Change in Enthalpy

Change in Enthalpy (Delta H)

• Every chemical reaction has a change in enthalpy, denoted as $\Delta H$, indicating that energy transfer occurs in the form of heat.

• Most substances have known enthalpy changes termed heat of formation, typically measured in kilojoules per mole (kJ/mol).

• The change in enthalpy is attributed to the breaking and forming of chemical bonds, leading to energy being absorbed during bond breakage and released during bond formation.

• The overall $\Delta H$ for a reaction is calculated as follows:
    - $\Delta H = \text{Energy absorbed (bonds breaking)} - \text{Energy released (bonds forming)}$
    - This differential value indicates whether a reaction is endothermic or exothermic.

Endothermic and Exothermic Reactions

• An endothermic reaction absorbs heat, resulting in a positive $\Delta H$.
    - Example: If the reaction is endothermic, then \Delta H > 0.

• Conversely, an exothermic reaction releases heat and has a negative $\Delta H$.
    - Example: For the reverse reaction, it is exothermic so \Delta H < 0.

• When reversing a reaction, the sign of its $\Delta H$ must change (multiply by negative one) while its absolute value remains the same.

• For a scaled reaction (multiplication or division), the same factor must apply to both the reaction coefficients and the $\Delta H$.

Hess' Law

• Hess' Law states that the total $\Delta H$ for a reaction is the sum of the $\Delta H$ values of the individual steps in the reaction pathway.

• To use Hess' Law:
    - Add and manipulate chemical equations, removing identical species on both sides.
    - Sum the corresponding $\Delta H$ values from each step.
    - Apply any necessary adjustments to coefficients in reactions, ensuring corresponding adjustments to $\Delta H$.

Reaction Examples and Steps

Example of the Combustion of Methane

1. Given the reaction of methane with oxygen resulting in carbon dioxide and water, we want to calculate the change in enthalpy.
     - Step 1: Identify the balanced chemical equation for the combustion of methane.
     - Step 2: Determine that methane (species A) appears in only one elementary step of the reaction. Since it needs to be a reactant, we reverse the half-step that currently makes it a product, changing $\Delta H$ from negative to positive.
     - Step 3: For subsequent elements in the reaction (like carbon dioxide), if they do not require manipulation, include them as is.
     - Step 4: If water needs to be a product, and two moles are needed, multiply by two. Update the corresponding $\Delta H$ by the same factor.
     - Step 5: Combine all manipulated reactions, cancel out substances that appear on both sides of the reaction, and sum the remaining equations to calculate the overall $\Delta H$.

Example of the Combustion of Methanol

1. For the combustion of methanol:
      - Similar steps apply as with methane: identify the balanced reaction and apply the principle of Hess' Law.

2. Reverse necessary reactions and adjust $\Delta H$ accordingly when manipulating elementary steps.

3. Cancel terms that show up on both sides of the reaction and verify the resulting equation matches initial requirements to finalize $\Delta H$ calculation.

Additional Notes

• Always check and verify the accuracy of the equations and calculations at the end of the process; this is crucial.

• Questions may arise post-calculation, such as assessing if the reaction is endothermic or exothermic and whether it is thermodynamically favorable.

• It is important to evaluate the stability of products, as their energy states can influence the overall reaction energy profile.