Enthalpy and the First Law

Introduction to Thermodynamics

  • Focus on enthalpy and the first law of thermodynamics in this session.

  • Note on notation: ( Δ E ) and ( Δ U ) may be used interchangeably depending on the edition of the textbook.

First Law of Thermodynamics

  • Key Equation: ( Δ U = Q + W )

    • Where ( W = -P Δ V )

    • Substitute for work: ( Δ U = Q - P Δ V )

Constant Volume Conditions

  • Under constant volume:

    • ( Δ V = 0 )

    • No work done: ( Δ U = Q )

Constant Pressure Conditions

  • Under constant pressure:

    • Work done: ( W = P Δ V )

    • Rearranging leads to: ( Q = Δ U + P Δ V )

    • Define enthalpy: ( Δ H = Δ U + P Δ V )

  • Definition of Enthalpy:

    • Enthalpy as a state function: ( H = U + PV )

  • Interpretation of enthalpy:

    • Enthalpy is heat at constant pressure conditions.

    • Many reactions in aqueous solutions occur at constant pressure.

Change in Enthalpy (( Δ H ))

  • Defined as: ( Δ H = Hfinal - Hinitial )

  • For a chemical reaction:

    • ( Δ H = H_{products} - H_{reactants} )

  • Exothermic Process:

    • Losing heat leads to negative ( \Delta H ). ( (H_{products} < H_{reactants}) )

  • Endothermic Process:

    • Gaining heat leads to positive ( \Delta H ). ( (H_{products} > H_{reactants}) )

Thermochemical Equations

  • Convention:

    • Write ( \Delta H ) as if reading from left to right.

  • Example of Exothermic Reaction: Combustion of hydrogen.

  • Example of Endothermic Reaction: Decomposition of mercury oxide.

Using Enthalpy in Reactions

  • Relationship between enthalpy and energy in reactions.

  • Most changes in internal energy (( \Delta U )) are in terms of enthalpy or heat, rather than work.

  • Activation Energy: Energy required to initiate reactions.

Calculating Enthalpy Changes

  • For a physical process like melting ice:

    • Heat of fusion: ( \Delta H = 6.01 \text{ kJ/mol} ).

    • Double for two moles: ( 12.02 \text{ kJ} ).

  • For freezing: ( \Delta H = -6.01 \text{ kJ/mol} ).

Summary of Thermochemical Rules

  • Coefficients correspond to moles in reactions.

  • If the reaction is reversed, ( \Delta H ) changes sign.

  • If an equation is multiplied/divided, adjust ( \Delta H ) accordingly.

  • Always specify states of matter for reactions since enthalpy depends on state.

Importance of States of Matter in Reactions

  • Gases are primarily considered for ( \Delta N ) in calculations.

  • Only changes in moles of gases do work.

Practical Example: Vaporization of Water

  • Calculate ( \Delta U ) for vaporization at 100°C:

    • ( \Delta H_{vaporization} = +40.66 \text{ kJ/mol} )

  • Use proper equations to solve for internal energy changes.

Other Considerations in Reactions

  • Look for balancing in combustion reactions and check for energy required to initiate reactions.

  • Work is often a small fraction of total energy changes in combustion.

Conclusion

  • Understand relationships between internal energy, enthalpy, and work in reactions.

  • Prepare for lab applications of these concepts and subsequent topics like calorimetry and Hess's law in future classes.