Energetics Flashcards
Enthalpy Change (AH)
This is the change in heat energy in a reaction at constant pressure. The majority of reactions occur at atmospheric pressure in which the pressure remains constant throughout and so this is the enthalpy change.
Standard Enthalpy Change (AH)
This is the enthalpy change that occurs under standard conditions of 100 KPa pressure, 298K temperature and any solution at a concentration of 1 moldm-3. Reactants and products being in their standard physical state ( solid etc) under these conditions.
By measuring enthalpy changes under standard conditions- they can be compared fairly from one reaction to another.
Standard Enthalpy of Formation (AH® +)
Definition - The standard enthalpy of formation is the enthalpy change when I mole of a substance is formed from its constituent elements under standard conditions. All substances are in their standard States.
Standard Enthalpy of Combustion (AH° c)
Definition - standard enthalpy of combustion is the enthalpy change when 1 mole of a sustance undergoes compete combustion in excess oxygen under standard conditions. All substances are in their standard States.
for organic compounds like hydrocarbons and alcohols - the products of complete combustion are water and carbon dioxide.
Note that the enthalpy of combustion of oxygen is zero as it cannot be combusted. Likewise the enthalpy of combustion of oxides such as, CO2 is zero as they have already undergone complete combustion.
Exothermic reactions
Exothermic reactions are ones that get hot so they give out energy to the surroundings. The products have less energy than the reactants and AH values are negative.
Many reactions that occur spontaneously are exothermic - in these the products of the reaction are more Stable as they possess less energy.
Endothermic reactions
Endothermic reactions are ones that get cold so they take in energy from the Surroundings
The products have more energy than the reactants and AH
values are positive
Hess’s law
This is a chemical representation of the first law of thermodynamics which states that energy can’t be created or destroyed.
Hess’s law states that the enthalpy change of a reaction is independent of the route taken.
Hess cycle formation example
Equation for formation
