Calculating Change in Enthalpy

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Calculating Change in Enthalpy (ΔH)
There are two ways to calculate enthalpy.
1. Enthalpy can be calculated by the heat of formation of the products minus the reactants using the
equation
ΔH = H (products) - H (reactants)
2. Enthalpy can also be calculated by the energy _________ to break bonds versus the energy ________
when bonds form.
ΔH = Energy of bonds broken – Energy of bonds formed
Example:
Methane burns in oxygen to form carbon dioxide and water. How much energy is released per mole of
methane burnt?
Step 1: Write a balanced chemical equation.
Step 2: Draw a Lewis structure for each substance.
Step 3: Using the bond energies below, calculate the energy needed to break bonds. (Endo or exo?)
Step 4: Using the bond energies below, calculate the energy needed to form bonds. (Endo or exo?)
Step 5: Calculate the energy change.
The bond energies are:
C—H
435KJ/mole
O=O
498KJ/mole
O—H
464KJ/mole
O=C
805KJ/mole
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