General Chemistry for Engineers - CHEN 1201

General Chemistry for Engineers (CHEN 1201)

Course Information

  • Instructor: Carolyn Kohlmeier

  • Department: Chemical and Biological Engineering

Week 7 Lecture 17 Overview

  • Chapter: 7.4

  • Topics Covered:

    • Review of last lecture on Energy, Heat, and Work

    • Focus for today: Quantifying heat and work

Key Concepts Related to Heat and Work

Energy Changes in a System

  • Scenario: When heat transfer is absent (q = 0), the options for the system energy are:

    • A) Increases

    • B) Decreases

    • C) Stays the same

Definitions

  • Heat (q):

    • Definition: The exchange of thermal energy between a system and its surroundings.

  • Temperature (T):

    • Definition: A measure of the average thermal energy of particles in a substance.

  • Thermal Equilibrium:

    • Definition: A state where the system and surroundings have the same temperature, resulting in no net heat exchange (q = 0).

  • Work (w):

    • Definition: The energy transferred when a force acts over a distance.

    • Example: Pressure-volume work (w = P imes ext{change in volume}).

Specific Heat Capacity

  • Basic Concepts:

    • Heat Capacity:

    • Extensive property of a system reflecting the heat required to change its temperature.

    • Relationship: q = C imes riangle T where C is the heat capacity and riangle T is the change in temperature.

    • Specific Heat Capacity (C_s):

    • Intensive property reflecting heat required to raise the temperature of 1 gram of a substance by 1 °C.

    • Relationship: q = m imes C_s imes riangle T where m is the mass of the substance.

    • Molar Heat Capacity (C_m):

    • Heat required to raise the temperature of 1 mole of a substance by 1 °C.

    • Relationship: q = n imes C_m imes riangle T where n is the number of moles.

Applied Concepts

Graphing Heat Absorption

  • The lecture includes a graphical representation of heat absorbed per temperature for three systems. The system with the larger molar heat capacity is determined by the maximum heat absorbed per mole of material.

Example Problem 1: Mass of Copper Block

  • Scenario: A block of copper at 65.4 °C is placed in 95.7 g of water at 22.7 °C, reaching a thermal equilibrium final temperature of 24.2 °C. The objective is to find the mass of the copper block using heat transfer calculations and the concepts of specific heat.

Example Problem 2: Temperature Change in Isolated Substances

  • Given: Two substances, A and B, initially at different temperatures are allowed to come into contact. The mass of A is twice that of B, and the specific heat capacity of B is four times that of A. Analyzing the changes in temperature ( riangle T) for both substances is required to find which will undergo the larger change in temperature.

Work Calculation in Chemical Reactions

  • Example: Calculate the work done by the chemical reaction when the volume increases from 3.0 L to 4.0 L at a constant external pressure of 5.0 atm. Formula for work: w = -P imes riangle V.

Ideal Gas Internal Energy Change

  • Scenario: Comparing two identical piston/cylinders containing one mole of an ideal gas at 300 K and 1 MPa, each heated with 5 kJ of heat, one at constant pressure and the other at constant volume. Questions assess which process undergoes the largest change in internal energy.

  • Options:

    • A. Same for both

    • B. Constant pressure

    • C. Constant volume

Example Problem 3: Burning Fuel in a Piston

  • Scenario: In a cylinder, fuel is burned causing the volume to expand from 0.255 L to 1.45 L under an external pressure of 1.02 atm. Additionally, 875 J is emitted as heat. The task is to calculate the change in energy ( riangle E) for this process using the first law of thermodynamics. Formula: riangle E = q + w, where w is calculated as the negative product of pressure and change in volume.