THERMOCHEMISTRY - Enthalpy, Calorimetry and Bond Energies
9 important questions on THERMOCHEMISTRY - Enthalpy, Calorimetry and Bond Energies
What are the three ways to calculate for ΔH?
- Hess Law
- Standard Enthalpy of formation
- Bond Dissociation
What is Hess law?
- It is the change in enthalpy of a reaction
- Sum of ΔH of all series of steps in the reaction
- sum of ΔH of any reaction that summed to give overall reaction
- Example:
How to solve for Hess Law?
- Work backwards from the net reaction
- Multiple and reverse reactions needed
- Apply relevant values to Δ
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What is the enthalpy of formation of stable elements?
- O2
- H2
- N2
- Cl2
- F2
- Group 18 (ex: argon)
- Br2
- I2
- C (graphite)
- Metals (Example: zinc)
- P4
- S8
How do you find the enthalpy of formation?
products - reactants
What is bond dissociation energy?
Increasing bond energy =
- Multiple bonds
- shorter bond length
- higher bond strength
How do you calculate the reaction enthalpy from bond dissociation energies?
For bond dissociation energies, why can you only use gas phase reactants and products?
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