Thermochemistry

Thermochemistry Summary

Chemical Hand Warmers

• Hand warmers release heat from the slow oxidation of iron:
  $4 ext{Fe} (s) + 3 ext{O}<em>2 (g) → 2 ext{Fe}</em>2 ext{O}_3 (s)$

• Temperature rise depends on size of hand warmer and clothing.

Nature of Energy

• Energy capacity to perform work or transfer heat.

• Work = force × distance; Heat = flow of energy due to temperature differences.

Units of Energy

• Joule (J): energy to move 1 kg 1 meter: $1 ext{J} = 1 ext{N} imes ext{m} = 1 ext{kg} imes rac{m^2}{s^2}$

• Calorie (cal): energy to raise 1 g water by 1 °C; 1 kcal = raising 1000 g by 1 °C.

Energy Uses Table

• Joules required for various activities compared to calories and kilowatt-hours.

First Law of Thermodynamics

• Energy conservation: total energy constant; energy can be converted, not created/destroyed.

• Example of energy in reactions: $ext{CH}<em>4 (g) + ext{O}</em>2 (g) → ext{CO}<em>2 (g) + ext{H}</em>2 ext{O} (g) + ext{heat}$.

Energy as a State Function

• Path independence in energy changes.

Endothermic & Exothermic Reactions

• Endothermic: absorbs energy (+∆H); Exothermic: releases energy (−∆H).

Enthalpy (∆H)

• Reaction enthalpy change: $∆H<em>{rxn} = H</em>{products} - H_{reactants}$.

• ∆H is an extensive property; sign changes for reverse reactions.

Calorimetry

• Heat transfer occurs until thermal equilibrium forms.

• Heat flow calculated for various systems.

Specific Heat Capacity

• Heat needed to raise 1 g of a substance by 1 °C; varies by material.

• Water has high specific heat capacity, used commonly for cooling.

Bomb Calorimetry

• Measures heat evolved in reactions.

• $q<em>{rxn} = -C</em>{cal} imes ΔT$ for calorimeter measurements.

Hess's Law

• Enthalpy change for a stepwise process equals sum of enthalpy changes of steps.

Energy in Chemical Reactions

• Energy to break bonds (positive) and released when formed (negative).