Enthalpy and enthalpy changes Enthalpy and enthalpy change (ΔH)

Enthalpy and enthalpy changesEnthalpy 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 + Wwith:W = P × VE = 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, enthalpy, enthalpy can not be measured, we can only measure the change in enthalpy (ΔH).ΔH = Hp - Hrwith:ΔH = change in enthalpyHp = enthalpy of productsHr = enthalpy of the reactants or reagentsa. 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 is negative, which means that 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) + PΔVH = (q - PΔV) + PΔVH = qThus, at constant pressure, the change in enthalpy (ΔH) is equal to the heat (q) absorbed or released (James E. Brady, 1990).A wide range 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 of the reaction bonding in the molecule H2 elements are:H2 → 2 H + a = ΔH kJEndothermic with ΔH is positive (+).b. The chemical reaction that releases heat is called exothermic reactions.Example:Bond formation reactions in the H2 molecule elements are:2H → H2 kJ ΔH =-aExothermic reaction with ΔH marked (-).Diagram enthalpy (energy level diagram)

 Enthalpy change for the combustion of 1 mole of a substance is measured at 298 K, 1 atm is called the standard enthalpy of combustion (standard enthalpy of combustion), expressed by ΔHc0. Enthalpy of combustion is also expressed in kJ mol -1.
Price enthalpy of combustion of various types of materials at 298 K, 1 atm are given in Table 3 below.
Table 3. Enthalpy of combustion of various types of materials at 298 K, 1 atmgb18
Burning gasoline is exothermic process. If the gas thought to be composed of isooctane, C8H18 (gasoline components) to 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 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 = 700 g mol gram/114 mol-1 = 6.14 mol. So the heat released in the combustion of 1 liter of gasoline is: 6:14 x 5460 kJ mol = 33524.4 kJ mol -1.Enthalpy of DecompositionDecomposition reaction is the reverse of the formation reaction. Therefore, in accordance with the principle of conservation of energy, equal to the value of the enthalpy of formation enthalpy of decomposition, but opposite in sign.



 Determination of Enthalpy Changes and Hess's Law
To determine the enthalpy changes in chemical reactions commonly used tools such as the calorimeter, thermometer and so on that may be more sensitive.
Calculation: DH reaction = S DHfo products - S reactants DHfo

Hess Law
"The amount of heat required or released in a chemical reaction does not depend on the course of the reaction but is determined by the initial state and the end."
Example: C (s) + O2 (g) ® CO2 (g), DH = x kJ ® 1 stageC (s) + 1/2 02 (g) ® CO (g), DH = y kJ ® 2 stagesCO (g) + 1/2 O2 (g) ® CO2 (g) DH = z kJ-------------------------------------------------- ---------- +C (s) + O2 (g) ® CO2 (g) DH = y + z kJ
According to Hess's Law: x = y + z



Reaction enthalpy change in release or absorbed only depends on the initial state and the final state. The higher the reaction temperature the faster the rate of reaction.
           Changes in heat on a substance or perubhan system is determined by the temperature, time and substance specific heat, specific heat is the heat required to raise the temperature of 1 gram of a substance as high as 1 k.
           Calculating the amount of heat released or absorbed berdsarkan temperature in solution time and turns the heat capacity of materials specified calories.
Enthalpy is a term in thermodynamics that states the amount of internal energy of a thermodynamic system plus the energy used to do work. Enthalpy can not be measured, which can be calculated is the value changes. Mathematically, the enthalpy change can be formulated as follows:
ΔH = ΔU + PΔV
where:
• H = enthalpy of the system (joules)
• U = internal energy (joules)
• P = pressure of the system (Pa)
• V = volume of the system ()