Unit 2 - Chemical Energy

Question 1

Enthalpy

Answer 1

Enthalpy is a measure of the chemical energy stored in a substance. Enthalpy is given the symbol H

Question 2

Exothermic

Answer 2

A reaction or process that releases heat energy is exothermic

In an exothermic reaction, enthalpy change is negative

Question 3

Endothermic

Answer 3

A reaction or process that takes in heat is endothermic

In an endothermic reaction, enthalpy change is positive

Question 4

Enthalpy of Combustion

Answer 4

The enthalpy of combustion of a substance is the enthalpy change when one mole of the substance burns completely in oxygen.

Combustion reactions are exothermic, so enthalpy change values are negative.

Question 5

Calorimeter

Answer 5

A calorimeter can be used to determine the enthalpy of combustion.
* To minimise heat loss the sample being burnt is surrounded by water and the container is sealed
* Constant stirring to ensure temperature change is accurate
* Pure O2 to ensure complete combustion

Question 6

Hess’s Law

Answer 6

Hess’s Law tells us that the total enthalpy change of a chemical reaction depends only on the chemical nature and the physical state of the reactants and products. It is independent of any intermediate steps.

According to Hess’s Law, ΔH1 = ΔH2 + ΔH3

Question 7

(Molar) Bond Enthalpy

Answer 7

(Molar) Bond enthalpy is the energy required to break one mole of bonds in a diatomic molecule.

Energy is required to break the bond so ΔH is positive - bond breaking = endothermic and bond forming is exothermic (negative)

Question 8

Mean (Molar) Bond Enthalpy

Answer 8

Mean bond enthalpy is the average energy required to break one mole of bonds, for a bond that occurs in a number of compounds.