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:

Reaction (1): . .Reaction (2): . .

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 .

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer

Cause its being chemically applied?