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.