CHM 141 Fall 2025 Lecture Notes

CHM 141 Fall 2025 Lecture Notes

General Information

• Lecture Details:

  • Lecture 28 on Friday, November 14th, 2025
  • Sections covered: 10.3 – 10.4

• Homework Deadlines:

  • Lecture 27 HW due: Friday, November 14th at 11:59 PM
  • ALEKS Chapter 9 Module due: Sunday, November 16th at 11:59 PM
  • Lecture 28 HW due: Monday, November 17th at 11:59 PM

• Exams and Assignments:

  • Exam 4 scheduled for: Wednesday, November 19th
  • Extra Credit assignment due: Sunday, November 23rd

Kinetic Molecular Theory (KMT)

Postulates of Kinetic Molecular Theory

• Postulate 1: Gas particles are tiny with large spaces between them.
• Postulate 2: Gas particles are in constant, random, straight-line motion except when they collide with each other or with the walls of the container.
  • Collision Type: Collisions are elastic, meaning energy is transferred but not lost in these collisions.
• Postulate 3: Between collisions, gas particles do not exert attractive or repulsive forces on each other.
• Postulate 4: The average kinetic energy of gas molecules in a sample is proportional to the absolute temperature (T).
  • Key Concept: At a given temperature (T), all gases have the same average kinetic energy.

Observed Physical Characteristics of Gases

• Shape and Volume: Gases assume both the shape and volume of their container.
• Flow: Gases flow freely.
• Compressibility: Gases are compressible (volume changes significantly with pressure).
• Temperature Effect: Gas volume changes significantly with temperature.
• Density: Gases have relatively low densities (large volumes, small masses).
• Mixing: Gases form homogenous mixtures (solutions) in any proportions (miscible).

Gas Laws

Combined Gas Law

• Description: The combined gas law illustrates how various gas properties (pressure, volume, temperature) affect each other.
  • Ideal Gas Law Expression: $PV = nRT$
  • Combined Form: $\frac{P<em>1 V</em>1}{n<em>1 T</em>1} = \frac{P<em>2 V</em>2}{n<em>2 T</em>2}$
  • Variables:
  • R (Universal gas constant) = 0.0821 atm·L/(mol·K)
  • Units: Volume (V) in L, Pressure (P) in atm, Temperature (T) in K.

Standard Conditions for Gases

• Definition of STP: Standard temperature and pressure (STP) specify conditions of 1 atm (760 torr) pressure and 0°C (273.15 K).
  • Standard Molar Volume: 22.4 L for gases at STP.

Gas Density and Molar Mass Relation

• Density Equation: The density (d) of a gas is given by: $d = \frac{m}{V}$,
  • Where:
  • m = mass of the gas
  • V = volume of the gas
• Relationship Between Density and Molar Mass:
  • Directly proportional to molar mass (M), inversely proportional to temperature (T): $d = \frac{M P}{RT}$.
• Variable Relationships:
  • $m = nM$
  • $PV = nRT$
  • Rearrangement yields: $M = \frac{dRT}{P}$

Stoichiometry with Gases

• Typical Gas Relations:
  • For gas A and gas B at the same conditions of P, V, T:
  • $PV = n_A R T$
  • $PV = n_B R T$
  • At STP, 1 mole of gas occupies: 22.4 L.
• Moles and Mass Relationship:
  • $n = \frac{m}{M}$, where:
  • n = amount in moles
  • m = mass in grams
  • M = molar mass (g/mol)

Example Problems

1. Oxygen Generators Reaction:
   • Reaction: $\text{NaClO}<em>3(s) + \text{Fe}(s) \rightarrow \text{O}</em>2(g) + \text{NaCl}(s) + \text{FeO}(s)$
   • Determine grams of $\text{NaClO}<em>3$ needed to produce 125 L of $\text{O}</em>2$ gas under conditions of 1.00 atm and 20.00°C.
2. Density Calculation:
   • Find the density of $\text{O}_2(g)$ at room conditions (20.0°C and 1.00 atm).

Exam 4 Group Practice Questions

• Resonance Structures:

  • Sulfur dioxide has two resonance structures. Draw them and include any formal charge considerations.
  • Define resonance in terms of delocalized electrons and bond order.
  • Discuss molecular geometry, bond angle, and hybridization of the central atom.

• Pressure Increase: List three methods to increase the pressure of a gas.

• Gas Volume Change: Calculate the new volume of nitrogen gas at 1.20 atm if its initial pressure is 745 torr and its initial volume is 37.4 L at 298K.

• Molecular Structures:

  • Draw the Lewis structure of water, detailing bond polarity and overall molecular polarity (including appropriate molecular dipoles and δ+ and δ- charges).

• Combustion Enthalpy:

  • Calculate the ΔH_rxn for the combustion of ethanol using bond enthalpies from your textbook or ALEKS.

Upcoming Assignments and Reminders

• Homework Schedules:
  • Lecture 27 HW due: November 14th at 11:59 PM
  • ALEKS Chapter 9 Module due: November 16th at 11:59 PM
  • Lecture 28 HW due: November 17th at 11:59 PM
• Exam 4 Prep: Scheduled on November 19th
• Extra Credit Assignment: Due on November 23rd.