Chemistry Problems
Thermodynamics
Blase Ferraris (did these problems for
Gangluff)
Final Project
1996B and 1997D
1997 D
PCl5(g)  PCl3(g) + Cl2(g)
What is the sign of ΔS for the reaction? Explain.
What change, if any, will occur with ΔG for the
reaction as the temperature is increased?
Explain your reasoning in terms of
thermodynamic principles.
If He gas is added to the original reaction
mixture at constant volume and temperature,
what will happen to the partial pressure of Cl2?
Explain.
If the volume of the reaction mixture is
decreased at constant temperature to half the
original volume, what will happen to the number
of moles of Cl2 in the reaction vessel? Explain.
What is the sign of ΔS for the reaction?
Explain.
Entropy increase will be observed in a reaction
in which there is an increase in the moles of gas
in the product
PCl5(g)  PCl3(g) + Cl2(g)
Moles products > moles reactants
ΔS is positive
What change, if any, will occur with ΔG for the
reaction as the temperature is increased? Explain
your reasoning in terms of thermodynamic
principles.
ΔG° = ΔH° - TΔS°
Since ΔS° is positive, increasing T will result in a
larger term being subtracted from ΔH°.
Or
ΔG°= -RTlnK as T increases, ΔG° gets more
negative
ΔG° will decrease
If He gas is added to the original reaction mixture
at constant volume and temperature, what will
happen to the partial pressure of Cl2? Explain.
Obviously, Helium is not a part of the overall
reaction
PCl5(g)  PCl3(g) + Cl2(g)
(where’s He? Exactly, NO WHERE)
Therefore, adding Helium to the reaction
wouldn’t affect anything in the reaction at all.
*note* Remember, the partial pressure of Cl2 is its pressure in
proportion to the other parts of the chemical equation. Since Helium
is not a part of the equation, adding it will affect the overall pressure,
but not Cl2’s partial pressure. It won’t tip the reaction in favor of the
products or the reactants therefore the reaction itself will remain
unaffected.
If the volume of the reaction mixture is decreased
at constant temperature to half the original volume,
what will happen to the number of moles of Cl2 in
the reaction vessel? Explain.
Decrease in Volume leads to an increase in
Pressure
When pressure increases, the reaction shifts to
side of the equation with lesser moles of gas.
PCl5(g)  PCl3(g) + Cl2(g)
Moles Reactants < Moles Products
Therefore PCl5 will increase and Cl2 will
decrease
MR GANGLUFF DANCING AT PROM