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AP Chemistry 3.6 Deviation from Ideal Gas Behavior Notes - New Syllabus 2024-2025

AP Chemistry 3.6 Deviation from Ideal Gas Behavior Notes- New syllabus

AP Chemistry 3.6 Deviation from Ideal Gas Behavior Notes – AP Chemistry –  per latest AP Chemistry Syllabus.

LEARNING OBJECTIVE

Explain the relationship among non-ideal behaviors of gases, interparticle forces, and/or volumes.

Key Concepts: 

  • Non-Ideal Behavior of Gases

AP Chemistry-Concise Summary Notes- All Topics

 Relationship Among Non-Ideal Gas Behaviors, Interparticle Forces, and Particle Volumes

 The Ideal Gas Law, \( \mathrm{PV = nRT} \), assumes that gas particles have no volume and no intermolecular forces. In reality, these assumptions break down under certain conditions, and gases exhibit non-ideal behavior. Real gases deviate most significantly at high pressures (small volume) and low temperatures (strong attractive forces).

1. Causes of Non-Ideal Behavior:

a) Interparticle Attractions:

  • Real gas molecules experience weak attractive forces (dispersion, dipole–dipole, or hydrogen bonding).
  • When these attractions become significant, gas particles collide with container walls less forcefully → the observed pressure is lower than predicted by \( \mathrm{PV = nRT} \).
  • These effects are strongest at low temperatures and moderate pressures, where molecules move more slowly and can interact longer.

b) Finite Particle Volume:

  • Gas molecules occupy physical space; at high pressures, the volume of particles is no longer negligible compared to the container volume.
  • This causes the available free space for motion to be smaller than the measured container volume → the observed volume is larger than predicted by the ideal gas law.

2. Conditions for Ideal vs. Non-Ideal Behavior:

ConditionGas BehaviorExplanation
High Temperature, Low PressureNearly IdealParticles are far apart; IMF effects negligible; volume negligible.
Low TemperatureNon-Ideal (Attractive Forces Important)Particles move slowly; IMF effects cause reduced pressure.
High PressureNon-Ideal (Volume Effects Important)Particles are crowded; finite size reduces free volume.

3. van der Waals Equation (Correction for Real Gases):

To account for real gas behavior, the van der Waals equation modifies the ideal gas law by introducing correction terms for intermolecular attractions (\(a\)) and finite volume (\(b\)):

  • \( \mathrm{a} \): accounts for attractive forces (adds to pressure).
  • \( \mathrm{b} \): accounts for finite volume of particles (subtracts from available volume).

Interpretation:

  • The term \( \mathrm{a\frac{n^2}{V^2}} \) corrects for intermolecular attractions that reduce pressure.
  • The term \( \mathrm{nb} \) corrects for finite particle volume that reduces free space for motion.

4. Graphical Representation of Deviations:

a) PV/nRT vs. P Plot:

  • For an ideal gas: the plot is a horizontal line at 1.0.
  • For a real gas:
    • At low pressure → ratio < 1 (due to attractive forces lowering P).
    • At very high pressure → ratio > 1 (due to finite molecular volume).

Comparison of Ideal vs. Real Gas Behavior

FeatureIdeal GasReal Gas
Particle VolumeNegligibleFinite; significant at high pressures
Intermolecular ForcesNoneAttractive and repulsive forces exist
Pressure BehaviorExact prediction from \( \mathrm{PV=nRT} \)Lower due to IMF; higher at very high P (volume effects)
Conditions of ValidityHigh T, Low PLow T, High P (deviations large)

Example :

Explain why \(\mathrm{CO_2}\) gas deviates from ideal behavior more than \(\mathrm{He}\) under the same conditions of temperature and pressure.

▶️ Answer/Explanation

Step 1: The degree of non-ideality depends on intermolecular forces and molecular size.

Step 2: \(\mathrm{CO_2}\) molecules are larger and exhibit stronger London dispersion forces due to more electrons and greater polarizability.

Step 3: Helium atoms are small, nonpolar, and have very weak dispersion forces.

Step 4: Thus, \(\mathrm{CO_2}\) molecules experience greater attractions, reducing their measured pressure more significantly at low temperatures or moderate pressures.

Final Answer: \(\mathrm{CO_2}\) deviates more from ideal gas behavior because of stronger intermolecular attractions and larger molecular volume compared to \(\mathrm{He}\).

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