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A level chemistry > 5.2 Energy > Flashcards

5.2 Energy Flashcards

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1
Q

Define lattice enthalpy

A

enthalpy change associated with the formation of one mole of an ionic lattice from its gaseous ions under standard conditions K+(g) +Cl- –> KCl(s)

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2
Q

Define enthalpy change of formation

A

The enthalpy change associated with the formation of one mole of a compound from its constituent elements in their standard conditions

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3
Q

How do lattice enthalpy values indicated the relative strength of ionic bonds

A

More exothermic lattice enthalpy values mean stronger ionic bonds and higher melting and boiling point
It also indicates smaller ions and larger charges

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4
Q

What can we use to measure lattice enthalpy

A

Born - Haber cycles because we can’t measure lattice enthalpy directly

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5
Q

What are the key features of a born haber cycle

A

continuous cycle
includes on step that shows the formation of one mole of the solid ionic lattice from the gaseous atoms
the remaining steps show intermediate changes
the lattice enthalpy can be calculated by applying hess’ law

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6
Q

Define the enthalpy change of atomisation

A

The enthalpy change associated when one mole of gaseous atoms is formed from its element in its standard state
K(s) –> K(g)

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7
Q

Define first ionisation energy

A

When one mole of gaseous 1+ ions is formed from its gaseous atoms
K (g)- e –> K+(g)

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8
Q

Define second ionisation energy

A

When one mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions
Mg+(g) - e- –> Mg2+(g)

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9
Q

Define the first electron affinity

A

when one mole of gaseous 1- ions is formed from gaseous atoms
Cl (g)+ e- –> Cl- (g)

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10
Q

Define second electron affinity

A

When one mole of gaseous 2- ions is formed from one mole of gaseous 1- ions

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11
Q

Describe the steps in a born haber cycle to determine lattice enthalpy of sodium chloride

A

atomisation of sodium
atomisation of chlorine
ionisation of sodium
electron affinity of chlorine

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12
Q

What happens when a solid dissolves

A

The ionic lattice breaks down
the free ions become a part of the solution (hydration )

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13
Q

Define the standard enthalpy of solution

A

The enthalpy change that take place when one mole of a solute is completely dissolved in water under standard conditions
KCl (s) = + aq –> K+ (aq) + Cl- (aq)

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14
Q

What is the enthalpy change associated with the breakdown of an ionic lattice

A

Lattice enthalpy of dissociation

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15
Q

How do lattice enthalpies differ energetically

A

Association is exothermic
Dissociation is endothermic

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16
Q

What dictates the size of the lattice enthalpy

A

the size of the ions involved
the charge on the ions

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17
Q

How does ionic radius affect lattice enthalpy

A

smaller ions can get closer together and will attract one another more strongly and give rise to a more exothermic lattice enthalpy values

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18
Q

How does ionic charge affect lattice enthalpy

A

Ions with higher charges cause greater electrostatic attraction and in turn more exothermic lattice enthalpy values

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19
Q

hat ions have the most negative lattice enthalpy values

A

The smallest most highly charged ions will give rise to the most exothermic lattice enthalpies as they can pack closer to the oppositely charged ions with a higher attraction

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20
Q

Define the standard enthalpy of hydration

A

The enthalpy change that takes place when dissolving one mole of gaseous ions in water under standard conditions
K+ (g) –> K+(aq)

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21
Q

What occurs during hydration

A

The positive ions will be attracted to the slightly negative oxygen
The negative ions will be attracted to the slightly positive hydrogen
the water molecules will completely surround the ions

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22
Q

What kind of process is hydration

A

Exothermic

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23
Q

What dictates enthalpy of hydration

A

Ionic radius
ionic charge

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24
Q

How does ionic radius affect enthalpy of hydration

A

ions with a smaller radii are able to get closer to the water molecule and are able to attract them more strongly so on hydration more energy is released and becomes more exothermic

25
Q

How does ionic charge affect enthalpy of hydration

A

The higher the charge on an ion the greater the attraction it will have on the water molecule giving a more exothermic value

26
Q

define entropy

A

the quantitative measure of the degree of disorder in a system

27
Q

Define standard entropy

A

The entropy content of one mole of a substance under standard conditions

28
Q

define standard entropy change of a reaction

A

the entropy change that accompanies a reaction in the molar quantities expressed in the chemical reaction under standard conditions

29
Q

what are the units for entropy

A

J/K/mol

30
Q

what figure is entropy

A

Always a positive figure

31
Q

When is entropy 0

A

At 0K in perfect crystals

32
Q

What state has the highest entropy

A

gas

33
Q

Why do gases have a higher entropy

A

more spreading out of energy and more random arrangement of molecules

34
Q

How is entropy affected by temperature

A

entropy increases as temperature increases

35
Q

How does entropy change when dissolving ionic solids

A

the reaction results in the products having more disorder
Ions can spread out and the positions of the ions are more disordered within the lattice

36
Q

How does the number of gas molecules affect entropy

A

An increase in the number of gas molecules causes an increase in entroy

37
Q

How do you calculate entropy change

A

entropy of products - entropy of reactants

38
Q

How do you calculate total change in entropy

A

Entropy change of the system + entropy change of the surroundings

39
Q

What do positive and negative entropy changes indicate

A

+ve - the change made the system more random

-ve - made the system more ordered

40
Q

What is the Gibbs equation

A

ΔG = ΔH-TΔS

41
Q

What must the value of ΔG must be for a reaction to occur spontaneously

A

negative

42
Q

What do we assume Δ G is when calculating free energy

A

0

43
Q

What are the limitations of using ΔG

A

whether or not a reaction happens also depends on factors such as
activation energy and the reaction might be very slow

44
Q

What is a half cell

A

comprises of an element in its two oxidation states

The simplest has a metal placed in its aqueous sloution of ions

45
Q

What is a hydrogen half cell

A

Contains hydrogen gas in contact with hydrogen ions

46
Q

Why is a platinum electrode used in a hydrogen half cell

A

No solid piece of metal can act as the electrode except platinum as it is inert and does not react at all
Its sole purpose is to be in contact with both the H2 and H+ ions and allow transfer of electrons in and out of the half cell via a connecting wire

47
Q

What are the standard conditions in a half cell

A

298k
100kPa
1mol/dm

48
Q

What is the standard electrode potential of a half cell

A

the e.m.f. of a half cell compared to a standard hydrogen half cell measured at 298k with solution concentrations of 1mol/dm at 100kPa

49
Q

what are metal ion/ metal ion half cells

A

This type of cells contain ions of the same element in different oxidation states for example Fe 3+ and Fe2+
this type of half cell requires a platinum electrode

50
Q

Define equimolar

A

solution containing ions at same concentrations

51
Q

How do we measure standard electrode potentials using a hydrogen half cell

A

the tendency for different half cells to accept or release electrons is measured as an electromotive force which is measured in volts
The hydrogen half cell can be used as a reference as it has an emf of 0V

52
Q

What is a salt bridge

A

This connects the two solutions and allows ions carrying charge to be transferred between the half cells

Filter paper soaked in KNO3 or NH4NO3

53
Q

What is the relationship between the E value and the metal

A

The more negative the value the greater the tendency towards the half cell undergoing oxidation (loss of electrons) so the reaction shifts to the left
Fe2+ + 2e- ⇌ Fe

whereas Cl2 +2e- ⇌ 2Cl- is positive so it is more likely to be reduced and shifts to the right

54
Q

How do you make a cell from two half cells

A

Two half cells with different electrode potentials are joined using a wire and salt bridge
One half cell releases electrons and the other gains electrons

55
Q

How would you determine which half cell is being reduced and oxidised

A

the half cell with the more negative value releases electrons more readily

56
Q

How is a reading aquired in a cell

A

The negative terminal releases electrons into the wire and they flow to the other electrode

The reading of the voltmeter measures the difference between electrode potentials of the half cells,
The bigger the value the further away from the equilibrium position the reaction moves

57
Q

How would you calculate E of the cell

A

E of positive terminal - E of negative terminal

58
Q

What can we find out from reactions by using standard electrode potentials

A

a species will react with the species below it

A level chemistry (14 decks)

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