Chapter 15: Thermochemistry
15.2: Enthalpy Changes in Chemical Reactions
- Enthalpy Change (ΔH)
- The energy absorbed or released during a chemical reaction under constant pressure.
- Endothermic Reaction
- A reaction that absorbs energy from its surroundings, resulting in a positive ΔH.
- Exothermic Reaction
- A reaction that releases energy to its surroundings, resulting in a negative ΔH.
Enthalpy Diagrams
- Shows the relative energies of reactants and products.
- Endothermic Diagram: Reactants have lower energy than products.
- Exothermic Diagram: Reactants have higher energy than products.
Heat of Reaction
- Standard Enthalpy of Formation (ΔHf)
- The change in enthalpy when 1 mole of a compound is formed from its elements in their standard states.
- Heat of Combustion (ΔHcomb)
- The energy released when 1 mole of a substance is completely burned in oxygen.
- Heat of Fusion (ΔHfus)
- The energy required to convert 1 mole of a solid to a liquid at its melting point.
- Heat of Vaporization (ΔHvap)
- The energy required to convert 1 mole of a liquid to a gas at its boiling point.
Hess's Law
The total enthalpy change of a reaction is the sum of the enthalpy changes of its individual steps.
Formula:
- Allows indirect calculation of ΔH using known reactions.
- Enthalpy changes are additive regardless of the reaction path.
Applications of Enthalpy Changes
- Industrial Processes: Energy calculations for production efficiency.
- Combustion Reactions: Power generation and energy sources.
- Environmental Impact: Predicting heat release and energy absorption in chemical reactions.
Questions for Students
- Define enthalpy change. How does it differ between endothermic and exothermic reactions?
- What is Hess's Law, and why is it useful in thermochemistry?
- Explain the difference between ΔHf, ΔHcomb, ΔHfus, and ΔHvap.
- Draw and label an enthalpy diagram for both endothermic and exothermic reactions.
- Provide an example of an application of enthalpy changes in daily life or industry.