The Behavior of Gases
Chemistry
Chapter 14
Properties of Gases
• Gases are easily compressed, or squeezed, into a
smaller space.
– Compressibility is a measure of how much the volume
of matter decreases under pressure.
• Gases are easily compressed because of the empty space
between the particles in a gas.
• The amount of gas- moles (n), volume – liter (V),
and temperature – kelvin (T) are factors that
affect gas pressure – kilopascal (P).
Variables & Pressure
The Gas Laws
• The laws that describe how gases behave depending
upon the four variables:
* Temp (T)
* Pressure (P)
* Volume (V)
* Amount of gas (n)
• The four gas laws we will be discussing:
• Boyle’s Law
• Charles’ Law
• Gay-Lussac’s Law
• The Combined Gas Laws
Boyle’s Law
• How would an increase in pressure affect the
volume of a contained gas?
– If the temperature is constant, as the
pressure of a gas increases, the volume
decreases. In turn as the pressure decreases the
volume increases.
– Boyle’s Law states that for a given mass of gas at
constant temp, the volume of gas varies inversely
with the pressure.
P1 × V1 = P2 × V2
Charles’ Law
• As the temperature of an enclosed gas
increases, the volume increases, if the
pressure is constant.
– Charles studied effects of temp on the volume of gas at
constant pressure.
– Observations are summarized in Charles’ Law,
which states, that the volume of a fixed mass of gas is
directly proportional to its Kelvin
temperature if the pressure is kept constant.
Gay-Lussac’s Law
• As the temperature of an enclosed gas increases, the
pressure increases, if the volume is constant.
– Gay-Lussac’s Law states that the pressure of a gas is
directly proportional to the Kelvin temp if the
volume remains constant.
Because Gay-Lussac’s law
involves direct proportions, the
ratios P1/T1 and P2/T2 are equal.
Thus Gay-Lussac’s law can be
written:
The Combined Gas Laws
• Combines Boyle’s, Charles’, and Gay-Lussac’s
Laws into a single expression.
– This allows you to do calculations for situations in
which only the amount of gas (moles) is
constant.
Ideal Gases
• To calculate the number of moles of a contained
gas requires an expression that contains the
variable n.
– The combined gas law can be modified to include # of
moles.
This equation shows that (P x V) / (T x n) is a constant.
• This constant holds for ideal gases (gases that conform
to the gas laws).
The Ideal Gas Constant (R)
• If you know the values for P, V, T, and n for one set
of conditions, you can calculate a value for the
constant.
– Recall that 1 mol of every gas occupies 22.4 L at STP
(101.3 kPa and 273 K). You can use these values to find
the value of the constant, which has the symbol R and
is called the ideal gas constant.
– Insert the values of P, V, T, and n into (P × V)/(T × n).
• The ideal gas constant (R) has the value 8.31
(L·kPa)/(K·mol).
The Ideal Gas Law
• The gas law that includes all four (4)
variables (P, V, T, and n) is called the Ideal
Gas Law.
PxV=nxRxT
Or
PV = nRT
Real vs. Ideal Gases
• Ideal gases don’t exist!
– However, at many conditions of temp &
pressure real gases behave very much like an
ideal gas.
• Real gases differ most from an ideal gas at
low temps and high pressures.
Mixtures of Gas
• In a mixture of gases, the contribution each gas makes to
the total pressure is called partial pressure.
• Dalton’s Law – at constant volume & temp, the total
pressure exerted by a mixture of gases is equal to
the sum of the partial pressures of each component
gas.
Ptotal = P1 + P2 + P3 +….
How Gases Move
• Diffusion is the tendency of molecules to move
toward areas of lower concentration, until
concentration is uniform.
• Effusion involves gas escaping through a tiny hole
in its container.
– Gases of lower molar mass diffuse & Effuse faster that
gases of higher molar mass.
• Graham’s Law of Effusion – the rate of effusion of
a gas is inversely proportional to the square root of
the gases molar mass.
– Another words the gas with the lower atomic
mass effuses fastest.
Graham’s Law (continued)
• Graham’s Law can be written as follows for two
gases, A and B:
• This expression compares the rates of effusion
of any two gases, at the same temp & pressure.