17.3. Heat and Changes of State http://www.ck12.org
large separation of the particles in the gas state. The values of the heats of fusion and vaporization are related to the
strength of the intermolecular forces. All of the substances listed above (Table17.2), with the exception of oxygen,
are capable of hydrogen bonding. Consequently, the heats of fusion and vaporization of oxygen are far lower than
the others.
Sample Problem 17.5: Heat of Vaporization
What mass of methanol vapor condenses to a liquid as 20.0 kJ of heat are released?
Step 1: List the known quantities and plan the problem.
Known
- ∆H = -20.0 kJ
- ∆Hcond=−35.3 kJ/mol
- molar mass of CH 3 OH = 32.05 g/mol
Unknown
- mass of methanol =? g
First, the amount of heat released in the condensation is multiplied by the conversion factor of (1 mol/−35.3 kJ) to
find the moles of methanol that condensed. Then, moles are converted to grams.
Step 2: Solve.
− 20 .0 kJ×
1 mol CH 3 OH
− 35 .3 kJ×
32 .05 g CH 3 OH
1 mol CH 3 OH= 18 .2 g CH 3 OHStep 3: Think about your result.
Condensation is an exothermic process, so the enthalpy change is negative. Slightly more than one half of a mole of
methanol has condensed.
Practice Problem- How much heat is absorbed when 1.00 g of liquid ammonia is vaporized at its boiling point?
Multi-Step Problems with Changes of State
In the chapterStates of Matter, you learned about heating curves and how they show the phase changes that a
substance undergoes as heat is continuously absorbed.
In the previous lesson, Thermochemical Equations, you learned how to use the specific heat of a substance to
calculate the heat absorbed or released as the temperature of the substance changes. It is possible to combine that
type of problem with a change of state to solve a problem involving multiple steps. The figure above (Figure17.8)
shows ice at−30°C being converted in a five-step process to gaseous water (steam) at 140°C. It is now possible
to calculate the heat absorbed during that entire process. The process and the required calculation is summarized
below.
- Ice is heated from−30°C to 0°C. The heat absorbed is calculated by using the specific heat of ice and the
equation∆H = m×cp×∆T.