Non-Ideal Behavior of Gases (College Board AP® Chemistry)

Non-Ideal Behavior of Gases

• The kinetic molecular theory and gas laws assume that molecules in the gaseous state have two important properties:

  • They exert no attractive or repulsive force on one another

  • They have negligible volumes compared to that of the container

• From the ideal gas equation, we can see what it means for a gas to have an ideal behavior by rearranging the ideal-gas equation to solve for n:

n = PV/RT

• This means that for 1 mol of an ideal gas, the quantity PV/RT equals 1 at all pressures

  • Hence, a graph plot of PV/RT against pressure should give a straight line parallel to the pressure axis

Graph Representation of an Ideal Behavior

A graph showing an ideal gas behavior for one mole of a gas at different pressures

Deviation from Ideal Behavior

• Although we can assume that real gases behave like an ideal gas, they do not do so under all conditions

  • For example, without intermolecular forces present, gases will not condense to form liquids

• Hence the ideal behavior described by the assumptions of the kinetic molecular theory and the ideal gas equation are only true under certain conditions

  • For real gases, this is true only at moderately low pressures and high temperature

• However, at high pressures and low temperatures, real gases deviate from the ideal behavior

  • At low temperatures, gas molecules have lower kinetic energy and begin to experience some intermolecular attraction

  • At high pressures, intermolecular attraction also comes into play because of the shorter distance seen when gas molecules are crowded

    • This means the combined volume of the gas molecules is not negligible relative to the container volume

Effect of High Pressure on Volume of Real Gases

At high pressure, the volume of molecules of real gases is not negligible

• For real gases, a graph plot of PV/RT against pressure does not have the same shape as that of an ideal gas

  • From the graph below, we can observe that at pressures below about 400 atm, cooling (lower temperatures) increases the extent to which a gas deviates from ideal behavior

    • This is because lower temperatures mean the gas molecules do not have sufficient kinetic energy to overcome the intermolecular force of attraction

    • This makes them stick to one another rather than bounce off each other

  • As temperature increases,

    • The negative deviation from the ideal value of 1 decreases, as seen in the graph from 200 K to 1000 K

    • This is because gas molecules begin to behave ideally as they have sufficient kinetic energy to overcome intermolecular attraction and their volumes become negligible

Deviation from Ideal Behavior

A graph showing the effect of temperature and pressure on real gases. The deviation is more significant at higher pressures and lower temperatures as seen by the direction of the curve away from the ideal gas.