Vapour Pressure 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 Lets review
Gas Pressure: Gas particles are in constant motion and exert a force through collisions. Air particles exert atmospheric pressure on everything 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 dont notice it 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. 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 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 Equilibrium will only be reached with:
1. 2. 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 Consider the equilibrium of a can of
Coke before you open it Vapour Pressure: Measured pressure of vapour at equilibrium in a closed system. 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 Compounds that easily vapourize (have low IMF) are volatile (volatility)
The opposite for stronger IMF low vapour pressure high vapour pressure 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 low vapour pressure high vapour pressure
Remember: high energy particles go first Vapour pressure is influenced by: Pressure 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:
(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 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 80oC weak IMF 80oC - boiling
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 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
Food takes longer to cook at high altitudes water isnt as hot when it boils 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 From the vapour pressure graph below: a. Which compound has the highest vapour pressure?
b. Which compound might have the strongest IMF? c. 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 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 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
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