HEAT IN CHANGES OFSTATE

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?

Terminologies

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?

Terminologies (contd)

the heat change caused by dissolution of 1 mole of substance. Exothermic or Endothermic?

Molar Heat of Solution or Solvation (ΔHsoln)

Examples of ΔHsoln

DIRECTION OF HEAT FLOW IN CHANGES OF STATE

ΔHfus = - ΔHsolid

ΔHvap = -ΔHcond

Molar Heat Relationship

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

Measuring Q or ΔH in Changes of States

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)

Table of Values for some substances

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

LOGIC

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

Heat math worksheet

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