Enthalpy and enthalpy change (ΔH)

Enthalpy (H) is the amount of energy that the system at a constant pressure. Enthalpy (H) is defined as the amount of energy contained in the system (E) and work (W).
H = E + W
with:
W = P × V
E = energy (joules)
W = work system (joules)
V = volume (liters)
P = pressure (atm)
The law of conservation of energy explains that energy can not be created and can not be destroyed, but can only be converted from one form of energy into another form of energy. Energy value of the material can not be measured, which can be measured is the change in energy (ΔE). Similarly, the enthalpy, enthalpy can not be measured, we can only measure changes in enthalpy (ΔH).
ΔH = Hp - Hr
with:
ΔH = change in enthalpy
Hp = enthalpy of products
Hr = enthalpy of the reactants or reagents
a. If the product H> H reactants, then ΔH is positive, it means that the absorption of heat from the environment to the system.
b. When the reactant H> H products, then ΔH are negative, meaning the release of heat from the system to the environment.
 
 Mathematically, enthalpy change (ΔH) can be derived as follows.
H = E + W (1)
At constant pressure:
ΔH = ΔE + PΔV (2)
ΔE = q + W (3)
Wsistem =-PV (4)
Substitution of equation (3) and (4) in equation (2):
H = (q + W) + PΔV
H = (q - PΔV) + PΔV
H = q
Thus, at a constant pressure, the change in enthalpy (ΔH) is equal to the heat (q) absorbed or released (James E. Brady, 1990).
Various kinds of chemical reactions based on the heat released / absorbed heat (Martin S. Silberberg, 2000):
a. Chemical reactions that require or absorb heat are called endothermic reactions.
example:
Termination reaction bonding in the molecule H2 elements are:
H2 → 2 H + a kJ ΔH =+aKJ
Endothermic reaction with ΔH is positive (+).
b. The chemical reaction that liberates heat is called an exothermic reaction.
example:
Bond formation reactions at the molecular elements of H2 are:
2H → H2 kJ ΔH =-aKJ
Exothermic reaction with ΔH marked (-).
Diagram enthalpy (energy level diagram)


 The kind of enthalpy change
 The magnitude of the change in enthalpy of a reaction depends on the amount of a substance that reacts, states of matter, temperature, and pressure, the enthalpy change is calculated based on the standard state is the state at standard temperature and pressure at a temperature of 250C (2980 K) and a pressure of 1 atm. Enthalpy change of the reaction was in the form of changes in enthalpy of formation (DHf0), enthalpy change of decomposition (DHd0), enthalpy change of combustion (DHc0), and the enthalpy change of neutralization (DHn0).

a. Standard enthalpy change of formation (DHf0)

Standard enthalpy change of formation, (DHf0) of a substance is the enthalpy change that occurs in the formation of one mole of a substance from its elements measured at standard conditions.
example:
1) Changes in enthalpy of formation of AgCl is the enthalpy change of the reaction:


























 2) Changes KMnO4 enthalpy of formation is the enthalpy change of the reaction:





 DHf0 depend on the states of matter produced, for example:
 ]
 DHf0 water in liquid form in contrast to DHf0 water in the solid state. Under the agreement, DHf0 element = 0 at all temperatures, for example:
 
 b. Standard enthalpy change of decomposition DHd0

Standard enthalpy change of decomposition is the reverse of the enthalpy change of formation. DHd0 a substance is the enthalpy change that occurs in the decomposition reaction of one mole of a substance into the elements in the standard state.

example:


 Marquis de Laplace of France's research found that the amount of heat released in the formation of a compound from its elements equal to the amount of heat required in the decomposition of the compound into its elements. This statement is known as the Law of Laplace.

example:

 c. Enthalpy change of Combustion DHc0

Enthalpy change of combustion, 􀀨 Hc is the enthalpy change that occurs in the combustion of 1 mole of an element or compound at standard conditions.
example:




 d. Enthalpy change of Neutralization DHn0

Enthalpy change of neutralization is the enthalpy change that occurs during the reaction between an acid with a base either by moles per mole of acid or base.

example: