ENTHALPY

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 =
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 =-a
Exothermic reaction with ΔH marked (-).
Diagram enthalpy (energy level diagram)


Types of Enthalpy
 
1. Enthalpy of Formation
Enthalpy change for the formation of one mole of a substance directly from its elements called molar enthalpy of formation or the enthalpy of formation. If the measurement is made at the standard state (298 K, 1 atm) and all of its elements in the form of the standard, the enthalpy change is called the standard enthalpy of formation (ΔHf 0). Enthalpy of formation is expressed in kJ per mole (kJ mol -1).

So that there is uniformity, then it should set the standard state, the temperature 25 0 C and a pressure of 1 atm. Thus thermochemical calculations based on the state standards.

In general, in thermochemical equation stated:

AB + CD ----> AC + BD Δ H0 = x kJ / mol

Δ H0 is the epitome of the enthalpy change in circumstances. What is meant by the standard form of an element is the most stable form of the element at standard conditions (298 K, 1 atm).

Example: enthalpy of formation of ethanol (C2H5OH) (l) is -277.7 kJ per mole. This means: the formation of 1 mole (46 grams) of ethanol from elements in standard form, the carbon (graphite), hydrogen gas and oxygen gas, measured at 298 K, 1 atm 277.7 kJ released by termokimianya equation is :
2 C (s, graphite) + 3H2 (g) + ½ O2 (g) -> C2 H5 OH (l) ΔH = -277.7 kJ

Value of enthalpy of formation of various substances and the formation reaction thermochemical equations given in Table 2 below.

Table 2. Value of the enthalpy of formation of various substances and thermochemical reaction equation formation

2. Enthalpy of Combustion

The reaction of a substance with oxygen reaction called combustion. Substances that are combustible elements carbon, hydrogen, sulfur, and various compounds of these elements. Said to be perfect if the combustion of carbon (c) burned to CO2, hydrogen (H) burned into H2O, sulfur (S) burned to SO2.

Enthalpy change for the combustion of 1 mol of a substance is measured at 298 K, 1 atm is called the standard enthalpy of combustion (standard enthalpy of combustion), which is expressed by ΔHc0. Enthalpy of combustion is also expressed in kJ mol -1.

Price enthalpy of combustion of various substances at 298 K, 1 atm are given in Table 3 below.

Table 3. Enthalpy of combustion of various substances at 298 K, 1 atm

Burning gasoline is an exothermic process. If gasoline is considered consisting of isooctane, C8H18 (a component of gasoline) determine the amount of heat released in the combustion of 1 liter of gasoline. Given the enthalpy of combustion of isooctane = -5460 kJ mol-1 and the density isooktan = 0.7 kg L -1 (H = 1 and C = 12).

Answer:
Enthalpy of combustion of isooctane is - 5460 kJ mol-1. The mass of 1 liter of gasoline = 1 liter x 0.7 kg L-1 = 0.7 kg = 700 grams. Isooctane mole = 700 grams gram/114 mol-1 = 6.14 mol. So the heat is released in the combustion of 1 liter of gasoline is: 6.14 x 5460 kJ mol = 33524.4 kJ mol -1.

3. Enthalpy of Decomposition
Decomposition reaction is the opposite of a reaction formation. Therefore, in accordance with the principle of conservation of energy, equal to the value of the enthalpy of decomposition enthalpy of formation, but opposite sign.

Example:

Known ΔHf 0 H2O (l) = -286 kJ mol -1, the enthalpy of decomposition of H2O (l) into hydrogen gas and oxygen gas is + 286 kJ mol-1

H2O (l) -> H2 (g) + ½ O2 (g) ΔH = + 286 kJ