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Chemistry > 17. Thermodynamics > Flashcards

17. Thermodynamics Flashcards

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1
Q

Define the standard molar enthalpy of formation

A
  • the enthalpy change
  • when one mole of a compound is formed from its constituent elements under standard conditions
  • all reactants and products in their standard states
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2
Q

Define the standard molar enthalpy of combustion

A
  • enthalpy change
  • when one mole of substance is completely burnt in oxygen
  • all reactants and products in their standard states
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3
Q

Define the standard enthalpy of atomisation

A
  • the enthalpy change
  • which accompanies the formation of one mole of gaseous atoms
  • from the element in its standard state
  • under standard conditions
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4
Q

Write an equation for the formation of water

A

H2 (g) + 1/2 O2 (g) —> H2O (l)

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5
Q

Write an equation for the combustion of methane

A

CH4 (g) +2O2 (g) —-> CO2 (g) + H2O (l)

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6
Q

Write an equation for the atomisation of magnesium and then chlorine

A

Mg (s) —-> Mg (g)

1/2Cl2 (g) —-> Cl (g)

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7
Q

Define the first ionisation energy

A
  • standard enthalpy change
  • when one mole of electrons
  • is removed from one mole of gaseous atoms
  • leaving one mole of +1 ions
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8
Q

Write an equation for the first ionisation of Mg

A

Mg (g) —> Mg+ + e-

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9
Q

Define the second ionisation energy

A
  • standard enthalpy change
  • when one mole of electrons
  • is removed from one mole of gaseous +1 ions
  • forming one mole of +2 ions
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10
Q

Write an equation for the second ionisation energy of Mg

A

Mg+ (g) —-> Mg2+ (g) + e-

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11
Q

Define first electron affinity

A
  • standard enthalpy change
  • when a mole of electrons
  • is added to a mole of gaseous atoms
  • forming one mole of -1 ions
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12
Q

Define second electron affinity

A
  • standard enthalpy change
  • when a mole of electrons
  • is added to a mole of gaseous -1 ions
  • forming one mole of -2 ions
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13
Q

Write an equation for the first electron affinity of oxygen

A

O (g) + e- —-> O- (g)

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14
Q

Write an equation for the second electron affinity of oxygen

A

O- (g) +e- —–> O2- (g)

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15
Q

Define lattice enthalpy of formation

A
  • the standard enthalpy change
  • when one mole of solid ionic compound
  • is formed from its gaseous ions
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16
Q

Write an equation for the lattice enthalpy of formation of NaCl

A

Na+ (g) + Cl- (g) —->NaCl (s)

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17
Q

Define enthalpy of lattice dissociation

A
  • standard enthalpy change
  • when one mole of solid ionic compound
  • dissociated into its gaseous ions
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18
Q

Write an equation for the lattice dissociation of NaCl

A

NaCl (s) —> Na+ (g) + Cl- (g)

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19
Q

Define enthalpy of hydration

A
  • standard enthalpy change
  • when water molecules surround
  • one mole of gaseous ions
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20
Q

Write an equation for the hydration of Na+

A

Na + (g) +aq —> Na+ (aq)

21
Q

Define enthalpy of solution

A
  • standard enthalpy change
  • when one mole of solute dissolves completely in sufficient solvent
  • to form a solution in which the molecules or ions are far enough apart
  • to not interact with each other
22
Q

Write an equation for the enthalpy of solution of NaCl

A

NaCl (s) + aq —> Na+ (aq) + Cl- (aq)

23
Q

Define mean bond enthalpy

A
  • standard enthalpy change
  • when one mole of gaseous molecules
  • each break a covalent bond to form
  • two free radicals
  • averaged over a range of compound
24
Q

What is a Born-Haber cycle

A

A thermochemical cycle that includes all the enthalpy changes involved in the formation of an ionic compound

25
Q

Write all the equation that has the equivalent enthalpy to the formation of NaCl (Na (s) + 1/2Cl2 (g) —->NaCl (s)

A

Atomisation of Na:
Na (s) + 1/2Cl2 (g) —> Na (g) + 1/2Cl2 (g)
Atomisation of Cl:
Na (g) + 1/2Cl2 (g) —–> Na(g) + Cl (g)
First ionisation energy of Na
Na(g) + Cl (g) —-> Na + (g) + e- + Cl (g)
First electron affinity of Cl:
Na + (g) + e- + Cl (g) —-> Na+ (g) + Cl- (g)

26
Q

What is the trend in lattice enthalpies (ion size)

A

Larger ions lead to smaller lattice enthalpies

27
Q

Why does larger ions lead to smaller lattice enthalpies

A

The oppositely charges do not approach each other as closely when the ions are larger so less attraction

28
Q

What is the trend in lattice enthalpies (charge size)

A

Lattice enthalpy increases with the size of the charge

29
Q

Why does lattice enthalpy increase with the size of the charge

A

Ions with higher charge give out more energy when they come together

30
Q

Enthalpy of hydration has the same enthalpy trend as what

A

Lattice enthalpy

31
Q

The dissolving of ionic compounds in water is the sum of what three processes

A
  • breaking the ionic lattice (to give separate ions- the lattice dissociation enthalpy has to be put in)
  • hydrating the positive ions (enthalpy of hydration is given out)
  • hydrating the negative ions (enthalpy of hydration is given out)
32
Q

Write the all the equations equivalent enthalpy change for the solution of NaCl (NaCl (s) + aq —> Na+ (aq) + Cl- (aq) )

A

NaCl (s) —> Na + (g) + Cl- (g)
Na+ (g) + aq + Cl- (g) —> Na+ (aq) + Cl- (g)
Na + (aq) + Cl- (g) + aq —> Na+ (aq) + Cl- (aq)

33
Q

Why in some compounds are there large discrepancies between the experimental and theoretical values for lattice formation enthalpy

A

Covalent character

34
Q

Describe how covalent character occurs

A
  • when the positive ion is relatively small and high charge
  • and the negative ion is relatively large and has a high negative charge
  • the positive ion can approach more closely to the negative ions electron cloud
  • and distort them towards the positive ion
  • the positive ion can easily distort the negative as the negative has a large size so electrons far from nucleus -and large negative charge means lots of negative charge to distort
  • this means more electrons than expected are concentrated between the two ions
  • the negative ion is said to be polarised
35
Q

What factors increase polarisation and therefore covalent character

A

positive ion: small size, high charge

negative ion: large size, high charge

36
Q

What do the words feasible or spontaneous describe

A

Describes reactions which could take place of their own accord

37
Q

Reactions often occur spontaneously when the enthalpy change is

A

Negative

38
Q

What is the order of randomness of the states

A

Solid

39
Q

What is entropy

A

A numerical measure of disorder in a chemical system

40
Q

Apart from negative enthalpy change what else makes a reaction feasible

A

An increase (positive) entropy

41
Q

How do you calculate entropy change

A

Entropy of the products - entropy of the reactants

42
Q

What two factors govern the feasibility of a reaction

A
  • enthalpy change

- entropy change

43
Q

What is Gibbs free energy

A

A quantity that combines enthalpy and entropy change

44
Q

If the change in G is negative…

A

then the reaction is feasible

45
Q

If the change in G is positive

A

then the reaction is not feasible

46
Q

What is the equation for ΔG

A

ΔG= ΔH -TΔS

47
Q

What happens when ΔG is 0

A

This is the temperature which the reaction is just feasible

48
Q

Why might spontaneous reactions not occur (such as the oxidation of graphite)

A
  • the reaction takes place so slowly that for practical purposes it does not occur at all
  • when this happens we say graphite is thermodynamically unstable but kinetically stable

Chemistry (27 decks)

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