Enthalpy means thr energy content that is stored in a chemical system.
Exothermic refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.
Endothermic refers to a reaction in which the enthalpy of the products is bigger than the enthalpy of the reactants, resulting in heat being taken from the surroundings.
(Enthalpy profile diagram of exothermic)
Standard conditions are a pressure of 100kPa (1 atmosphere), a stated temperature, usually 298K (25°C) and a concentration of 1.0mol (for aq solutions).
Standard state is the physucal state of a substance under the standard conditions of 100 kPa and 298K.
Enthalpy change means the exchange of energy between a reaction mixture (system) and its surrounding when the reaction takes place at constant pressure.
Enthalpy change of combustion is the enthalpy change when one mole of a compound burns completely in oxygen. The compound and the products of burning must be in their normal stable states.
Enthalpy change of formation is the enthalpy change when one mole of compound forms from the elements in their normal stable states.
Enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.
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