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AQA A Level Physical Chemistry > 4. Energetics > Flashcards

4. Energetics Flashcards

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1
Q

What is energy transfer during a reaction due to?

A

The breaking and forming of bonds

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2
Q

What happens to energy when bonds are broken?

A

Energy is taken in

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3
Q

What happens to energy when bonds are formed?

A

Energy is released

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4
Q

In terms of bonds, when is energy taken in?

A

When bonds are broken

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5
Q

In terms of bonds, when is energy released?

A

When bonds are formed

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6
Q

What is stored energy known as?

A

Enthalpy

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7
Q

What is the symbol for enthalpy?

A

H

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8
Q

What is enthalpy?

A

Stored energy

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9
Q

Can enthalpy be measured?

A

No

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10
Q

How can enthalpy changes be found?

A

By measuring the change in temperature during reactions - at a constant pressure

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11
Q

What must be constant when measuring change in temperature to work out enthalpy change?

A

Pressure

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12
Q

What is the mnemonic to remember energy when breaking/making bonds?

A

Bendy Mexicans

breaking = endo, making = exo

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13
Q

What happens to energy in an endothermic reaction?

A

It is taken in

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14
Q

When energy is taken in during an endothermic reaction, what form does it usually take?

A

Heat

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15
Q

What happens to the temperature of the surroundings during an endothermic reaction?

A

It decreases

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16
Q

What is higher in an endothermic reaction; the energy to make bonds or to break them?

A

Energy to break bonds is higher than to make them

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17
Q

What does it mean, in terms of energy, that endothermic reactions need more energy to break bonds than to make them?

A

There is an overall need for energy for the reaction to take place = positive ΔH value

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18
Q

Why do endothermic reactions have a positive ΔH value?

A

There is an overall need for energy in order for the reaction to take place - so the system gains energy

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19
Q

Do endothermic reactions have a positive or negative ΔH value?

A

Positive

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20
Q

What happens to energy in an exothermic reaction?

A

It is released

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21
Q

When energy is released during an endothermic reaction, what form does it usually take?

A

Heat

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22
Q

What happens to the temperature of the surroundings during an exothermic reaction?

A

It increases

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23
Q

Why does the temperature of the surroundings increase during an exothermic reaction?

A

Energy is transferred to the surroundings

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24
Q

What is higher in an exothermic reaction; the energy to make bonds or to break them?

A

The energy when making bonds

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25
Q

What does it mean, in terms of energy, that exothermic reactions need less energy to break bonds than to make them?

A

There is an overall release of energy when the reaction takes place = negative ΔH value

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26
Q

Why do exothermic reactions have a negative ΔH value?

A

There is an overall release of energy when a reaction takes place - so the system loses energy

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27
Q

Does a system lose or gain energy during an endothermic reaction?

A

Gains energy

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28
Q

Does a system lose or gain energy during an exothermic reaction?

A

Loses energy

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29
Q

What do values for enthalpy change depend on?

A

The pressure and temperature at which the reaction is carried out at

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30
Q

What is the standard enthalpy change of a reaction?

A

The heat energy exchange with its surroundings at a constant pressure and temperature (under standard conditions)

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31
Q

What are the standard conditions for standard enthalpy change?

A
  • pressure - 100kPa

* temperature - 298K

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32
Q

What is standard enthalpy change measured in?

A

kJ mol-1

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33
Q

What is the standard enthalpy change of formation?

A

The enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, with all substances being in their standard states

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34
Q

What is the symbol for standard enthalpy change of formation?

A

ΔHf

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35
Q

What is the ΔHf of an element?

A

Zero

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36
Q

What is standard enthalpy change of combustion?

A

The enthalpy change when 1 mole of a substances undergoes complete combustion under standard conditions, with all substances being in their standard states

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37
Q

What is the symbol for standard enthalpy change of combustion?

A

ΔHc

38
Q

Which experimental technique is used to measure the standard enthalpy of combustion?

A

Calorimetry

39
Q

What is calorimetry?

A

An experimental technique used to measure standard enthalpy change

40
Q

What has to be 1 mole during an equation for standard enthalpy change of combustion?

A

The reactant

41
Q

What has to be 1 mole during an equation for standard enthalpy change of formation?

A

The product

42
Q

What is the equation for the standard enthalpy change of formation of methane?

A

C + 2H₂ -> CH₄

43
Q

What is the equation for the standard enthalpy change of combustion of methane?

A

CH₄ + 2O₂ -> CO₂ + 2H₂O

44
Q

What is calorimetry?

A

The process of measuring enthalpy changes of reactions

45
Q

What is a calorimeter?

A

Any container used to measure enthalpy change

46
Q

Is combustion an endo or exothermic reaction?

A

Exothermic

47
Q

Is energy released or taken in during combustion?

A

Released

48
Q

How is the enthalpy of combustion measured?

A

By measuring the heat transferred (when fuel is burnt) to a given volume of water in the calorimeter

49
Q

Can the enthalpy of combustion be measured directly?

A

No - instead it is measured by the heat transferred to a given volume of water

50
Q

Should a calorimeter be a good conductor?

A

Yes

51
Q

Why should a calorimeter be a good conductor?

A

Enthalpy is measured by the heat transferred to a given volume of water in the calorimeter

52
Q

For neutralisation and displacement, should the calorimeter be a good conductor?

A

No - it should be an insulator

53
Q

What can be used as a calorimeter in neutralisation and displacement reactions?

A

A polystyrene cup

54
Q

What can be used to measure more accurately the volume of acids and alkalis for neutralisation reactions?

A

A burette

55
Q

What is specific heat capacity?

A

The amount of energy required to raise the temperature of 1g of a substance by 1K

56
Q

How much energy is required to raise the temperature of one gram of water by one kelvin?

A

4.18J

57
Q

What is the specific heat capacity of water?

A

4.18 J g⁻¹ K⁻¹

58
Q

What is the equation used to calculate heat released from a reaction?

A

q=mcΔT

59
Q

What does the ‘q’ stand for in q=mcΔT?

A

Heat released (in Joules)

60
Q

What does the ‘m’ stand for in q=mcΔT?

A

Mass of substance being heated - often water (in grams)

61
Q

What does the ‘c’ stand for in q=mcΔT?

A

Specific heat capacity in J g⁻¹ K⁻¹

62
Q

What is the equation used to calculate enthalpy change?

A

ΔH=q/n

63
Q

What is the unit for enthalpy change?

A

kJmol⁻¹

64
Q

What does the ‘q’ stand for in ΔH=q/n?

A

Heat released in kJ

65
Q

What is the heat released in the equation ΔH=q/n measued in?

A

kJ

66
Q

What does the ‘ΔH’ stand for in ΔH=q/n?

A

Enthalpy change

67
Q

What does the ‘n’ stand for in ΔH=q/n?

A

The number of moles of substance

68
Q

What is the mass of water equal to?

A

The volume of water being heated

69
Q

What equations are used to calculate enthalpy change?

A
  • q=mcΔT

* ΔH=q/n

70
Q

How is the number of moles of fuel (used to calculate enthalpy change of combustion) calculated?

A
  • if fuel or solid: n=mass/mr

* if solution: n=cv (v in dm³)

71
Q

When working out the enthalpy of neutralisation, what are the density and specific heat capacities of the acid and base taken as?

A

The same as for pure water

72
Q

When measuring the energy transferred for other reactions (other than combustion), what is the mass of water assumed to be equal to?

A

The total volume of the solution used

73
Q

Which substance is used to calculate the enthalpy change of a reaction?

A

Whichever substance is not in excess

74
Q

What are you required to do on graphs when reagents have just been mixed so a temperature cannot be recorded?

A

The graphs must be extrapolated

75
Q

What can you calculate after extrapolating graphs with axes time and temperature?

A

The maximum temperature change

76
Q

What law enables us to calculate enthalpy changes uses known enthalpy changes?

A

The law of conservation, stating that energy cannot be created or destroyed

77
Q

What does Hess’s law state?

A

That the overall enthalpy change for a reaction is independent of the route the reaction takes

78
Q

What is the enthalpy of formation of an element?

A

0 kJ mol⁻¹

79
Q

What is the enthalpy of combustion of carbon equal to?

A

The enthalpy of formation of CO2

80
Q

What is the enthalpy of combustion of hydrogen equal to?

A

The enthalpy of formation of H2O

81
Q

Does breaking bonds require or release energy?

A

Requires energy

82
Q

Is breaking bonds exo or endothermic?

A

Endothermic

83
Q

Does making bonds require or release energy?

A

Releases energy

84
Q

Is making bonds exo or endothermic?

A

Exothermic

85
Q

What is mean bond enthalpy?

A

The average bond dissociation enthalpy for a particular bond in a range of different compounds

86
Q

Does the bond enthalpy of the same bond ever vary?

A

Yes, slightly

87
Q

Why might the bond enthalpy of the same bond vary?

A

Depending on the molecule it is in

88
Q

What are mean bond enthalpies used to calculate?

A

An approximate value for enthalpy changes of a reaction

89
Q

Why do enthalpy values calculated using mean bond enthalpies differ from those determined using Hess’s law?

A

Mean bond enthalpies are approximate values whereas those determined using Hess’s law are actual values

90
Q

What method of calculating enthalpy change should be used if you are given enthalpies of formation/combustion?

A

Hess cycle

91
Q

How to calculate enthalpy change using mean bond enthalpies?

A
  1. Add up values for each bond in reactants to calculate total energy required to break bonds.
  2. Add up values for each bond in products to calculate total energy released when bonds formed
  3. Use equation BB - MB = ΔH

AQA A Level Physical Chemistry (12 decks)

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