Energetics

Question 1

Standard enthalpy change of combustion (ΔH_c^θ)

Answer 1

The enthalpy change when 1 mole of a substance is completely burnt in excess oxygen under standard condition of 298K and 1 bar

Exothermic and negative

Question 2

Standard enthalpy change of formation (ΔH_f^θ)

Answer 2

The enthalpy change when 1 mole of a pure substance is formed from its constituent element in their standard states, under standard condition of 298K and 1 bar

Can be either exo or endothermic

Question 3

Standard enthalpy change of neutralisation (ΔH_neut^θ)

Answer 3

The enthalpy change when 1 mole of water is formed when an acid neutralizes a base and is carried out in an infinitely dilute aqueous solution, under standard condition of 298K and 1 bar

Usually ΔH_neut^θ = -53 KJ mol^-1 (exothermic)

Question 4

Lattice energy (ΔH_LE)

Answer 4

The energy released when one mole of an ionic solid is formed from its isolated gaseous ions from an infinite distance apart

ΔH_LE ∝ q⁺q^- / r⁺ + r^-

Exothermic and negative

Question 5

First ionisation energy (1^st I.E.)

Answer 5

The energy required to remove one mole of electrons from one of gaseous atoms to give one mole of gaseous ions with a single positive charge

Usually exothermic and negative

Question 6

Bond energy (B.E.)

Answer 6

Energy absorbed when 1 mole of covalent bonds between 2 atoms in a gaseous molecule are broken

Endothermic and positive

Question 7

Standard enthalpy change of atomisation (ΔH_at^θ)

Answer 7

The enthalpy change when 1 mole of isolated gaseous atoms are produced from the element in its standard state, under standard condition of 298K and 1 bar

Endothermic and positive

Question 8

Electron affinity (ΔH_ea)

Answer 8

The enthalpy change when 1 mole of gaseous atoms or negatively charged ions gain one mole of electrons

The first electron affinity is always exothermic and negative

Question 9

Standard enthalpy change of hydration (ΔH_hyd^θ)

Answer 9

The enthalpy change when 1 mole of gaseous ions is dissolved when a large amount of water, under standard condition of 298K and 1 bar

Exothermic and negative

Affected by ionic size and charge

ΔH_hyd^θ ∝ q⁺/r⁺
ΔH_hyd^θ ∝ q^-/r^-

Question 10

Standard enthalpy change of solution (ΔH_sol^θ)

Answer 10

The enthalpy change when 1 mole of a solute is completely dissolved in a solvent to form an infinitely dilute solution, under standard condition of 298K and 1 bar

Can be either endothermic or exothermic

ΔH_sol^θ = -ΔH_LE + ∑ΔH_hyd^θ

Question 11

Calculation of ΔH from experimental data

Answer 11

q = mcΔT

Question 12

Calculation of ΔH_r from bond energy

Answer 12

ΔH_r = ∑B.E. of reactants - ∑B.E. of products

Question 13

Calculation of ΔH_r from ΔH_c^θ

Answer 13

ΔH_r = ∑ΔH_c^θ of reactants - ∑ΔH_c^θ of products

Question 14

Calculation of ΔH_r from ΔH_f^θ

Answer 14

ΔH_r = ∑ΔH_f^θ of products - ∑ΔH_f^θ of reactants

Question 15

Entropy S

Answer 15

Measure of the degree of discord of a system
Units: J mol^-1 K^-1

Question 16

Factors affecting entropy

Answer 16

Entropy increases when disorder of a system increases. Disorder of a system increases when the orderly arrangement of particles breaks down or when there are more ways of arranging particles or energy quanta

1. Change in temperature
2. Change in phase
3. Change in number of particles (esp for gases)
4. Mixing of particles

always link back to more/less ways of arranging the particles

Question 17

Gibbs Free Energy ΔG^θ

Answer 17

ΔG^θ = ΔH^θ - TΔS

If ΔG^θ < 0, reaction is thermodynamically favourable/spontaneous