Question

define enthalpy of formation and give one example

Answer 1

The standard enthalpy of formation is a measure of the energy released or consumed when one mole of a substance is created under standard conditions from its pure elements. The symbol of the standard enthalpy of formation is ΔHf.

Δ = A change in enthalpy
o = A degree signifies that it's a standard enthalpy change.
f = The f indicates that the substance is formed from its elements
The equation for the standard enthalpy change of formation (originating from Enthalpy's being a State Function), shown below, is commonly used:

ΔHoreaction=∑ΔHof(products)−∑ΔHof(Reactants)(1.1)(1.1)ΔHreactiono=∑ΔHfo(products)−∑ΔHfo(Reactants)
This equation essentially states that the standard enthalpy change of formation is equal to the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants.