Detailed summary of the A level topic, Energetics. Enthalpy change is described and definitions are given in an easy-to-learn way. The calculations for Hess's law are shown in an easy and understandable way. The different calorimetry practicals are also described as well as the extraction of titani...
Enthalpy change (delta H) is the heat change of a substance measured under
conditions of constant pressure
Standard enthalpy changes refer to standard conditions of 100kPa and a stated
temperature (usually 298K) - often shown as a degree sign with a line through the
middle.
Enthalpy of reaction
DEFINITIONS
Enthalpy of combustion: deltaHc
Enthalpy change when one mole of a substance is completely burned in air or
oxygen with all reactants and products in standard states under standard
conditions.
Enthalpy of formation: deltaHf
Enthalpy change when one mole of a substance is formed from its constituent
elements with all reactants and products in standard states under standard
conditions.
Enthalpy of neutralization: deltaHn
Enthalpy change when one mole of water is formed in a reaction between and acid
and an alkali under standard conditions.
Hess's law
The enthalpy change accompanying a chemical change is independent of the route
by which the chemical change occurs. In other words it is an alternate pathway. EG:
ENERGETICS 1
, when we can't calculate the enthalpy of reaction directly through an experiment, we
use an alternate pathway by calculating the combustion or formation values
Endo/exothermic reactions
Energy profile diagram
Endo/exo
Endothermic Exothermic
Reactions that absorb energy from the Reactions that release energy to the
surroundings surroundings.
EG: thermal decomposition EG: combustion
Bonds broken Bonds made
To brake a bond, energy needs to be
When bonds are formed, energy is released
absorbed.
ENERGETICS 2
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