1.5.4Additional lessons
Real Gases — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 18 of 98
Part 1 of 6The Ideal Gas Model
The equation pV = nRT only holds exactly if four assumptions are true; real gases break at least two of them.
Point particles
- An ideal gas treats particles as having no volume at all, so the whole container volume is free space for movement.
No attractions
- Ideal particles neither attract nor repel one another, so each collision with the wall is as forceful as it would be alone.
Elastic collisions
- No kinetic energy is lost when particles collide, so the sample never slows down by itself.
Temperature link
- Absolute temperature is proportional to the average kinetic energy of the particles in the sample.
Part 2 of 6Why Real Gases Deviate
The two causes become important under different conditions.
Why Real Gases Deviate
Finite volume
- Real molecules occupy space, so the volume available for movement is less than the container volume, mainly at high pressure.
Attractive forces
- Intermolecular forces pull molecules inward,
- so they strike the walls less often and less hard,
- giving a lower pressure than predicted.
High pressure
- Squeezing the gas packs molecules close together, so their own volume becomes a large fraction of the total volume.
Low temperature
- Slower molecules spend longer near neighbours, so attractions matter more and the gas may even condense to a liquid.
Part 3 of 6Conditions for Ideal Behaviour
1
Start from a gas at room temperature and atmospheric pressure, where molecules are far apart and the ideal equation works well.
2
Raise the pressure at constant temperature and the molecules are forced close together, so their own volume stops being negligible.
3
Lower the temperature and molecules move more slowly, so intermolecular attractions have more effect on collisions with the wall.
4
Near the condensing point the attractions are strong enough to pull molecules together and the measured pressure falls well below the ideal value.
5
Return to low pressure and high temperature and the molecules are far apart and fast, so the gas again behaves almost ideally.
| Condition | Effect on deviation | Reason |
|---|---|---|
| High pressure | Larger | Molecular volume is no longer negligible |
| Low temperature | Larger | Attractions become significant |
| Low pressure | Smaller | Molecules far apart |
| High temperature | Smaller | Kinetic energy outweighs attractions |
Part 4 of 6Which Gases Are Most Ideal
Helium and hydrogen
- Small, non-polar molecules with very weak forces behave nearly ideally even at fairly low temperatures.
Gases with strong forces
- Polar molecules or those with many electrons, such as ammonia, deviate more because their attractions are stronger.
Large molecules
- Bigger molecules occupy more volume and have stronger dispersion forces, so they deviate sooner.
Approximate use
- In most IB calculations at ordinary conditions the ideal gas equation is accepted as accurate enough.
Part 5 of 6Worked Example
Question Calculate the pressure predicted by the ideal gas equation for 1.00 mol of gas in a 0.500 dm³ container at 300 K, then comment on its reliability.
- Convert the volume0.500 dm³ = 0.500 × 10⁻³ m³.
- Rearrange pV = nRT to p = nRT / V.
- Substitutep = (1.00 × 8.31 × 300) / (0.500 × 10⁻³) = 4.99 × 10⁶ Pa.
Answer
- p ≈ 4.99 × 10⁶ Pa (about 4.99 MPa).
- At this high pressure molecular volume matters, so the real pressure would differ from this value.
Part 6 of 6Exam Checklist & Tips
1.5.4 – Key Points for the Exam
- Ideal gas A hypothetical gas obeying all assumptions of kinetic theory exactly, with pV = nRT true at all conditions.
- Real gas A gas whose particles have volume and attractions, so it follows the ideal equation only approximately.
- Intermolecular forces Attractions between neighbouring molecules that reduce the pressure a real gas exerts.
- Deviation The difference between the measured behaviour of a gas and that predicted by the ideal gas equation.
Common Mistakes
- Saying real gases deviate most at high temperature;
- deviations are largest at low temperature and high pressure.
- Forgetting that BOTH effects (volume and attractions) must be named when asked to explain deviation.
- Stating that attractions increase the pressure; they reduce the pressure compared with the ideal prediction.
- Using Celsius temperatures in pV = nRT instead of kelvin.
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