Thermochemistry

Understanding Thermochemistry

  • Thermochemistry is a branch of chemistry that concerns the study of the heat energy associated with chemical reactions.
  • It involves the concepts of enthalpy (H), entropy (S) and Gibbs free energy (G).
  • It is a crucial part of chemical energetics, facilitating calculations for energy values of various substances.

The First Law of Thermodynamics

  • The first law of thermodynamics states that energy cannot be created or destroyed, only transferred.
  • When dealing with chemistry, the two forms of energy to consider are heat (q) and work (w).
  • The energy change in a system is equal to the heat added to the system minus the work done by the system: ΔU = q – w .

Hess’s Law

  • Hess’s law states that the total energy change during a chemical reaction is the same regardless of the pathway or the number of steps taken for the reaction to occur.
  • Hess’s Law is incredibly useful in calculating enthalpy changes where the direct reaction pathway is difficult to measure experimentally.

Enthalpy Changes

  • Enthalpy changes occur during a chemical reaction when energy is either taken in from the surroundings (endothermic reactions) or energy is lost to the surroundings (exothermic reactions).
  • The equation ΔH = Hproducts - Hreactants is essential in calculating the enthalpy change for a reaction.
  • When energy is absorbed during the course of a chemical reaction, this change is represented as a positive value (ΔH > 0). Conversely, when energy is released during a chemical reaction, enhtalpy change is shown as a negative value (ΔH < 0).

Entropy and Gibbs Free Energy

  • Entropy (S) is a measure of the disorder of a system. An increase in entropy favours a chemical reaction.
  • Gibbs free energy (G) combines both enthalpy and entropy into a single value. The equation for calculation is ΔG = ΔH - TΔS.
  • If ΔG is negative, the reaction is spontaneous. If ΔG is positive, the reaction is non-spontaneous.

Measurement of Heat Changes

  • Highly accurate devices, called calorimeters, are used to measure heat changes in chemical reactions.
  • A simple calorimeter can be set up using a polystyrene coffee cup. More advanced, constant-pressure calorimeters are used in professional settings.
  • Calorimetry is vital for determining the heats of reactions, formation, combustion, and solution.