Enthalpy:

Enthalpy is a measure of internal energy of a system.

If a chemical reaction occurs in an open container the heat energy is lost or gained. The flow of heat occurs between the system and the surroundings until both of them attain the temperature. Enthalpy is internal energy of a system This is used to do the mechanical work. Under constant pressure the heat absorbed or released in a reaction is called enthalpy. It is denoted by Delta H. Enthalpy cannot be measured accurately. But the heat energy before and after a reaction can be measured using these results the change in heat energy can be found

Delta H = H (final) H (initial)

If the value is higher ie. If the value of delta H is positive then it is an endothermic reaction that is heat is absorbed

If the above value is negative then the reaction is exothermic heat is released. At constant pressure when a reaction occurs heat is released or absorbed.This amount of heat released or absorbed is called enthalpy change.

DH = H (product) - H (reactant)

Enthalpy change will be given for a mole of a substance.. Change in energy is measured at 1 atmospheric pressure and 25degree Celsius.

Exothermic reaction

In exothermic reaction the heat is released. Energy is the product of the reaction. temperature inside the reaction vessel increases

Energy of the reactants is greater than the energy of the product.

DH = Energy of product - energy of the reactant

= negative.

N2 + 3H2 ---------- 2NH3 + 92.4 KJ

For exothermic reaction The enthalpy Change is negative.

Endothermic reaction

Heat is absorbed during this reaction

2NH3 +92.4 KJ ------- N2 +3H2

The reaction vessel will be cooler. Temperature inside the vessel decreases.. The energy of the reactant is less than the energy of the product.

DH = H (product) - H (reactant)

= Positive.

For endothermic reaction the heat content enthalpy will be positive.

If two or more elements combines and give a compound the increase or decrease of enthalpy is called heat formation of a compound. The change of enthalpy when a solid changes to liquid is called heat of fusion.

The change of enthalpy when a liquid changes to vapour is called heat of vaporizations (latentheat) By seeing the change in enthalpy we can say whether heat is to be added more or less for a reactiob to occur. For example the formation of ammonia from nitrogen and hydrogen is exothermic. So heat is released during this reaction enthalpy is negative. so to produce more of ammonia more heat should be added to the reactants.