Learn & Review: THERMOCHEMISTRY in One Shot | Complete Chapter Of Physical Chemistry | NEET

Summary of Thermochemistry Concepts

This summary outlines key concepts in thermochemistry, focusing on state functions, the laws of thermochemistry, and various types of enthalpies.

1. State Functions and Enthalpy of Reaction

• State Functions: Values of state functions are used to calculate other state function values.
• Enthalpy of Reaction ($\Delta H_{rxn}$): Generally calculated as the enthalpy of the products minus the enthalpy of the reactants ($\Delta H_{rxn} = H_{products} - H_{reactants}$).
• Entropy of Reaction ($\Delta S_{rxn}$): Calculated as the entropy of the products minus the entropy of the reactants ($\Delta S_{rxn} = S_{products} - S_{reactants}$).

2. Laws of Thermochemistry

• Reversing a Reaction: If a reaction is reversed, the sign of its enthalpy change is reversed (e.g., if $\Delta H = -200$ kJ, reversing it makes $\Delta H = +200$ kJ).
• Multiplying a Reaction: If a reaction is multiplied by a factor, its enthalpy change is multiplied by the same factor (e.g., if $\Delta H = 200$ kJ, multiplying the reaction by 2 makes $\Delta H = 400$ kJ; dividing by 2 makes $\Delta H = 100$ kJ).
• Adding Reactions (Hess's Law): If multiple reactions can be added to form an overall reaction, the overall enthalpy change is the sum of the individual enthalpy changes.

3. Types of Enthalpies and Their Definitions

• Enthalpy of Formation ($\Delta H_f^\circ$):

  • Defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states.
  • Elements in their standard state have an enthalpy of formation of zero.
  • The reaction must be balanced for one mole of the product.
  • $\Delta H_{rxn}$ can be calculated using enthalpies of formation: $\Delta H_{rxn} = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants})$.

• Enthalpy of Combustion ($\Delta H_c^\circ$):

  • Defined as the enthalpy change when one mole of a substance undergoes complete combustion.
  • Combustion reactions typically involve reaction with oxygen to produce $\text{CO}_2$ and $\text{H}_2\text{O}$.
  • $\Delta H_{rxn}$ can be calculated using enthalpies of combustion: $\Delta H_{rxn} = \sum \Delta H_c^\circ (\text{reactants}) - \sum \Delta H_c^\circ (\text{products})$.
  • Exception: For combustion enthalpy calculations, the formula is reactants minus products, unlike other standard enthalpy of reaction calculations.

• Bond Dissociation Enthalpy ($\Delta H_{BD}$):

  • The energy required to break one mole of a specific bond in the gaseous state.
  • This is an endothermic process, so $\Delta H_{BD}$ is positive.
  • For a polyatomic molecule, the average bond dissociation enthalpy is often used.
  • When calculating the enthalpy of reaction using bond dissociation enthalpies: $\Delta H_{rxn} = \sum (\text{Bonds broken in reactants}) - \sum (\text{Bonds formed in products})$.

• Bond Formation Enthalpy:

  • The energy released when one mole of a specific bond is formed.
  • This is an exothermic process, so the enthalpy change is negative.

• Enthalpy of Neutralization ($\Delta H_{neut}$):

  • The enthalpy change when one mole of water is formed from the reaction of a strong acid and a strong base.
  • For strong acid-strong base reactions, the value is approximately fixed at -57.1 kJ/mol (or -13.7 kcal/mol).
  • For reactions involving weak acids or bases, the enthalpy of neutralization is less exothermic because some energy is absorbed to ionize the weak species.

• Resonance Energy:

  • The difference between the observed heat of formation (or combustion) and the calculated heat of formation (or combustion) based on bond energies.
  • It represents the extra stability due to electron delocalization (resonance).
  • Calculated as: Observed $\Delta H$ - Calculated $\Delta H$.

4. Other Enthalpy Types Mentioned

• Enthalpy of Fusion: Heat absorbed during melting.
• Enthalpy of Vaporization: Heat absorbed during boiling.
• Enthalpy of Sublimation: Heat absorbed during sublimation.
• Enthalpy of Hydrogenation: Heat change during the addition of hydrogen.