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three states of matter

I. Gas Laws describe how gases behave - Interrelating temperature, pressure and volume - NOTE: FOR QUANTITATIVE CALCULATION NEED TO USE TEMPERATURE IN KELVIN

Gas laws apply to ideal gases, which are characterized by the following statements.

1. The molecules have no volume.  They are infinitely small.  This of course cannot be true, but the size of molecules (atoms) is generally small relative to the distance between them in gases so this is close enough.

 2. There are no intermolecular interactions – molecules (atoms) of a gas are not attracted or repelled to one and other.  In other words, they do not know that there are other molecules (atoms) around them.

1. Boyles law, pressure-volume relation, for a given mass of gas at constant T, the volume varies inversely with pressure.

T constant, 

increase volume, decrease pressure

decrease volume, increase pressure

Mathematically:  P1V1 = P2V2 for constant temperature

Ex: A gas initially has a volume of 22.4L at a pressure of 1 atm.  It is compressed to a final volume of 11.2 L, what is the final pressure of the gas.

P1 = 1 atm, V1 = 22.4 L
P2 = ?, V2 = 11.2 L

Solve for P2 and plug in:

P2 = P1V1 / V2 = (1 atm)(22.4 L) / (11.2 L) = 2 atm.  Does this make sense??

2. Charles law, Volume-Temperature relation, for a pressure constant, the volume of a fixed mass of gas varies in direct proportion with temperature

P constant

decrease T, volume goes down

increase T, volume goes up

V1 / T1 = V2 / T2 for constant pressure

3. Temperature-pressure relation

V constant

increase T, P increases
decrease T, P decreases

P1 / T1 = P2 / T2

4. Combined gas law - puts the three laws above together.

P1V1 / T1 = P2V2 / T2

5. Ideal gas law - interrelates the pressure, volume, temperature, and number of moles of a gas.  Above laws always dealt with the same amount of gas.

PV = nRT

n = number of moles,  R = 0.0821 L atm mol-1 K-1

What volume does 0.5 moles of gas occupy at 273K and 1 atm?

Solve the ideal gas law for V and solve.

V = nRT / P = (0.5 mol)(0.0821 L atm mol-1 K-1)(273K) / (1 atm) = 11.2 L

Remember earlier we said 1 mol of gas at STP occupies 22.4 L

(0.5 mol)(22.4 L / mol) = 11.2 L  so this makes sense!

 6. Dalton’s law of partial pressure: 

If we double the number of molecules in a container of fixed volume at constant temperature, what happens to the pressure?

 It doubles!

 

With ideal gases, the nature of the particles does not matter!  If ideal gases, He and Ne behave exactly the same, then what happens in the following situation:

 The pressure doubles.

 Ptot = P­A + P­B + PC …..

 The pressure of a mixture of gases is the sum of the partial pressures of the individual gases.

 

II. Liquids

Ideal gases have no intermolecular forces.  It is these intermolecular forces that cause liquids and solid to form.

A. Types of weak intermolecular forces (review section 5.9).

1. Dipole interactions - attractions between polar molecules

Hydrogen bonding - special type, particularly important in water.  Gives water many of its special properties.

 

2. Dispersion forces - very weak. sloshing of electrons create temporary dipole moments.  Why non-polar molecules condense

Stronger the intermolecular interactions, the more likely something is to be a liquid or solid.

B. Phase changes - vaporization and condensation

  vaporization  
Liquid vapor
  condensation  

 

  endothermic  
Liquid + heat vapor
  exothermic  

Why do molecules evaporate, vaporize?

->motion of the atoms or molecules overcome intermolecular forces

Vapor pressure - the pressure exerted by a vapor in equilibrium with its liquid

Vapor pressure is a strong function of temperature

For water

Temperature  (K) Vapor pressure (mm Hg)
323 K 92.5
353 K 355
373 K 760 ( = 1 atm)

Boiling point - the temperature at which the vapor pressure of a liquid is equal to the external pressure. 

The pressure of the gas leaving the liquid is sufficient to push back the atmosphere and liquid is converted to solid throughout the liquid (thus bubbling), not just at the surface. 

The vapor pressure is a property of a particular substance whereas the boiling point depends on the atmospheric pressure.  The higher the altitude, the lower the atmospheric pressure, the lower the boiling point.   On top of Mt. Everest (28,000ft), the boiling point of water is 76.5oC.

Normal boiling point - the boiling point of a liquid at 1 atm pressure (see level)

substance boiling point
butane (non-polar) 0oC
Ethyl alcohol (polar) 78.5oC
water (polar, hydrogen bonds) 100oC
 

Heat of vaporization (endothermic) - the amount of heat required to convert 1 g of a liquid to a gas at 1 atm

substance heat of vaporization
butane (non-polar) 34.6 cal / g
Ethyl alcohol (polar) 78.5 cal / g
water (polar, hydrogen bonds) 540 cal / g
 

Energy input to a liquid at its boiling point goes into vaporizing the liquid as opposed to raising the temperature.  Very difficult to raise a liquid to a temperature higher than its boiling point. 

Vaporization is a cooling process!  The evaporation of our sweat helps cool us off.  Heat from our bodies goes into vaporization.

Condensation is exothermic, exact opposite of vaporization.

III. Solids -

Atoms, molecules or ions that are held in an ordered arrangment

As a solid is heated, the particles vibrate more rapidly.

  fusion  
solid liquid
  solidification  

 

  endothermic  
solid + heat liquid
  exothermic  

melting point - temperature that a solid changes to a liquid

Heat of fusion (endothermic) - heat needed to completely transform 1 g of a solid at its melting point to 1 g of liquid.

substance heat of fusion
butane 19.2 cal / g
Ethyl alcohol 26.1 cal / g
water 79.7 cal / g