Step 2:Decide which bonds need to be formed on the product side of the reaction. Add up all
the bond energies for the bonds that are formed. Call this subtotal ∆H 2. Assign ∆H 2 a negative
value because when energy is released, bonds are formed. In the example given, two CH
bonds and a CC bond need to be formed. This becomes (2 ×413 kJ/mole) + 348 kJ/mole, or
1174 kJ/mole. Remember to assign a negative sign, which makes ∆H 2 = −1174 kJ/mole.
Step 3:Apply Hess’s law: ∆H°= ∆H 1 + ∆H 2
This becomes 1050 kJ/mole + ( −1174 kJ/mole) = −124 kJ/mole.
- According to the law of Dulong and Petit, the best prediction for the specific heat of
technetium (Tc), MM = 100., is
A. 0.10 J/g ⋅°C
B. 0.25 J/g ⋅°C
C. 0.50 J/g ⋅°C
D. 0.75 J/g ⋅°C
E. 1.0 J/g ⋅°C
Answer: B
The law of Dulong and Petit states that
molar mass ×specific heat ≈25 J/mole ⋅°C
You know that technetium has an atomic mass of 100., and substituting this into the law of
Dulong and Petit gives you
100. g/mole ×xJ/g ⋅°C ≈25 J/mole ⋅°C
x≈0.25 J/g ⋅°C
- How much heat is necessary to convert 10.0 grams of ice at −10.0°C to steam at 150°C?
The specific heat capacity of ice is 0.500 cal/g ⋅°C. The heat of fusion of ice is
76.4 cal/g. The specific heat capacity of water is 1.00 cal/g ⋅°C. The heat of vaporization
of water is 539 cal/g. The specific heat capacity of steam is 0.482 cal/g ⋅°C.
A. 2500 cal
B. 4433 cal
C. 7445 cal
D. 8255 cal
E. 9555 calThermochemistry