***UPDATED*** (WILL THUMBS UP SOLUTION) When we calculate how much energy is needed to break a chemical bond, it makes a

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***UPDATED*** (WILL THUMBS UP SOLUTION)
When we calculate how much energy is needed to break a chemical
bond, it makes a difference if we are talking about a single
molecule in a vacuum, or lots of molecules contained in a fluid.
The latter situation is more realistic and less abstract, but also
requires that we consider not just the energy to break the bond,
but the energy required to “deal with” the environment. If, for
example, as is common in chemistry (or biology), a reaction takes
place at a constant pressure and temperature, then the energy we
put into the reaction is used not only to break apart bonds but
also to do work – the volume must be increased to make room for the
additional particles that are created so the pressure remains
constant. This is why we might use enthalpy as our energy needed to
cause a reaction rather than just the energy needed to break the
bond itself: enthalpy also includes the amount of energy required
to change the volume, i.e., the work. The energy needed to simply
pull the bond apart is called the dissociation energy (it’s the
negative of the binding energy). In more complicated reactions, of
course, we can both break and form bonds, and so there could be a
net input or output of energy. In either case, enthalpy would still
be the relevant quantity for a reaction taking place at constant
pressure, and the change in the enthalpy is simply the heat that is
added or removed from a system at constant pressure. Let’s consider
the following example: To break apart a single 𝐻2 molecule, 𝐻2 ⟶
2𝐻, the dissociation energy is 4.52 𝑒𝑉.
a. How much energy would be required (in 𝑘𝐽) to dissociate 1 𝑚𝑜𝑙
of hydrogen molecules? (1 𝑒𝑉 = 1.602 × 10−19 𝐽 and there are 6.02 ×
1023 molecules in 1 𝑚𝑜𝑙) Suppose we are putting in energy to
dissociate a bubble of 1 𝑚𝑜𝑙 of 𝐻2 molecules at STP (𝑃 = 105 𝑃𝑎 and
𝑇 = 300 𝐾). We’ll consider the bubble as our system and assume the
pressure and temperature remain at STP.
b. How many moles of 𝐻 atoms are in the bubble after we have
dissociated all the hydrogen molecules?
c. What are the initial and final volumes of the bubble? Has the
bubble expanded or contracted?
d. Does the thermal energy of the system increase, decrease, or
stay the same in this process? Explain your reasoning. If the
thermal energy changes, calculate Δ𝐸𝑡ℎ.
e. Does the chemical energy of the system increase, decrease, or
stay the same in this process? Explain your reasoning. If the
chemical energy changes, calculate Δ𝐸𝑐ℎ𝑒𝑚.
f. What is the change in internal energy Δ𝑈𝑖𝑛𝑡 of the system for
this process?
g. Calculate the work for this process. Is the work transferring
energy in or out of the system? How can you tell?
h. Using the first law of thermodynamics, Δ𝑈𝑖𝑛𝑡 = 𝑊 + 𝑄,
calculate the heat for this process. Is the heat transferring
energy in or out of the system? How can you tell?
i. Using the definition of enthalpy, Δ𝐻 = Δ𝑈𝑖𝑛𝑡 + 𝑃Δ𝑉, calculate
the change in enthalpy for this process. Is the enthalpy increasing
or decreasing? How does the enthalpy change compare to the heat you
calculated in part h