CHEM 1410: Enthalpy

Enthalpy and Internal Energy

  • Relationship Between Liter Atmospheres and Joules
      - Liter atmospheres (L·atm) are units of energy.
      - Joules (J) is the SI unit of energy.
      - Liter atmosphere can be expressed as an energy term and is part of the internal energy equation.

  • Internal Energy (U)
      - Defined as the total energy contained within a system that is not related to work done on or by the system.
      - The internal energy combined with pressure-volume work leads to the definition of total enthalpy (H).

Understanding Enthalpy (H)

  • Definition of Enthalpy
      - Enthalpy (H) of a reaction is the total heat released at constant pressure.
      - Formula:
        ΔH=ΔU+PΔV\Delta H = \Delta U + P\Delta V
        - Where:
          - ΔH\Delta H = Change in enthalpy
          - ΔU\Delta U = Change in internal energy
          - PΔVP\Delta V = Work done by the system due to change in volume
          - Heat (q) is interchangeable with enthalpy under constant pressure conditions.

  • Heat and Work in Internal Energy
      - Relationship:
        ΔU=q+w\Delta U = q + w
        - Where:
          - q = heat
          - w = work
        - For work done by the system:
          w=−PΔVw = -P\Delta V

  • Derivation of Enthalpy Changes
      - Substituting work into the enthalpy formula:
        ΔH=q+(−PΔV)+PΔV\Delta H = q + (-P\Delta V) + P\Delta V
      - This implies:
        - At constant pressure, the heat of the reaction can be expressed as the enthalpy change.

Enthalpy Diagrams

  • Concept of Enthalpy Diagrams
      - An enthalpy diagram illustrates the energy changes during a chemical reaction.
      - The y-axis represents energy, and the x-axis represents the reaction coordinate (progress from reactants to products).

  • Exothermic Reactions
      - Energy is released when the reactants convert to products.
      - The activation energy is the energy needed to initiate the reaction.
      - The difference in energy between reactants and products indicates the total energy released.

  • Endothermic Reactions
      - Energy is absorbed when products are formed from reactants.
      - Activation energy must be supplied for the reaction to occur; once surpassed, the reaction can proceed and absorb energy from the surroundings.

Calculating Enthalpy Changes

  • Change in Enthalpy of a Reaction
      - Formula:
        ΔHreaction=Hproducts−Hreactants\Delta H_{reaction} = H_{products} - H_{reactants}
      - Important to note: Products minus reactants to compute the energy change.

  • Example: Combustion of Hydrogen
      - Reaction:
        2H2(g)+O2(g)→2H2O(l)2H_{2(g)} + O_{2(g)} \rightarrow 2H_{2}O_{(l)}
      - Heat released: −572 kJ/mol-572 \, kJ/mol
      - To find heat released for 0.5 moles of O₂:
        Heat=12mol×(−572 kJ/mol)=−286 kJ\text{Heat} = \frac{1}{2} mol\times(-572 \, kJ/mol) = -286 \text{ kJ}
        

Thermochemical Equations

  • Stoichiometry in Enthalpy Calculations
      - Understanding stoichiometric coefficients is essential since they dictate the amount of heat produced or consumed per mole in reactions.

  • Principles of Thermochemical Reactions
      - When coefficients of a reaction are multiplied or divided, $
      \Delta H$ must be adjusted proportionally.
      - When the reaction is reversed, the sign of $\Delta H$ also changes (e.g., an exothermic reaction has a positive enthalpy when reversed).

Hess's Law

  • Hess's Law Statement
      - States that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps irrespective of the pathway taken.

  • Application of Hess's Law
      - To find an unknown reaction enthalpy, combine known reactions and their associated enthalpies.
      - Example using hypothetical reactions:
        - If two steps lead to the final desired reaction, their enthalpies can be summed to provide total enthalpy change.

  • Example Calculation
      - Given equations for research:
        1. 2CLF+O2→CL2O+OF2 (ΔH=+205.6kJ)2 CLF + O_2 \rightarrow CL_2O + OF_2 \, (\Delta H = +205.6 kJ)
        2. 2OF2→O2+2F2 (ΔH=−49.4kJ)2 OF_2 \rightarrow O_2 + 2F_2 \, (\Delta H = -49.4 kJ)
        3. CLF3+O2→12CL2O+32OF2 (ΔH=+266.7kJ)CLF_3 + O_2 \rightarrow \frac{1}{2} CL_2O + \frac{3}{2} OF_2 \, (\Delta H = +266.7 kJ)
      - Rearranging to find the overall enthalpy change leads to the complete answer.

Conclusion

  • Understanding thermodynamics, enthalpy changes, and reaction stoichiometry is crucial for predicting the energy changes involved in chemical processes.

  • Continual practice with calculations and diagrams will solidify these concepts as they relate to heat, work, and chemical behavior.