Standard Form and Standard Decomposition Enthalpy
The enthalpy (H) of a substance is determined by the amount of energy and all forms of energy possessed by a substance whose amount cannot be measured. Changes in heat or enthalpy that occur during the process of receiving or releasing heat are expressed as "change in enthalpy (ΔH)". For example in the change of ice into water, it can be written as follows:
Δ H = H H20 (l) -H H20 (s)
If we observe the reaction of combustion of gasoline in the engine motor. Some of the chemical energy contained in gasoline, when gasoline burns, is converted to heat energy and mechanical energy to drive the motor. Likewise with the mechanism of action of battery cells. When the battery cell is working, chemical energy is converted into electrical energy, the heat energy used to burn gasoline and the combustion reaction of gasoline produces gas, moving the piston so that the motor wheel moves.
The actual enthalpy of a substance cannot be determined or measured. But ΔH can be determined by measuring the amount of heat absorbed by the system. For example in the change of ice into water, which is 89 calories / gram. At the change of ice into water, ΔH is positive, because the enthalpy of change results, the enthalpy of water is greater than the enthalpy of ice.
Thermochemistry is a part of chemistry that studies the enthalpy changes that accompany a reaction. In chemical changes there is always a change in enthalpy. The magnitude of the change in enthalpy is as large as the difference between the enthalpy of the reaction product and the amount of enthalpy of the reactant.
In endothermic reactions, the enthalpy after the reaction becomes larger, so ΔH is positive. Whereas in exothermic reactions, the enthalpy after the reaction becomes smaller, so ΔH is negative. The change in enthalpy in a reaction is called the reaction heat. The reaction heat for typical reactions is also called a unique name, for example heat of formation, heat of decomposition, heat of combustion, heat of dissolution and so on.
Standard Form Enthalpy (piH◦f)
The standard enthalpy of formation of a compound states the amount of heat needed or freed for the process of forming 1 mole of a compound from its elements which is stable under standard conditions (STP). The enthalpy of standard formation is given the symbol (ΔH◦f), the symbol f is derived from the word formation which means formation. Examples of elements that are stable under standard conditions, namely: H2, O2, C, N2, Ag, Cl2, Br2, S, Na, Ca, and Hg.
Standard Decomposition Enthalpy (ΔH◦d)
The standard decomposition enthalpy of a compound states the amount of heat needed or freed for the process of decomposing 1 mole of a compound from its elements that are stable under standard conditions (STP). The standard decomposition enthalpy is given the symbol (ΔH◦d) d symbol derived from the word decomposition which means decomposition.
According to Laplace's Law, the amount of heat released in the formation of compounds from its elements is the same as the amount of heat needed in the decomposition of these compounds into its elements. So, the decomposition enthalpy is the opposite of the enthalpy of formation of the same compound. Thus the number of heat is the same but the sign is opposite because the reaction is in the opposite direction.