Thermodynamics

Question 1

Define standard enthalpy of formation:

Answer 1

The enthalpy change when one mole of a substance is formed from its elements, under standard conditions, with all substances in their standard state.

Question 2

Define standard enthalpy of combustion:

Answer 2

The enthalpy change when one mole of a substance is burnt completely in oxygen, under standard conditions, with all substances in their standard state.

Question 3

State Hess’s Law:

Answer 3

The enthalpy change (at constant pressure) in a reaction is independent of the route taken (and depends only on the initial and final states).

Question 4

Define enthalpy of atomisation:

Answer 4

The enthalpy change when 1 mole of gaseous atoms are formed from its element in its standard state.

Question 5

Define enthalpy of bond disassociation:

Answer 5

The standard enthalpy change to break 1 mol of covalent bonds in a gaseous molecule to form gaseous atoms.

Question 6

Define electron affinity:

Answer 6

The first electron affinity is the enthalpy change when 1 mole of gaseous atoms gain one mole of electrons to form one mole of gaseous atoms with a -1 charge.

Question 7

Define second electron affinity:

Answer 7

The enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.

Question 8

Why is the first electron affinity exothermic and the second endothermic?

Answer 8

The first is exothermic because there is an attraction between the nucleus and the electrons (making bonds). That nuclear attraction was stronger than the repulsion felt by the electron.

The second is endothermic because it takes energy to overcome the repulson between the negative ion and the electron.

Question 9

Define enthalpy of lattice formation:

Answer 9

The standard enthalpy change when 1 mole of a solid ionic lattice is formed from its constituent ions in gaseous form.

Question 10

Why is the enthalpy of formation always exothermic?

Answer 10

When a lattice forms, many electrostatic attractions between oppositely charged ions are made. Making bonds is exothermic.

Question 11

Define enthalpy of lattice disassociation:

Answer 11

The standard enthalpy change when 1 mole of a solid ionic lattice is disassociated into its constituent ions in gaseous form.

Question 12

Why is the enthalpy of lattice disassociation always endothermic?

Answer 12

This reaction is overcoming many electrostatic attractions between oppositely charged ions. Breaking bonds is endothermic.

Question 13

The strength of a lattice enthalpy of formation depends on the following factors:

Answer 13

1. The charge on the ion
   The greater the charge on the ion, the greater the attraction between the ions so the stronger the lattice enthalpy
2. The size of the ion
   The larger the ions, the weaker the lattice. As ions increase in size, their charge density decreases so the ions have a weaker attractive force between them.

Question 14

Which one has a higher lattice enthalpy of formation and why?
Out of NaF and LiF?

Answer 14

LiF

Na+ is bigger than Li+ so Na+ has a smaller charge density than Li+.

So when the Na+ forms a bond with F-, less energy is released compared with when Li+ joins with F-.

NaF forms a weaker ionic bond, so a smaller lattice enthalpy of formation.

Question 15

Why is the lattice enthalpy of formation for Na2O more exothermic than NaF

Answer 15

The oxide ion has a higher charge than the fluoride ion therefore the oxide ion has a higher charge density. Thus, more energy is released when a sodium ion bonds to an oxide ion, compared to when it bonds to fluoride.

Question 16

What do theoretical lattice enthalpies assume?

Answer 16

A perfect ionic model where the ions are 100% ionic and spherical and the attractions are purely electrostatic.

Question 17

What does it mean to be ionic with covalent character?

Answer 17

Means the charge cloud of the negative ion is distorted.

Question 18

Here are some lattice enthalpy of formation data for AgCl:

Experimental data = -890 kjmol-1
Theoretical data = -769kjmol-1

Why are the 2 values different?

Answer 18

• The theoretical value does not take into consideration covalent character
• The experimental value is more exothermic suggesting that there are more bonds formed than just ionic.
• The more covalent character, the bigger the difference between the 2 values.T

Question 18

Differences between theoretical and Born Haber (experimental) lattice
enthalpies:

Answer 18

The Born Haber lattice enthalpy is the real experimental value.
When a compound has some covalent character- it tends towards giant covalent so the lattice is stronger than if it was 100% ionic. Therefore the Born-Haber value would be larger than the theoretical value.

There are more bonds formed so there’s more exothermic enthalpy of lattice formation.

Question 19

There is a tendency towards covalent character in ionic substances when:

Answer 19

• The positive ion is small
• The positive ion has a greater positive charge
• The negative ion is large
• The negative ion has a greater negative charge

Question 20

What does polarizing power mean?

Answer 20

The ability of a cation to distort an anion is known as polarizing power.

Question 21

Why do larger anions such as iodide have a greater polarising power than smaller ions such as fluroide?

Answer 21

Iodide ions have a lower charge density.
Their electrons are further from the nucleus as they have more shielding therefore they don’t have very strong nuclear attraction and their negative ion can get distorted easily.

Question 22

Define enthalpy of solution:

Answer 22

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.e

Question 23

Define enthalpy of hydration:

Answer 23

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

Na+ (g) –> Na+ (aq)

Question 24

What causes sodium chloride to dissolve?

Answer 24

The polar ends of the water molecules are attracted to the cations and the anions in the sodium chloride ionic lattice.
This attraction disrupts the attraction between the sodium and chloride ions in the lattice so the ions are then surrounded by water molecules.

Question 25

Why is it that as the ionic radius increases, the enthalpy of hydration gets less exothermic?

Answer 25

The ions have a lower charge density so there is weaker electrostatic attraction between the positive cation and the partially negative oxygen in the water molecule.

Question 26

Why is enthalpy of hydration always negative?

Answer 26

We’re always forming attractions with gaseous ions, exothermic process.

Question 27

What does it mean if a reaction is spontaneous?

Answer 27

If it occurs with the addition of external energy.
It just happens randomly.DE

Question 28

Define entropy:

Answer 28

A measure of disorder to the system
The more disorder, the greater the system
More disorder means entropy change is positive.

Units are J/mol/k

Question 29

What are the physical processes that could cause an increase in entropy (increase the disorder)?

Answer 29

• If solids melt
  Solids are in a regular arrangement of particles, when they melt, there’s less regular arrangement so there’s more disorder
• Liquids boil
  This is much bigger increase in entropy than when solids melt
• Ionic solids dissolve in water
  There’s regular arrangement in ionic solids. upon dissolving, it becomes more disordered.
• The moles of a substance increases.
  The more particles, the more disorder.
• The temperature increases
  Lower temperature means less energy. Higher temperature means more energy and more disorder.

This is why the units include k, because entropy is temperature dependent.

Question 30

What does it mean for a reaction be feasible?

Answer 30

Gibb’s energy is zero or negative
Feasible reaction will occur spontaneously.