Energetics I & II

Question 1

What is the standard enthalpy change of combustion?

Answer 1

the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions (298K, 100kPa), with all reactants and products in their standard states

Question 2

What is the firm rule when talking about energy changes?

Answer 2

Energy can be stored and transferred, but never ‘created’ or destroyed

Question 3

What is the definition of enthalpy?

Answer 3

the change in heat under at constant pressure.

Question 4

What is the standard enthalpy change of formation?

Answer 4

the enthalpy change when one mole of a substance forms from its constituent elements under standard conditions (298K, 100kPa), with all reactants and products in their standard states

Question 5

What is the standard enthalpy change of neutralisation?

Answer 5

the enthalpy change when one mole of water is produced in a neutralisation reaction between an acid and an alkali under standard conditions (298K, 100kPa). [note: not standard states because will be aqueous]

Question 6

What is the standard enthalpy change of reaction?

Answer 6

the enthalpy change when the number of moles of substances shown by an equation as written react under standard conditions (298K, 100kPa), with all reactants and products in the states given.

Question 7

What is Hess’ Law?

Answer 7

the enthalpy change for a given reaction is the same, independent of the route by which this is achieved (provided the temperatures, pressures, and physical states of products and reactants are the same).
(If you can’t go by A –> B, go by C)

Question 8

What is bond enthalpy?

Answer 8

the energy required to break ONE MOLE of bonds, in the GASEOUS STATE.

Question 9

What is mean bond enthalpy?

Answer 9

the energy required to break one mole of bonds, in the gaseous state, AVERAGED ACROSS A RANGE OF COMPOUNDS.

Question 10

What is the standard enthalpy of atomisation of an element?

Answer 10

the amount of energy
required to produce one mole of gaseous atoms from an element in its standard state under standard conditions.
FOR EXAMPLE: Na(s) -> Na(g)

Question 11

What is lattice enthalpy of formation?

Answer 11

the enthalpy change when one mole of an ionic compound is formed from its constituent gaseous ions.
(hence LATTICE)
(it will always be exothermic because of MAKING bonds)
FOR EXAMPLE: Ca2+(g) + Cl2-(g) -> CaCl2(s)

Question 12

What is lattice enthalpy of dissociation?

Answer 12

the enthalpy change when one mole of an ionic compound is split up into its constituent gaseous ions.
(hence LATTICE)
(it will always be endothermic)
FOE EXAMPLE: CaCl2(s) -> Ca2+(g) + Cl2-(g)

Question 13

What is first electron affinity?

Answer 13

the enthalpy change when one mole of electrons is added to one mole of atoms in the gaseous state to form one mole of gaseous 1- ions.
(this is like the opposition of ionisation)
FOR EXAMPLE: Cl(g) + e- -> Cl-(g)

Question 14

What is second electron affinity?

Answer 14

the enthalpy change when one mole of electrons is added to one mole of 1- ions in the gaseous state to form one mole of gaseous 2- ions.

Question 15

What is the enthalpy of hydration?

Answer 15

the enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions.
FOR EXAMPLE: Ca2+(g) + aq -> Ca2+(aq)

Question 16

What is the standard enthalpy change of solution?

Answer 16

the enthalpy change when one mole of substance in its standard state is dissolved in water to form a solution where the ions are far enough apart not to interact, under standard conditions.
FOR EXAMPLE: NaCl(s) + aq -> Na+ (aq) + Cl- (aq)

Question 17

What is happening to the individual chemical species step-by-step in this reaction:
Na(s) + 1⁄2 Cl2(g) -> NaCl(s)

Answer 17

• atomisation of Na (s)
• first ionisation of Na (g)
• atomisation of 1/2 Cl2 (g)
• first electron affinity of Cl (g)
• so now we have Na+ (g) and Cl- (g)
• lattice enthalpy of formation to form NaCl2 (s)