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OFFICIAL OCR A-Level Chemistry COPY > Chapter 22 - Enthalpy and Entropy > Flashcards

Chapter 22 - Enthalpy and Entropy Flashcards

Lattice enthalpy, Enthalpy changes in solution, Factors affecting lattice enthalpy and hydration, Entropy, Free energy. (54 cards)

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1
Q

Define lattice enthalpy

A

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

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2
Q

What kind of energy change will lattice enthalpy always be?

A

Exothermic

Enthalpy change is negative

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3
Q

What is the Born-Haber cycle?

A

A way of indirectly calculating lattice enthalpy from other known energy changes.

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4
Q

What is the formation of gaseous atoms and what energy change is it?

A

Changing elements in their standard states into gaseous atoms.
Endothermic as it involves bond breaking.

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5
Q

What is the formation of gaseous ions and what energy change is it?

A

Changing gaseous atoms into positive and negative gaseous ions.
Endothermic.

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6
Q

What is the lattice formation and what energy change is it?

A

Changing gaseous ions into the solid ionic lattice.

Exothermic.

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7
Q

Define standard enthalpy change of formation

A

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

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8
Q

Define standard enthalpy change of atomisation

A

The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.

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9
Q

Define first ionisation energy

A

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

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10
Q

Why is ionisation energy always endothermic?

A

Energy is required to overcome the attraction between a negative electron and a positive nucleus.

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11
Q

Define first electron affinity

A

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

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12
Q

Why is first elctron affinity always exothermic?

A

Electron being added is attracted in towards the nucleus.

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13
Q

Why are successive electron affinities endothermic?

A

The negative ion repels the additional electron, so energy is required to force the electron onto it.

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14
Q

Define standard enthalpy change of solution

A

The enthalpy change that takes place when one mole of a solute dissolves in a solvent.
Can be exothermic or endothermic

If the solvent is in water the ions from the ionic lattice finishes up surrounded with water molecules as aqueous ions

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15
Q

What processes take place when a solid ionic compound dissolves in water?

A

Ionic lattice breaks up

Water molecules are attracted to and surround the ions.

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16
Q

What are the energy changes involved in these processes of a solid ionic compound dissolving in water?

A

Ionic lattice is broken up forming separate gaseous ions.
Separate gaseous ions interact with polar water molecules to form hydrated aqueous ions.
Called enthalpy change of hydration.

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17
Q

Define enthalpy change of hydration

A

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

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18
Q

Describe the effect of ionic size on lattice enthalpy and melting point

A

Ionic radius increases
Attraction between ions decreases
Lattice enthalpy becomes less negative
Melting point decreases

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19
Q

Describe the effect of ionic charge on lattice enthalpy and melting point

A

Ionic charge increases
Attraction between ions increases
Lattice enthalpy becomes more negative
Melting point increases

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20
Q

Describe the effect of ionic size on hydration enthalpy

A

Ionic radius increases
Attraction between ion and water molecules decreases
Hydration energy less negative

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21
Q

Describe the effect of ionic charge on hydration enthalpy

A

Ionic charge increases
Attraction with water molecules increases
Hydration energy becomes more negative

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22
Q

Why is hydration enthalpy an inaccurate method of predicting solubility?

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A

Many compounds with endothermic enthalpy changes of solution are soluble.

23
Q

Define entropy

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A

A measure of the dispersal of energy within the chemicals in a chemical system.

Measure of disorder in a system ( particles like disorder because it’s more energetically stable)

Entropy = S

24
Q

What are the units of entropy?

25
How does entropy relate to physical states?
In order of size: Gases > liquids > solids for entropy value
26
Define standard entropy
The entropy of one mole of a substance, under standard conditions. (Always positive and always J K-1 mol-1)
27
How is an entropy change calculated for a reaction?
Change in entropy is the sum of the entropies of the products minus the sum of the entropies of the reactants.
28
What is feasibility?
Whether a reaction is able to happen and is energetically feasible (or spontaneous).
29
What is the equation for free energy change?
ΔG = ΔH - TΔS
30
What is the requirement for the free energy change such that a reaction is feasible?
ΔG < 0 | A decrease in free energy.
31
Even though a reaction may have a negative value for ΔG, why might the reaction not take place at room temperature?
High activation energy resulting in a very slow rate.
32
Born haber cycle
Draw one out and label
33
How to calculate enthalpy change of soloution
1) calculate energy change (q) in the solution in kJ = (q=mc(change in)T 2) calculate in moles of the ionic lattice that dissolved 3) calculate standard enthalpy change of soloution in kj mol-1
34
MUST COMPLETE SOME EXAM QUESTIONS ON CALCULATING ENTHALPY SHIT SUCH AS
Calculating enthalpy change of hydration etc,,,..
35
What’s the enthalpy of hydration of an ion
The enthalpy change that occurs when warm all of the gaseous ions completely hydrated by water Conditions of 25°C and 101KPA
36
What determines dissolving
- A small highly charged ion will exert the strongest force so give out the most energy during hydration - A substance will only dissolve if the energy released is roughly the same or greater than the energy taken in - lattice enthalpy similar in size to hydration enthalpy =A chance it will dissolve Entropy also plays a part as dissolving usually causes an increase in entropy. However for small highly charged ions there may be a drop in entropy as being surrounded by water molecules can make things more orderly. Entropy changes are very small but can sometimes make the difference.
37
What happens if a system changes to become more random
Energy can be spread out more thus there will be an entropy change which will be positive
38
Calculating entropy changes
Standard entropies can be used to calculate the entropy change of a reaction Entropy change of a reaction = sum of products-reactants
39
Using enthalpy cycles to determine 1) enthalpy of solution 2) lattice formation enthalpy 3) enthalpy of hydration
Left top to right top - enthalpy of solution Bottom to left top - lattice formation enthalpy Bottom to right top - enthalpy of hydration (IF LATTICE DISSOCIATION ENTHALPY USED = where lattice formation enthalpy is but the arrow reversed so left top to bottom)
40
Solubility = what does higher charge density do?
Better attracting water = more energy released= more exothermic Thus smaller ions = more exothermic as higher charge density
41
Why is barium sulphate basically insoluble
- barium ion is larger so has a weaker lattice enthalpy = easier to breakdown. But this larger ion also means that when it’s hydrated less energy is given out - it is a balance between these two that decides if enthalpy of solution is exothermic or endothermic
42
What makes an entropy change positive and is a entropy change favourable and what happens to entropy at 0k
- if the products have more entropy than the reactants - yes because it allows reactions occurring which lowers the activation energy - no enthalpy thus all substances would have an entropy value of zero
43
Which has most entropy
N204 = 2NO2 Products - there is more particles = potential for more randomness =more entropy
44
Calculating entropy changes
(Change in sysyem) = sum of (entropy of products) - ( sum of) entropy of reactants -so if products have more entropy than reactants then you will get a positive entropy value.
45
Calculating entropy changes
- a feasible (spontaneous) reaction Will just happen by it self - can get spontaneous and endothermic reactions (would normally need to provide energy fo these) - The energy requirement is covered by the increase in entropy
46
Free energy
- This helps us to combine enthalpy and entropy to work out if a reaction is feasible at room temp Free energy (Gibbs free energy) has symbol G
47
How to work out change in free energy
(Change in) free energy = (change in) enthalpy - temperature x (change in) entropy change of system Temp = Kelvin Change in entropy of system = kj K-1 mol-1 Free energy = kj mol-1
48
Work out feasibility of reaction at different temperature
Step 1 = make units consistent With ones in equation Step 2 = use gibbs equation to calculate change In free energy at the first temp given Step 3 = use Gibbs equation to do it at second temp
49
(Change in)stotal
Change in ssytem + change in ssurroundings
50
Change in ssystem
Sum of entropy of products - sum of entropy of reactants
51
Change in ssurroundings
Change in H (J/mol) / T- K
52
4 scenarios and possibility of feasibility
- Exothermic and disorder = always spontaneous - exothermic and order = spontaneous at lower temps - endothermic and disorder = spontaneous at higher temps - endothermic and order = never spontaneous
53
Min temp for feasibility
1) calculate change in entropy 2) calculate enthalpy change 3) use Gibbs equation to calculate change in temp at 25°C 4) calculate minimum temp for thermal decomposition to occur(rearrange equation) Change in free energy =enthalpy change - temp x entropy change = 0 Temp = change in enthalpy/ change in entropy And convert to Degrees Celsius from Kelvin
54
Why don’t some reactions occur that should due to predictions made the feasibility
Activation energy

OFFICIAL OCR A-Level Chemistry COPY (36 decks)

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