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AQA A Level Physical Chemistry > 4. Energetics > Flashcards

4. Energetics Flashcards

(91 cards)

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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
What does it mean, in terms of energy, that exothermic reactions need less energy to break bonds than to make them?
There is an overall release of energy when the reaction takes place = negative ΔH value
26
Why do exothermic reactions have a negative ΔH value?
There is an overall release of energy when a reaction takes place - so the system loses energy
27
Does a system lose or gain energy during an endothermic reaction?
Gains energy
28
Does a system lose or gain energy during an exothermic reaction?
Loses energy
29
What do values for enthalpy change depend on?
The pressure and temperature at which the reaction is carried out at
30
What is the standard enthalpy change of a reaction?
The heat energy exchange with its surroundings at a constant pressure and temperature (under standard conditions)
31
What are the standard conditions for standard enthalpy change?
* pressure - 100kPa | * temperature - 298K
32
What is standard enthalpy change measured in?
kJ mol-1
33
What is the standard enthalpy change of formation?
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
34
What is the symbol for standard enthalpy change of formation?
ΔHf
35
What is the ΔHf of an element?
Zero
36
What is standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substances undergoes complete combustion under standard conditions, with all substances being in their standard states
37
What is the symbol for standard enthalpy change of combustion?
ΔHc
38
Which experimental technique is used to measure the standard enthalpy of combustion?
Calorimetry
39
What is calorimetry?
An experimental technique used to measure standard enthalpy change
40
What has to be 1 mole during an equation for standard enthalpy change of combustion?
The reactant
41
What has to be 1 mole during an equation for standard enthalpy change of formation?
The product
42
What is the equation for the standard enthalpy change of formation of methane?
C + 2H₂ -> CH₄
43
What is the equation for the standard enthalpy change of combustion of methane?
CH₄ + 2O₂ -> CO₂ + 2H₂O
44
What is calorimetry?
The process of measuring enthalpy changes of reactions
45
What is a calorimeter?
Any container used to measure enthalpy change
46
Is combustion an endo or exothermic reaction?
Exothermic
47
Is energy released or taken in during combustion?
Released
48
How is the enthalpy of combustion measured?
By measuring the heat transferred (when fuel is burnt) to a given volume of water in the calorimeter
49
Can the enthalpy of combustion be measured directly?
No - instead it is measured by the heat transferred to a given volume of water
50
Should a calorimeter be a good conductor?
Yes
51
Why should a calorimeter be a good conductor?
Enthalpy is measured by the heat transferred to a given volume of water in the calorimeter
52
For neutralisation and displacement, should the calorimeter be a good conductor?
No - it should be an insulator
53
What can be used as a calorimeter in neutralisation and displacement reactions?
A polystyrene cup
54
What can be used to measure more accurately the volume of acids and alkalis for neutralisation reactions?
A burette
55
What is specific heat capacity?
The amount of energy required to raise the temperature of 1g of a substance by 1K
56
How much energy is required to raise the temperature of one gram of water by one kelvin?
4.18J
57
What is the specific heat capacity of water?
4.18 J g⁻¹ K⁻¹
58
What is the equation used to calculate heat released from a reaction?
q=mcΔT
59
What does the 'q' stand for in q=mcΔT?
Heat released (in Joules)
60
What does the 'm' stand for in q=mcΔT?
Mass of substance being heated - often water (in grams)
61
What does the 'c' stand for in q=mcΔT?
Specific heat capacity in J g⁻¹ K⁻¹
62
What is the equation used to calculate enthalpy change?
ΔH=q/n
63
What is the unit for enthalpy change?
kJmol⁻¹
64
What does the 'q' stand for in ΔH=q/n?
Heat released in kJ
65
What is the heat released in the equation ΔH=q/n measued in?
kJ
66
What does the 'ΔH' stand for in ΔH=q/n?
Enthalpy change
67
What does the 'n' stand for in ΔH=q/n?
The number of moles of substance
68
What is the mass of water equal to?
The volume of water being heated
69
What equations are used to calculate enthalpy change?
* q=mcΔT | * ΔH=q/n
70
How is the number of moles of fuel (used to calculate enthalpy change of combustion) calculated?
* if fuel or solid: n=mass/mr | * if solution: n=cv (v in dm³)
71
When working out the enthalpy of neutralisation, what are the density and specific heat capacities of the acid and base taken as?
The same as for pure water
72
When measuring the energy transferred for other reactions (other than combustion), what is the mass of water assumed to be equal to?
The total volume of the solution used
73
Which substance is used to calculate the enthalpy change of a reaction?
Whichever substance is not in excess
74
What are you required to do on graphs when reagents have just been mixed so a temperature cannot be recorded?
The graphs must be extrapolated
75
What can you calculate after extrapolating graphs with axes time and temperature?
The maximum temperature change
76
What law enables us to calculate enthalpy changes uses known enthalpy changes?
The law of conservation, stating that energy cannot be created or destroyed
77
What does Hess's law state?
That the overall enthalpy change for a reaction is independent of the route the reaction takes
78
What is the enthalpy of formation of an element?
0 kJ mol⁻¹
79
What is the enthalpy of combustion of carbon equal to?
The enthalpy of formation of CO2
80
What is the enthalpy of combustion of hydrogen equal to?
The enthalpy of formation of H2O
81
Does breaking bonds require or release energy?
Requires energy
82
Is breaking bonds exo or endothermic?
Endothermic
83
Does making bonds require or release energy?
Releases energy
84
Is making bonds exo or endothermic?
Exothermic
85
What is mean bond enthalpy?
The average bond dissociation enthalpy for a particular bond in a range of different compounds
86
Does the bond enthalpy of the same bond ever vary?
Yes, slightly
87
Why might the bond enthalpy of the same bond vary?
Depending on the molecule it is in
88
What are mean bond enthalpies used to calculate?
An approximate value for enthalpy changes of a reaction
89
Why do enthalpy values calculated using mean bond enthalpies differ from those determined using Hess's law?
Mean bond enthalpies are approximate values whereas those determined using Hess's law are actual values
90
What method of calculating enthalpy change should be used if you are given enthalpies of formation/combustion?
Hess cycle
91
How to calculate enthalpy change using mean bond enthalpies?
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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