Thermodynamics - Definitions

Question 1

what are the standard conditions

Answer 1

100kPa + 298K

Question 2

enthalpy of formation

Answer 2

enthalpy change when 1 mole of a compound is formed form its constituent elements, under standard conditions with all reactants and products in their standard states

Question 3

enthalpy of combustion

Answer 3

enthalpy change when 1 mole of a substance is completely burnt in oxygen

Question 4

enthalpy of atomisation

Answer 4

enthalpy change which accompanies the formation of 1 mole of gaseous atoms from the element in its standard state under standard conditions
e.g. Mg(s) –> Mg(g)

Question 5

first ionisation energy

Answer 5

standard enthalpy change when 1 mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge.
eg. Mg(g) –> Mg+(g) + e-

Question 6

second ionisation energy

Answer 6

the loss of a mole of electrons from a mole of singly positively charged ions
Mg+(g) –> Mg2+(g) + e-

Question 7

first electron affinity

Answer 7

standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions each with a single negative charge
O(g) + e- –> O-(g)

Question 8

second electron affinity

Answer 8

enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form ions each with 2 negative charges.
O-(g) + e- –> O2-(g)

Question 8

lattice enthalpy of formation

Answer 8

standard enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions
Na+(g) + Cl-(g) –> NaCl(s)

Question 9

is the enthalpy change for lattice enthalpy of formation always negative or positive and why

Answer 9

ALWAYS NEGATIVE - when a lattice forms new bonds are made so energy is given out - exothermic

Question 10

enthalpy of lattice dissociation (+ve or -ve)

Answer 10

standard enthalpy change when 1 mole of solid ionic compound dissociates into its gaseous ions
enthalpy change always positive
same value as LEOF

Question 11

enthalpy of hydration

Answer 11

standard enthalpy change when 1 mole of gaseous ions are converted into aqueous ions/ aqueous solution
Na+(g) + aq –> Na+(aq)

Question 12

enthalpy of solution

Answer 12

standard enthalpy change when 1 mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other.
NaCl(s) + aq –> Na+(aq) + Cl-(aq)

Question 13

mean bond enthalpy

Answer 13

enthalpy change when 1 mole of gaseous molecules each breaks a covalent bond to form 2 free radicals, averaged over a range of compounds
CH4(g) –> C(g) + 4H(g)