Unit 1: Section 6 - Thermodynamics

Question 1

What is enthalpy change?

Answer 1

The heat energy transferred in a reaction at constant pressure

Question 2

What is enthalpy change of formation?

Answer 2

ΔfH - the enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standard states - exothermic for most substances
2Na(s) + 1/2O2(g) -> Na2O(s)

Question 3

What is enthalpy of combustion?

Answer 3

ΔcH - When 1 mole of a substance undergoes complete combustion in oxygen with all substances in their standard states - exothermic
H2 (g) + 1/2O2(g) -> H2O(l)

Question 4

What is enthalpy of neutralisation?

Answer 4

ΔneutH - when 1 mole of water is formed in a reaction betweeen an acid and alkali under standard conditions - exothermic
1/2H2SO4(aq) + NaOH(aq) -> 1/2Na2SO4(aq) + H2O(l)

Question 5

What is ionisation enthalpy?

Answer 5

ΔieH
1st ionisation enthalpy - enthalpy change whe each atom in 1 mole of gaseous atoms loses 1 electron to form 1 mole of gaseous 1+ ions - endothermic
Mg(g) -> Mg+(g) +e-
2nd ionisation enthalpy - enthalpy change when each ion in 1 mole of gasous 1+ ions loses 1 electron to form 1 mole of gaseous 2+ ions - endothermic
Mg+(g) -> Mg2+ +(g) +e-

Question 6

What is electron affinity?

Answer 6

ΔeaH
1st electron affinity - enthalpy change when each atom in 1 mole of gaseous atoms gains 1 electron to form 1 mole of gaseous 1- ions - exothermic for many non-metals
O(g) + e- -> O-(g)
2nd electron affinity - enthalpy change when each ion in 1 mole of gasous 1- ions gains 1 electron to form 1 mole of gasoues 2- ions - endothermic
O-(g) + e- -> O2-(g)

Question 7

What is enthalpy of atomisation?

Answer 7

ΔatH - when 1 mole of gaseous atoms is produced from an element in its standard state - endothermic
1/2I2(s) -> I(g)

Question 8

What is Hydration enthalpy?

Answer 8

ΔhydH
When 1 mole of gaseous atoms dissolve in water (become hydrated) - exothermic
Mg2+(g) + aq -> Mg2+(aq)

Question 9

What is enthalpy of solution?

Answer 9

ΔsolH
When 1 mole of an ionic solid disolves in an amount of water large enough so that the dissolved ions are well separated and don’t interact with each other - varies
MgCl2(s) + aq -> Mg2+(aq) + 2Cl-(aq)

Question 10

What is bond dissociation enthalpy?

Answer 10

ΔdisH
When 1 mole of covalent bonds is broken in the gaseous state - endothermic
I2(g) -> 2I (g)

Question 11

What is lattice enthalpy of formation?

Answer 11

ΔLEFH
When 1 mole of a solid ionic compound is formed from its consistuent ions in the gas phase - exothermic
Mg2+(g) + 2Cl-(g) -> MgCl2(s)

Question 12

What is lattice enthalpy of dissociation?

Answer 12

When 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase - endothermic
MgCl2(s) -> Mg2+(g) + 2Cl-(g)

Question 13

What is enthalpy of vaporisation?

Answer 13

ΔvapH
Whe 1 mole of a liquid is turned into a gas - endothermic
H2O(l) -> H2O(g)

Question 14

What is enthalpy of fusion?

Answer 14

ΔfusH
When 1 mole of a solid is turned into a liquid - endothermic
Mg(s) -> Mg(l)

Question 15

What are Born-Haber cycles used for?

Answer 15

Used to calculate lattice enthalpies

Question 16

What is the perfect ionic model of a lattice?

Answer 16

Assumes that all the ions are spherical, and have their charge evenly distributed around them

Question 17

Why is the perfect ionic model often not accurate?

Answer 17

• Ions are not perfectly spherical
• Polarisation often occurs when small positive ions or large negative ions are involved
• Some lattices are not regular and the crystal structure can differ

Question 18

How do we know about covalent character?

Answer 18

1. The lattice enthalpy found experimentally is often not the theoretical lattice enthalpy
2. This is evidence that most ionic compounds have some covalent character

Question 19

How does covalent character arise?

Answer 19

1. The positive and negative ions in a lattice aren’t exactly spherical
2. Positive ions polarise neighbouring negative ions to different extents
3. The more polarisation there is, the more covalent the bonding will be

Question 20

What characteristics does covalent character affect?

Answer 20

1. Solubility - makes the compound less soluble
2. General conductivity

Question 21

What 2 things happen when a solid ionic lattice dissolves?

Answer 21

1. The bonds between the ions break to give free ions - endothermic
2. Bonds between the ions and the water are made - exothermic
   The enthalpy change of solution is the overall effect on the enthalpy of these 2 things

Question 22

When do substances dissolve?

Answer 22

If the energy released is roughly the same, or greater than the energy taken in

Question 23

What is entropy?

Answer 23

1. A measure of the number of ways that particles can be arranged and the number of ways the energy can be shared out between the particles in a system
2. The more disordered the particles are, the higher the entropy is

Question 24

What affects entropy?

Answer 24

Physical state - solids have the lowest entropy as they just vibrate about a fixed position but gases have the highest entropy as they have the most disordered arrangements
More particles - more ways they can be arranged and the more energy there is

Question 25

What makes a substance more stable?

Answer 25

1. Substances tend towards disorder - they're more energtically stable when there's more disorder 2. This is why some reactions are feasible even when the enthalpy change is endothermic

Question 26

How can you calculate the entropy change for a reaction?

Answer 26

ΔS = Sproducts - Sreactants

Question 27

What is the standard entropy of a substance?

Answer 27

The entropy of 1 mole of that substance under standard conditions

Question 28

What is free energy change?

Answer 28

ΔG, a measure used to predict whether a reaction is feasible If ΔG is negative or equal to 0 then the reaction might happen by itslef

Question 29

How do you calculate free energy?

Answer 29

ΔG = ΔH - TΔS Make sure the units are all the same

Question 30

How does temperature affect the feasibility of reactions?

Answer 30

[https://www.savemyexams.com/a-level/chemistry/aqa/17/revision-notes/5-advanced-physical-chemistry-a-level-only/5-1-thermodynamics-a-level-only/5-1-8-reaction-feasibility/](http://) | The table - summary for temperature and Gibbs free energy

Question 31

How to calculate the temperature at which a reaction becomes feasible?

Answer 31

1. When ΔG=0 a reaction is just feasible 2. Rearrange the equation to T=ΔH/ΔS 3. In an exam question write down the og equation and the replacement of ΔG with 0 to gain all marks

Question 32

What affects lattice enthalpy?

Answer 32

Size of the ion - the bigger the ion, the lower the magnitude of lattice enthalpy Charge - higher the magnitude of charge, the bigger the magnitude of lattice enthalpy

Question 33

When is ΔH and ΔS favourable?

Answer 33

ΔH - as negative as possible ΔS - as positive as possible

Question 34

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

Answer 34

1.Use known data to predict certain values of theoretical compounds 2.See if these compounds would be thermodynamically stable

Question 35

What does it mean if a reaction is spontaneous and feasible mean?

Answer 35

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

Question 36

What are the limitations of using G as an indicator of whether a reaction will occur?

Answer 36

* Gibbs free energy only indicates if a reaction is feasible * It does not take into account the rate of reaction (the kinetics of the reaction) * In reality, many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occuring