DEFINITIONS OF ENTHALPIES

Question 1

Hess’s law

Answer 1

states that total enthalpy change is independent of the route taken

Question 2

standard molar enthalpy of formation

Answer 2

• the enthalpy change when one 1 mole of a species is formed from its constituent elements, under standard conditions and with all the products and reactants in their standard states
• enthalpy change is negative; exothermic

Question 3

standard enthalpy change of atomisation

Answer 3

• the enthalpy change that takes place for the formation of 1 mole of gaseous atom from its element in standard state, under state conditions
• enthalpy change is positive; endothermic

Question 4

first ionisation energy (first IE)

Answer 4

• the enthalpy change required to remove 1 electrons from each atom in 1 mole of gaseous atoms to form 1 mole of 1+ charged gaseous ion
• enthalpy change is positive; endothermic

Question 5

second ionisation energy (second IE)

Answer 5

• the enthalpy change required to remove 1 electron from each atom in 1 mole of 1+ charged gaseous ion to form 1 mole of 2+ charged gaseous ion
• enthalpy change is positive; endothermic

Question 6

first electron affinity

Answer 6

• the enthalpy change that takes place when 1 electron is added to 1 mole of gaseous atom to form 1 mole of a 1- charged gaseous ion
• enthalpy change is negative; exothermic

Question 7

second electron affinity

Answer 7

• the enthalpy change that takes place when 1 electron is added to 1 mole of 1- charged gaseous ion to form 1 mole of a 2- charged gaseous ion
• enthalpy change is positive; endothermic

Question 8

lattice enthalpy of formation

Answer 8

• the enthalpy change that accompanies the formation of 1 mole of a solid ionic compound from its gaseous ions, under standard conditions
• enthalpy change is negative; exothermic

Question 9

lattice enthalpy of dissociation

Answer 9

• the enthalpy change that accompanies the dissociation of 1 mole of a solid ionic compound to its gaseous ions, under standard conditions
• enthalpy change is positive; endothermic

Question 10

standard enthalpy of hydration

Answer 10

• the enthalpy that takes place when 1 mole of gaseous ion dissolves in water to form 1 mole of aqueous ion
• enthalpy change is negative; exothermic

Question 11

standard enthalpy change of solution

Answer 11

• the enthalpy change that takes place when 1 mole of solute is dissolved in sufficient amount of solvent to form a solution where molecules or ions are far enough apart to not interact with each other, all under standard conditions
• enthalpy change is positive; endothermic

Question 12

mean/average bond enthalpy

Answer 12

• the enthalpy change accompanied by breaking 1 mole of covalent bond to give separated atoms - everything being in the gas state
• enthalpy change is positive; endothermic

Question 13

what are the standard conditions

Answer 13

• 298K

- 100kPa

Question 14

why is the second IE greater than the first

Answer 14

because it takes more energy to remove an electron from a positively charged ion than from a neutral atom, this is because of the strong electrostatic attractions that must be overcame

Question 15

why is the enthalpy change of the first electron affinity always negative

Answer 15

energy is given out to remove an electron attracted to the positively charged nucleus, from a neutral atom

Question 16

why is the enthalpy change of the second electron affinity always positive

Answer 16

energy must be put in to overcome the repulsion b/w an electron and a negatively charged ion

Question 17

why is the enthalpy change of lattice formation negative

Answer 17

when a lattice is formed, new bonds are formed; energy is being given out

Question 18

why is the enthalpy change of lattice dissociation positive

Answer 18

bonds are being broken, therefore energy is needed to be put in
however, it has the same numerical value as LEF