Question

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.

Answer 1

Answer:

Choice C. In Hess's Law, $\Delta H_\text{reaction}$ from reaction steps are combined to determine an unknown

Explanation:

Consider this example: the $\Delta H_\text{reaction}$ for the two reactions are given:

• Reaction (1): $\rm A + B \to C + D$. $\Delta H_\text{reaction} = \Delta H(1)$.
• Reaction (2): $\rm C + D \to E + F$. $\Delta H_\text{reaction} = \Delta H(2)$.

The goal is to find the $\Delta H_\text{reaction}$ for reaction (3):

$\rm A + B \to E + F$.

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

$\rm A + B + C + D \to C+ D + E + F$.

Eliminate the intermediary species $\rm C$ and $\rm D$ to get:

$\rm A + B \to E + F$.

That's the same as reaction (3). With the help of Hess's Law, it is possible to find the $\Delta H_\text{reaction}$ for reaction (3) without taking any measurement.

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

$\begin{aligned}& \Delta H_\text{reaction}(3)\\ &= \Delta H_\text{reaction}(1) + \Delta H_\text{reaction}(2)\end{aligned}$.

No Gibbs Free energy $\Delta G$ 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 $\Delta H_\text{reaction}$ of reactions that are hard to carry out, control, or measure. That includes finding the lattice enthalpy of ionic compounds, and finding the $\Delta H_\text{reaction}$ of the hard-to-control reaction $\displaystyle \rm C\, (\text{s, graphite}) + \frac{1}{2} \,O_2\, (g) \to CO\, (g)$.

Answer 2

Answer:a

Explanation: