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AQA Physical Chemistry > Energetics > Flashcards

Energetics Flashcards

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1
Q

What is enthalpy?

A

The heat energy stored within a chemical system

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2
Q

Where is heat energy stored?

A

Within chemical bonds and IMFs

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3
Q

What is enthalpy change?

A

The heat energy exchange with the surroundings in a reaction at STP

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4
Q

What is STP?

A

100KPa (1 atm) and 298K (25 degrees celcius)

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5
Q

What happens in an exothermic reaction?

A

Heat energy is given out so temperature is increased (surroundings)

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6
Q

What is the value of enthalpy change for an exothermic reaction?

A

-ve value because heat energy is lost

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7
Q

What happens in an endothermic reaction?

A

Heat energy is drawn in so temperature decreases (surroundings)

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8
Q

What is the value of enthalpy change for an endothermic reaction?

A

+ve because energy is drawn in

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9
Q

What are the units of enthalpy change?

A

KJ per mole

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10
Q

What is the enthalpy change of a reaction?

A

The enthalpy change of a reaction at STP

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11
Q

What is STP?

A

100KPa (1 atm) and 298K (25 degrees celcius)

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12
Q

What are the units of enthalpy change of a reaction?

A

KJ per mole

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13
Q

Describe the enthalpy change of a reversible reaction

A

The enthalpy change of the forward reaction is equal and opposite to the enthalpy change of the reverse

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14
Q

What is the enthalpy change of formation?

A

The enthalpy change where 1 mole of a compound is produced from its constituent elements in their standard states at STP

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15
Q

What is STP?

A

100KPa (1 atm) 298K (25 degrees celcius)

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16
Q

What are the units of enthalpy change of formations?

A

KJ per mole

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17
Q

What is required for enthalpy change of formation?

A
  • Elements must be in standard states - 1 mole of substance is being produced
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18
Q

What is the enthalpy change of combustion?

A

The energy change where 1 mole of a substance is burnt in excess oxygen under STP

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19
Q

What is STP?

A

100KPa (1 atm) and 298K (25 degrees celcius)

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20
Q

What are the units of enthalpy change of combustion?

A

KJ per mole

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21
Q

What is required for enthalpy change of combustion?

A

Must be 1 mole of a substance

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22
Q

What is the enthalpy change of neutralisation?

A

The energy change where one mole of water is produced from neutralisation at STP

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23
Q

What are the units of enthalpy change of neutralisation?

A

KJ per mole

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24
Q

What is required for enthalpy change of neutralisation?

A

Must be 1 mole of water

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25
Q

What is STP?

A

100KPa (1atm) and 298K (25 degrees celcius)

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26
Q

What do energy profile diagrams show?

A

The enthalpy changes that occur during a chemical reaction

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27
Q

What is the x axis of an energy profile diagram?

A

Reaction progress

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28
Q

What is the y axis of an energy profile diagram?

A

Energy (KJ)

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29
Q

What is the enthalpy change of an exothermic reaction?

A

-ve

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30
Q

What does an exothermic energy profile look like?

A

Products lower than reactants

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31
Q

What does the activation energy of the reverse reaction equal (exothermic)?

A

Activation energy of the forward reaction + total enthalpy change

32
Q

What happens to enthalpy change and activation energy and enthalpy change when a catalyst is added?

A

Activation energy is lowered but enthalpy change remains the same because reactant and product molecules still have the same energy the catalyst just provides an alternate reaction pathway with a lower activation enetgy

33
Q

What is the enthalpy change of an endothermic reaction?

A

+ve

34
Q

What does an endothermic energy profile look like?

A

Reactants lower than the products

35
Q

What does the activation energy of the reverse reaction equal (endothermic)?

A

Activation energy of the forward reaction - enthalpy change

36
Q

What is calorimetry?

A

The process that allows us to calculate the energy change of a reaction by measuring the temperature change of a known volume of water/solution

37
Q

What is needed for the calorimetry set up?

A
  • Thermometer- Solution/water- Colorimeter
38
Q

What are the two different types of calorimeter?

A
  • Polystyrene cup- Copper (metal) can
39
Q

For what type of calorimetry would you use a polystyrene cup?

A

A reaction that takes place in solution

40
Q

For what type of calorimetry would you use a copper (metal) can?

A

Where a fuel is being burnt underneath the calorimeter to heat water

41
Q

What is the calorimetry equation?

A

Q=mc (change in)T

42
Q

In calorimetry, what is Q?

A

Energy change (J)

43
Q

In calorimetry, what is m?

A

Mass (g)

44
Q

In calorimetry, what is c?

A

Specific heat capacity (J per g per k)

45
Q

In calorimetry, what is the change in T?

A

Change in temperature in degrees celcius or degrees kelvin

46
Q

How can you convert between grams and cm?

A

1 cm^3 = 1g

47
Q

What is the equation that links enthalpy change and calorimetry?

A

Enthalpy change = Energy change / number of moles

48
Q

In the equation that links enthalpy change and calorimetry what is enthalpy change measured in?

A

Kj per mole

49
Q

In the equation that links enthalpy change and calorimetry what is energy change measured in?

A

Kj

50
Q

In the equation that links enthalpy change and calorimetry what is n?

A

The number of moles of the limiting reactant

51
Q

What is the alternative calorimetry calculation?

A

Q = -(mc (change in) T)

52
Q

Why do we use an alternate calorimetry calculation?

A

To ensure the enthalpy change has the correct sign

53
Q

When calculating the enthalpy change of neutralisation what are you doing?

A

Calculating the energy change involved in a neutralisation reaction in Kj per mole

54
Q

What equipment is needed when calculating the enthalpy change of neutralisation?

A
  • Digital thermometer- Calorimeter (polystyrene)- Solution- Magnetic stirrer
55
Q

When calculating the enthalpy change of neutralisation why do we use a digital thermometer?

A
  • Minimises human error- If an error occurs it is likely the same error will be carried across all results therefore the results will be proportional
56
Q

When calculating the enthalpy change of neutralisation why do we use a magnetic stirrer?

A

To keep the solution homogeneous

57
Q

Why can you pick up inaccuracies when calculating the enthalpy change of neutralisation?

A
  • Reaction not done at STP (lab conditions)- Heat can be lost from the calorimeter (no lid)
58
Q

When measuring and calculating the enthalpy change of combustion what are you doing?

A

Calculating the enthalpy change involved in the combustion of a fuel in KJ per mole (change in enthalpy for the fuel)

59
Q

What equipment is used to measure and calculate the enthalpy change of combustion?

A
  • A thermometer- Calorimeter (metal can)- Fuel- Water- Spirit burner
60
Q

When measuring and calculating the enthalpy change of combustion how do you get the mass of fuel used?

A

Weight the spirit burner before and after

61
Q

When measuring and calculating the enthalpy change of combustion, why is the value calculated different to the data book?

A
  • Reaction carried out in lap conditions not stp- Heat will be lost (more endothermic than expected/less exothermic than expected)
62
Q

When is Hess’ Law used?

A

When a reactions enthalpy change cannot be measured directly

63
Q

What does Hess’ Law state?

A

The enthalpy change for a reaction is independent of the route taken

64
Q

How can the enthalpy change of the reaction be calculated?

A

The sum of the enthalpies in the alternate route

65
Q

What can be said about enthalpy change values in a reversible reaction?

A

The enthalpy change values are equal and opposite

66
Q

What is the enthalpy change of formation?

A

The enthalpy change when 1 one of a substance is formed from its constituent elements in standard stapes at STP

67
Q

What is STP?

A

100KPa (1 atm) and 298K (25 degrees celcius)

68
Q

What are the units for enthalpy change?

A

KJ per mole

69
Q

What is the enthalpy change for elements in their standard states?

A

0 KJ per mole

70
Q

What are the 7 steps for working out enthalpy change from enthalpy of formation values

A

1) Balence equation 2) Constituent elements as intermediates3) Arrows all pointing up4) Input the enthalpy of formation values5) Account for moles6) Invert reactant arrows and values7) Add up all the values

71
Q

What is the enthalpy change of combustion?

A

The enthalpy change when 1 mole of a substance is burned completely in oxygen

72
Q

What is the enthalpy change of combustion of oxygen?

A

0

73
Q

What are the steps when calculating the enthalpy change of combustion?

A

1) Balence2) Products of combustion as intermediates3) Arrows down4) Add values5) Account for moles6) Invert values and arrows for products7) Add up

74
Q

What is a mean bond enthalpy?

A

The average energy required/released when 1 mole of covalent bonds are broken/formend

75
Q

What happens to the reactants?

A

Bonds are broken, endothermic, + enthalpy change

76
Q

What happens to the products?

A

Bonds are formed, exothermic, - enthalpy change

77
Q

Are average bond enthalpies more or less accurate than enthalpy of combustion/formation values?

A

Less accurate

AQA Physical Chemistry (12 decks)

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