Calculate the ΔH in joules for:
C(graphite)→C(diamond)
from the data:
C(graphite)+O2(g)⟶CO2(g);ΔHo=−393.5kJ
C(graphite)+O2(g)⟶CO2(g);ΔHo=−395.4kJ
Calculate the ΔH in joules for:
C(graphite)→C(diamond)
from the data:
C(graphite)+O2(g)⟶CO2(g);ΔHo=−393.5kJ
C(graphite)+O2(g)⟶CO2(g);ΔHo=−395.4kJ
The correct option is D 1.9kJ
C(s,graphite)+O2(g)→CO2(g)ΔH∘=−393.5kJC(s,diamond)+O2(g)→CO2(g)ΔH∘=−395.4kJ
C(s,graphite)+O2(g)→CO2(g)ΔH∘=−393.5kJ
CO2(g)→C(s,diamond)+O2(g)ΔH∘=+395.4kJ
Look at the enthalpy for the second equation, the one reversed. Notice how the sign has changed. This is an absolute requirement of using Hess' Law: reversing an equation means reversing the sign on the enthalpy value.
The reason is that the first, unreversed equation is exothermic. We know this from the negative in front of the 395.4. That means that the opposite, reverse equation is endothermic. Putting in enthalpy (endothermic) is the reverse, the opposite of exothermic (giving off enthalpy). Hence, we change the sign EVERY time we reverse an equation.
Add the equations together. Also, add the enthalpies together. Here is the added equation without anything taken out:
CO2(g)+C(s,graphite)+O2(g)→CO2(g)+C(s,diamond)+O2(g)
ΔH∘=(−393.5kJ)+(+395.4kJ)
Notice the items which are the same on both sides and remove them:
C(s,graphite)→C(s,diamond)ΔH∘=+1.9kJ
C(s,graphite)+O2(g)→CO2(g)ΔH∘=−393.5kJ
C(s,graphite)+O2(g)→CO2(g)ΔH∘=−393.5kJ CO2(g)→C(s,diamond)+O2(g)ΔH∘=+395.4kJ Look at the enthalpy for the second equation, the one reversed. Notice how the sign has changed. This is an absolute requirement of using Hess' Law: reversing an equation means reversing the sign on the enthalpy value. The reason is that the first, unreversed equation is exothermic. We know this from the negative in front of the 395.4. That means that the opposite, reverse equation is endothermic. Putting in enthalpy (endothermic) is the reverse, the opposite of exothermic (giving off enthalpy). Hence, we change the sign EVERY time we reverse an equation. Add the equations together. Also, add the enthalpies together. Here is the added equation without anything taken out:
Notice the items which are the same on both sides and remove them:
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