Thermochemistry Notes

Thermochemistry

Study of Heat Transfer

• Deals with heat transfer in both chemical and physical processes.

  • Physical Example: H2O(l) + energy \rightarrow H2O(g)

  • Chemical Example: 2H2(g) + O2(g) \rightarrow 2H_2O(g) + energy

Heat

• Measure of the total energy in a system.

• Measure of the total energy transferred.

  • Measured in Joules (J) which is equivalent to (N x m) or Kilojoules (KJ).

Temperature

• Measure of the average kinetic energy (KE) of particles in a system.

  • Measured in degrees Celsius (^{\circ}C) or Kelvin (K).

• Example to consider:

  • Which has more heat? (Which would melt more ice?)

    • A teacup filled with 100 ^{\circ}C water.

    • A swimming pool filled with 25 ^{\circ}C water.

    • The swimming pool contains more heat due to its larger volume, even though the temperature is lower.

Specific Heat

• The amount of energy required to raise the temperature of 1 gram of a substance by 1 ^{\circ}C.

• Represented as SH or C_p.

  • Cp(H2O(l)) = 4.18 \frac{J}{g^{\circ}C} : It takes 4.18 J to raise the temperature of 1g of H_2O(l) by 1 ^{\circ}C.

• What takes more energy (J) to raise the temperature (^{\circ}C) of one gram of wood or metal?

• Metals have low C_p values.

  • C_p(Fe) = 0.449 \frac{J}{g^{\circ}C}; Ferum

  • C_p(Au) = 0.129 \frac{J}{g^{\circ}C}; Aurum

  • It takes less energy to heat Au.

  • Au is a better conductor of heat.

• Insulators have high C_p values.

Enthalpy Changes

• \Delta H = H{\text{products}} – H{\text{reactants}}

  • Quantity of energy transferred as heat during a chemical reaction.

Exothermic Reaction

• Formation of products releases energy.

• Products exist at a lower energy level than reactants.

• Example: $$H2(g) + \frac{1}{2}O2(g) H_2O(g) + 241.8 KJ

Endothermic Reaction

• Formation of products absorbs energy.

• Reactants exist at a lower energy level.

• Example: 2H2O(g) + 483.6KJ —> 2H2(g) + O_2(g)