Liquids

States of Matter We are familiar with three states of matter:

Kinetic Theory The behavior of matter under different conditions of temperature and pressure is explained by kinetic theory. We cover the gas laws, which are the foundation of kinetic theory, in the second part of the 10-week class. After a brief description of the kinetic theory of gases, we consider liquids and solids.

Kinetic Theory of Gases Under conditions of high temperature and low pressure, many substances are gases. In “ideal” gases, the size of the molecules is incredibly small relative to the distance between them. Gas molecules move in straight lines, and collide with each other like billiard balls. There are negligible attractive forces between molecules in ideal gases.

Kinetic Theory of Gases When these assumptions hold, certain properties of gases can be easily explained: - gases are compressible - gases take the shape of the container - increasing pressure decreases the volume - increasing temperature increases volume and pressure of gases

Conditions for Forming Liquids When particles are in conditions of low temperature and/or high pressure, inter- particle attraction becomes a factor, and gas changes into a liquid. At low temperatures, molecules move more slowly. When they collide, they no longer travel in straight lines, and often “stick” together.

High Pressure Under conditions of high pressure, molecules are no longer separated by great distances. Molecules are confined in small spaces, and their actual volumes become more significant At high pressures, gases become incompressible, and become more like liquids

Properties of Liquids Liquids have definite boundaries, and take the shape of their containers. Because they have boundaries, they have definite volumes, unlike gases. Liquids do not fill every container that they are in! Liquids, like gases, are able to flow. They are both fluids. Increasing pressure on a liquid has little or no effect – liquids are incompressible.

More Properties of Liquids Increasing temperature produces only minor increases in volume Liquids resist flow (viscosity), depending on the attractive forces between particles. Large, complex particles generally have high resistance. Higher temperature results in increased kinetic energy, reduced viscosity, and increased flow.

Surfaces of liquids behave like a stretched membrane which can support objects that are more dense than the liquid. This is called surface tension, and it results from unequal distribution of attractive forces at the surface (none at the top). Liquids evaporate spontaneously – they change into gas at any temperature.

Evaporation Liquids are like gases in that not all molecules have the same kinetic energy. Particles with high KE may escape the surface – evaporation, or vaporization.

When particles with high KE escape, what happens to the temperature of the liquid? In confined spaces, gas particles escape the liquid and are trapped in the space above. The vapor particles have pressure – vapor pressure.

When liquid is added to a sealed container, initially vapor pressure = 0 Evaporation begins, increasing vapor pressure Slowly condensation starts happening, until evaporation = condensation

Vapor Pressure Equilibrium When evaporation = condensation, A state known as vapor-liquid equilibrium exists. The space is saturated with vapor, and will hold no more at that temperature. When vapor pressure = atmospheric pressure, what happens? The liquid boils. The temperature at which this happens is called the boiling point temperature.

Vapor Pressure and Intermolecular Forces Non-polar molecules, such as diatomic elements (O 2, N 2 ) or hydrocarbons (CH 4 ) have weak attractive forces – they vaporize easily Non-polar molecules have high vapor pressures at a given temperature compared to polar molecules Polar molecules, like water or ethanol, have low vapor pressures because their attractive forces are greater.

Table H Worksheet What is the normal boiling point of each substance? At what temperature will ethanol boil if the pressure is 50 kPa? Which compound has the strongest intermolecular forces? Which compound has the weakest intermolecular forces?

What is the change in the boiling point of water when the pressure is increased from 1 atm (101kPa) to 2 atm (202 kPa)? What is the vapor pressure of each liquid at 75ºC? When we say that a compound has a “high vapor pressure”, what does that mean in terms of intermolecular forces? When a compound has high vapor pressure, it is sometimes called “volatile”. Why? When propanone boils at standard pressure, what is the vapor pressure of water? When boiling water in a kettle is cooled to room temperature (25ºC), what is the change in the vapor pressure?

When water boils at standard pressure, what is the vapor pressure of ethanoic acid? At 50ºC, what is the vapor pressure of each substance? propanone ethanol water ethanoic acid What temperature will each substance boil if the atmospheric pressure is 80 kPa? propanone ethanol water ethanoic acid

Let’s Compare the Four Molecules

Large atoms or molecules have greater attractive forces than smaller ones Larger atoms can induce temporary dipoles when they encounter each other. These are called dispersion forces (Van der Waals forces) Larger molecules have greater dispersion forces than smaller molecules.

Lab 9 Vapor Pressure and Intermolecular Forces Materials Liquids: propanone (acetone), ethanol, water; thermometer, cotton, rubber band, timer

Procedure Take a thermometer and wrap a piece of cotton around the bulb. Fasten it with a rubber band, with the band as high as possible. Get a 50 ml beaker and add a few ml of each liquid. On a separate sheet of paper, make a data table with time and temperature columns. Take the initial time as 0 and write the temperature in your table. Then dip the thermometer into the liquid, remove it, and measure the temperature every 20 seconds until it stops decreasing. Repeat the procedure for the other three liquids.

Results Construct an appropriate data table and a line graph with a line for each substance. Your graph should have 3 lines, each clearly labeled. Plot time on the x axis and temperature on the y axis. Make sure your graph is labeled, with an appropriate title.

Lab Report Write a complete lab report. It should include an objective, introduction (in your own words, not like those above), a results section and a graph, and the follow up questions.

Lab Questions Which molecule has the strongest intermolecular forces? Which one has the weakest intermolecular forces? Make references to your data in your answer. Define the following terms a) evaporation b) vapor pressure c) volatility d) boiling point Using Table H, answer the following questions – What are the normal boiling points of the three liquids we studied today? – Water boils at 82  C at the top of Mt Everest. What is the atmospheric pressure there? – What temperatures would propanone and ethanol boil at on Mt Everest? – At what temperatures would the three liquids boil at if they were under 2 atmospheres (202.6 kPa) of pressure?

Potential Energy and Kinetic Energy When the temperature of a liquid is at its boiling point, the distance between molecules (potential energy) increases, while kinetic energy remains the same. The amount of heat required to change liquid to gas is called the heat of vaporization, expressed in J/gram.

How much heat, in joules, is needed to vaporize a 18.0 gram sample of water at 100°C? A coffee cup filled with boiling water loses 10,000 J at 100°C. How much water is in the cup?