1.3 - Hess's Law

disadvantages of calorimetry

1. reactions release too much energy to be performed safely with a simple calorimeter

2. simple calorimeters are only useful for processes involving dilute solutions

3. many reactions are too violent or happen too slowly for the calorimetric method to be practical

Hess’s Law

enthalpy change of a physical or chemical process depends only on the initial and final conditions of the process

standard enthalpy of formation

energy associated with formation of a substance per mole with reactants in their natural state

bond energy

the quantity of energy required to break a chemical bond

bond breaking is ___

endothermic

bond making is ___

exothermic

Nature favours stability. Unbonded atoms have high potential energy and when they form a bond, they move to a more stable, lower energy state and excess energy is released to surroundings.

When atoms bond, they achieve a more stable electron configuration, meaning their energy decreases.

formula to calculate enthalpy change using bond energies

sum of energy from bonds broken - sum of energy from bonds formed (gives an approximate value)

formula to calculate enthalpy change using standard enthalpies of formation

sum of energy of formation for products - sum of energy of formation for reactants

ways to calculate change in enthalpy

1. calorimetry

2. Hess’s Law

3. standard heats of formation

4. bond energies (approximation)