Q. What is Standard Enthalpy of Formation?
Ans.
It is really difficult to measure the total energy content or absolute entropy of a substance. In order to quantify entropy we put it on a sliding scale, like temperature. Everyone agrees upon a zero point and we all work from there.
Scientists call the zero value of enthalpy the standard state. For any substance in its standard state we define the enthalpy to be zero. The definition of standard is pretty complicated, but for now think of the standard state of the substance s its familiar form the way you would most likely encounter it in your daily life.
The standard state of oxygen is a diatomic solid at room temperature. Water is a liquid and aluminum is a solid. So for all these standard states of the substances the enthalpy value is zero.
An enthalpy change will occur in any temperature or pressure change, and any change of state.
Once we have defined the reference point for the enthalpy of a substance, we can determine the enthalpy of a compound. We do this by measuring how much energy is used in the formation of the compound from its component elements in their standard state.
The energy change for the reaction is called the standard enthalpy of formation. It is represented by the symbol
ΔHof
The 0 tells us that the calculation originates form the standard state enthalpy of the substances. The f tells us that we are interested in the enthalpy of formation of one mole of compound.