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AQA Chemistry A Level > Thermodynamics > Flashcards

Thermodynamics Flashcards

Enthalpy changes + perfect ionic model

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1
Q

Exothermic processes

A

Bond forming
– enthalpy change

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2
Q

Endothermic processes

A

Bond breaking
+ enthalpy change

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3
Q

Enthalpy change of formation
+ formation of NaCl

A

The enthalpy change when 1 mole of a substance is formed from its component elements in their standard states

Na(s) + 1/2Cl2(g) –> NaCl(s)

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4
Q

Enthalpy change of lattice formation + Formation of NaCl

A

The enthalpy change when 1 mole of an ionic compound is formed from its component ions in a gaseous state

Na+(g) + Cl-(g) –> NaCl(s)

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5
Q

What factors affect strength of enthalpy change of lattice formation

A

Charge on ion
Size of ion

greater charge + smaller ion = more exothermic

Charge > size

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6
Q

What energy change is lattice formation

A

Exothermic, forms ionic bond

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7
Q

Enthalpy change of atomisation + atomisation of Na + Cl

A

The enthalpy change when 1 mole of gaseous atoms are FORMED

Na(s) –> Na(g)
1/2Cl2(g) –> Cl(g)

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8
Q

What type of energy change is atomisation

A

Endothermic, breaking bonds of attractions

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9
Q

1st ionisation energy
+ 1st ionisation energy of Na

A

The enthalpy change to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

Na(g) –> Na+(g) + e-

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10
Q

What type of energy change is ionisation energy

A

Endothermic, overcoming electrostatic bond between nucleus and outer electron

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11
Q

What factors affect strength of ionisation energy

A

Nuclear charge, size of ion, electron shielding

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12
Q

Bond dissociation energy + Bond dissociation energy of Cl2

A

The enthalpy change when 1 mole of covalent bonds are broken in the gaseous state

Cl2(g) –> 2Cl(g)

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13
Q

1st electron affinity + 1st electron affinity of Cl

A

The enthalpy change of to add 1 mole of electrons to 1 mole of gaseous atoms to make 1 mole of gaseous ions with a 1- charge

Cl(g) + e- –> Cl-(g)

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14
Q

What type of energy change is 1st electron affinity

A

Exothermic, forming electrostatic attraction between nucleus and ion

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15
Q

2nd electron affinity + 2nd electron affinity of O-

A

The enthalpy change of to add 1 mole of electrons to 1 mole of gaseous atoms with a 1- charge to make 1 mole of gaseous ions with a 2- charge

O-(g) + e- –> O^2-(g)

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16
Q

What type of energy change is 2nd electron affinity

A

Endothermic, although a bond is being formed, negatively charged electrons are being added to a negatively charged ion which causes forces of repulsion, requiring energy to form the bond

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17
Q

What are perfect ionic compounds

A

Ionic compounds with :
Perfect spheres
No covalent character

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18
Q

What is meant by covalent character?

A

Where the electron clouds in an ionic bond overlap, forming a weak covalent bond

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19
Q

If covalent character is present in an ionic bond, what will happen to the lattice enthalpy?

A

It will become more exothermic

20
Q

What 2 factors affect the strength of an ionic bond?

A

Ionic charge
Ionic radius

21
Q

What does theroretical lattice enthalpy assume?

A

That the ionic compounds resemble the perfect ionic model

22
Q

What does it mean if the theoretical and experimental value of lattice enthalpy are similar?

A

Compound obeys the perfect ionic model

23
Q

What does it mean if the theoretical and experimental value of lattice enthalpy are not similar?

A

The bonding is stronger and so contains covalent character, does not obey perfect ionic model

24
Q

Why do some ionic compounds contain covalent character?

A

If the positve ion is polarising, the electron cloud is pulled from the negative ion, distorting the ion and overlapping clouds

25
Q

What factors increase the polarising ability of the + ion in an ionic compound?

A

Small ionic radius
Large ionic charge

26
Q

What factors increase the polarisability of the - ion in an ionic compound?

A

Larger ionic radius
Low ionic charge

27
Q

When considering factors affecting the polarising of an ionic bond, is the charge or radius of the ions more significant?

A

Charge is a more significant factor to consider.

28
Q

What is entropy?

A

The measure of disorder in a system

29
Q

What factors affect the entropy of a system

A

States - Gas>liquid>solid (gas causes greater entropy)

Greater Mr = greater entropy

Number of species/ more species = greater entropy

30
Q

How is entropy change calculated?

A

Entropy of products - reactants

31
Q

What is the gibbs free - energy equation?

A

ΔG = ΔH - TΔS

T must be in kelvin
ΔS must be KJ K-1 mol-1

32
Q

What does ΔG mean + its units

A

If the value is 0 or below, a reaction is feasible at that temperature

Kjmol^-1

33
Q

Why may some reactions not occur even if the reaction is deemed feasible?

A

Activation energy is too high

34
Q

How can you use a graph to calculate gibbs free energy?

A

ΔG = ΔH - T ΔS

Y = -mx + c
ΔG = - ΔST + ΔH
Where -ΔS = gradient

35
Q

What is enthalpy of solution + Enthalpy of solution of MgCl2

A

The enthalpy change when 1 mole of an ionic substance is completely dissolved in water to infinite dilution

MgCl2(s) –> Mg^2+(aq) + 2Cl-(aq)

36
Q

What is enthalpy of hydration + enthalpy of hydration of Mg^2+

A

The enthalpy change when 1 mole of gaseous ions is diluted to 1 mole of aqueous ions

Mg^2+(g) –> Mg^2+(aq)

37
Q

What is lattice dissociation enthalpy + equation using MgCl2

A

The enthalpy change when 1 mole of an ionic solid is converted into gaseous ions

MgCl2(s) –> Mg^2+(g) + 2Cl-(g)

38
Q

What is the formula to find enthalpy change of solution?

A

ΔH solution = ΔH L.D + ΔH hydration of both ions

39
Q

What does it mean if the enthalpy change of L.D is more than hydration?

A

Solution enthalpy will be endothermic

40
Q

What does it mean if the enthalpy change of L.D is less than hydration

A

Solution enthalpy will be exothermic

41
Q

What factors can affect hydration enthalpy?

A

Size of ion - smaller means more exothermic
Charge of ion - greater means more exothermic

42
Q

What factors can affect lattice dissociation

A

SIze of ion - smaller means more endothermic (stronger electrostatic forces need more energy to overcome)

Charge of ion - larger means more endothermic (requires more energy to overcome)

43
Q

Why are ionic compounds with very endothermic lattice dissociation enthalpies insoluble?

A

Lots of energy needed to break the electrostatic forces.

44
Q

What is the nature of attraction when talking about enthalpy of hydration

A

The forces between the partial charges on the water molecule and the ions

45
Q

How does the enthalpy of solution affect the solubility of the compound?

A

If enthalpy of solution exothermic, solubility decreases as temperature increases

If enthalpy of solution is endothermic, solubility increases with temperature

AQA Chemistry A Level (43 decks)

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