(634) A Level Chemistry Revision "Introduction to Enthalpy Cycles"

Hess's Law

  • Definition: Hess's Law states that if a reaction can be carried out by two different pathways, then the total enthalpy change for these two pathways will be the same, provided that the starting and final conditions are the same.

    • Example:

      • Chemical A converts directly to Chemical C.

      • Alternatively, Chemical A can convert to Chemical B, and then Chemical B to Chemical C.

  • Application: Since both pathways start with Chemical A and end with Chemical C, the enthalpy change for the direct reaction (reaction 1) is equal to the sum of the enthalpy changes for the two-step pathway (reaction 2 + reaction 3).

  • Term: This scenario is often referred to as an enthalpy cycle or thermochemical cycle.

Importance of Enthalpy Cycles

  • Useful for measuring enthalpy changes for reactions that are difficult to measure directly, such as those that do not easily occur under normal conditions.

  • By finding alternative pathways, scientists can measure other reactions' enthalpy changes and apply Hess's Law for calculations.

Standard Enthalpy Changes

  • Common types of data used in Hess's Law problems:

    • Standard enthalpy change of formation

    • Standard enthalpy change of combustion

  • Focus here is on standard enthalpy change of formation.

Definition of Standard Enthalpy Change of Formation

  • Definition: The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions, with all chemicals in their standard states.

Example Problem: Calculation using Standard Enthalpy Change of Formation

  • Reaction of Interest: Methane (CH4) + Chlorine (Cl2) → Chloromethane (CH3Cl) + Hydrogen Chloride (HCl)

  • Important Note: The standard enthalpy change of formation of an element (like Cl2) is zero since it does not apply to elements.

Elements Used in Calculation

  • Carbon (C) - Standard state: solid

  • Hydrogen - 4H atoms: represented as 2H2 molecules

  • Chlorine - 2 Cl atoms: represented as Cl2 molecule

Filling in Data

  • Standard enthalpy of formation of:

    • Methane (CH4): -74.9 kJ/mol

    • Chloromethane (CH3Cl): -83.7 kJ/mol

    • Hydrogen Chloride (HCl): -92.3 kJ/mol

Direction of Arrows in the Cycle

  • Important to note that when using standard enthalpy change of formation data in cycles, the arrows must point upwards from elements to products.

    Why do arrows point upwards?

    When using ΔHf∘\Delta H_f^\circΔHf∘​, the cycle typically represents the formation of a compound from its elements.

    • Arrows pointing up indicate the energy change when elements combine to form the compound.

    • If the reaction goes in the opposite direction (e.g., breaking the compound into elements), the arrow would point downward, and the sign of ΔHf∘​ must be reversed.

  • If an arrow points in the reverse direction, the sign for that standard enthalpy change must be reversed for calculations.

Final Calculation of Enthalpy Change of Reaction

  • Total standard enthalpy of formation for the right-hand side (products): -176 kJ/mol

  • Calculation:

    • Using: +74.9 kJ/mol (reversed sign for methane) + (-176 kJ/mol)

    • Final result: -101.1 kJ/mol