Standard State

• Standard enthalpy (ΔH): the enthalpy change at standard conditions

○ ⁰ = recorded in standard state

• Standard State conditions:

○ For a gas: pressure is 1 atm

○ For a pure substance in a condensed state (liquid or solid) = pure liquid or solid

○ For a solution, concentration is 1 molar

○ Temp is at exactly 25 C

• Remember: of a pure element is 0
• Standard heats of formation: the amount of heat needed to form 1 mole of a compound from its elements in their standard states (ie. how it exists in nature → oxygen is O2)

○ Unit: kJ

○ Have to make one mole of product to meet the definition

○ SS of an element is the form in which the element exists under 1 atm and 25 C

■ Not 0 = not how the element exists in nature

• Ex: Write the equation for the formation of methanol (CH3OH)
1. . Step 1: write the elements (in standard state)

● Note: The elements that exist as diatomic molecules are hydrogen (H2), nitrogen (N2), fluorine (F2), oxygen (O2), iodine (I2), chlorine (Cl2) and bromine (Br2)

2. Step 2: Balance

• Formula: (on RFS)

○ Use chart → add up all of heats of formation of products – all the heats of formation of reactants