Concepts regrding Hess's Law

[105764441]

Hess's Law:
Enthalpy changes are additive:
Enthalpy is a STATE FUNCTION, so enthalpy therefore must be additive.
The enthalpy changes in a multistep reaction can be added to give the total enthalpy change.

The first method, Hess's method just breaks down one large reaction into each of the smaller reactions that led to the large reaction, then all the enthalpies of each reaction are added up and that gives the total enthalpy.

The second method that can be used is the utilization of bond enthalpies to calculate the delta H of the reaction.


Some things to note when calculating enthalpy of a system:
1. Tables of bond enthalpies refer to breaking bonds in GASES, thus they are always positive because as mentioned, energy is required to break a bond, so these tables are displaying the endothermic reaction.
This means that you must add enthalpy of phase change for liquids and solids.
Br2(l) --> 2Br(g)
for this reaction, you must add the enthalpy of vaporization for Br, and you must also add the bond enthalpy, as a was broken.

remember=enthalpy is the study of heat being released and absorbed in chemistry.

2.Bond enthalpies of diatomic molecules are accurate (measured for those molecules)
3.All others= averages from many different types of molecules
so, using bond enthalpies is the most accurate representation of the enthalpy of a reaction!

Standard reaction enthalpies:
***Note, since R and P can be in different states, and rxns can occur at different pressures, the enthalpy of a rxn can be different.

When all R and P are in their standard state at 1 ATM, rxn enthalpy is called standard rxn enthalpy delta H.
-standard state for a gas= 1 ATM
For a solution = 1M(@1ATM of pressure)
For a pure liquid or solid, it is the pure L or S for an element it is the most stable phase @ 1ATM and the temp of interest, (usually 25 degrees celsius).

Most rxns are given in their standard state, delta H.
The standard RXN enthalpy delta H for the formation of one mole of a substance from its elements in their most stable state is called standard enthalpy of formation, and the units are kJ/moll.
By deffinition, the stANDARD RXN ENTHALPY OF FORMATION OF AN ELEMENT IN ITS MOST STABLE FOR IS ZERO.

If bond enthalpies are not available, one can calculate the standard rxn enthalpy by using the standard rxn enthalpy of formation of all reactant.

[Audrey2B]

For the second method, what is the equation to use?

[Tiffany Wang 2G]

For the second method, the equation you can use is (enthalpies of bonds broken) + (enthalpies of bonds formed). The enthalpy of breaking a bond will always be positive and the enthalpy for bonds forming is always negative. You basically add all the enthalpies of all the bonds being formed/broken together.