2 Thermochemical Equations PowerPoint

Overview of Reactions

  • Exothermic Reactions: Release energy; characterized by the equation:

    • Reactants → Products + Energy

  • Endothermic Reactions: Absorb energy; characterized by the equation:

    • Reactants + Energy → Products

Enthalpy

  • Definition:

    • Enthalpy (H) is the heat content of a substance at constant pressure.

  • Measuring Enthalpy:

    • Enthalpy cannot be measured directly; instead, we measure the change in enthalpy, denoted as ΔH.

  • Change in Enthalpy (ΔH):

    • Represents the amount of heat absorbed or released under constant pressure during a reaction.

    • Formula: ΔH = H_products - H_reactants

Heat of Reaction

  • Definition:

    • The heat of reaction is the amount of heat absorbed or released when a chemical reaction occurs.

  • Relationship:

    • ΔH can indicate the nature of the reaction:

      • Positive ΔH: Endothermic (heat absorbed)

      • Negative ΔH: Exothermic (heat released)

Thermochemical Equations

  • Definition:

    • Shows the thermal energy change alongside the chemical equation.

    • Example: For the formation of water:

      • 2H2(g) + O2(g) → 2H2O(g) + 483.6 kJ

      • Energy as part of products indicates energy release (exothermic).

      • 2H2O(g) + 483.6 kJ → 2H2(g) + O2(g) indicates energy was absorbed (endothermic).

Methods to Indicate Heat in Reactions

  • Beside the equation format:

    • Example:

      • 2H2(g) + O2(g) → 2H2O(g) ΔH = -483.6 kJ

      • Exothermic indicates energy was released.

    • Additional notations for clarity:

      • ΔH = – 483.6 kJ (Exothermic, energy lost)

      • ΔH = + 483.6 kJ (Endothermic, energy absorbed)

Heat of Reaction & Moles

  • Heat release or absorption correlates with the number of moles indicated in the reaction equation.

    • Example: 2H2(g) = –483.6 kJ indicates that for every 2 moles of H2, 483.6 kJ of energy is lost.

Practical Application Examples

  • Benzene Combustion:

    • Reaction: 2C6H6 + 15O2 → 12CO2 + 6H2O, ΔH = -98.0 kJ

    • Calculation: Calculate heat released by 1.75 mol C6H6:

      • Calculation Result: –85.8 kJ

  • Sulfur Reaction:

    • Reaction: C(s) + 2S(s) → CS2(l), ΔH = 89.3 kJ

    • Energy required for 1.40 mol S: 62.5 kJ

  • Iron Reaction:

    • Reaction: 4Fe + 3O2 → 2Fe2O3, ΔH = -1652 kJ

    • Energy released by 7.00 mol Fe: -2890 kJ

    • Calculate heat released when 22.75 g O2 is used: -391.5 kJ

  • Propane Combustion:

    • Reaction: C3H8 + 5O2 → 3CO2 + 4H2O, ΔH = -2220 kJ

    • Calculation of energy released by 425 kg of propane yields: -2.14 × 10^7 kJ

    • Determine oxygen required for energy production in reactions.

Conclusion

  • This section provides an understanding of thermochemical equations, energy changes in reactions, and practical calculations related to enthalpy, helping to contextualize the role of energy in chemical processes.