1.5.4Additional lessons
Real Gases — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 21 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5The Ideal Gas Assumptions
An ideal gas obeys PV = nRT at every pressure and temperature; real gases only approximate this.
No volume
- No volume The model treats gas particles as points whose own volume is negligible compared with the container.
No attractions
- No attractions Ideal particles exert no forces on each other, so they collide perfectly elastically.
Random motion
- Random motion Particles move continuously and randomly, and their average kinetic energy depends only on temperature.
Real behaviour
- Real behaviour Real particles have a finite size and attract each other, so both assumptions fail to some degree.
Section 2 of 5Deviation at High Pressure
Deviation at High Pressure
Compressed gas
- Compressed gas Squeezing the gas into a small volume brings the particles close together.
Particle volume
- Particle volume The space taken by the particles themselves becomes a significant part of the total volume.
Effect on volume
- Effect on volume The real volume is larger than the ideal calculation predicts, since the particles cannot be squashed to nothing.
Result
- Result The product PV is greater than nRT, so the measured behaviour lies above the ideal line.
Section 3 of 5Deviation at Low Temperature
Deviation at Low Temperature
Slower particles
- Slower particles At low temperature particles have less kinetic energy and move more slowly.
Attractions matter
- Attractions matter Intermolecular forces now have time to pull particles together during collisions.
Effect on pressure
- Effect on pressure Particles strike the walls less forcefully, so the real pressure is lower than the ideal value.
Extreme case
- Extreme case If the temperature is low enough the attractions condense the gas into a liquid.
Section 4 of 5When Is a Gas Most Ideal?
1
Gases behave most ideally at low pressure, where the particles are far apart and their volume is negligible.
2
They also behave most ideally at high temperature, where fast particles overcome weak attractions.
3
Small non-polar molecules such as helium and hydrogen have very weak London forces, so they are closest to ideal.
4
Larger or polar molecules such as ammonia or water vapour have stronger attractions and deviate much more.
5
Plotting PV/nRT against pressure shows an ideal gas as a horizontal line at 1, with real gases curving away from it.
Section 5 of 5Exam Checklist & Tips
1.5.4 – Key Points for the Exam
- Real gas A gas whose particles have volume and attract one another, so it departs from PV = nRT.
- Intermolecular forces Attractions between molecules that lower the pressure of a real gas.
- Ideal conditions Low pressure and high temperature.
- Deviation The difference between the measured and the ideal behaviour of a gas.
Common Mistakes
- Saying that real gases deviate because their particles are different from ideal ones in mass rather than size and attraction.
- Stating that ideal behaviour is best at high pressure.
- Mixing up the causesparticle volume matters at high pressure, attractions matter at low temperature.
- Claiming that helium deviates most; its tiny weak-force atoms make it nearly ideal.
Last updated:
\n