ENERGY AND CHEMICAL REACTION

ENERGY AND CHEMICAL REACTION

        A chemical reaction is a process of breaking and bond formation. The process is always accompanied by energy changes. The energy required to break chemical bonds, thus forming free radicals called the bond energy. For complex molecules, the energy required to break the molecule to form free atoms called atomization energy.
        Bond energy is defined as the energy required to break 1 mole of bonds of a molecule in the form of gas. Bond energies expressed in kilojoules per mole (kJ mol -1)
       Atomization energy is the energy required to sever all ties 1 mole of molecules into free atoms in the gas state. Atomization energy = total bonding atoms in 1 mole of the compound.

Energy price range of molecular bonds (kJ / mol)

 

Based on Hess's Law,  H determination can be made via:
1). Enthalpy change  H for a reaction is calculated by the sum of several reaction enthalpy changes associated.
2). Enthalpy change  H for a reaction is calculated based on the difference in enthalpy of formation ( Hf °) between products and reactants.            Association for Calculating Energy Changes Reaction Enthalpy

The chemical reaction would involve breaking the bond energy between atoms bond formation between the reactants and reaction products atom. The difference between the energy for breaking and bond formation is ΔH. Can be formulated as follows:
ΔHreaksi = Σ (bond energy reactants) - Σ (bonding energy of the reaction product)

In general, the relationship between ΔH reaction with Σ (bond energy of the reactants) and Σ (bond energy of the reaction product) for endothermic reactions and reactions eksterm can be seen in the following figure.

Endothermic reaction energy level diagram
Exothermic reaction energy level diagramtype-type enthalpy change• The enthalpy change measured at 25 ° C and a pressure of 1 atm (standard state) is called the standard enthalpy change (represented by the sign of DHO or DH298).• The enthalpy change that does not refer to the condition expressed by the symbol DH measurement alone.• molar enthalpy = enthalpy change per mole of a substance (kJ / mol).• The enthalpy change, include:a. Standard enthalpy change of formation (DHF o) = heat of formationIs the enthalpy change that occurs in the formation of one mole of a compound from its elements at standard temperature and pressure (25 ° C, 1 atm). Enthalpy can be released or absorbed. Its units are kJ / mol.Standard form of an element is the most stable form of the element in the standard state (298 K, 1 atm).If the change in enthalpy of formation was not measured at the standard state is denoted by DHFNote:• DHF free element is zero• In the enthalpy of formation, the amount of substance produced is 1 mol.• Formed from elements in standard form.
b. Standard enthalpy change of decomposition (DHD o)Is the enthalpy change that occurs in the decomposition of 1 mole of a compound into its constituent elements in their default state.If the measurements are not performed at the standard state, denoted by the DHD. Unit = kJ / mol.Standard enthalpy change of decomposition is the reverse of the standard enthalpy change of formation, its value would be opposite in sign.According to the Marquis de Laplace, "the amount of heat released in the formation of compound constituent elements = amount of heat needed to decomposition of the compound into its constituent elements. "This statement is called the Law of Laplace.Example:Given DHF o H2O (l) = -286 kJ / mol, the enthalpy of decomposition of H2O (l) into hydrogen gas and oxygen gas is +286 kJ / mol.
c. The standard enthalpy change of combustion (DHC o)Is the enthalpy change that occurs in the burning of one mole of a substance completely on the state standards.If the measurements are not performed at the standard state, it is denoted by the DHC. Unit = kJ / mol.d. The standard enthalpy change of neutralization (DHN o)Is the enthalpy change that occurs in 1 mole of acid neutralization by alkali or alkaline by 1 mole of acid in the standard state.If the measurements are not performed at the standard state, then denoted by DHN. Unit = kJ / mol.Example:DHN reaction = -200 kJDHN NaOH = -200 kJ / 2 mol = -100 kJ / molDHN H2SO4 = -200 kJ / 1 mol = -200 kJ / mol
e. Evaporation enthalpy changes Standards (DHovap)Is the enthalpy change that occurs in the evaporation of one mole of a substance in the liquid phase to the gas phase at standard conditions.If the measurements are not performed at the standard state, then denoted by DHvap. Unit = kJ / mol.
f. Standard enthalpy change of fusion (DHofus)Is the enthalpy change that occurs in the melting / smelting 1 mole of a substance in the solid phase a substance in the liquid phase at standard conditions.If the measurements are not performed at the standard state, then denoted by DHfus. Unit = kJ / mol.
g. Standard Sublimation enthalpy changes (DHosub)Is the enthalpy change that occurs in sublimation 1 mole of a substance in the solid phase a substance in the gas phase at standard conditions.If the measurements are not performed at the standard state, then denoted by DHsub. Unit = kJ / mol.
h. Standard Dissolution enthalpy changes (DHosol)Is the enthalpy change that occurs when one mole of a substance dissolves in a solvent (usually water) at standard conditions.If the measurements are not performed at the standard state, then denoted by DHsol. Unit = kJ / mol.