Youtube Video on Hess Law
Understanding Enthalpy Changes
Enthalpy Change Definition: The amount of heat energy released or taken in per mole of a substance during a physical or chemical change.
Exothermic: When overall heat energy is released, resulting in an increase in the surroundings' temperature.
Endothermic: When overall heat energy is absorbed, resulting in a decrease in the surroundings' temperature.
Types of Enthalpy Changes
Enthalpy Change of Formation: Energy change when one mole of a compound is formed from its elements in their standard states.
Enthalpy Change of Combustion: Energy change when one mole of a substance is completely burned in oxygen.
Bond Enthalpy: Energy change when one mole of gaseous covalent bonds is broken.
Hess' Law
Law Definition: The enthalpy change during a chemical process is independent of the steps taken, as long as the reactants and products are the same.
Implication: The overall enthalpy change will remain constant regardless of the pathway between reactants and products.
Hess Cycle Introduction
Hess Cycle: A visual representation of Hess' Law used to calculate unknown enthalpy changes.
Typical usages: formation enthalpies, combustion enthalpies, or bond enthalpies.
Example 1: Hess Cycle with Formation Enthalpies
Reaction Example: Complete combustion of methane.
Set Up: Create a box under the reaction, representing the elements involved.
Draw arrows:
From the box (elements) to reactants (methane and oxygen).
From the box to products (carbon dioxide and water).
Entropies to Include:
Methane (formation): -75 kJ/mol
Carbon Dioxide (formation): -394 kJ/mol
Water (formation): -286 kJ/mol (multiply by 2 for two moles).
Calculation:
Route 1 (from box to products) = -75 + ? (enthalpy of combustion for methane) = Route 2 (from box directly to products).
Final formula: ? = -394 + (2 * -286).
Result: Enthalpy change of combustion for methane = -891 kJ/mol.
Example 2: Hess Cycle with Combustion Enthalpies
Reaction Example: Formation of benzene.
Set Up: Create a box under the reaction representing combustion products.
Draw arrows:
From reactants to box (products of combustion).
From products to the box (benzene).
Entropies to Include:
Multiply combustion enthalpy of carbon by 6 (for 6 moles) and hydrogen by 3 (for 3 moles).
Calculation:
Follow arrows to equate the two routes:
Route from reactants to box vs. route from products to box.
Result: Enthalpy change of formation for benzene = +45 kJ/mol.
Example 3: Hess Cycle with Bond Enthalpies
Reaction Example: Complete combustion of ethanol.
Set Up: Create a box of gaseous atoms formed by breaking bonds.
Draw arrows:
From reactants to box and from products to box.
Additional Energy: Include enthalpy of vaporization for ethanol (as it's liquid in this case).
Bonds in Reactants:
Identify all bond types in the ethanol molecule and their enthalpies.
Bonds in Products:
Carbon Dioxide: 2 C=O bonds (4 total)
Water: 3 moles of H2O = 6 O–H bonds.
Calculation:
Set up using two routes while following the direction of the arrows.
Result: Enthalpy of combustion for ethanol = -1,015 kJ/mol.
Key Tips for Hess Cycles
Finding Routes: Always opt for two routes that follow the direction of arrows to simplify calculations.
Avoid Sign Changes: Ensuring continuity of direction helps prevent mistakes that often arise from needing to change signs of enthalpy values.