishes off first)

Both the reactions rise to the same height eventially- both produce the same amount of product.

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21
Q

In the expeirment showing the effect of concentration on rate of reaction between calcium carbonate and HCl, describe the difference between 2 M HCl and 1M HCl

A

The Higher conc of HCl - higher conc reaction has a greater rate of reaction. The higher concentration reaction levels off first (finishes first) Reactions with double concentration of reactant rise to twice the height - produce twice as much product

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22
Q

How does temperture affect the rate of a reaction and explain the reason.

A

Increasing temerture increases the rate of reaction.
Decreasing temperture decreases the rate of an reaction

1.Increasing temp increases the energy of the particles (More effective collisions - more collisions reach required activation energy)
2. Increasing temp also increases velocity of collisions (greater frequency of collisions will occour - more reach reuired activation energy)

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23
Q

Given the two reasons why temperture increases the rate of reaction, which is more significant? Explain

A

Increasing the energy of the particles.
The increase in velocity is small compared to the substantial increase in energy in the particles ( which cause more effective collisions)

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24
Q

What effect does increasing/decreasing temperature have on the activation energy of a reaction?

A

None

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25
Q

Describe the procedure to study the effect of concentration on reaction rate using sodium thiosulfate and hydrocloric acid

A

using a graduated cylinder, add a fixed volume of sodium thiosuldate to concial flask. The conical flask has a ** ‘X’ drawn on the bottom** with a marker. Then add dilute HCl quickly to the sodium thiosulfate. The conical flask is swirled and stopwatch is immediately started.
While looking down through the solution, time the time is takes for the yellow sulfur precipitate to obscure the cross. Expeirment repeated w/ different conc of sodium thiodulfate. Graph of rate vs conc is plotted

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26
Q

How is rate caluclated

A

rate of reaction is the reciprocal/ inverse of time
rate = 1/time

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27
Q

plot a graph for the reaction between sodium thiosulfate and HCl

A

best fit line through the origin

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28
Q

explain the relationship between rate of reaction and concentration of solution

A

rate of reaction is directly porportional to concentration of solution.
A more concentrated solution has a greater number of particles.
A greater number of particles mean greater frequency of collisions will occour.
so more effective collisions

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29
Q

Why is it essential the same volume and concentration of HCl are used during each reachtion

To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid

A

only one variable can be changed in order for the expeirment to be a faur test. All other vairables must be fixed

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30
Q

Identify the precipate in this reaction? Descibe its apperance

To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid

A

Sulfur - precipates as a fine pale yellow powder

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31
Q

describe how the reaction time in measued in this expeirment? describe how you could determine when the same mass of sulfur has been formed

To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid

A

The conical flask has a X drawn on bottom.
The sodium thiosulfate and HCl are mixed and looking down through the solution, using a stopwatch, the time for how long it takes the yellow sulfur precipate to obscure the cross is recorded

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32
Q

explain the signifinance of using the same shaped conica flask during each run

To study the effect of concentration on reaction rate using sodium
thiosulfate and hydrochloric acid

A

Only one vairable can be change to ensure a fair test.
other vairables must remain fixed

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33
Q

Describe this procedure

To study the effect of temperature on reaction rate using sodium
thiosulfate and hydrochloric acid

A

Using a graduated cylinder, add a fixed volume of sodium thiosulfate to a conical flask.
The conical flask has a cross drawn on the bottom
the conical flaks is placed in a water bath on a hot plate and use thermoeter set it to a specific temperture
Add a fixed volume of dultue hydrovloric acid.
The conical flask is swirled and stopwatch is immediately stated While looking down through the solution, time the time it takes for the yellow sulfur precipitate to obscure the cross is recorded.
using a hot plate, water bath and theromometer, the expeirment is repeated at different tempertures
results are entered in a table and a graph of rate vs temperture is plotted

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34
Q

Explain the relationship between rate of reaction and temperture

To study the effect of temperature on reaction rate using sodium
thiosulfate and hydrochloric acid

A

as temperture increases, the rate of reaction increases exponentially.
1. Increasing temp increases energy of particles.more effective collisions- more collision reach the required activation energy
2. increases velocity-greater frequency of collisions will occour-more effective collisions

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35
Q

Why is a water bath preferable to bunsen burner/directly heating with hot plate

To study the effect of temperature on reaction rate using sodium
thiosulfate and hydrochloric acid

A

it is easier to obtain a desired temperture using a water bath- allows for more gentle heating

36
Q

What is a catalyst

A

a substance that alters the rate of a chemical reaction without being used up in the reaction

37
Q

explain why the presence of catalyst can speed up the rate of reaction

A

a catalyst lowers the activation energy required for an effective collision providing an alternative pathway for a reaction to occour.
lower activation energy means more effective collisions- more collisions reach the required activation energy.

38
Q

compared to an energy profile of an exothermic reaction,describe the changes that occour with a catalyst

A

the activation energy is lowered, same amount of peroducts, The ΔH value does not change. -ΔH

39
Q

List the properties of catalysts

A

1.Chemically unchanged at the end of an reaction
2.Only required in small amounts
3.specific to a certain type of chemical reactions.
4.can be inactivated by catalytic poisons
5.Brings a reversible reaction to eqilibrium more quickly but does not affect the state of equilibrium

40
Q

What is an enzyme

A

biological catalyst that is protein in nature

41
Q

Name two examples of enzymes

A

Catalayse
Amylase

42
Q

How could you reduce the rate of a chemical reaction that takes place in solution

A

lowering the temperture- cooling mixture
lowering conc of reactants- dilution the reaction micture
adding an inhibitor

43
Q

Name the three types of catalysis that can occour

A

homogenous catalysis
heterogenous cataysis
Autocatalysis

44
Q

What is homogenous catalysis and name an example

A

in which the catalyst is in the same phase as the reactants and products.
eg esterification

45
Q

what is hetrogenous catalysis and name an example

A

in which the catalysis is in a different phase to the reactants and products
eg. haber process

46
Q

What is auto catalysis and name an example

A

in which one of the products of the reaction acts as a catayst for the reaction.
eg. titrating potassium manganate against iron II solution

47
Q

Breifly describe the procedure for

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

1.Heat sprial of platium wire in a numsem burner and heat the methanol to 60C in a fumehood.
2.Place a cardboard t-shape covered in aluminium foil in a conical flask. With a sprial of platinum wire above methanol

48
Q

Describe the observations during the reaction

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A
  1. The sprial of platinum goes through cycles of glowing red hot and then dimming
  2. Popping sounds are heard
  3. the smell of methanal (formaldehyde)
49
Q

What is the purpose of the cardboard T piece and why is it wrapped in luminium foil

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

the cardboard T piece acts as a chimney allowing waste gasses formed to be swept away
its wrapped in aluminium foil to prevent the cardboard from being burned

50
Q

explain why the sprial of platinum glows red hot

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

the reaction is exothermic

51
Q

Explain why the sprial of platinum glows then ceass glowing for a while before becoming red hot again

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

Carbon Monoxide is a waste gas formed from the incomplete combustion of methanol- it poisons the platinum catalyst
when the carbon monoxide gas is swept away, the catalyst can begin working again

52
Q

Explain the popping or mini explosions sounds that can be heard

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

the formation of hydrogen gas (hydrogen ignited with a pop)

53
Q

Give three major products of this reactions

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

Methanal (formaldehyde)
Water
Hydrogen gas

54
Q

What type of catalysis is involved in this reaction?Explain

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

Heterogenous catalysis- the solid platinum catalyst is in a different phase to the reactants and products

55
Q

Explain one way in whcih the presence of the platinum catalyst speeds up the oxidation of the hot methanol

Observing the effect of a catalyst – oxidation of methanol using platinum wire as catalyst

A

the catalyst lowers the activation energy required for an effective collision providing an alternative pathway for a reaction to occour.
Lower activation energy means more effective collisions- more collision reach the required activation energy

56
Q

Name the two mechanisms of catalysis

A
  1. Internediate formation theory
  2. Surface adsorption theory
57
Q

Describe the mechanism for intermediate formation theory

A
  1. One of the reactants reacts with the catalysts to form an intermediate compound
  2. the intermediate compound is not stable and tempoary- it reacts with the other reactant to form the final product with the catalysts regenerated
58
Q

What does intermediate formation apply to

A

homogenous catalysis (reactions in solution)

59
Q

brefiely explain the procedure to demostrate intermediate theory - oxidation of potassium sodium tartate by hydrogen peroxide catalysed by cobalt II ions

A

set up the conical flask with cobalt chloride crystas to potassium tartrate soltuion in it. plate on hot plate. note pink colour. Add some hydrogen peroxide to the beaker.

60
Q

State obervations

Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions

A
  1. Pink colour turns green
  2. Effervesence/fizzing is observed
  3. the fizzing ceases and the green colour returns to pink
61
Q

EXPLAIN how these obervations provide evidence for intermediate formation theory

Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions

A

initial pink caused by Co2+ ions
pink to green to pink again suggests an intermediate substance is formed which is tempoary.
The effervesence of the green solution suggests that the internediate substance is reacting and forming product

62
Q

What product is cause this effervesence

Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions

A

CO2

63
Q

What property of catalysts is shown by the solution returning to a pink colour?

Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions

A

Catalysis are chemically unchanged by the end of the reaction-Co2+ iosn still present

64
Q

What type of catalysis is involved in this reaction?Explain

Evidence for the intermediate formation theory - oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions

A

Homogenous catalysis- the catalyst is in solution ie. the same phase as the reactants and the products

65
Q

What is adosrption

A

the attachment of liquid or gaseous molecules to a solid surface

66
Q

Describe the mechanism of surface adsorption theory

A
  1. Adsorbtion-the reactants attach onto the surface of the catalysts, forming temporart bond
  2. Reaction- the reactants are now in high conc on the surface of the catalysts and are more likely to collide
    the Bonds within the reactant molecules are weakened and the collision of reactants are more likely to be effective
  3. Desorption- the product is formed and leave the surface of the catalysts
67
Q

What type of catalysis is the mechanism surface adsorption

A

Heterogenous catalysis

68
Q

Explain why the reaction rate is greater when powered platinum is used instead of larger pieces of platinum

Surface adsorption theory

A

A powdered platinum has a larger surface area exposed - greater frequency of collisions will occour- more effective collisions

69
Q

Explain how a catalyst poison makes a catalyst inactive

A

a catalyst poison is preferentially adsorbed onto to catalyst surface and forms a permanent bond with it.
It is unable to desorb and blocks the reactants from adsorbing on the surface preventing their reaction

70
Q

What is a catalytic converter

A

A catalytic converter is a devide in the exhause of a motor vehicle that uses catalysts to convert harmful gases in the exhausts into less harmful gases

71
Q

Name the three elements used as catalysts in catalytic converters

A

Platinum (Pt)
Palladium (Pd)
Rhodium (Rh)

72
Q

Comment on the shape of a catalytic converter

A

honeycomb shape for larger surface area

73
Q

Give the name of three harmful substances that enter a car’s catalytic converter and name the substances that they are converted to by the catalysts mentioned

A

Carbon monoxide - Carbon Dioxide
NOx compounds (eg Nitrogen monoxide)- Nitrogen gas
Hydrocarbons (CxHy) - Carbon Dioxide (Co2) + water (H2O)

74
Q

Give three environental benefits of a catalytic converter in cars

A
  1. carbon monoxide, nitrogen monoxide and nitrogen dioxide - poisonous
    2.Nitrogen Monoxide contributed to acid rain
    3.Hydrocarbons - cause smog and contribute to greenhouse effect
75
Q

Name two substances that poisions the catalysis in a catalytic converter

A

Lead compounds
Sulfur compounds

76
Q

What type of catalysis is involved in a catalytic converter? Explain

A

Heterogenous catalysis- the solid catalysis is in a different phase to the reactants and products

77
Q

Breifly explain the procedure of monitoring the rate of production of oxygen from hydrogen peroxide using manganese dioxide as a catalyst

A

Magnesium Oxide added to Hydrogen peroxide from a small test tube in conical flask.
mixture is shaken and simultaneouly, a stopwatch is started. The Oxygen gas produced is collected over water using a beehive shelf.
vol of O2 measured at regular intervals

78
Q

Explain the general trend in the curved obtained when a graph of volume of oxygen vs time is plotted

A

Beginning- Slope steepest – reaction occurring at its fastest rate, most volume of gas produced per time
Middle- Slope becomes less steep - reaction slows down, less volume of gas produced per time
End- Slope is 0 - reaction is finished, no more gas produced

79
Q

Write a balanced equation for the catalysed decomposition of hydrogen peroxide using
manganese dioxide

To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst

A

H2O2 –(MnO2)–> H2O + 1/2O2

80
Q

what is the apperance of the maganese dioxide catalyst

To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst

A

Black powder

81
Q

How is it ensured the stopwatch is started at excatly the same time as the reaction starts

To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst

A

The Manganese dioxide is added to the hydrogen peroxide from a small test tube in the conical flask
The mixture is shaken, and the stopwatch is started at excatly the same time

82
Q

What chemical hazard symbols are associated with i) hydrogen peroxide ii) manganese dioxide?
Explain their meaning and suggest how you could manage their risk

A

i) Oxidiser- keep away from other substances
ii) Irritant- wear gloves

83
Q

What effect would increasing the mass of the catalyst have on the rate of reaction? Explain

A

no effect, they are only required in small amounts

84
Q

Wat type of catalysis is involved in this reaction? Explain

To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst

A

Heterogenous catalysis- the solid magnesium dioxide catalyst is in a different phase to the reactants and products

85
Q

Name and describe the mechanism for the action of the manganese dioxide in speeding up the
decomposition of hydrogen peroxide

To monitor the rate of production of oxygen from
hydrogen peroxide using manganese dioxide as a catalyst

A

Surface adsorbtion theory
Adsorption - The hydrogen peroxide reactants are adsorbed onto the surface, forming
temporary bonds
Reaction - The hydrogen peroxide reactants are now in high concentration on the
surface of the catalyst and are more likely to collide
The bonds within the hydrogen peroxide molecules are weakened and collisions of
reactants are more likely to be effective i.e. the activation energy is lowered
iiDesorption - The water and oxygen products are formed and leaves the surface of the
catalyst

Chemistry (30 decks)

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