The ideal gas model treats a gas as moving particles with negligible volume and no intermolecular forces. At both IB Chemistry SL and HL, an ideal gas obeys under all conditions, although real gases only approximate this behaviour.
Assumptions of the Ideal Gas Model
| Assumption | Meaning |
|---|---|
| Particles have negligible volume | The particles are treated as points, so their own volume is insignificant compared with the container's volume. |
| No intermolecular forces act | Particles neither attract nor repel one another except during collisions. |
| Particles move constantly and randomly | They travel in straight lines between collisions and move in all directions. |
| Collisions are perfectly elastic | Collisions between particles and with container walls cause no net loss of kinetic energy. |
| Collision time is negligible | Collisions occur instantaneously compared with the time particles spend moving freely. |
| Average kinetic energy depends on temperature | The average kinetic energy of particles is proportional to the absolute temperature in kelvin. |
Gas pressure results from particles colliding with the container walls. Raising the temperature increases their average kinetic energy, so collisions become more frequent and forceful. This explains the pressure-temperature relationship when volume is constant.
Real gases deviate most from ideal behaviour at high pressure and low temperature. At high pressure, particles are close together, so their volume is no longer negligible. At low temperature, particles have less kinetic energy, making intermolecular attractions more significant.
A common misconception is that ideal gas particles have no volume at all. The model assumes their volume is negligible relative to the container, not that real particles are physically volumeless.
Exam Technique
For Structure 1.5 questions, state the relevant assumption and connect it directly to the conditions. If asked to explain deviation, write: high pressure makes particle volume significant, while low temperature makes intermolecular attractions significant. In calculations using , convert temperature to kelvin and ensure the pressure and volume units match the chosen value of .