Topic 1.4 - Energetics

Question 1

Define an exothermic reaction

Answer 1

Energy is release into the surroundings

Question 2

Define an endothermic reaction

Answer 2

Energy is absorbed / taken in from surroundings

Question 3

Define enthalpy

Answer 3

A measure of the heat energy of a substance

Question 4

Define enthalpy change

Answer 4

Change in heat energy at a constant pressure

Question 5

What is the formula to calculate energy required for enthalpy change?
(includes mass, specific heat capacity and temperature change)

Answer 5

q (J) = m (g) x c (Jg-1K-1) x T (K or C)

Question 6

What is the formula to calculate enthalpy change?
(Includes energy and moles)

Answer 6

H (kJmol-1)= q (J) / moles (mol)

• check is H is positive/negative because of type of reaction (endothermic or exothermic)

Question 7

Define enthalpy change of formation

Answer 7

Enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions

Question 8

Define enthalpy change of combustion

Answer 8

Enthalpy change when one mole of a substance is completely burned in oxygen in their standard states under standard conditions

Question 9

What is Hess’ law ?

Answer 9

The total enthalpy change for a reaction is independent of the route taken

Question 10

Define mean bond enthalpy

Answer 10

Enthalpy change when one mole of covalent bonds are broken, averaged over a range of compounds in a gas phase