Thermochemistry Lecture Notes

Key Concepts

• Thermochemistry: Study of the energy and heat associated with chemical reactions.
• Internal Energy (9E): Change in the internal energy of a system is given by the equation 9E = Q + W.

Heat and Work

• Q (Heat Energy):

  • Represents heat energy entering or leaving the system.
  • Heat flows from hot to cold (e.g., system at 1006C, surroundings at 506C).
  • Exothermic Process: Q is negative when heat is released by the system.
  • Endothermic Process: Q is positive when heat is absorbed by the surroundings.
  • Conversions:
    • 1 kJ = 1,000 J
    • 1 calorie = 4.184 J
    • 1 Calorie (food) = 1,000 calories

• W (Work):

  • Work is defined as W = P9V, where P is pressure and 9V is change in volume.
  • Compression: Work done on a system (W is positive).
  • Expansion: Work done by a system (W is negative).
  • Conversion: 101.3 J = 1 Latm

Example Problem: Calculating Internal Energy

1. Given:
   • 300 J of heat absorbed
   • Gas expands from 2 L to 3 L at 5 atm
2. Solution:
   • Calculate W using P9V: 5 atm * (3 L - 2 L) = 5 L*atm
   • Convert to J: 5 * 101.3 J/L*atm = 506.5 J
   • 9E = Q + W = 300 J - 506.5 J = -206.5 J

Specific Heat Capacity

• Equation: Q = mC9T
  • m = mass
  • C = specific heat capacity
  • 9T = change in temperature
• Example: Heating 50 g of water from 256C to 756C requires 10,460 J.

Phase Changes

• Use Q = m9H for phase changes.
• Heat of Fusion: Amount of energy to convert solid to liquid without temperature change.
• Example: Melting 54 g of ice (using 6 kJ/mol for water's heat of fusion) requires 18 kJ.

Thermochemical Equations

• Describes heat changes during reactions.
• Balance chemical equations for accurate energy calculations.
• Example: Combustion of propane releases 12,200 kJ.

Hess's Law

• States that the total enthalpy change is the same, regardless of the path taken.
• Use Hess's Law to determine the enthalpy change of reactions by using known enthalpies of other reactions.

Enthalpy of Formation

• 9H_f: Heat change when one mole of a compound is formed from its elements.
• Calculate the enthalpy of a reaction using the formula:
  • 9H = sum of 9H_f (products) - sum of 9H_f (reactants)

Example Calculations

• Combustion Reactions: Convert grams to moles to kJ.
• Reverse Reactions: Reverse reaction signs and add for combined enthalpy.
• Hess's Law Application: Modify and combine given reactions to find unknown enthalpy values.

Conclusion

• Thermochemistry involves understanding heat transfer and work in chemical reactions.
• Important to be familiar with key equations and concepts for problem-solving in this area.

Key Formulas

• 9E = Q + W
• W = P9V
• Q = mC9T
• Q = m9H (phase changes)
• 9H = sum of 9H_f (products) - sum of 9H_f (reactants)

Study Tips

• Practice conversion between units (cal, J, kJ).
• Understand the concept of endothermic vs. exothermic reactions.
• Use Hess's Law strategically to simplify complex enthalpy calculations.