Gas Pressure

Pressure = Force/AreaEg: lbs/in2, kg/m2, g/cm2

Force applied to a reduced area will INCREASE Pressure

Increased Force applied to same Area will INCREASE Pressure

Measuring Pressure

• Atmospheric pressure:• Gases have MASS..that is pressing down on you.• High Pressure compresses air leading to better

weather.• Low Pressure allows for condensation. RAIN..Clouds.

Barometer: Measures atmospheric pressure using mercury.

1 atm = 760 mm Hg. = 760 torr. (Evangelista Torricelli) = 14.7 lb/square in. ( English Units)

Converting Units of Pressure

• Convert to atmospheres

• 385 mmHg

• 570 torr

More Conversions

• What is equivalent of 0.930 atm in mm mercury and in torr.?

Measuring Diff. Pressure

• Manometer: used to measure the pressure of gas in a closed container.

• Differential Manometer; measures pressure by revealing the difference between two pressures.

• Gas Gauge: Membrane separates gas and a springlike mechanical device exerts a force opposing the gas . This force is calibrated to read Pounds Per Squre inch and mm. Hg

The Gas Laws

• The Physical Properties of gases depends on 4 Variables:– Pressure (P)– Volume (V)– Temperature (T)– Moles (n)

The Laws

• Varying 2 properties and keeping 2 constant allows us to observe many behaviours of gases.

• Boyle’s Law: Vary pressure and volume• Charle’s Law: Temperature and Volume• Gay Lussac’s Law: Temperature and Pressure• Avogadro’s Law: moles and Volume

Boyles Law

• The product of a gas pressure multiplied by it’s volume is CONSTANT.

• Experimentation reveals:– As pressure decreases, the volume increases, or as

the pressure increases, the volume decreases.– The P x V product is Constant.– Therefore:– P1V1 = P2V2

Breathing

• As lung volume increases, gas pressure in lungs decreases, and air will flow into the lungs.

• As lung volume decreases, gas pressure increase and air will flow out of the lungs.

Examples

• A 712 mL sample of gas at 505 torr is compressed at constant temperature until its final pressure is 825 torr. What is it’s final volume.

Another example

• A 2.0 L sample of gas at 0.800 atm must be compressed to 1.6L at constant temperature. What pressure in atmospheres must be exerted to bring it to that volume?

Charles Law

• V1/T1 = V2/T2

A 512mL sample of a gas, in a cylinder with a moveable piston at 0 deg. C is heated at a constant pressure of 0.800 atm to 41 deg. C.

What is its final volume?

Gay Lussac’s Law

• As temperature increases, pressure increases• The ration P/T is a constant, the the ration P/t

is not constant. (t is in deg. C, T is in Kelvin)• Kelvin = Celsius + 273

• P1/T1 = constant = P2/T2

Example

• A constant volume sample of gas at 27 deg. C and 1.00 atm. Is heates so that its pressure increases to a final value of 1.5 atm. What is its final temperature?

Avogadros’s Law

• V = constant = n

• Volume of gases at the same temperature and pressure contain equal numbers of molecules (n)

• V1/n1 = V2/n2

Combined Gas Law

• P1V1/T1 = P2V2/T2

What is the new pressure of a gas when both temperature and volume are changed simultaneously?

What is the new T of a gas when P and V are changed?

What is the new V of a gas when T and P are changed?

Ideal Gas Law

• PV / nT = constant• PV = nRT

• R is the universal gas constant.• At 1 atm, 273K, 1.0 mol, 22.4 L • R = 0.0821 Latm/Kmol

What gas is better?

• To fill your tires with?• To fill your cushions?

Ideal Gas Law and Molar Mass

• n = g/M ;

• Number of moles (n) = sample in grams/Molar Mass

• (g)/ (g/mol) = mol• M = gRT/PV

Dalton’s Law of Partial Pressures

• Ptotal = Pa + Pb

Air Pressure = 760 mm Hg or Torr, Partial Pressure of Oxygen is 160 Torr, What is Nitrogen?

Henry’s Law

• The amt. of gas that will dissolve in a liquid depends manly on the gas pressure and is described by Henry’s Law.

• As gas pressure increases, more gas will dissolve.

• (Volume of gas dissolved)/(volume of liquid) = CH x gas pressure

Chapter 6

• Solids: fixed volume and shape• Liquids: Fixed volume but no shape• Gases: volume depends on the the volume of

container. A gas will always occupy all of its container.

Kinetic Energy at Molecular level

• Kinetic Energy: energy of a moving body, or energy of motion. On a molecular level, kinetic energy depends on temperature.

• T increases, so does Kinetic energy.• Molecules move faster• Kinetic Energy tends to override attractive

forces.

Phase Transitions

• Melting: solid transforms into a liquid with the addtion of sufficient heat.

• Freezing: reverse process• MP and FP of a solid are identical.• Vaporization: transition from liquid to gas.• Condensation: reverse process

• Phase transitions are reversible and occur in either direction.

Physical Properties

• Molar Heat of Fusion: The heat required per mole of solid to melt it .

• Molar Heat of Vaporization: The heat per mole of liquid required for vaporization.

Attractive Forces between Molecules

• Van der Waals Forces, or intermolecular , or secondary forces… all the same.

• They cause gases to condense and liquids to freeze into solids.

• They are electrical and they arise from molecular polarity.

London Forces of Interaction

• Even for molecules that have no permanent polarity, temporary dipoles exist for brief moments.

• Temporary dipoles are caused by erratic motions of electrons that result in uneven distributions of electric charge .

• The attractive force resulting from this temporary dipole is called a London Force.

Dipole – Dipole Interactions

• Molecules with permanent dipole moments, or POLAR Molecules also attract different molecules possessing dipole moments.

Dipole – Dipole Forces

• The attractive forces between 2 polar molecules or mixtures of the 2 are called dipole-dipole forces.

• Substances composed of polar molecules have higher melting and boiling points then those composed of molecules possessing no dipole moment.

The Hydrogen Bond

• The difference between Hydrogen and each of 3 elements (F, O2, N2) electronegativity is very large. This polarity leads to a unique strong attractive force called : Hydrogen Bond.

• Eg. H20, NH3, HF

The Hydrogen Bond (Water)

Surface TensionResisting the expansion of the liquid’s

surface

Surface-Active Agentsaka: Surfactants

• Reduce surface tension of water. – Soaps– Detergents– Creation of stable foams.– Consist of both ionic and– Non polar segments

Amphipathic MoleculesHydrophilic- Water Loving and Hydrophobic – Water Hating

How a Surfactant reduces surface tension.

• Surfactants tend to stay at surface of water• At the interface, hydrophobic end can escape

the water.• Hydrophilic end remains nestled in water.• The concentration of the water at the surface

is reduced. Therefore so are the attractive forces between water molecules.

• Surface tension is reduced.

Molecular Mixtures

• Solution: a mixture of substances with different kinds of molecules (or ions) uniformly distributed visually throughout the mixture.

• Secondary forces are key to the formation of solutions.

• Like dissolves like (similar secondary forces)

Molecule A & B in Solution

Predicting Solutions

• Can water and ethanol go into solution?• 1. Draw Structural formula• 2. Are they polar or nonpolar or both?• 3. They are both polar. • 4. Do they have any Hydrogen Bonds? Yes OH

group.• 5. Secondary forces are about the same, they will

easily bond with each others secondary forces. Therefore …YES it will go into soluton!

More Examples

• Carbon Tetrachloride and Water?• Weak 2ndary forces will not combine with

strong secondary forces.

• Hexane and Water?• Hexane is in liquid state from London forces• Water is Hydrogen bonding.• NO…solution

The Vaporization of Liquids

• Vapor: the gaseous part of a liquid and gas simultaneously present.

• Gas in a gas station ( the smell is a gas vapor)• Oxygen vapor exists at -200 deg. C• To vaporize the molecule must free itself of

neighbor’s attractive forces and enter the gas phase.

Influence of Secondary Forces on Vapor Pressure

• VP measures the escaping tendency of a substance’s molecules.

• The smaller the forces, the greater tendency of molecules to escape & the greater the vapor pressure.

• The larger the forces, the smaller the vapor pressure.

Boiling• Bubbles of vapor form throughout the liquid in an open

container.• A liquids molecules must overcome the opposing force of

the atmospheric pressure to enter the gas space over the liquid.

• When the temperature of the liquid is such that it’s vapor pressure is less than atmospheric pressure, vapor will leave only at its’ surface.

• When VP is equal to atmospheric pressure, bubbles of vapor form throughout the bulk of the liquid. BOILING

• It’s all about VaporPressure of Liquid and the Atmospheric pressure being equal….it changes with atomospheric pressue.

Normal Boiling Point

• Temperature at which boiling occures under an external pressure of exactly 1 atm.

• We can use normal boiling point as a good qualitative indicator of the secondary forces in a liquid.

Using Molecular structure to predict boiling points

• H2, CH4, NH3, HF• 1) Are there any polar compounds?• Polar compounds have higher boiling points then non-polar

compounds.– Why? Non-polar compounds are only influenced by London

Forces.• 2. Do any of the Polar Compounds have H-bonds? The one

with the highest electronegativity will have the highest boiling point.

• 3. If they have similar forces and electronegativities, then which molecule is bigger/heavier, and more electrons and therefore will interact with stronger London forces then a smaller molecule.

Stronger Bonds, Higher Boiling Point

• What intramolecular force is stronger?• Ionic Bond, or Covalent Bond?

• Covalent Bonds are stronger. • Ionic bonds will dissassociate into ions in water.

They are not actual molecules.

• What is stronger NaCl or Graphite (long non-polar carbon chain?)

EquilibriumIn a closed system, the rate of vaporization = rate of condensation

Vapor pressure in equilibrium is called Equilibrium Vapor Pressure.

Dynamic Equilibrium: Liquid + Heat -----------vapor vaporization

Vapor --------------liquid + heat condensation

Evaporation

• Vaporization in an Open System.• There will be an imbalance with no

condensation, so the system is not in equilibrium.

• Therefore the liquid must continuously absorb heat from the surroundings. If the flow of heat is restricted, the temp. of the liquid will drop.

• Evaporation is a cooling process.

Using Secondary forces to predict boiling points

• H20(water) vs H2S (hydrogen sulfide)• Boiling point of water is 100 deg. C• Boiling point of H2S is -61 deg. C

• H bonds only happen with O, N, and F.

• H2S is limited to interact with weaker secondary forces.

example

• Which has a higher boiling point?

• Ammonia (NH3) or Phosphine (PH3)

Attractive Forces and The Structure of Solids

• Crystalline Solids:– Solids form crystals arranged in regular patterns.

They can exist in well-defined shapes, like prisms, cubes…

– They have sharp edges or flat faces because it is that highly ordered inner surface that is exposed.

• Amorphous Solids:– Lacking crystalline structure.– Eg. Non crystalline form of SiO2(glass)

Crystalline

Crystalline VS Amorphous

Photo 1 is Quartz Photo 2 is glass formed from molten quartz

Melting Points

• Covalent Crystals: Held together by covalent bonds.

• Ionic Crystals: formed by ionic bonds.• Molecular Crystals: held together by

secondary forces, like Hydrogen bonds, London forces, and dipole-dipole forces.

Table of Melting Points