Energetics

Question 1

Enthalpy Change

Answer 1

The heat energy change measured under conditions of constant pressure

Question 2

Mean bond enthalpy

Answer 2

Enthalpy change when one mole of a covalent bond is broken into two gaseous atoms/ free radicals

Question 3

Standard enthalpy of formation

Answer 3

The enthalpy change when 1 mol of a substance is formed from its constituent elements under standard conditions, all reactants & products in their standard states
• EXO (for most substances)
• 2Na(s) + 1/2O2(g) —> Na2O(s)

Question 4

Standard enthalpy of combustion

Answer 4

The enthalpy change when 1 mol of a substance is completely burnt it excess oxygen with all other reactants being in their standard states under standard conditions

Question 5

Hess’s law

Answer 5

The enthalpy change for a reaction is independent of the route.
Given that the initial and final conditions remain the same

Question 6

Enthalpy Change equation

Answer 6

Make bonds - Break bonds
(+) (-)

Question 7

Specific heat capacity eq and definition

Answer 7

q= mcT
Energy required to raise 1g of substance by 1k without change of state
1cm3 of water = 1g of water

Question 8

Combustion Hess’s law

Answer 8

Arrows point towards central product
Always H2O and CO2

Question 9

Endo and Exo

Answer 9

Endo: + energy
~ temp decreases

Exo: - energy
~ temp increases

Question 10

Standard enthalpy of atomisation

Answer 10

•Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state
•Endothermic
•1/2I2(s) —> I(g)

Question 11

First ionisation energy

Answer 11

Enthalpy change when atoms in one mole of gaseous atoms each lose one e to form one mole of gaseous 1+ ions
• ENDO
• Mg(g) —> Mg+(g) + e

Question 12

Second IE

Answer 12

Enthalpy change when ions in one mole of gaseous +1 ions each lose one e to form one mole of gaseous 2+ ions each
• ENDO
• Mg+(g) —> Mg2+(g) + e

Question 13

1st e affinity

Answer 13

Enthalpy change when one mole of gaseous atoms each gain an e to form 1 mole of gaseous 1- ions
• EXO
• O(g) + e —> O-(g)

Question 14

2nd e affinity

Answer 14

Enthalpy change when one mole of gaseous 1- ions each gain one e to make gaseous 2- ions
•ENDO
• O-(g) + e —> O2-(g)

Question 15

Lattice enthalpy of formation

Answer 15

•Standard enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions
• EXO
• Mg2+(g) + 2Cl-(g) —> MgCl2(s)

Question 16

Lattice enthalpy of dissociation

Answer 16

• Standard enthalpy change when one mole of a solid ionic lattice is broken up into its constituents gaseous ions
• ENDO
• MgCl2(s) —> Mg2+(g) + 2Cl-(g)

Question 17

Enthalpy of hydration

Answer 17

• Enthalpy change when 1 mole of gaseous ions become aqueous ions / hydrated
• EXO
• Mg2+(g) —> Mg2+(aq)

Question 18

Enthalpy of solution

Answer 18

Standard enthalpy change when one mole of an ionic solid dissolves completely in sufficient amount of water to form a solution where the ions are too far apart to interact with each other
• ENDO OR EXO
• MgCl2(s) —> Mg2+(aq) + 2Cl-(aq)