Energetics

Energetics
Endothermic ­ take in heat
Exothermic ­ give out heat

Enthalpy change is heat energy change measured under conditions of constant pressure.
Standard enthalpy changes refer to standard conditions (ie. 100kPa) and stated
°
temperature (eg H298 )

Standard Enthalpy of Combustion, C H°
= enthalpy change when 1…

Hess's Law
Total enthalpy change for reaction is independent to route taken. Total enthalpy change for one
route is the same as another.

Good to find enthalpy changes that you can't find directly by doing experiment.












Using Enthalpies of Formation

Need to know fH° for all reactants and product that…

Using Enthalpies of Combustion
Use enthalpy changes of combustion instead of enthalpy changes of formation.






Bond Enthalpies
Atoms in molecules held by strong covalent bonds. Takes energy to break them (endothermic so
H is +ve) and energy is given out when new bonds formed (exothermic so H is ­ve). Bond…

Enthalpy change of reaction = total energy absorbed ­ total energy released
To calculate overall enthalpy change for reaction calculate total energy needed to break all
bonds in reactants.
To find total energy released by reaction calculate total energy needed to form all new
bonds in products.
The overall enthalpy…