5.1e Enthalpy change of a reaction

Standard condition. Symbol Ѳ 

Standard state refers to the normal, most pure stable state of a substance measured at 100 kPa. Temperature is not a part of the definition of standard state, but 298 K is commonly used.

Example: standard state of CO2 is gas, of butter is solid

ΔHfѲ of CO2 is -393.5 kJ/mol

What is enthalpy change?

Note: there is NO UNIT on the y-axis because you cannot know what is the enthalpy of the reactants and products, BUT you can know the difference or change in enthalpy (∆H)

Standard enthalpy change of formation ΔHf

Is the energy change upon the formation of 1 mol of a substance at standard state.

ΔHfѲ = Σ(ΔHfѲ products) - Σ(ΔHfѲ reactants)

Example: 2 H2O  -->  2 H2 + O2

ΔHfѲ = (2* enthalpy H2 + enth O2 ) - 2 * enthalpy H2O

= 0 - (-285.8 Kj/mol)*2 = +571.6 kJ

Standard enthalpy change of combustion ΔHc

Is the heat involve in burning of 1 mol of a substance.

Example:

2 H2 + O2 --> 2 H2O

ΔHcѲ =  -286 KJ/mol

For this equation ΔHcѲ = - 572 KJ

Enthalpy change per mole

Example: above H20 example. To find enthalpy per mole. Divide enthalpy to the # of moles.