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Chemistry > CH17 Thermodynamics > Flashcards

CH17 Thermodynamics Flashcards

1
Q

What is Hess’s law

A

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

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2
Q

Standard enthalpy of formation

A

Enthalpy change when 1 mole of a compound is formed from its constituent elements in standard conditions

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3
Q

What is standard enthalpy of an element

A

0 by definition

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4
Q

Define standard enthalpy of combustion

A

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

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5
Q

Define standard enthalpy of atomisation

A

Enthalpy change when 1 mole of gaseous atoms is formed from a compound in its standard state in standard conditions

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6
Q

Define first ionisation energy

A

Enthalpy change when 1 mole of electrons is removed from one mole of gaseous atoms to form 1 mole of gaseous 1+ ions

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7
Q

Define second ionisation energy

A

Enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions

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8
Q

Define first electron affinity

A

Enthalpy change when 1 mole of gaseous atoms gains 1 mole of electrons to form 1 mole of gaseous 1- ions

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9
Q

Define second electron affinity

A

Enthalpy change when 1 mole of gaseous 1- ions gains 1 mole of electrons to form 1 mole of gaseous 2- ions

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10
Q

Define lattice enthalpy of formation

A

Enthalpy change when 1 mole of solid ionic lattice is formed from its constituent gaseous ions

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11
Q

Define lattice enthalpy of dissociation

A

Enthalpy change when 1 mole of solid ionic lattice is dissolved into its gaseous ions

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12
Q

Define enthalpy of hydration

A

Enthalpy change when 1 mole of gaseous ions become hydrated to infinite dilution

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13
Q

Define enthalpy of solution

A

Enthalpy change when 1 mole of solute dissolves completely in a solvent to infinite dilution

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14
Q

Define mean bond enthalpy

A

Enthalpy change when 1 mole of covalent bonds is broken, with all species in the gaseous state

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15
Q

Example equations for:
Standard enthalpy of formation
Standard enthalpy of combustion

A

Standard enthalpy of formation - Mg (s) + 1/2 O2 (g) –> MgO (s)
Standard enthalpy of combustion - CH4 (g) + 2O2 (g) –> CO2 (g) + 2H2O (l)

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16
Q

Example equation for:
Standard enthalpy of atomisation
First ionisation energy

A

Standard enthalpy of atomisation - 1/2 I2 (g) –> I(g)

First ionisation energy - Li (g) –> Li+ (g) + e-

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17
Q

Example equations for:
Second ionisation energy
First electron affinity

A

Second ionisation - Mg+ (g) –> Mg2+ (g) + e-

First electron affinity - Cl (g) + e- –> Cl- (g)

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18
Q

Example equations for:
Second electron affinity
Lattice enthalpy of formation

A

Second electron affinity - O- (g) + e- –> O2- (g)

Lattice enthalpy of formation - Na+ (g) + Cl- (g) –> NaCl (s)

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19
Q

Example equations for:
Lattice enthalpy of dissociation
Enthalpy of hydration

A

Lattice enthalpy of dissociation - NaCl (s) –> Na+ (g) + Cl- (g)
Enthalpy of hydration - Na+ (g) –> Na+ (aq)

20
Q

Example equations for:
Enthalpy of solution
Mean bond dissociation enthalpy

A

Enthalpy of solution - NaCl (s) –> Na+ (aq) + Cl- (aq)

Mean bond dissociation enthalpy - Br2 (g) –> 2Br (g)

21
Q

What is a Born-Haber cycle

A

Thermochemical cycle showing all the enthalpy changes involved in the formation of an ionic compound. State with elements in their standard states

22
Q

What factors affect the lattice enthalpy of an ionic compound

A

Size of the ions

Charge of the ions

23
Q

How can you increase lattice enthalpy of a compound - why does this increase it

A

Smaller ions - charge centres will be closer together so greater electrostatic forces of attraction

24
Q

How can Born-Haber cycles be used to see if compounds could theoretically exist

A

Use known data to predict certain values of theoretical compounds, and then see if these compounds will be thermodynamically stable

25
Q

What happens when a solid is dissolved in terms of interactions with ions of water molecules

A

The lattice is broken into gaseous ions - dissolve each gaseous ion in the water. The aqueous ions are surrounded by water molecules

26
Q

What is the perfect ionic model

A

Assumes that ions are perfectly spherical and that there is an even charge distribution. Act as point charges

27
Q

Why is this often not accurate

A

Ions are not perfectly spherical. Polarisation occurs

28
Q

Which kind of bonds will be mostly ionic

A

Between large positive ions and small negative ions

29
Q

Define terms spontaneous and feasible

A

If a reaction is spontaneous and feasible, it will take place of its own accord; does not take account of rate of reaction

30
Q

Is a reaction with a positive or negative enthalpy change more likely to be spontaneous

A

Negative - exothermic

31
Q

Define entropy

A

Disorder/randomness of a system

32
Q

Units of entropy

A

JK-1 mol-1

33
Q

What is the second law of thermodynamics

A

Entropy always increases, s it is overwhelmingly more likely for molecules to be disordered than ordered

34
Q

Is a reaction with positive or negative entropy change more likely to be spontaneous

A

Positive - Reactions always try and increase the amount of disorder

35
Q

Compare the general entropy values for solids, liquids and gases

A

Solids < liquids < gases

36
Q

How would you calculate the entropy change for a reaction

A

Entropy change = sum of product’s entropy - sum of reactant’s entropy
ΔS = ΣS (products) - ΣS (reactants)

37
Q

Define Gibbs free energy using an equation

A 
ΔG = ΔH - TΔS
G = Gibbs free energy
H = Enthalpy change
S = Entropy change 
T = Temperature
38
Q

What does the value for Gibbs free energy of a reaction show

A

If G < 0, reaction is feasible. If G = 0, reaction is just feasible. If G > 0, reaction is not feasible

39
Q

Significance of temperature at which G = 0

A

Temperature at which reaction becomes feasible

40
Q

Limitations of using G as an indicator of whether a reaction will occur

A

Only indicated if reaction is feasible. Doesnt take into account rate of reaction

41
Q

If reaction is exothermic what is value of G

A

G is always negative, so reaction is feasible

42
Q

If reaction is endothermic and entropy decreases, what is value of G

A

G always positive, so reaction is never feasible

43
Q

If reaction is exothermic and entropy decreases, what is value of G

A

Temperature dependant

44
Q

If the reaction is endothermic and entropy increases what is value of G

A

Temperature dependant

45
Q

Why is entropy 0 at 0K

A

No disorder - molecules/atoms not moving

46
Q

2 key things to look out for to decide if entropy decreases, increases or stays the same

A

number of moles - more moles made –> increase in entropy

Going from solid - liquid or liquid - gas

Chemistry (18 decks)

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