The study of energy transfers between reacting chemicals and their surroundings.
Exothermic and Endothermic
Exothermic is the release of heat energy.
Enothermic is the taking of heat energy.
Bond breaking requires energy whilst bond making releases energy.
The energy released or taken from the surrounding is known as enthalpy change of reaction.
The reaction in which the changes are happening are known as the system. - Everything else is known as the surroundings.
Energy cannot be created or destroyed. The total energy of the universe is constant.
Measuring energy changes
Enthalpy is represented by the letter H.
H = H(products) - H(reactants).
For exothermic reactions, Hproducts < Hreactants
For endothermic reactions, Hproducts > Hreactants
H = mcT
Energy Transferred (J) = Mass (kg) x Specific heat capacity (J kg-1 K-1) x Temperature change (K).
Specific heat capacity, c is the amount of energy in joules (J) needed to raise the temperature of one kilogram (kg) of a particular substance by 1 kelvin (K).
HOWEVER, some heat is still transferred to the surroundings. - Leads to inaccuracy and unreliability.
Standard Enthalpy of Combustion
The enthalpy change when one mole of a substance is completley burnt in oxygen under standard coniditions.
Standard Enthalpy of Formation
The enthalpy change when one mole of the compound is formed from its elements, under standard conditions.
Standard Enthalpy of Atomisation
The enthalpy change when one mole of its atoms in the gaseous state is formed from the element under standard conditions.
Standard Enthalpy of Neutralisation
The enthalpy change when an acid and a base react to form one mole of water under standard conditions.
The total enthalpy change for a reaction is independant of the route taken.
A + B → C + D
Energy change in a reaction involving covalent bonds.
The energy needed to break a particular covalent bond, or energy released when the bond is broken is bond dissociation energy.
Mean bond enthalpy - the average value of the bond dissociation enthalpy of a particular bond over a wide range of compounds.