Enthalpy Definitions
Standard enthalpies of formation and combustion are the building blocks of thermodynamics.
Standard Enthalpy of Formation ()
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Example (Ethanol):
*Note: The Formation Enthalpy of any ELEMENT in its standard state is ZERO.
Standard Enthalpy of Combustion ()
The enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions.
Example (Methane):
*Always Exothermic (Negative).
Putting it into Practice
Writing Formation Equations
Paper 2 StyleWrite the thermochemical equation for the standard enthalpy of formation of Sulfur Trioxide ().
Rule: You must form exactly 1 mole of the compound from its elements in their standard states.
- Product: (Must be 1 mole)
- Elements: Sulfur is , Oxygen is .
- Balance: We need 1 S and 3 O atoms.
Notice we use fractions for the elements to keep the product as 1 mole.
Practice: Standard States
Identify the standard state (solid, liquid, or gas) for the element Bromine at 298 K.