of vaporization, boiling point and intermolecular forces. © IBO 2007
To see the connection between enthalpy of vaporization, ∆Hvap,
boiling point and intermolecular forces, let us consider the changes that occur as a liquid is heated.
As the liquid is heated, its temperature increases and the molecules of the liquid move with increasing energy. We saw earlier that the proportion of molecules that are in the vapour phase increases with temperature and the equilibrium vapour pressure also increases (see figure 6.1.3). When the equilibrium vapour pressure is equal to the external pressure over the liquid (usually 1 atm pressure), the liquid boils. The temperature at which this occurs is the boiling point. The energy that is required for this transition (boiling) to occur is called the enthalpy of vaporization.
Since the strength of intermolecular forces governs the amount of energy needed to break bonds between the molecules of a liquid, the boiling point and the enthalpy of vaporization both increase as the strength of the intermolecular forces increases.
For example, mercury and water are both liquids at room temperature. The bonding between mercury atoms is metallic bonding, while that between water molecules is hydrogen bonding. Although hydrogen bonding is the strongest type of intermolecular force, metallic bonding is more strictly an intramolecular force and as such is stronger. In table 6.1.1 we can see that the boiling point of mercury is 357°C and its enthalpy of vaporization is 59.0 kJ mol−1, while the boiling point of water is 100°C and its enthalpy of vaporization is 40.8 kJ mol−1. With its stronger intermolecular forces, mercury has a higher enthalpy of vaporization and a higher boiling point than water.
ThEory of knowlEdgE
Why do we believe the claims made by scientists? Usually we believe something because we have a good reason or a justification for doing so. We tend to agree more with scientific claims because the justification used involves sound reasoning and empirical observation. Consider, for example, the explanations given for the relationship between the enthalpy of vaporization and the strength of the intermolecular forces between molecules. You accept the reasoning because you can figure it out logically; it makes sense considering what you know. You find further
justification by designing an experiment to test the idea and find that, as expected, more heat energy is required to vaporize water, with its stronger
intermolecular forces, than propanone.
• If we are more likely to believe knowledge claims that are justified by reasoning or empirical observation, what sorts of justification are we less likely to believe?
• Can you think of anything you have heard or read in Chemistry that seemed so outrageous that you found it hard to believe? What made it so hard to believe?
• Comment on the claim ‘We tend to agree more with scientific claims because the justification used involves sound reasoning and observation.’ Are you more likely to believe the claims made in Chemistry than those made in your other subjects? Explain. • Identify the following arguments as either
inductive or deductive reasoning.
1 Alcohol A absorbs heat energy when it vaporizes. Alcohol B absorbs heat energy when it vaporizes. Alcohol C absorbs heat energy when it vaporizes. Therefore all alcohols absorb heat energy when they vaporize.
2 All alcohols absorb heat energy when they vaporize. A is an alcohol; therefore, A absorbs heat energy when it vaporizes.
Figure 6.1.6 As energy is added to a sample of water, the temperature increases, except at the times when a change of state is occurring. vaporization melting liquid water temperatur e
CHAPTER 6
equilibrium
Chem CompLement
boiling temperature and pressure
Most of us have never boiled water on top of a high mountain; however, it is well known that the atmospheric pressure at the top of a high mountain is lower than that at sea level. On top of Mt Everest, the tallest mountain in the world, water boils at only 69.57°C! Since the boiling temperature is the temperature at which the vapour pressure of a liquid is equal to the external pressure, the lower external pressure on top of Mt Everest of 0.302 atm (30.6 kPa) is easily matched by the vapour pressure as the water is heated, so water boils at a much lower temperature than at sea level.
Under conditions of high pressure, the boiling point of water (and other liquids) is greater than normal. This is the basis on which pressure cookers
work. When cooking commences, the pressure is increased inside the strongly built pressure cooker and the small amount of water inside the cooker boils at a temperature above 100°C. This higher temperature increases the rate at which the food cooks, producing a speedy meal!
Figure 6.1.7 A pressure cooker cooks foods at pressures above atmospheric pressure and the resulting higher boiling temperature of the water inside decreases cooking time considerably.
1 Define the term equilibrium vapour pressure.
2 Describe the processes that are occurring when the equilibrium vapour pressure has been reached at a given temperature.
3 Consider the following boiling points.
liquid Ethanol Water Ethoxyethane
boiling point (°C) 78 100 35
List these three liquids in order of increasing equilibrium vapour pressures at 20°C and explain the order that you have chosen. 4 Explain how the equilibrium vapour pressure depends on the:
a temperature
b intermolecular forces in the liquid c volume of the liquid.
5 Sketch a graph to show the relationship between the equilibrium vapour pressure and temperature of a volatile liquid X.
6 Considering equilibrium vapour pressure, explain the following observation: Wet clothes dry more quickly on a hot dry day than on a hot, humid day. 7 Heat energy must be supplied to boil a liquid. Explain what happens to
this heat energy.
8 a Explain, in terms of kinetic theory, why for any given liquid the vapour pressure is greater at high temperatures than at low temperatures. b Draw two Maxwell–Boltzmann curves to illustrate your answer to part a. 9 What can be understood about the intermolecular forces in a liquid from
its enthalpy of vaporization?