🟫 TOPIC 13: Energetics II

Question 1

Enthalpy of atomisation
(Definition, endo/exo, examples)

Answer 1

Enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions
(Endothermic as bonds must be broken)
Eg/ Na(s) —> Na(g)
0.5Cl2(g) —> Cl(g)
Br2(l) —> 2Br(g)

Question 2

First electron affinity

Answer 2

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
(Opposite to ionisation energy)
(Exothermic because electron is attracted to nucleus)
Eg/ S(g) + e- —> S-(g)
O(g) + e- —> O-(g)

Question 3

Is second electron affinity endothermic or exothermic, and why?

Answer 3

First electron affinity is exothermic because the electron is attracted to the nucleus, but the second electron affinity is endothermic because the electron is repelled by the 1- ion, which must be overcome

Question 4

Lattice formation enthalpy (/lattice energy)
(Definition, endo/exo, example)

Answer 4

Energy change when 1 mole of an ionic solid lattice is formed from its constituent gaseous ions under standard conditions (provides a measure of ionic bond strength)
(Exothermic because energy is given out when ionic bonds are formed from gaseous ions)
Eg/ Na+(g) + F-(g) —> NaF(s)

Question 5

Are 1st/2nd ionisation energies endothermic or exothermic and why?

Answer 5

Both endothermic as the electron lost has to overcome the attraction from the nucleus. 2nd ionisation energy is more endothermic than 1st as the +1 ion is smaller than the atom, so greater attraction from the nucleus, so more energy required to overcome the force

Question 6

Enthalpy of hydration
(Definition, endo/exo, example)

Answer 6

The enthalpy change when 1 mole of a gaseous ion is completely dissolved in water under standard conditions to give 1 mole of aqueous ions and a solution of infinite dilution
(Exothermic - forming forces of attraction)
Eg/ Na+(g) —> Na+(aq)
Cl-(g) —> Cl-(aq) [Hydrate separately]

Question 7

Enthalpy of solution
(Definition, endo/exo, example)

Answer 7

Enthalpy change when 1 mole of an ionic substance dissolves in excess water to give a solution of infinite dilution under standard conditions
(Endothermic / exothermic dependent on balance)
Eg/ NaCl(s) —> Na+(aq) + Cl-(aq)
[remember to balance equation for eg/ MgCl2]

Question 8

Infinite dilution

Answer 8

So dilute (lots of water) so all substance is dissolved - if any more water is added it is so dilute that concentration will not change

Question 9

Covalent character

Answer 9

Occurs in ionic bonds when the positive ion is highly charge dense and can polarise the anion causing electrons to be shared between the two ions rather than electrons being completely localised on the anion

Question 10

What conditions make covalent character at its maximum?

Answer 10

• small cation with high charge (high charge density)
• large anion with high charge (more easily polarised)

[make sure if comparing two different compounds that you are COMPARING]

Question 11

What conditions make lattice energy at its maximum?

Answer 11

The greater the charge density of both the cation and anion, the stronger the attraction to the anion, so the more exothermic it is

Question 12

Experimental lattice energy

Answer 12

ie. The real lattice energy - calculated using Born-Haber cycle and other energy changes which are determined experimentally

Question 13

Theoretical lattice energy

Answer 13

ie. In theory (not reality) - lattice energy obtained from electrostatic theory assuming charge on ion is evenly distributed ie. Doesn’t take covalent character into account

Question 14

What is the difference (value wise) between experimental lattice energy and theoretical lattice energy? Why?

Answer 14

Experimental lattice energy is more exothermic (more negative) than theoretical lattice energy due to a degree of covalency and polarisation/ distortion of the anion which the theoretical lattice energy does not take into account

Question 15

What does a significant difference between experimental and theoretical lattice energy suggest?

Answer 15

That there is considerable covalent character

Question 16

What does it mean if enthalpy of solution is exothermic?

Answer 16

It is more likely to be soluble as it releases energy when it dissolves rather than needing energy to dissolve

Question 17

Entropy

Answer 17

A measure of the disorder in a chemical system, ie. the randomness of its arrangement
(in J/K/mol whereas enthalpy are all in kJ/mol)
[Things tend towards entropy, the more energy a system contains, the greater the entropy]
(Since some endothermic reactions can occur at room temperature it must be entropy that allows this)

Question 18

When do you get an increase in entropy?

Answer 18

• solid —> liquid (melting)
• liquid —> gas (boiling)
• solid —> aqueous (dissolving)
• increase in number of moles (molecules)
• if in same state, an increase in number of atoms
• if same number of atoms, an increase in electrons (ie. bigger atom)

Question 19

Why may a reaction that is thermodynamically spontaneous/feasible not actually take place?

Answer 19

• It may have a high activation energy, so very few, if any, collisions result in reaction (we say that the reactants are kinetically stable)
• non-standard conditions - (the value [and sign] of 🔺G will change if conditions are changed (eg. higher temperatures, concentrations or pressures)

Question 20

Polarised ion

Answer 20

An ion in which the even distribution of its electron density is distorted due to the high charge density of the ions

Question 21

What are the factors that affect enthalpy of hydration?

Answer 21

(Hydration depends on the ability of an ion to attract water so) the greater the ION charge density (greater charge and smaller IONIC radius) the greater the attraction to the water giving a greater exothermic change (more -ve)

Question 22

Why will change in entropy at a lower temperature always be greater than change in entropy at a higher temperature?

Answer 22

If temperature of surroundings is high, the air molecules already have high state of disorder so input of heat energy will only make small difference to its disorder.
If temperature is low an input of heat energy will cause a much bigger increase in disorder

Question 23

The magnitude of 🔺S Surroundings will always ___________ with increasing temperature

Answer 23

Decrease

Question 24

First ionisation energy

Answer 24

The energy required to remove an electron from each atom in 1 mole of atoms in the gaseous state
X(g) —> X+(g) + e-

(The first ionisation energy of helium is almost double that of hydrogen because it has double the nuclear charge - increased shielding does not have much of an affect)

Question 25

Second ionisation energy

Answer 25

The energy required to remove an electron from each singly charged positive ion in one mole of positive ions in the gaseous state
X+(g) —> X2+(g) + e-

Question 26

Why might a carboxylic acid such as ethanoic acid have quite a small value for enthalpy change of reaction?

Answer 26

This is because it is a weak acid and some energy is needed to break the O-H bonds to release H+ ions (as don’t dissociate fully to produce H+) (and hence less energy is released out of the system)