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Chapter 12: Gases
• Kinetic-Molecular Theory 1.Gases are composed of tiny particles, with little or no attractive forces, separated by empty space.
2.Gas particles are in constant, random, straight-line motion, with perfectly elastic collisions .
• A perfectly elastic collision passes 100% of energy.
• KE= 1/2 mv2 where average temperature tells KE.
• Gases are compressible and take the shape of their container.
• Gases effuse and diffuse based upon Graham’s law of diffusion…
Graham’s Law Example
• Compare the rate of diffusion of hydrogen and oxygen.
• Look up the GAM of helium and the GMM of nitrogen…. H2=2.0 g/mole and O2=32.0 g/mole.
• So…the rate of H2 is 4x O2
Gas Pressure
• Pressure is defined as the force exerted per unit of area…for gas pressure, this pressure is exerted by a gas, for air pressure, it is exerted by the atmosphere… • Think of a column of air 1 inch by 1 inch from the ground up to space…that column would weigh 14.7 pounds per square inch… also • 14.7 p.s.i=760 mmHg=101.3 kPa=760 Torr.
• Let’s look at these units a little closer…
Inches or mm of Mercury
A mercury-filled glass tube, with a vacuum in the top is inverted into a dish of mercury…the height of the column is measured to measure the atomospheric pressure… 29.93 inches of mercury=1atm=760mmHg=760 millibars. This device is called a barometer.
More on Units of Pressure
• The SI unit of pressure is the Pascal.
• 1 atm=760 mmHg=29.93 inchesHg=14.7 p.s.i.
• 760 mmHg=760 Torr.=760 millibars • Like a barometer measures the atmospheric pressure, a manometer measures a sealed container’s pressure.
Math with Pressure Units
• Convert pressure units using dimensional analysis.
• Example… Convert 788 mmHg to atm… • Example2… Convert 788 mmHg to kPa….
Answers
• UK: atm • K: 788 mmHg • 788 mmHg x 1atm = 1.03
6842 atm • 1 760 mmHg • Round to 3 sd… . 1.04 atm • UK: kPa • K: 788 mmHg • • 788 mmHg x 101.3 kPa = 105 .0321 kPa 1 760 mmHg • Round to 3 sd… . 105 kPa
Dalton’s Law
• Dalton’s law of partial pressures states that the partial pressures in a space or container add up to the total pressure of the space or container.
• PTOTAL= P1+P2…..+PFINAL • Example1 CO2 pressure = .55 atm O2 pressure = .75 atm • What is the total pressure of these two gases in a sealed container?
• .55 atm + .75 atm = 1.3 atm
12-2 Intramolecular Attractions
• +/- attraction between a cation and anion make ionic bonds some of the strongest.
• Covalent bonds are made by sharing of electrons.
• Metallic bonds are caused by the free-moving sea of electrons (mobile electrons) which hold the metal + ions together.
12-2 Forces of Attraction Between Molecules
• The attractions between molecules are called intermolecular attractions.
• For molecular compounds, these attractions are ….
1.Dispersion forces….electron motion causes attraction 2.Dipole attractions….+/- attraction of molecules 3.Hydrogen bonding…+/- attraction of H+ and an anion.
• Ionic compounds are solids at room temperature because they have strong +/- attractions.
Properties of Compounds and Metals
• Most molecular compounds are gas or liquid, and have a low melting/boiling point.
• Ionic compounds have very high melting/boiling points.
• Metals have high melting/boiling points.