Chemical Energetics

Question 1

what are the standard conditions for thermochemical measurement (temperature, pressure, concentration)

Answer 1

298K, 1 bar, 1 mol dm^-3

Question 2

standard enthalpy change of reaction

Answer 2

the energy change when molar quantities of reactants as specified by the chemical equation react to form products under standard conditions

Question 3

standard enthalpy change of formation of a substance

Answer 3

energy change when 1 mole of substance is formed from its constituent elements, under standard conditions (1 bar, 298K)

Question 4

what is the standard enthalpy change of an element

Answer 4

zero

Question 5

standard enthalpy change of combustion

Answer 5

energy evolved when 1 mole of substance is completely burnt in oxygen under standard conditions

Question 6

hess’ law

Answer 6

enthalpy change accompanying a chemical reaction is same regardless of route by which chemical change occurs, provided initial and final stages are the same

Question 7

standard enthalpy change of neutralisation

Answer 7

energy evolved when 1 mole of water is formed from neutralisation between an acid and base under standard conditions

Question 8

why is standard enthalpy change of neutralisation lower in reactions involving weak acid/base i.e slightly lower than -57 kJ mol^-1

Answer 8

weak acid/base are only slightly dissociated in aqueous solution.

Ionisation, which involves bond breaking, is an endothermic process.

Some of the energy evolved from neutralisation process is used to further dissociate weak acid/base completely.

enthalpy change of neutralisation involving weak acid/base less negative

Question 9

specific heat capacity of a substance, units

Answer 9

quantity of heat required to raise temperature of 1g of substance by 1K, J g^-1 K^-1