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Aqa A Level Chemistry > Kinetics, Equilibra And Redox Reactions > Flashcards

Kinetics, Equilibra And Redox Reactions Flashcards

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1
Q

Formula for rate of reaction

A

Rate of reaction=

Amount of reactant used or product formed/ time

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2
Q

What are particles in liquids and gases always doing?

A

Moving and colliding with each other

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3
Q

Do they react every time they collide?

A

No only when conditions are right

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4
Q

What two things must be true according to collision theory for a reaction to occur?

A

Collide in the right direction they need to be facing each other in the right way
Collide with at least the activation energy required

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5
Q

What’s activation energy?

A

The minimum amount of kinetic energy that particles need to react

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6
Q

Why is activation energy required to start off a reaction?

A

Need at least the energy needed to break their bonds and start the reaction

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7
Q

What reactions happen easily?

A

Ones with low activation energy

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8
Q

What reaction happen don’t happen easily?

A

Ones with a high activation energy so you need to give particles extra energy by heating them

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9
Q

How much energy do molecules in a gas have?

A

Some don’t have much kinetic energy and move slowly

Others have loads of kinetic energy and whizz along but molecules are sometimes in between

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10
Q

If you plot a graph of number of molecules in a gas with different kinetic energy what do you get?

A

Maxwell-Boltzmann distribution

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11
Q

Area under Maxwell-Boltzmann distribution?

A

The total number of molecules

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12
Q

Where does the curve of maxwell-Boltzmann begin?

A

(0,0) no molecules have zero energy

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13
Q

Peak of curve maxwell-Boltzmann?

A

Represents most likely energy of any single molecule

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14
Q

Mean (average) energy is where?

A

All the molecules a bit right to a peak

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15
Q

Which molecules react?

A

Some molecules have more than the activation energy. These are the only ones that can react

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16
Q

Increasing the temperature of a reaction does what? Maxwell-Boltzmann

A

The particles will on average have more kinetic energy and move faster

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17
Q

What does a higher temperature mean for the proportion?

A

Greater proportion of molecules will have at least the activation energy and be able to react
Changes shape of maxwell-Boltzmann distribution curve pushing it over to the right

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18
Q

Total number of molecules is the same for?

A

Each of these reactions meaning the area under the curve must be the same

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19
Q

Why does a slight increase in temperature result in quite a large increase in reaction rate?

A

Molecules move faster so collide more
More molecules have the activation energy required
Together can lead to large increase in reaction rate

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20
Q

What does increasing pressure do the rate of reaction?

A

Increase concentration of reactants in a solution the particles will be on average closer together
If closer they will collide more often. If collisions occur more frequently they’ll have jute chance to react meaning increasing concentration increases reaction rate

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21
Q

Pressure does what to the rate of reaction?

A

If a reaction involved gases, increasing pressure works in just the same way as increasing concentration
Raising pressure pushes all gas Particles together making them more likely to collide so collisions take place more frequently and reaction rate increases

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22
Q

What’s a catalyst?

A

A substance that increases the rate of reaction providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged at the end of the reaction

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23
Q

Why are catalysts great?

A

They don’t get used up so you only need a tiny bit of catalyst to catalyse a huge amount of stuff. They do take part in reactions but they’re remade at the end

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24
Q

Why are catalyst fussy about which reactions they catalyse?

A

They often only work on a single reaction

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25
Q

Why do catalysts save heaps of money in industrial processes?

A

Allow you to make the same amount of product faster and often at a lower temperature too

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26
Q

Why can a catalyst work?

A

The catalyst lowers the activation energy meaning there’s more particles with enough energy to react when they collide. It does this by allowing the reaction to go via a different route so certain amount of time more particles react

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27
Q

How is rate of reaction defined?

A

As the change in concentration (or amount) or reactant or product over time

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28
Q

How can you follow the rate of reaction?

A

Either by how fast reactants are used up or how fast the products are formed.

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29
Q

Three methods to measure reaction rate?

A

Timing how long a precipitate takes to form
Measuring a decrease in mass
Measuring volume of gas given off

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30
Q

Timing how long a precipitate takes to form when can you use this method?

A

When the product is a precipitate which clouds a solution.

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31
Q

Timing how long a precipitate takes to form what do you do?

A

Watch a mark through the solution and time how long it takes to be obscured.

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32
Q

Timing how long a precipitate takes to form how can you compare it?

A

If the same observer uses the same mark each time you can compare the rate of reaction because roughly the same precipitate will have formed when the mark becomes obscure

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33
Q

Timing how long a precipitate takes to form what’s wrong with this method?

A

Method is subjective- different people may not agree on the exact same moment the mark disappears

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34
Q

Measuring a decrease in mass when can this be done?

A

When or more products is a gas you can measure the rate of formation using a mass balance

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35
Q

Measuring a decrease in mass how does it work?

A

As gas is given off the mass of the reaction mixture decreases

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36
Q

Measuring a decrease in mass disadvantage?

A

Method is accurate and easy to do but does release gas into the atmosphere so usually done in a fine cupboard

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37
Q

Measuring volume of gas given off involves?

A

Using a gas syringe to measure volume of gas being produced

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38
Q

When can you use measuring the volume of gas given off?

A

Only when one or more of the products is a gas

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39
Q

How accurate is measuring the volume of gas given off?

A

Gas syringes usually give volumes to the nearest 0.1cm3 so this method is accurate

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40
Q

What are sodium thiosulfate and hydrochloric on their own?

A

Clear, colourless solutions

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41
Q

What colour are sodium thiosulfate and hydrochloric acid when they react together?

A

They form a yellow precipitate of sulfur

42
Q

How can you measure the reaction between sodium thiosulfate and hydrochloric acid?

A

Amount of time that it takes for the precipitate to form as a measure of the rate of the reaction

43
Q

What is reaction between sodium thiosulfate and hydrochloric acid often demonstrate?

A

The effect of increasing temperature on reaction rates

44
Q

Step 1 for reaction between sodium thiosulfate and hydrochloric acid?

A

Measure out fixed volumes of sodium thiosulfate and hydrochloric acid using a measuring cylinder

45
Q

Step 2 for reaction between sodium thiosulfate and hydrochloric acid?

A

Use water bath to gently heat both solutions to desired temperature before you mix them

46
Q

Step 3 for reaction between sodium thiosulfate and hydrochloric acid?

A

Mix solutions in conical flask. Place flask over a black cross which can be seen through the solution. Watch the black cross disappear through cloudy sulfur and time how long it takes

47
Q

Step 4 for reaction between sodium thiosulfate and hydrochloric acid?

A

The reaction can be repeated for solutions at different temperatures. The depth of liquid must be kept the same each time. The concentration of solutions must also be kept the same

48
Q

Step 5 for reaction between sodium thiosulfate and hydrochloric acid?

A

The results should show that higher temperature the faster the reaction rate and therefore the less time it takes for the mark to disappear

49
Q

What are a lot of chemical reactions?

A

Reversible they go both ways

⇌

50
Q

What happens as the reactants are used up?

A

The forward reaction slows down as more product is formed the reverse reaction speeds up

51
Q

What’s dynamic equilibrium

A

The forward reaction going at the same rate as the backwards reaction so amount of product and reactants won’t be changing

52
Q

What happens at equilibrium?

A

Concentration of reactants and products stay constant

53
Q

When can dynamic equilibrium happen?

A

Only in a closed system

54
Q

What happens if you change the concentration, pressure or temperature of a reversible reaction?

A

You will alter the position of equilibrium. Meaning you end up with different amounts of reactants and products at equilibrium

55
Q

What happens if the equilibrium moves to the left?

A

You’ll get more reactants

56
Q

What happens if the position of equilibrium moved to the right?

A

You get more products

57
Q

What does Le Chatelier’s principle say?

A

If a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change

58
Q

What does the Le Chatelier’s principle basically mean?

A

If you raise the temperature the position of equilibrium will shift to try to cool things down. If you raise the pressure or concentration the position of equilibrium will shift to try to reduce it again.

59
Q

What can you use the Le Chatelier’s principle to work out?

A

What effect changing the concentration, pressure or temperature will have on the position of equilibrium.

60
Q

What does the Le Chatelier’s principle apply to?

A

Homogeneous equilibria meaning reactions where every species is in the same physical state (all liquid or all gas)

61
Q

What happens if you increase the concentration of a reactant?

A

The equilibrium tries to get rid of extra reactant. It does this by making more product. So the equilibrium shifts to right.

62
Q

What happens if you increase the concentration of the product?

A

The equilibrium tries to remove extra product making the reverse reaction go faster. The equilibrium shifts to the left

63
Q

What happens if you decrease the concentrations?

A

It will have the opposite effect

64
Q

Pressure when does this change anything?

A

Only affects equilibria involving gases

65
Q

Increasing the pressure does what?

A

Shifts the equilibrium to the side with fewer fas molecules reducing pressure

66
Q

Decreasing the pressure does what?

A

Shifts the equilibrium to the side with more gas molecules raising pressure

67
Q

Increasing the temperature means what?

A

Adding heat

68
Q

What happens if the temperature is increased?

A

The equilibrium shifts to the endothermic (positive 🔼H) direction to absorb this heat

69
Q

Decreasing the temperature means what?

A

Removing heat

70
Q

What does decreasing temperature do?

A

Shifts the equilibrium in the exothermic (negative 🔼H) direction to produce more heat in order to counteract the drop in temperature

71
Q

What will the reverse reaction be if the forward reaction is endothermic?

A

Exothermic

72
Q

What will the reverse reaction be if the forward reaction is exothermic?

A

Endothermic

73
Q

What will catalysts do to equilibrium?

A

Nothing on the position of equilibrium
Can’t increase yield
But do mean equilibrium is reached faster

74
Q

What do companies have to think about?

A

How much it costs to run a reaction and how much money they can make from it. Meaning they have a few factors to think about when choosing the right conditions

75
Q

Give example

A

Ethanol can be produced via a reversible exothermic reaction between ethene and stream
C2H4(g) + H2O(g) C2H5OH (g)
🔼H=-46kJmol-1
Reaction is carried out at pressures of 60-70 atomspheres and a temperature of 300 degrees with a phosphoric acid catalyst

76
Q

Perfect temperature

A

Exothermic reaction lower temperature favours forwards reaction meaning at lower temperature more ethene and stream converted to ethanol. You get a better yield.

77
Q

Compromise temperature

A

Means slower rate of reaction
No point getting a very high yield of ethanol if it takes you 10 years so 300 degrees is a compromise between a reasonable yield and a faster reaction

78
Q

Perfect pressure

A

High pressure shifts equilibrium to the side with fewer molecules which favoured the forwards reaction here. High pressure increases rate of reaction so pressure of 60-70 atomspheres is used

79
Q

Compromise pressure

A

Very high pressure are very expensive to produce and you need string pipes and containers to withstand high pressures
So 60-70 atomspheres is a compromise giving a reasonable yield for lowest possible cost

80
Q

If you know the molar concentration of each substance at equilibrium what can you work out?

A

The equilibrium constant Kc

81
Q

Equilibrium constant equation

A

aA+bB dD+eE

Kc=(D)^d(E)^e/ ((A)^a(B)^b)

82
Q

What can you use the equilibrium constant equation for?

A

Equilibrium constant and it’s units

83
Q

Kc valid for?

A

One particular temperature

84
Q

If you change the temperature of the system?

A

You will also change the equilibrium concentrations of the products and reactants so Kc will change

85
Q

If the temperature means more product?

A

Kc will rise

86
Q

If temperature means less product at equilibrium?

A

Kc will decrease

87
Q

Changing the concentration of a reactant or product?

A

Will not affect Kc

88
Q

Catalyst does what to Kc?

A

Nothing
They’ll speed up the reaction in both directions by the same amount so they just help the system to reach equilibrium faster

89
Q

Loss of electron

A

Oxidation

90
Q

Gain of electron

A

Reduction

91
Q

Reduction and oxidation happening stimultaneously?

A

Redox reaction

92
Q

What does an oxidising agent do?

A

Accepts electrons and gets reduced

93
Q

A reducing agent does what?

A

Donates electrons and gets oxidised

94
Q

What does the oxidation state of an element tell you?

A

The total number of electrons it has donated or accepted

95
Q

Rules you can use to work out oxidation state of an atom when it’s in a compound, in an ion or on its own

A

1) uncombined elements like He and Ar have oxidation of 0
2) elements just bonded to identical atoms have oxidation state of 0
3) oxidation state of a simple monatomic ion is same as its charge
4) compound ions the overall oxidation state is the ion charge
5) sum of oxidation states for a neutral compound is 0
6) combined oxygen is nearly always -2 except in peroxide where it’s -1
7) combined hydrogen is +1 except in metal hydrides where’s -1

96
Q

What are Roman numberials in the chemical name

A

Oxidation number

97
Q

What do ionic half-equations show?

A

Oxidation or reduction

98
Q

What do you show in an ionic half-equation?

A

Electrons that have been lost or gained in a half- equation

E.g. Na->Na+ + e-

99
Q

What can you do with half-equations?

A

Combine them for different oxidising or reducing agents together to make full equations for redox

100
Q

Magnesium burns in oxygen to form magnesium oxide

A 
Oxygen reduced to O2-
O2+4e- => 2O2-
Magnesium oxidised to Mg 2+
Mg=> Mg2+ +2e-
You need both equations to contain same number of electrons so double everything in the second equation 
2Mg=> 2Mg2+ + 4e-
Combine to make 
2Mg+ O2 -> MgO
101
Q

Aluminium reacts with chlorine to form?

A 
Aluminium chloride 
Aluminium oxidised to Al3+
Al-> Al3+ + 3e-
Cl2+ 2e- => 2Cl-
Make sure equations have same number of electrons 
Combine the two equations 
2Al+3Cl2=> 2AlCl2

Aqa A Level Chemistry (12 decks)

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