5.3c Energy, Changes of State, and Heating Curves

Back
Next

As stated earlier, when an object is heated, it can get warmer, it can undergo a phase change, and it can undergo a chemical change. We have discussed the energy changes associated with warming and cooling objects. We now turn to the second possible outcome: a phase change.

If you heat a sample of ice at -10 °C at 1 atm pressure, it becomes warmer. If you heat ice at 0 °C (and 1 atm), however, initially it does not become warmer. Instead, the ice melts while maintaining a constant temperature of 0 °C, the melting point of water.

When a solid such as ice is heated and melts, the heat energy is used to overcome the forces holding the water molecules together in the solid phase. The heat energy is therefore changed into chemical potential energy. This potential energy can be reconverted to thermal energy when the forces between water molecules form again, that is, when liquid water freezes to form ice (Interactive Figure 5.3.5).

Due:
1/1/20 11:59 PM
Score: 0
Melting ice involves overcoming forces between water molecules.

The energy to effect a phase change has been measured for a number of substances. The enthalpy (heat) of fusion (ΔHfus) for a substance is the energy needed to melt 1 g or 1 mol of that substance. The enthalpy (heat) of vaporization (ΔHvap) is the energy needed to vaporize 1 g or 1 mol of that substance.

We now have the tools needed to calculate both the amount of thermal energy required to warm an object in a particular physical state and the amount of thermal energy required to change the physical state of a substance. Interactive Figure 5.3.6 explores the energy involved in heating 1-g samples of benzene and water over a large temperature range.

Due:
1/1/20 11:59 PM
Score: 0
Heating curves for benzene and water

When 1-g samples of the solids at -20 °C are heated, the following processes occur between the indicated points on the heating curves:

  • A to B: Temperature increases as added heat warms the solid to its melting point.
  • B to C: Temperature remains constant as solid is converted to liquid at the melting point.
  • C to D: Temperature increases as added heat warms the liquid to its boiling point.
  • D to E: Temperature remains constant as liquid is converted to vapor at the boiling point.
  • E to F: Temperature increases as added heat warms the vapor above its boiling point.

As shown in the following example, the amount of heat required to heat a substance can be calculated when specific heat capacities, enthalpy of fusion, and enthalpy of vaporization values are known.

Top
Back
Next