Thermodynamics

Question 1

Enthalpy change definiton

Answer 1

Heat energy change at constant pressure.

Question 2

Enthalpy of formation definition

Answer 2

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states. EXOTHERMIC

H2(g) + ½ O2(g) →H2O(l)

Question 3

Enthalpy of combustion definition

Answer 3

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states.
EXOTHERMIC

CH4(g) + 2O2(g) →CO2(g) + 2H2O(l))

Question 4

Enthalpy of neutralisation definition

Answer 4

Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions
EXOTHERMIC

Question 5

Enthalpy of atomisation definition

Answer 5

Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.
ENDOTHERMIC

Question 6

First ionisation energy definition

Answer 6

Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions.

Mg(g) →Mg+(g) + e-

ENDOTHERMIC

Question 7

Second ionisation energy definition

Answer 7

Enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions.

Mg+(g) →Mg2+(g) + e-
ENDOTHERMIC

Question 8

First electron affinity definition

Answer 8

Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1– ions.

O(g) + e- → O-(g)
EXOTHERMIC

Question 9

Second electron affinity definition

Answer 9

Enthalpy change when each ion in one mole of gaseous 1– ions gains one electron to form one mole of gaseous 2– ions.

O-(g) + e- → O2-(g)
ENDOTHERMIC

Question 10

Lattice enthalpy of formation definition

Answer 10

Enthalpy change when one mole of a solid ionic compound is formed from into its constituent ions in the gas phase.

Na+(g) + Cl- (g) → NaCl(s
EXOTHERMIC

Question 11

Lattice enthalpy of dissociation definition

Answer 11

Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase
ENDOTHERMIC

Question 12

Enthalpy of hydration definition

Answer 12

Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water).

Na+(g) + aq → Na+(aq
EXOTHERMIC

Question 13

Enthalpy of solution definition

Answer 13

Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.

NaCl(s) + aq → Na+(aq) + Cl-(aq)
ENDO/EXO

Question 14

Bond dissociation enthalpy definition

Answer 14

Enthalpy change when one mole of covalent bonds is broken in the gaseous state.
ENDOTHERMIC

Question 15

Mean bond enthalpy definition

Answer 15

Enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds.

CH4 (g) → C(g) + 4H(g) = +1664kJ mol-1 / 4

ENDO/EXO

Question 16

Enthalpy of vaporisation definition

Answer 16

Enthalpy change when one mole of a liquid is turned into a gas
ENDOTHERMIC

Question 17

Enthalpy of fusion definition

Answer 17

Enthalpy change when one mole of a solid is turned into a liquid
ENDOTHERMIC

Question 18

How to draw a born-haber cycle?

Answer 18

1) Start with solid on bottom line

2)Enthalpy of formation ( writing the elements in their standard states)

3)enthalpy of atomisation of the elements ( one mole of gaseous atoms)

4) ionisation energy (mainly metal first)

5) 2nd ionisation energy/affinity depending on the q’s

Question 19

What areas of born-haber cycle are negative?

Answer 19

enthalpy of formation (bottom left)

(left hand side)

Question 20

How is a lattice enthalpy calculated?

Answer 20

using a born-haber cycle

Question 21

what determines the theoretical and experimental values of a lattice enthalpy?

Answer 21

it depends on how purely ionic the compound is.

Question 22

What can data referring to a perfectly ionic model assume?

Answer 22

the theoretical lattice enthalpy

Question 23

What makes a perfect ionic model perfect?

Answer 23

• Ions that are perfectly spherical
• The charge is evenly distributed in this sphere (point charges)

Question 24

Why is the experimental value different from the theoretical value of lattice enthalpy?

Answer 24

• Suggests that the compound being experimented on doesn’t follow the perfectly ionic model and has covalent characteristics.
• The positive ion distorts the charge distribution in the negative ion ( positive ion polarises the negative ion)

Question 25

What does more polarisation indicate?

Answer 25

The more covalent character there will be

Question 26

What can lattice enthalpy values tell us?

Answer 26

How much a substance is 'purely ionic'

Question 27

What does bigger lattice enthalpy suggest?

Answer 27

more polarisation so the greater the covalent character in compound.

Question 28

What process occurs when enthalpy change of solution and when can it happen?

Answer 28

The ionic lattice (in solid form) reacts with water to form free moving ions. There are new bonds formed between ions and water. The ions are hydrated. It can only happen if the new bonds formed are the same strength or greater than those broken. If this isn't the case then it's very unlikely to dissolve. Soluble substances tend to have exothermic enthalpies of solution for this reason.

Question 29

What is entropy?

Answer 29

Entropy is the measure of disorder in a system. Its the number of ways energy can be shared out between particles.

Question 30

What does more disorder indicate about the level of entropy?

Answer 30

The more disorder the higher the level of enthalpy.

Question 31

what are the levels of disorders in solids, liquids and gases?

Answer 31

solids have the lowest level of disorder as particles are arranged in rows. Liquids and gases are more disordered.

Question 32

What factor affects entropy change?

Answer 32

The number of particles also affect entropy change. If there is an increase in moles then entropy increases as there are more ways for energy to be distributed.

Question 33

Increasing entropy is....

Answer 33

energetically favourable and some reactions that are enthalpically unfavourable (endothermic) can still spontaneously react of changes in entropy overcome changes in enthalpy.

Question 34

What type of reaction is enthalpically unfavourable?

Answer 34

Endothermic reactions

Question 35

What type of reaction is entropically favourable?

Answer 35

When there is an increase in moles from reactants --> products when the state of product changes to one of increased disorder

Question 36

How to calculate enthalpy of solution?

Answer 36

- using a hess cycle basically down arrow from breaking up solid lattice ( enthalpy of lattice dissociation) up arrow when gaseous ions are dissolved in water (enthalpy of hydration)

Question 37

What is the Gibbs free energy equation and what does it tell us?

Answer 37

It tells us if a reaction is feasible or not. ΔG=ΔH-TΔS Gibbs free energy = Enthalpy change (mol-1) - temperature (k) x entropy change (jk-1mol-1)

Question 38

What makes a reaction feasible in theory?

Answer 38

If ΔG (Gibbs free energy) is negative or 0

Question 39

What is the rearranged equation and method to calculate temperature of which a reaction becomes feasible?

Answer 39

T= ΔH/ ΔS Calculate entropy change ΔS: S.Products- S.reactants