AP Chemistry Unit 6: Thermochemistry

Introduction

• Lecturer: Jeremy Krug
• Focus: Thermochemistry
• Resource recommendation: Ultimate Review Packet

Key Concepts

Exothermic vs. Endothermic Processes

• Endothermic Processes

  • System gains energy from surroundings
  • Surroundings get colder

• Exothermic Processes

  • System loses energy to surroundings
  • Surroundings get warmer

Energy and Bond Formation

• Formation of a bond releases energy
• Breaking a bond absorbs energy
• Dissolving ionic compounds can be exothermic or endothermic depending on bond strength

Energy Diagrams

• Show changes in energy from reactants to products
• Exothermic: Net loss of potential energy
• Endothermic: Net gain of potential energy

Temperature and Heat Transfer

• Average kinetic energy = temperature
• Heat transfers from warmer to cooler object until thermal equilibrium is reached

Calculating Heat Transfer (Q = MCΔT)

• Q: Heat transferred (Joules)
• M: Mass (grams)
• C: Specific heat capacity (J/g°C)
• ΔT: Change in temperature (°C)
• Heat gained by one system equals heat lost by another

Specific Heat Capacity

• Resistance of material to temperature change
• Can be expressed in Joules per mole degree Celsius

Phase Changes

• Melting & Boiling: Endothermic (temperature constant during process)
• Condensation & Freezing: Exothermic
• Opposite processes: Melting/Freezing and Boiling/Condensation

Chemical Reactions and Enthalpy

• Enthalpy of Reaction (ΔH)
  • Example: 2Na + Cl2 -> 2NaCl releases -822 kJ
  • Doubling reactants doubles heat released

Bond Enthalpies

• Calculate ΔH by bonds broken - bonds formed
• May require algebra to solve for unknown bond enthalpy

Standard Enthalpies of Formation

• ΔH = Sum of products' enthalpies - Sum of reactants' enthalpies
• Enthalpy of formation for elements in natural state = 0 kJ/mol

Hess's Law

• Add reactions to find ΔH of new reaction
• May require flipping or multiplying reactions

Conclusion

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• Upcoming Unit 7 on Chemical Equilibrium

• Tip: Check playlists for more detailed videos on each unit.
• Next: Review of Chemical Equilibrium in Unit 7.