Chemistry 112
Chapter 6 Class Notes

PROPERTIES AND PATTERNS


Although gases have different chemical properties, gases have remarkably similar physical properties.
            (1) Gases always fill their containers (recall solids and liquids). No definite shape and volume
            (2) Gases are highly compressible: Volume decreases as pressure increases
                 Volume increases as pressure decreases
            (3) Gases diffuse (move spontaneously throughout any available space).
            (4) Temperature affects either the volume or the pressure of a gas, or both.

Therefore a definition for gas is: a substance that fills and assumes the shape of its container, diffuses rapidly, and mixes readily with other gases.

THREE GAS LAWS:

(I) Bovle’s Law

Pressure -- force of colliding particles per unit area

According to the kinetic theory, gases exert pressure due to the forces exerted by gas particles Colliding with themselves and the sides of the container.

SI unit for pressure is Kilopascals - kPa
Atmospheric pressure — pressure exerted by air  

SATP — 100    kPa at 25 °C
STP—    1O1.3 kPa at   0 °C

Boyle’s Law - as pressure on a gas increases, the volume of the gas eases proportionally as the temperature is held constant.

                                                P1V1 = P2V2

(2) Charles’ Law — Temperature

Temperature - the average kinetic energy of the particles making up a substance
Kelvin Temp Scale: based of absolute zero — all kinetic motion stops

                        273°C =     0 K
                            0°C = 273 K
                          30°C  =303 K
                        -20°C  = 253 K

                        °C = K - 273
                           K= °C+273

Charles’ Law — the volume increases proportionally as the temperature increases, if the pressure is held Constant

                                    V1 = V2
                                    T1     T2

 

(3) Combined Gas Law

This law combines Boyle’s and Charles’ Laws

                                    P1V1 = P2V2

                                    T1            T2

AVOGADRO’S THEORY AND MOLAR VOLUME

·        The kinetic molecular theory is strongly supported by experimental evidence.

·        The K M theory explains why gases, unlike solids and liquids, are compressible.

·        The K M theory explains the concept of gas pressure.

·        The K M theory explains Boyle’s Law — Increase volume \ decrease pressure

·        The KM theory explains Charles’ Law Increase volume \ increase temperature

 

      In 1808 - Joseph Guy - Lussac stated:

Law of Combining Volumes — when measuring at the same temp and pressure, volumes of gas reactants and products of chemical reactions are always in simple ratios of whole numbers.

            In 1810 — Amadeo Avogadro stated:

Avogadro’s Theory — equal volumes of gases at the same temperature and pressure contains equal number of molecules

                        If T1 = T2 and  P1 = P2 and  V1 = V2 then  MM1 = MM2     (MM – molar mass)

 

Molar Volume of Gases

Let’s integrate Avogadro’s Theory with the mole concept.
A mole has a specific number of particles (6.02 x 1023 particles)

At STP              T1+2      =        0 °C
                        P1+2      =    101.3 kPa
                        MM1+2     =      I mol
                        V1        =      V2

At SATP            T 1+2    =       25 °C    
                        P 1+2   =      100 kPa
                        MM 1+2 =      1 mol

                               V1  =       V2

 

The volume of 1 mol of gas at STP/SATP is known as MOLAR VOLUME.
            At STP              22.4 L/mol
            At SATP            24.8 L/mol

Conversion step – factor label method  (1 mol of any gas = molar volume at STP or SATP)

n = moles (mol)
v total volume (L)
V molar Volume (L/mol) (22.4 L/mol at SIP 24.8 L/mol at SATP)

 

IDEAL GAS EQUATION

·        Ideal Gas — is a hypothetical gas that obeys all the gas laws perfectly under all conditions. It is composed of particles with no attraction to each other. (Real gas particles do have a tiny attraction)

·        We assume ideal gases always.

                                    Equation - PV = nRT

            P = pressure (kPa)

            V = volume (L)
            n = moles (mol)
            R = ideal gas constant (8.31  kPa*L )
                                                       Mol * K
            T = temperature (K)

            Sometimes the n must be converted to mass after the equation is completed. If this is necessary, use a conversion.