theromodymics defentions

Question 1

Enthalpy of lattice formation

Answer 1

It is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its
constituent ions in gaseous form.

essentially charged gases form an ionic solid

Na+ and Cl- (g = NaCl

Question 2

What is enthalpy of hydration (ΔhydHo)?

Answer 2

• enthalpy change
• when one mole of gaseous ions
• dissolves in water
• to form one mole of aqueous ions

Question 3

What affects lattice enthalpy?

Answer 3

strength of ionic bonding

• charge/size ratio of ions
• polarisation of ions

Question 4

What is lattice enthalpy of formation (ΔLHo)?

Answer 4

enthalpy change

• when one mole of a solid ionic compound
• is formed from its gaseous ions

Question 5

What is lattice enthalpy of dissociation (ΔLHo)?

Answer 5

• enthalpy change
• when one mole of a solid ionic compound
• is broken up into its gaseous ions

Question 6

Why can lattice enthalpies be greater than expected?

Answer 6

• ions are not perfectly spherical
• ions are polarised
• bonding has some covalent character
• stronger than ionic bonding

Question 7

What assumption is made when calculating lattice enthalpy in a Born-Haber cycle?

Answer 7

• ionic compound has no covalent character

- ions are perfectly spherical

Question 8

What is second electron affinity (ΔeaHo)?

Answer 8

• enthalpy change
• when one mole of gaseous 1- ions
• gains one mole of electrons
• to form one mole of 2- ions

Question 9

Why is first electron affinity exothermic?

Answer 9

electrostatic attraction between nucleus and electron

- electron loses energy to occupy orbital

Question 10

What is enthalpy of solution (ΔsolHo) ?

Answer 10

• enthalpy change
• when one mole of solute
• dissolves completely in a sufficient volume of solvent
• so that ions do not interact with each other

Question 11

Define bond dissociation enthalpy. (3)

Answer 11

• enthalpy change required
• to break a covalent bond
• with all species in the gaseous state

Question 12

What is first electron affinity (ΔeaHo)?

Answer 12

• enthalpy change
• when one mole of gaseous atoms
• gains one mole of electrons
• to form one mole of 1- ions

Question 13

Define standard enthalpy of formation

Answer 13

Define standard enthalpy of formation

Question 14

Define mean bond enthalpy. (3)

Answer 14

• energy required
• to break a particular covalent bond
• averaged over a range of compounds

Question 15

Why are mean bond enthalpy calculations inaccurate?

Answer 15

• average values

- not specific to compound

Question 16

Define enthalpy change

Answer 16

• heat energy exchange between reaction mixture and surroundings
• at constant pressure

Question 17

What is the standard enthalpy of atomisation (ΔatHo)?

Answer 17

• enthalpy change
• when one mole of gaseous atoms of an element
• is formed from its standard state
• under standard conditions of 298K and 100kPa

Question 18

Define standard enthalpy of combustion

Answer 18

• enthalpy change
• when one mole of a substance undergoes complete combustion in excess oxygen
• under standard conditions with all reactants and products in their standard states