Gases

Volume, Pressure & Temperature

Volume: Total space occupied by a substance

The volume of a gas can either increase (expansion) or decrease (contraction)

Volume of a gas increases when the number of moles of gas increases

Volume of a gas increases when temperature increases

Volume of a gas increases when pressure decreases



Usually measured in L (liquids and gases)

1 cm3 = 1 mL

1 dm3 = 1 L = 1000 mL

1 m3 = 1000 L



Indicate, in each pair, which object has a higher gas volume, and why (same gas always).


Pressure: Force applied per unit of area (P = F/A)

The pressure of a gas results from the collisions of the gas molecules with the walls of the container

Pressure of a gas increases when the number of moles of gas increases

Pressure of a gas increases when temperature increases

Pressure of a gas increases when volume decreases



Can be measured in different units

1 atm = 101.3 kPa *Standard Pressure

1 atm = 760 mm Hg

1 atm = 1013 mbar

Can be measured with a barometer (atmospheric pressure) or a manometer (gas pressure in closed container)



If the same person is wearing these shoes when stepping on somebody’s foot: which heel would exert more pressure? Why?



How could you lower the pressure applied by this truck on the road? Explain.


Temperature: Manifestation of average kinetic energy of molecules of a gas

Which gas is at a higher temperature? Explain.


Boyle’s law: At constant temperature, the volume of a given quantity of gas is inversely proportional to its pressure

PV = k (k: constant)

or

P1V1 = P2V2


Example: You need to put the equivalent of 400 L of air at standard pressure to go scuba- diving. If your tank’s capacity is only 11.4 L, what pressure will be needed in it assuming that temperature is constant?


Kelvin’s absolute temperature:

At T = 0°C, T = 273K T(K) = T(°C) + 273

Freezing point of Water:

T(K) = T(°C) + 273

T(K) = 0°C + 273

T(K) = 273K

Boiling point of Water:

T(K) = T(°C) + 273

T(K) = 100°C + 273

T(K) = 373K


Charles’s law: At constant pressure, the volume of a given quantity of gas is directly proportional to its temperature

V/T (K) = k (k: constant)

or

V1 / T1(K) = V2 / T2 (K)

*T(K): temperature in Kelvin T(K) and n must be constant


Combining Boyle’s & Charles’s law: At constant volume, the pressure of a given quantity of gas is directly proportional to its temperature

P/T (K) = k (k: constant)

or

P1 / T1(K) = P2 / T2 (K)

*T(K): temperature in Kelvin T(K) and n must be constant


When temperature increases in a room (external factor):

_The volume (internal factor) of a gas trapped in a container increases, as molecules will move faster, collisions will increase and expansion will occur.

_The pressure (internal factor) will also increase because molecules will move faster and therefore collide with walls of recipient with higher force.

It all leads to an EXPLOSION!


Example: The volume of a given gas sample is 1.56L at -70°C. What will its volume be at 2°C if the pressure of the system remains constant?


Yes. The speed (and thus kinetic energy) of gas molecules has increased as a result of the increase in temperature

Volume does not change (can)

Pressure increased as the energy given to molecules (by the increase of temperature) increased their velocity (Ek) and so their collisions with walls

would increase, thus increasing pressure (P=F/A)

No, it does not. It only applies when volume is allowed to change as a result of a change in temperature, at constant pressure.

Gases

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