Chemistry 30S

1. Chemistry 30S Unit 1 – Properties of Matter

2. Learning Outcomes • C11-1-06 Operationally define vapour pressure in terms of observable and measurable properties. • C11-1-07 Operationally define normal boiling point temperature in terms of vapour pressure. • C11-1-08 Interpolate and extrapolate the vapour pressure and boiling temperature of various substances from pressure versus temperature graphs.

3. Recall: • Boiling or Vaporization • State change from liquid to gas • As you heat a liquid the particles absorb the energy and begin to move faster and further apart---eventually some particles have enough energy to escape the liquid • Boiling Point – temperature at which a substance transforms from liquid to gas • Normal Boiling Point – boiling point for a substance at standard pressure • Condensation • State change from gas to liquid • As the quickest & hottest particles escape the liquid the temperature of the remaining liquid is cooled  condensation

4. Heating Curve for a Liquid • If you heat a liquid you will see the temperature of that liquid increase until it reaches its boiling point • At this point the energy (heat) being put into the liquid is instead used to help the particles change state • Energy is used to break intermolecular attractions • Temperature will remain constant • The same applies to reaching melting point

5. Heating Curves • For this reason you can find the melting or boiling point of a substance based on the plateaus of graphs

6. Cooling Curves • Same, but opposite to heating curves • Plateau will again be shown at the melting and boiling point of the substance • Temperature remains the same because as the particles slow down and condense together energy (heat) is produced • Keeping the temperature from dropping further

7. Vapour-Liquid Equilibrium • If the temperature were to stay at the boiling point of the liquid the liquid and gas states of this liquid would remain in dynamic equilibrium • This requires a closed system • Particles would change back and forth continually so long as the system remained closed

8. Pressure • According to K-M Theory: • Particles of a gas are constantly moving in random straight-line motion • If the gas particles are in a container, the particles must eventually collide with the sides of the container or other gas particles • When the particles collide with the sides of the container, they exert a force upon the container’s walls • We call this force gas pressure

9. Pressure • Pressure is defined as force per unit area • The Earth's atmosphere is about 100 km thick • This means each object on or above the Earth has a column of air (the atmosphere) pushing down on it • The force created by this column of air is called air pressure or atmospheric pressure

10. Units of Pressure • KiloPascal(kPa) • Newton of pressure per square metre of area • SI unit of pressure • Millibar • Meteorological unit of atmospheric pressure • One bar is equal to standard atmospheric pressure or 1 atmosphere. • Atmosphere (atm) • Derived from standard atmospheric pressure at sea level • 1 atmosphere is equal to 760 mmHg, or 101.325 kPa • Mm of mercury • Not a common unit used today outside the laboratory, however, many barometers found in the home use both mm of mercury as well as another unit like kilopascals.

11. Measuring Pressure • Pressure can be measured using several devices or instruments. One device is the Manometer • Usually has a bulb or glass container on one end and can be open or closed on the other • A liquid, often mercury, is placed in a U-shaped tube • The pressure is measured by finding the difference in height on both sides of the tube. • The greater the pressure of the gas in the bulb, the greater the force on the mercury and the higher the level of the mercury on the right side of the U-shaped tube.

12. Liquid: Open Container vs. Closed Container • Liquid left in an open container will eventually evaporate to dryness • The particles on the surface of the liquid vaporize and leave the container • If the container is sealed, the vapour is trapped and cannot leave the container • Eventually, the space above the liquid becomes saturated with vapour and for every molecule that evaporates, another condenses Liquid in a closed container, at a constant temperature, reaches equilibrium with its vapour

13. Liquid in a Closed Container Liquid in a closed container, at a constant temperature, reaches equilibrium with its vapour rate of evaporation = rate of condensation • The pressure created by the vapour at equilibrium is known as the vapour pressure, abbreviated Pvap • Vapour pressure is a characteristic physical property because many substances have different vapour pressures

14. Vapour Pressure & Intermolecular Forces • Intermolecular forces determine the rate of evaporation • If the intermolecular forces are lower, the amount of vapour that accumulates increases because it is easier for the particles to escape the forces of attraction in the liquid and become gaseous Lower intermolecular forces = more vapour = higher vapour pressure

15. Example: Alcohol & Water • Example: alcohol vs. water • Water & rubbing alcohol on a countertop which one will disappear first? • The temperature of both liquids is the same (if both at room temperature), so their intermolecular forces must determine which evaporates faster • The alcohol has lower intermolecular forces than the water, so it evaporates faster.

16. Volatile Substances • Generally, the lower the forces of attraction, the higher the vapour pressure • Substances that evaporate easily, such as alcohol and acetone, are said to be volatile • The lower the forces of attraction, the greater the volatility • Often, volatile substances are also very flammable

17. Vapour Pressure & Temperature • Increase in temperature = Increase in vapour • The higher temperature causes more particles to have enough energy to overcome the forces of attraction of the liquid • This increases the rate of evaporation, increasing the number of vapour particles… • If the number of vapour particles increases, the pressure increases • Decrease in temperature = increase in condensation • The increase in condensation reduces the number of vapour particles, reducing the vapour pressure. Increase in temp = increase in number of vapour particles = increase in vapour pressure

19. Vapour Pressure & Boiling Point • Boiling - when vaporization occurs throughout the liquid and the temperature of the liquid remains constant • This is known as an operational definition of boiling • The boiling point of a substance is defined as the temperature at which the vapour pressure of a substance is equal to the atmospheric pressure. • Temperature of a liquid increases = increase in vapour pressure • Bubbles of vapour begin to form in the liquid and rise towards the surface • Once the pressure inside the bubble is great enough to overcome the atmospheric pressure, the bubble is able to release the vapour it contains • If the atmospheric pressure is greater than the vapour pressure, the bubble will be kept below the surface until the pressure increases.

21. Atmospheric Pressure & Boiling Point • Higher intermolecular forces = lower the vapour pressure • Therefore, the greater amount of energy needed to produce enough vapour, so that vapour pressure equals atmospheric pressure • Since boiling occurs when atmospheric pressure and vapour pressure are equal, any change in pressure will produce a change in the boiling point • The normal boiling point of a substance is defined as the temperature when vapour pressure equals standard pressure (1 atmosphere, 101.3 kPa, 760 mmHg, 760 torr).

22. Atmospheric Pressure & Boiling Point • Lower atmospheric pressure = lower boiling point • Since less vapour pressure is required, less heat is needed • Example: longer cooking time to prepare foods at higher altitudes • At higher altitudes, the atmospheric pressure is lower because the column of air is shorter • The lower the air pressure, the lower the boiling point • If we were to try to hard boil an egg at a high altitude, the water would boil at a much lower temperature and the egg would take much longer to cook • Pressure cookers are often used to speed up cooking • Increases the pressure inside the pot  increases boiling point  higher temperature at which water boils = shorter the cooking time.

24. Vapour Pressure Curves Sample Questions: • What is the normal boiling point of ethanol?

25. Vapour Pressure Curves Sample Questions: • What is the normal boiling point of ethanol? • If the pressure in a city is 90 kPa, at what temperature will water boil?

26. Vapour Pressure Curves Sample Questions: • What is the normal boiling point of ethanol? • If the pressure in a city is 90 kPa, at what temperature will water boil? • Which substance has the largest forces of attraction at 60°C?