How can an unknown A Heaction be determined using Hess's law?
A. The free energy of the reaction is used to determine the A H for
the reaction.
O
B. The reaction is repeated at different temperatures to determine
the A Hreaction
O
C. Enthalpies from reaction steps are added to determine an
unknown A Hreaction:
D. The unknown A Heaction is determined after the reaction is run in a
calorimeter.

Choice C. In Hess's Law, from reaction steps are combined to determine an unknown

Explanation:

Consider this example: the for the two reactions are given:

The goal is to find the for reaction (3):

.

Note that adding reaction (1) and (2) gives:

.

Eliminate the intermediary species and to get:

.

That's the same as reaction (3). With the help of Hess's Law, it is possible to find the for reaction (3) without taking any measurement.

Since Reaction (1) + Reaction (2) = Reaction (3), by Hess's Law,

.

No Gibbs Free energy is involved in this calculation. There's not even the need to carry out an experiment or take any new measurements. Because of that, Hess's Law can be very useful for finding the of reactions that are hard to carry out, control, or measure. That includes finding the lattice enthalpy of ionic compounds, and finding the of the hard-to-control reaction .

vanderwaal's forces of attraction are the electrostatic intermolecular forces that exist in a molecule due to the temporary or permanent dipoles that are developed in a molecule. in , the major intermolecular forces are dipole-dipole interactions as the molecule is polar (bent structure). so the vanderwaal's forces of interaction in between molecules are the permanent dipole-dipole interactions that develop due to the dipole that exists in between the polar h - s bond. the positively charged hydrogen atom is attracted towards the negative end of the dipole that is sulfur of the adjacent molecule. therefore, these forces of attraction are relatively stronger than that of london dispersion forces that exist in non polar molecules.