chem thermochem 1

Essential Questions

  • How do we describe energy in chemical processes?
  • What are the types of energy changes in chemical reactions?
  • How can we communicate the change in energy in a chemical reaction?
  • How do I calculate the enthalpy changes in chemical reactions experimentally?
  • How do I calculate the enthalpy changes in chemical reactions theoretically using collected data?

Thermochemistry Overview

Energy Dimensions

  • Energy Mostly Heat
    • Communicating Enthalpy
    • Exothermic vs Endothermic
    • Notation: ∆H
    • Potential Energy Diagrams
  • Temperature Change
  • Calorimetry
    • Importance in experimental measurements

Thermochemistry – Introduction to Terms

Curriculum Relations

  1. General Energy in Hydrocarbons
    • Stored energy in bonds originated from the sun
  2. SI Units and Significant Digits
  3. Chemical Reactions and Energy Changes
    • Bonds breaking/forming and changes in potential and kinetic energy

Basic Definitions

Terms and Definitions
  • Heat: Form of energy that flows between samples at different temperatures.
  • Temperature: Average kinetic energy of the molecules.
  • Thermal Energy: Total heat energy in a substance.
Example: Temperature vs Thermal Energy
  • Cup of Coffee (80°C) vs Bathtub Full of Water (50°C)
    • Average Kinetic Energy: Coffee molecules > Bath water molecules
    • Total Thermal Energy: Bath water has more due to greater mass.

Laws of Thermodynamics

First Law of Thermodynamics
  • Definition: Energy can be converted from one form to another but can’t be created or destroyed.
Second Law of Thermodynamics
  • Definition: Heat flows from hot to cold until thermal equilibrium is reached.
Mechanisms of Heat Transfer
  • Convection, Conduction, Radiation
  • Heat increases particle movement but not total energy involved.

Energy Definitions and Applications

Energy Measurement

  • Measured in Joules (J)
  • Types of Energy:
    • Potential Energy: Stored energy in bonds within and between molecules (chemical energy).
    • Kinetic Energy: Energy of motion of ions, atoms, molecules (includes translational, rotational, vibrational motion).

Energy Changes in Reactions

Definitions
  • Exothermic: Energy released to surroundings during chemical processes.
  • Endothermic: Energy absorbed from surroundings.
Key Messages
  1. Temperature Changes Indicate Kinetic Energy Changes.
  2. Potential Energy Changes Indicate Bonds Breaking or Forming.
  3. Energy is Conserved but in changed forms.
  4. No Process is 100% Efficient; some energy is often lost as heat.

Calculating Kinetic Energy

Calculating Enthalpy Changes

  • Application of Q = mcΔt
  • Definitions:
    • Kinetic Energy (E_k): Related to temperature changes of a substance.
    • Heat Capacity: Energy needed to raise the temperature of a specific mass by 1°C.
    • Molar Heat Capacity: Energy needed to raise 1 mole of a substance by 1°C.
    • Specific Heat Capacity: Energy required to raise 1 gram by 1°C.

Key Formula for Kinetic Energy

Ek=mc∆tE_k = mc∆t

  • Where:
    • EkE_k = Kinetic Energy (J)
    • mm = mass of substance (g)
    • cc = specific heat capacity (J/g·°C)
    • ∆t∆t = change in temperature (°C)
Example Problem
  1. Heating 150.0 g of water from 25.3°C to 75.0°C:
    • Apply formula to calculate energy needed.

Practice Problems: Kinetic Energy Calculations

  1. Water Cooling:
    • 5.00 x 10² g of water cooled from 41.5°C to 10.0°C.
  2. Find New Temperature:
    • 50.0 g of water at 40.0°C cooling, releasing 2.5 kJ of heat.

Energy Dynamics in Reactions

  • Bonds Breaking = Endothermic.
  • Bonds Forming = Exothermic.
Enthalpy Communication Methods
  1. Written in reaction: Energy as product (exothermic) or reactant (endothermic).
  2. ∆H Notation: Shows energy loss (negative) or gain (positive).
  3. Potential Energy Diagrams: Illustrate changes in energy through reaction progress.

Hess's Law Overview

Definition and Usage

  • Hess’s Law: If a reaction can be expressed as a sum of reactions, the enthalpy of the net reaction is the sum of the enthalpies for each contributing reaction.

Calorimetry Fundamentals

Experimental Energy Changes Measurement

  • Assumptions: Energy conserved; heat flows between hot and cold.
  • Types of Calorimeters:
    • Styrofoam Calorimeter: Best for aqueous reactions.
    • Metal Can Calorimeter: Useful for combustion reactions; includes metal in heat calculations.
    • Bomb Calorimeter: Used for high-pressure reactions; measures heat of combustion distinctly.

Example Calculations in Calorimetry

  1. Enthalpy of Solution: Calculate molar enthalpy change when ammonium bromide dissolves in water.
  2. Metal Burning: Use data from a reaction in calorimetry to determine molar enthalpy and adjust to balanced equation.

Additional Topics in Energy and Thermochemistry

Energy Efficiency

  • Definition: Efficiency ratio of useful output energy to energy input.
  • Example Calculation: Calculate energy outputs for propane combustion finding efficiency in real-world applications.

Diploma Examination Practice Questions

  • Comprehensively go through potential energy changes and review enthalpy correlations.

Practice and Application

  • Engage in numerous calorimetry and Hess's law practice problems to reinforce methodology and theory in thermochemistry.
  • Analyzed competitive questions from past diploma examinations to prepare effectively.