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A Level Chemistry > Module 3 Section 2: Physical Chemistry > Flashcards

Module 3 Section 2: Physical Chemistry Flashcards

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1
Q

What is enthalpy change

A

Enthalpy change ΔH, is the heat energy transferred in a reaction at constant pressure
The units of ΔH are kJ mol-1

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2
Q

What does writing ΔH mean

A

Shows that the measurements were made under standard conditions and that the elements were in their standard states (physical states under standard conditions)

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3
Q

What are the standard conditions

A

100kPa (about 1atm) pressure
298K (25°C)

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4
Q

What happens in Exothermic reactions

A

Reaction gives out energy
ΔH is negative
Examples are combustions of fuels and oxidation of carbohydrates

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5
Q

What happens in endothermic reactions

A

Reaction absorbs energy
ΔH is positive
Examples are thermal decompositions and photosynthesis

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6
Q

What do enthalpy profile diagrams show

A

Shows how the enthalpy (energy) changes during reactions

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7
Q

What is the activation energy

A

Symbol is Ea
It’s the minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction

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8
Q

What do enthalpy profiles looks like for exothermic and endothermic reactions

A

Endothermic reaction:
Reactants are on a lower enthalpy than products

Exothermic reaction:
Reactants are on a higher enthalpy than products

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9
Q

Why must you know the specific conditions for enthalpy changes

A

You can’t directly measure the actual enthalpy of a system
You only need to know the enthalpy change
This can be found by experiment or in data books
Enthalpy changes in data books are usually standard enthalpy changes, done under standard conditions
Enthalpy changes are affected by temperature and pressure

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10
Q

What are the different enthalpy changes

A

Standard enthalpy change of reaction, ΔrH
Standard enthalpy change of formation, ΔfH
Standard enthalpy change of combustion, ΔcH
Standard enthalpy change of neutralisation, ΔneutH

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11
Q

How does enthalpy change for making and breaking bonds

A

Energy is needed to break bonds so this is endothermic (ΔH is positive)
Energy is released when bonds are formed so this is exothermic (ΔH is negative)
Enthalpy change for a reaction is the overall effect of these two changes
If more energy is needed to break bonds than is released when bonds are made, ΔH is positive
If you need less, ΔH is negative

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12
Q

What is bond dissociation enthalpy

A

The energy needed to break oppositely charged ions apart in ionic compounds
The energy needed to separate positive nuclei to the negative charge of the shared electrons
The bond dissociation enthalpy is the amount of energy you need per mole
Always involve bond breaking in gaseous compounds

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13
Q

What is average bond enthalpy

A

The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds

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14
Q

Why are average bond enthalpies not exact

A

It’s an average for a much bigger range of molecules
E.g. O-H average bond enthalpy isn’t just for water, it includes alcohols and carboxylic acids

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15
Q

How to investigate enthalpy changes in a lab

A

Must know:
The number of moles for the chemical that’s reacting
The change in temperature

For exothermic reactions:
E.g. to find the enthalpy for a combustion, you burn it
The burning fuel heats water
Can work out the heat absorbed by water if you know the mass of water, the temperature change of the water, and the specific heat capacity of water
Ideally all the heat released by the fuel is absorbed by water but some is always lost
This is calorimetry

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16
Q

What is calorimetry

A

Can be used to calculate an enthalpy change for a reaction that happens in a solution, such as neutralisation or displacement

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17
Q

What does the equation q=mcΔT stand for

A

q= heat lost of gained (in joules). Same as enthalpy change is pressure is constant
m= mass of water in the calorimeter, or solution in the insulated container (g)
c= specific heat capacity of water (4.18 J g-1 K-1)
ΔT= change in temperature of the water solution (K)

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18
Q

In a laboratory experiment, 1.16g of organic liquid fuel was burned
The heat released raised the temperature of 100g or water from 295.3K to 357.8K
Calculate the standard enthalpy of combustion of the fuel
It’s Mr is 58.0

A

Calculate amount of heat in joules
q=mcΔT
q= 100 x 4.18 x (357.8-295.3) = 26,125J (26.125 kJ)

Standard enthalpy of combustion involves 1 mole of fuel, so find out how many moles of fuel produced this heat
n = 1.16/58 = 0.02 mole of fuels

Heat produced by 1 mole of fuel = -26.125/0.02 = -1310 kJ mol-1

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19
Q

What happens in calorimetry for neutralisation reactions

A

For a neutralisation reaction, known quantities of acid and alkali are combined in an insulated container, and the temperature change is measured

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20
Q

What is the standard enthalpy change of reaction

A

The enthalpy change for a reaction with the quantities shown in the balanced equation
The value should be quoted along with the equation

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21
Q

What are the standard conditions for enthalpy change

A

Standard pressure: 100kPa
Standard temperature: 298K
Standard concentration: 1 mol dm-3
Standard state: physical state of a substance under standard conditions

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22
Q

What is the standard enthalpy change of formation

A

The enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions of 298K and 100 kPa
Make sure 1 mole of substance in equation

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23
Q

Standard enthalpy change of combustion

A

The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions of 100kPa and 298K
Make sure 1 mole of substance in equation

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24
Q

Standard enthalpy change of neutralisation

A

Enthalpy change when one mole of water is formed from the reaction of an acid with a base or when 1 mole of water is formed from reaction H+(aq) + OH-(aq) = H2O(l) under standard conditions of 298K and 100kPa
Make sure 1 mole of water in equation

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25
Q

Why is the enthalpy change of neutralisation always -57 kJ mol-1

A

Same reaction occurring each in terms of ions
Always H+(aq) + OH-(aq) = H2O(l)

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26
Q

What is Hess’ law

A

If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy is the same for each route

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27
Q

How can Hess’ law be used to calculate enthalpy change

A

The two routes from the reactants to products are:
Route 1: A + B arrows
Route 2: C arrow
By Hess’ law the total enthalpy change for both routes is the same
A + B = C

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28
Q

Equation to find enthalpy change of formation

A

ΔrH = Σ ΔfH products - Σ ΔfH reactants

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29
Q

Calculate the ΔH for ZnCO3 = ZnO + CO2

ΔfH: ZnCO3: -812, ZnO: -348, CO2: -393

A

ΔrH = Σ ΔfH products - Σ ΔfH reactants

Σ ΔfH products: (1x-348) + (1x-393) = -741
Σ ΔfH reactants: (1x-812) = -812

-741 - -812 = 71kJ mol-1
Endothermic

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30
Q

Calculate the ΔH for 2CO + O2 = 2CO2

ΔfH: CO: -111, CO2: -393

A

ΔrH = Σ ΔfH products - Σ ΔfH reactants

Σ ΔfH products: (2 x -393) = -786
Σ ΔfH reactants: (2 x -111) = -222

-786 - -222 = -564 kJ mol-1
Exothermic

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31
Q

Equation for enthalpy change of combustion

A

ΔrH = Σ ΔfH reactants - Σ ΔfH products

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32
Q

How to work out missing bond enthalpy

A

Write out enthalpy arrow diagram, e.g. formation where arrows go into raw elements
Work out total enthalpy of each arrow including X
Using Hess’ law: X + other bond enthalpies on that side = other arrows
Rearrange to find X

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33
Q

Where do the arrows point in enthalpy diagrams for different enthalpy changes

A

Formation: arrows down into elements away from equation
Combustion: arrows down into CO2 and H2O

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34
Q

What does the rate of reaction measure

A

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

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35
Q

Definition of rate of reaction

A

The change in concentration of a reactant or a product in a given time

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36
Q

How to work out rate of reaction

A

Rate (mol dm-3 s-1) = change in concentration(mol dm-3) / s

37
Q

How to find rate of reaction on a graph

A

Draw tangent and find gradient of this to find rate

38
Q

When is the rate of reaction at its fastest

A

The rate of a reaction is fastest at the start of the reaction, as each reactant is at its highest concentration, line is steepest here

39
Q

Why does the rate of reaction decrease over time

A

Rate of reaction slows down as the reaction proceeds, because there reactants are being used up and their concentration decreases (meaning less frequent collisions), this is shown by the line becoming less steep and starting to curve

40
Q

Why does the rate of reaction graph level out

A

Once one of the reactants has been completely used up, the concentrations stop changing and the rate of reaction is 0, the line plateaus

41
Q

What factors affect the rate of reaction

A

Concentration (or pressure when reactants are gases)
Temperature
Use of a catalyst
Surface area of solid reactants

42
Q

What is collision theory

A

The collision theory states that two reacting particles must collide for a reaction to occur

43
Q

What is an effective collision

A

A collision that leads to a chemical reaction

44
Q

What leads to an effective collision

A

The particles collide with the correct orientation
The particles have sufficient energy to overcome the activation energy barrier of the reaction

45
Q

How does concentration affect rate of reaction

A

Increasing the concentration increases the overall collision frequency as there’s more particles per unit volume
Therefore in a given period of time there will be more effective collisions and an increased rate of reaction

46
Q

How does pressure affect rate of reaction for gases

A

Keeping a gas at a lower volume the pressure increases, therefore the rate of reaction increases
Concentration of the gas molecules increases as the same number of gas molecules occupy a smaller volume
Gas molecules are close together so will collide more frequency

47
Q

How to measure rate of reaction

A

Monitoring decrease in concentration of reactant
Following increase in concentration of a product

48
Q

What methods are used to measure the rate of reaction with reactions that produce gas

A

Monitoring the volume of gas produced at regular time intervals using gas collection
Monitoring the loss of mass of reactants using a balance

Volume of gas produced and mass loss are both proportional to the change in concentration of a reactant or product
So the change in volume with time or the mass loss with time both give a measure of the rate of reaction

49
Q

Practicals methods to measure the rate of reaction with gas

A

Inverted measuring cylinder filled with water
Gas syringe
Monitoring mass loss on a balance

50
Q

Advantages and disadvantages of inverted measuring cylinder method

A

Easier to read than gas syringe as it slower moving
Lower precision

51
Q

Advantages and disadvantages of gas syringe method

A

Precise measurements
Gas syringe can get stuck

52
Q

Advantages and disadvantages of mass balance method

A

Easy to set up and losing gas is no problem
CO2 may dissolve back into the solution

53
Q

What is activation energy for collision theory

A

Minimum amount of kinetic energy particles need to react
Particles need this much energy to break bonds to start the reaction

54
Q

How does activation affect how easily reactions occur

A

Reactions with low activation energies often happen easily
Whereas reactions with high activation energies occur less easily

55
Q

What does a Boltzmann distribution graph show

A

Shows the different proportions of kinetic energy of particles
Most particles moving at a moderate speed so the frequency of them is highest
Can mark activation energy to show how some particles will not have enough energy

56
Q

How does temperature affect the rate of reaction

A

Increasing temperature means particles have more kinetic energy and will move faster meaning a higher collision frequency
This means a greater proportion of molecules will have at least the activation energy and be able to react

57
Q

What are catalysts

A

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

58
Q

How many different chemical reactions does a catalyst work for

A

Many catalysts usually only work on a single reaction

59
Q

How do catalysts affect a Boltzmann distribution

A

The line for activation energy moves to the left towards the majority of particles

60
Q

How do catalysts affect the enthalpy profile

A

The curve for the activation energy is reduced with the catalyst

61
Q

What are heterogenous catalysts

A

This catalyst is in a different physical state from the reactants
The reaction happens on the surface of the heterogenous catalyst
Usually solids in contact with gaseous reactants or aqueous reactants

62
Q

What happens on a heterogenous catalyst

A

Reactant molecules are adsorbed (weakly bonded) onto surface of catalyst, where the reaction takes place
The reactant molecules leave the surface of the catalyst by desorption

63
Q

What are homogeneous catalysts

A

Homogenous catalysts are in the same physical state as the reactants
Usually a homogenous catalyst is an aqueous catalyst for a reaction between two aqueous solutions

64
Q

What happens on a homogenous catalyst

A

A homogenous catalyst works by forming an intermediate species
The reactants combine with the catalyst to make an intermediate species
This then reacts to form the products and reform the catalyst

65
Q

How are catalysts good for industries

A

Catalysts lower production costs
Provide more product in a shorter time
Help make better products

66
Q

Example of a catalyst used in industry

A

Iron is used as a catalyst in ammonia production
Without iron, the temperature would have to be raised to make the reaction happen quick enough
This would cost more and reduce the yield of ammonia

67
Q

How can catalysts change the properties of a product

A

E.g. poly(ethene)
Properties of poly(ethene) without a catalyst: less dense, less rigid
Properties of poly(ethene) made with a catalyst: more dense, more rigid, higher melting point

68
Q

How do catalysts benefit environmental sustainability

A

Using catalysts means that lower temperatures and pressures can be used
Energy is saved so less CO2 is released and fossil fuel reserves are preserved
Catalysts reduce wastes by allowing a different reaction to be used with a better atom economy

69
Q

How can catalytic converters act as environmentally beneficial catalysts

A

Catalytic converters on cars are made from alloys of platinum and palladium, they reduce the pollution released into the atmosphere by speeding up the reaction:
2CO + 2NO -> 2CO2 + N2

70
Q

Features of a Boltzmann distribution

A

No molecules have 0 energy - the curve starts at the origin
The area under the curve is equal to the number of molecules
There is no maximum energy for a molecule - the curve does not meet the x-axis at high energy (curve would need to reach infinite energy to meet the x-axis)

71
Q

Effect of increasing temperature on the Boltzmann Distribution

A

More molecules have an energy greater than or equal to the activation energy
Greater proportion of collisions will lead to a reaction, increasing the rate of reaction
Frequency of collisions will be increased as molecules are moving faster, but increased energy of molecules is more significant

72
Q

Effect of catalysts on the Boltzmann Distribution

A

Catalysts provide an alternative route with a lower activation energy
Greater proportion of molecules will have an energy greater or equal to the activation energy with the catalyst
Activation energy with catalyst moves to the left of the curve where there’s a greater proportion of molecules

73
Q

What is a reversible reaction

A

A reaction that can go in both ways
A + B ⇌ C
A + B = C
C = A + B

74
Q

How does a reaction reach dynamic equilibrium

A

As the reactants get used up the forwards reaction slows down and as more product is formed, the reverse reaction speeds up to reform reactants
These reactions can continue until the forwards reaction and reverse reaction are occurring at the same rate
The amounts of reactants and products won’t be changing any more so their concentrations will stay the same

75
Q

What is dynamic equilibrium

A

A reversible reaction where the forwards and reverse reactions are occurring at the same rate in a closed system

76
Q

What is a closed system

A

No substances can get in or out of where the reaction is taking place

77
Q

What happens when you change the concentration, pressure or temperature of a reaction at equilibrium

A

The position of equilibrium will shift
It will produce different amounts of reactants and products at equilibrium

78
Q

Depending on which way the position of equilibrium shifts, what happens to the amounts of reactants and products

A

Position of equilibrium moves to the right:
More products produced

Position of equilibrium moves to the left:
More reactants are produced

79
Q

Le Chatelier’s principle

A

If there’s a chance in concentration, pressure or temperature, the equilibrium will move to help counteract the change

80
Q

What happens to a reaction at equilibrium when the concentration is changed

A

Increasing the concentration of a reactant will result in the equilibrium trying to reduce the amount of the extra reactant
It makes more product and shifts position of equilibrium to the right

Increasing the concentration of a product means that the equilibrium will increase the rate of the reverse reaction to reduce the amount of extra product
Equilibrium shifts to the left

81
Q

What happens to a reaction at equilibrium when the temperature is changed

A

Increasing the temperature means that the product will shift in the endothermic direction to absorb the heat that has been added

Decreasing the temperature means that the equilibrium shifts in the exothermic direction to replace the heat

82
Q

What happens to a reaction at equilibrium when the pressure is changed

A

Increasing the pressure shifts the equilibrium to the side with fewer gas molecules to reduce the pressure
Decreasing the pressure shifts the equilibrium to the side with more gas molecules which increases the pressure again
This only applies to gases

83
Q

What effect do catalysts do to the position of equilibrium

A

Catalysts have no effect on the position of equilibrium
They speed up the forward and reverse reactions by the aame amount
Can’t increase yield but means that equilibrium is reached faster

84
Q

What does the equilibrium constant show

A

Shows how far to the left or right the equilibrium is

85
Q

Expression for the equilibrium constant

A

General reaction: aA + bB ⇌ dD + eE

Kc = [D]^d [E]^e / [A]^a [B]^b

Remember [ ]

86
Q

Putting the equation H2(g) + I2 (g) ⇌ 2HI (g) into equilibrium constant and finding Kc with correct units
HI = 0.8moldm-3, H2 = 0.1moldm-3, I2 = 0.1moldm-3

A

Kc = [HI(g)]^2 / [H2(g)]^1 [I2(g)]^1
Kc = 0.80^2 / 0.10 x 0.10 = 64

Units: (mol dm-3)^2 / (mol dm-3) (mol dm-3)
All units cancel out so there are no units

Kc = 64 no units

87
Q

How to estimate the position of equilibrium using Kc

A

The larger the value of Kc is, the further to the right equilibrium is and the more products there are relative to reactants

The smaller the value of Kc the further to the left equilibrium lies and the more reactants there are relative to product

88
Q

Why is the value from enthalpy change experiments never accurate

A

Heat is released to the surroundings
Incomplete combustion
Non standard conditions

A Level Chemistry (15 decks)

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