Lecture Notes: Heat Transfer and Thermochemistry

Review of Heat Transfer

• Recap from last lecture: Heat transfer between two objects, reaching final equilibrium temperature.
• Example: Coin in warm beer. Coin loses heat, beer absorbs heat, reaches same final temperature.

Heat Transfer in an Isolated System

• New scenario: More than two objects in isolated system.
• Example problem:
  • 50g of water at 20°C in a coffee cup.
  • 48g of water at 80°C added.
  • Final temperature: 47.8°C.
  • Calculate heat capacity of the coffee cup (Cp of water = 4.18 J/g°C).

Analysis of the System

• Cold water (20°C) and hot water (80°C) mixed, final temperature 47.8°C.
• Coffee cup is object number 3, involved in heat exchange.

Heat Exchange Calculation

• Use the equation: Q_lost + Q_gained = 0 in an insulated system.
• Hot water loses heat, cold water gains heat, cup gains heat.
• Q_hot water + Q_cold water + Q_cup = 0.
• Break down each Q into expressions: M * Cp * ΔT.

Solving the Problem

• Plug in numerical values:
  • M_hot = 48g, M_cold = 50g, Cp = 4.18 J/g°C.
  • ΔT calculations for each object.
• Simplify to find C_cup = 23.4 J/°C.

Introduction to Thermochemistry

• Transition from heat transfer to thermochemistry.
• Study of energy involved in chemical reactions.
• Importance of energy in chemistry, e.g., fuel combustion, battery reactions.

Enthalpy (H)

• Enthalpy: Energy involved in chemical reactions, a state function.
• Change in enthalpy (ΔH): Heat change during a reaction.
• ΔH > 0: Endothermic (absorbs energy), ΔH < 0: Exothermic (releases energy).

Thermochemical Equations

• Example: CH4 + 2O2 → CO2 + 2H2O, ΔH° = -127 kJ.
• Thermochemical equations combine chemical equations with energy changes.

Calorimetry

• Method to measure ΔH in chemical reactions using an insulated system.
• Example Problem:
  • 50g NaOH solution mixed with 50ml HCl solution, final temp 26.5°C.
  • Calculate ΔH of the neutralization reaction.
• Solution absorbs heat; exothermic reaction.

Calculating ΔH

• Use equation: ΔH + Q = 0.
• Q for solution: M * Cp * ΔT, Q for cup: C * ΔT.
• Example solution: ΔH = -2870 J or -2.87 kJ.*

Importance of Thermochemistry

• Provides insight into energy changes during reactions.
• Common chemical reactions have known ΔH values.
• Use calorimetry to find ΔH experimentally.

Closing Remarks

• Next lecture: Converting ΔH to standard values for thermochemical equations.