What is the standard enthalpy of formation (ฮH<sub>f</sub>)?
The enthalpy change when one mole of a compound is formed from its elements in their most stable state at standard conditions (25ยฐC and 1 atm).
What are standard conditions for enthalpy of formation?
25ยฐC (298 K) and 1 atm.
What is meant by 'most stable state'?
The form of an element that is most stable at standard conditions (e.g., O<sub>2</sub>(g), C(s, graphite)).
What is the ฮH<sub>f</sub> of an element in its standard state?
Zero.
Define ฮH<sub>rxn</sub>.
The enthalpy change for a chemical reaction.
How do you calculate ฮH<sub>rxn</sub> using ฮH<sub>f</sub> values?
ฮH<sub>rxn</sub> = ฮฃnฮH<sub>f</sub>(products) - ฮฃmฮH<sub>f</sub>(reactants), where n and m are stoichiometric coefficients.
What is the first step to calculate ฮH<sub>rxn</sub>?
Write out the balanced chemical equation.
What is the second step to calculate ฮH<sub>rxn</sub>?
Identify the ฮH<sub>f</sub> values for each reactant and product.
What is the third step to calculate ฮH<sub>rxn</sub>?
Apply the formula: ฮH<sub>rxn</sub> = ฮฃnฮH<sub>f</sub>(products) - ฮฃmฮH<sub>f</sub>(reactants)
What is the fourth step to calculate ฮH<sub>rxn</sub>?
Make sure to pay attention to the stoichiometric coefficients in the balanced equation when applying the formula.
What is the difference between H<sub>2</sub>O(l) and H<sub>2</sub>O(g) when calculating ฮH<sub>rxn</sub>?
They have different ฮH<sub>f</sub> values. Always use the value corresponding to the correct state.
