CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 17. Thermochemistry

The mass of ice is first converted to moles. This is then multiplied by the conversion factor of (6.01 kJ/1 mol) in
order to find the kJ of heat absorbed.

Step 2: Solve.

31 .6 g ice×

1 mol ice

18 .02 g ice

×

6 .01 kJ

1 mol ice

= 10 .5 kJ

Step 3: Think about your result.

The given quantity is a bit less than 2 moles of ice, so just under 12 kJ of heat is absorbed by the melting process.

Practice Problems

1. What mass of ice at 0°C can be melted by the addition of 559 J of heat?


2. What is∆H when 250. g of water is frozen?

Heat of Vaporization and Condensation

Energy is also absorbed when a liquid is converted into a gas. As with the melting of a solid, the temperature
of a boiling liquid remains constant, and the input of energy goes into changing the state. Themolar heat of
vaporization(∆Hva p)of a substance is the heat absorbed by one mole of that substance as it is converted from
a liquid to a gas. As a gas condenses to a liquid, heat is released. Themolar heat of condensation(∆Hcond)
of a substance is the heat released by one mole of that substance as it is converted from a gas to a liquid. Since
vaporization and condensation of a given substance are the exact opposite processes, the numerical value of the
molar heat of vaporization is the same as the numerical value of the molar heat of condensation, but opposite in sign.
In other words,∆Hva p=−∆Hcond.

When 1 mol of water at 100°C and 1 atm pressure is converted to 1 mol of water vapor at 100°C, 40.7 kJ of heat are
absorbed from the surroundings. When 1 mol of water vapor at 100°C condenses to liquid water at 100°C, 40.7 kJ
of heat are released into the surroundings.

H 2 O(l)→H 2 O(g) ∆Hvap= 40 .7 kJ/mol

H 2 O(g)→H 2 O(l) ∆Hcond=− 40 .7 kJ/mol

Molar heats of fusion and vaporization for some other substances are given in the table below (Table17.2).

TABLE17.2: Molar Heats of Fusion and Vaporization

Substance ∆Hf us(kJ/mol) ∆Hva p(kJ/mol)

Ammonia (NH 3 ) 5.65 23.4

Ethanol (C 2 H 5 OH) 4.60 43.5

Methanol (CH 3 OH) 3.16 35.3

Oxygen (O 2 ) 0.44 6.82

Water (H 2 O) 6.01 40.7

Notice that for all substances, the heat of vaporization is substantially higher than the heat of fusion. Much more
energy is required to change the state from a liquid to a gas than from a solid to a liquid. This is because of the