Enthalpy Definitions

Question 1

Enthalpy change

Answer 1

heat transferred in a reaction at constant pressure

Question 2

Exothermic

Answer 2

energy released to surroundings (-ve)

Question 3

Endothermic

Answer 3

takes energy from surroundings(+ve)

Question 4

standard enthalpy change of reaction

Answer 4

reaction happens in molar quantities shown in reaction

Question 5

standard enthalpy change of formation

Answer 5

1 mole of a compound is formed from its elements

Question 6

standard enthalpy change of combustion

Answer 6

1 mole of substance is completely burned in oxygen

Question 7

standard enthalpy change of neutralisation

Answer 7

1 moles of water is formed from neutralising H ions by OH ions

Question 8

standard enthalpy of atomisation

Answer 8

1 mole of an element is converted to gaseous atoms

Question 9

average bond enthalpy

Answer 9

enthalpy change when 1 mole of a bond is broken into separate gaseous atoms

Question 10

Hess’s Law

Answer 10

enthalpy change is independent o the route by which the reaction takes place

Question 11

Why is experimental combustion enthalpy less exothermic than that given in data books?

Answer 11

Any out of:

• Heat lost to surroundings
• Incomplete combustion
• Evaporation of fuel (after weighing)
• Incomplete transfer of heat
• Heat absorbed by calorimeter
• Measurements not carried out in standard conditions