chapter four_thermochemistry

Question 1

exothermic characteristics

Answer 1

negative h value

releases heat to the surroundings

Question 2

endothermic characteristics

Answer 2

positive h value

absorbs heat from the surroundings

Question 3

definition of standard enthalpy changes of reaction, formation, combustion and neutralisation

Answer 3

reaction: change in h when molar quantities of reactants react to form products at 1 bar and 298 k.
* formation: enthalpy change when 1 mole of substance is formed from its constituent elements in their standard states at 1 bar and 298 k
* combustion: the heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 1 bar and 298 k.
neutralisation: heat evolved when 1 mole of water is formed from the reaction between an acid and a base. (a weak acid and weak base will result in a smaller enthalpy change as they do not completely ionise in aqueous solution).

Question 4

alternative to hess’s law

Answer 4

sum of formation of products - sum of formation of reactants

Question 5

definitions of bond dissociation energy and bond energy

Answer 5

both are always endothermic, which means that the enthalpy change is positive

• bde: energy required to break 1 mole of a PARTICULAR covalent bond in a SPECIFIC molecule in the GASEOUS state.
• be: average energy required to break 1 mole of covalent bonds in the gaseous state

Question 6

formula for bde

Answer 6

sum of bonds broken-sum of bonds formed

Question 7

definition of lattice energy

Answer 7

the heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions.

Question 8

exothermic or endothermic?

formation
combustion
neutralisation
lattice

Answer 8

f: can be both
c: always exothermic
n: always exothermic
l: always exothermic

only be and bde are endothermic