Unit 6: Topic 8 - Enthalpy of Formation Flashcards
What is enthalpy of formation?
The enthalpy of formation is a measure of the energy released or absorbed when one mole of a specific substance is created from its pure elements. The enthalpy of formation of a pure element in its reference form (i.e. H2, O2) is 0 under standard conditions. The enthalpy of formation is also different for a specific substance in two different phases; the enthalpy of formation for gaseous water is -241.8 kJ/mol and for liquid water is -285.8 kJ/mol.
How can we use enthalpy of formation to calculate the change in enthalpy of a reaction?
Equation. In a chemical reaction, reactants undergo some chemical change to form products. Since the products are formed, the total enthalpy change for the products can be calculated as the sum of the enthalpy of formation of each of the individual products, accounting for coefficients. Since the reactants are used up, the enthalpy of formation of the reactants should be negated in the calculation of enthalpy change for the reaction. The change in enthalpy of the reaction is the sum of the enthalpy of formation of the product minus the sum of the enthalpy of formation of the reactants.
