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MOLAR HEAT: it is the enthalpy changes(ΔH)
for 1 mole of substance to change a state.
Molar Heat of Fusion (ΔHfus): the amount of
heat absorbed by 1 mole of a substance in
melting from solid to a liquid. Exothermic or
Endothermic?
Molar Heat of Solidification (ΔHsolid): the
amount heat lost when 1 mole of a liquid
solidifies or freezes at a constant
temperature. Exothermic or Endothermic?
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Molar Heat of Vaporization (ΔHvap):
the amount of heat needed to
vaporize 1 mole of a liquid.
Exothermic or Endothermic?
Molar heat of Condensation (ΔHcond):
the amount of heat released or lost
when 1 mole of vapor condenses or
cools down to form liquid. Exothermic
or Endothermic?
the heat change caused by
dissolution of 1 mole of
substance. Exothermic or
Endothermic?
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ΔHfus = - ΔHsolid
ΔHvap = -ΔHcond
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Q or q: stands for the amount of heat
absorbed or released for any given number of
moles of substance.
S.I. unit for Q or ΔH (amount of heat) is in
Joules (J)
The unit for molar heat: ΔHfus, or ΔHvap or
ΔHsoln is in kJ/mol. Also in cal/g or J/g
Remember: 1000J = 1kJ and 1J= 0.2390 cal
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Q or ΔH (kJ)= number of moles of a
substance (mol) x Molar heat of substance
(kJ/mol)
OR
Q or ΔH (kJ)= mass (g) x inverted molar mass
(mol/g) x Molar heat of substance (kJ/mol)
OR
Q or ΔH (cal)= mass (g) x Molar heat of
substance (cal/g)
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According to law of conservation of energy:
If Molar heat of fusion (ΔHfus) for water is
6.01kJ/mol therefore, the Molar heat of
Solidification (ΔHsolid) for water would be
-6.01kJ
1. assuming no temperature change? The heat
of fusion of copper is about 0.2 kJ/g.
a. 80 kJ
c. 88,000 kJ
b. 40 kJ
d. 1,000 kJ
2. What is Q when 6 grams of water freezes at
the freezing point, with no temperature
change?
a. 3240 joules
c. 0.0111 joules
b. –2004 joules
d. 0.018 joules
3. What is the latent heat (Q) when 90 grams of
water vapor condenses?
a. 48,600 joules c. 0.0398 joules
b. 0.1667 joules d. –203,400 joules
4. What is the change in thermal energy
content of 900 grams of water when it
vaporizes?
a. –0.6 calories
c. –2,034,000 calories
b. –0.3982 calories
d. 486,000 calories
5. An ice cube in a well-mixed glass of ice water is
melting at the melting point. How much of the
ice will melt if the ice cube gains 4,000 calories
of energy?
a. 2.7778 g
c. 0.02 g
b. 0.36 g
d. 50 g
6. A vat of liquid iron is at its boiling point, 2750
oC. The latent heat of vaporization of iron is 350
kJ/mol. How much of the 2750 oC liquid iron can
be turned into 2750 oC iron vapor if 70,000 kJ of
energy are given to the molten iron?
a. 3.5812 g
c. 11,169 g
b. 0.005 g
d. 0.27924 g