Vapour Pressure

C11-1-12 MOLECULAR PROPERTIES OUTCOME QUESTION(S): C11-1-12 MOLECULAR PROPERTIES Define normal boiling point temperature in terms of vapour pressure. Include: volatility, dynamic equilibrium Interpolate and extrapolate information from pressure versus temperature graphs and heating/cooling curves. Explain colligative properties like freezing-point depression and boiling-point elevation at the molecular level. Vocabulary & Concepts Closed system  

Standard atmospheric pressure (at sea level): Gas Pressure: Gas particles are in constant motion and exert a force through collisions. Air particles exert atmospheric pressure on everything Let’s review… Standard atmospheric pressure (at sea level): 1 atmosphere = 760 mmHg = 101.3 kPa Our bodies are built to withstand the force of air pressure pushing on us and so we don’t notice it…

Why is a closed container necessary to measure vapour pressure? Vapor Pressure: (l) (g) Endothermic - particles on the surface that have enough energy to overcome IMFs A measure of the natural evaporation rate of a substance Why is a closed container necessary to measure vapour pressure? Vapour Pressure: The measured pressure of vapour at equilibrium above the liquid in a closed system.

“Dynamic equilibrium” Vapour Pressure and Equilibrium: Begin: molecules vapourize filling the space. As more molecules evaporate, it becomes more likely for molecules to condense. End: evaporation rate = condensation rate. Amounts of each state remain constant VAPOUR “Dynamic equilibrium” changing balance Microscopically molecules are in flux but macroscopically quantities are constant. LIQUID

Vapour pressure is a characteristic property Vapour pressure can be measured by a manometer: Compares vapour pressure to normal atmospheric pressure Measures difference in height on both sides Pvap = Patm + h (vapour pressure > air pressure) Pvap = Patm – h (vapour pressure < air pressure) Vapour pressure is a characteristic property

Vapour pressure is influenced by: Vapour Pressure Equilibrium will only be reached with: Sealed container Constant condition ([ ], T, P, V) closed system Vapour pressure is influenced by: Intermolecular forces - stronger attraction restricts phase change Temperature - energy is required to phase change Pressure - pressure forces particles into liquid state Vapour Pressure: Measured pressure of vapour at equilibrium in a closed system. Consider the equilibrium of a can of Coke – before you open it…

Vapour pressure is influenced by: Intermolecular forces Follow the logic arrows… Weaker intermolecular forces  less energy needed to break IMF  easier for all particles to phase change (regardless of temperature)…  increasing rate of vapourization  increasing vapour pressure The opposite for stronger IMF… Compounds that easily vapourize (have low IMF) are volatile (volatility) low vapour pressure high vapour pressure

Remember: high energy particles go first Vapour pressure is influenced by: Temperature Increasing temperature  increases the kinetic energy of all particles  increasing the number of particles with the energy to break IMF…  increasing rate of vapourization  increasing vapour pressure Of course the opposite happens with cooling… Remember: high energy particles go first low vapour pressure high vapour pressure

Vapour pressure is influenced by: Increasing pressure  added force restricting particle motion  harder for all particles to phase change (regardless of temperature)...  decreasing rate of vapourization  decreasing vapour pressure An open system must contend with atmospheric pressure - air molecules colliding and pushing on the surface of the liquid making vaporization more difficult

Boiling: Boiling Point: (l) (g) Endothermic – rapid vaporization of a liquid Energized particles near the heat source spread out, forming bubbles that rise and enter the atmosphere Increased vaporization at the surface pushes against the air particles reducing the pressure on the liquid - making it easier for vapour to form inside the liquid state - bubbles low vapour pressure high vapour pressure Boiling Point: Point (temperature) where the vapour pressure produced equals air pressure. boiling

The opposite for stronger IMF… Boiling Point and IMF: Weaker intermolecular forces  less energy needed to break free  increased vaporization pushes against atmospheric pressure…  easier for internal vapour to form  lower boiling point (temperature) The opposite for stronger IMF… strong IMF weak IMF 80oC 80oC - boiling

Where do we see changes in atmospheric pressure? Boiling Point and Atmospheric Pressure: Any change in air pressure will produce a change in the boiling point Less atmospheric pressure  less force pushing on the surface  easier for surface vapour to equalize the air pressure…  less energy needed for internal vaporization  Lower boiling point high air pressure low air pressure Where do we see changes in atmospheric pressure? 80oC 80oC - boiling

Atmospheric pressure decreases with altitude: Less pressure, less energy needed to vaporize “Boils” (vapour bubbles form) at lower temperature Boiling Point of Water Outer Space ? 10 000 ft 90oC 7500 ft 92oC 5000 ft 95oC 2000 ft 98oC Sea level 100oC Food takes longer to cook at high altitudes – water isn’t as hot when it “boils” Since there is NO atmosphere in space the lack of pressure massively reduces the boiling point of liquids - your gruesome fate: your blood, your eyeballs will boil furiously…

If air pressure is reduced boiling point is reduced… Vapour Pressure Graphs and Boiling Point: Graph of vapour pressure vs. temperature Standard air pressure is 101.3 kPa Substance boils at point where vapour pressure equals atmospheric pressure If air pressure is reduced boiling point is reduced… Boiling Point

Pick any temperature to compare the compounds From the vapour pressure graph below: Which compound has the highest vapour pressure? Which compound might have the strongest IMF? What is the boiling point for ethanol at an air pressure of 85 kPa? Pick any temperature to compare the compounds a. chloroform b. acetic acid c. 73oC

Boil – when vapour pressure A heating curve test of Acetic Acid was done and the results are graphed below: Using the vapour pressure graph information as well, at what pressure must the heating test have been conducted? The test was done at 90 kPa Boil – when vapour pressure equals air pressure

C11-1-12 MOLECULAR PROPERTIES CAN YOU / HAVE YOU? C11-1-12 MOLECULAR PROPERTIES Define normal boiling point temperature in terms of vapour pressure. Include: volatility, dynamic equilibrium Interpolate and extrapolate information from pressure versus temperature graphs and heating/cooling curves. Explain colligative properties like freezing-point depression and boiling-point elevation at the molecular level. Vocabulary & Concepts Closed system