Use the bond energies from the table in the reference sheet to determine the approximate enthalpy change of the following reaction. H2(g) + Br2(g) --> 2HBr(g)
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Use the bond energies from the table in the reference sheet to determine the approximate enthalpy change of the following reaction.
H2(g) + Br2(g) --> 2HBr(g)
![### Introduction to Bond Energies and Formal Charge Calculations
**Formal Charge Calculation:**
The formal charge of an atom within a molecule is calculated using the formula:
\[ \text{Formal Charge} = \# \text{valence } e^- - \text{nonbonding } e^- - \frac{1}{2} \text{bonding } e^- \]
**Enthalpy Change of a Reaction (\( \Delta H_{rxn} \)):**
The enthalpy change for a reaction can be determined through the bond energies using the equation:
\[ \Delta H_{rxn} = \sum \left( \Delta H \text{ (bonds broken)} \right) + \sum \left( \Delta H \text{ (bonds formed)} \right) \]
**Bond Energy Values:**
The bond energies for various bonds are given below:
| Bond | Bond Energy (kJ/mol) |
|-------|----------------------|
| C-H | 415 |
| O-H | 464 |
| H-H | 436 |
| C=O | 350 |
| \( \text{C} \equiv \text{O} \) | 1080 |
| Br-Br | 190 |
**Example: H-Br Bond Energy:**
The bond energy for an H-Br bond is 370 kJ/mol.
### Molecular Orbital Diagram
The diagram below provides molecular orbital (MO) diagrams for homonuclear diatomic molecules from Lithium (Li) to Neon (Ne).
- **Vertical Axis (E):** Represents energy levels.
- **Horizontal Segments:** Represents each diatomic molecule (Li₂, Be₂, B₂, C₂, N₂, O₂, F₂, and Ne₂).
Each segment outlines the relative energy levels of the molecular orbitals for the respective diatomic molecules.
**Molecular Orbitals:**
- **\( \sigma_{2px} \):** This orbital is shown as a solid black line for each molecule.
- **\( \pi_{2px}, \pi_{2py} \):** Represented by dashed lines, indicating degenerate (equal energy) orbitals.
- **\( \pi_{2py^*, \pi_{2pz^*} \):** Antibonding π orbitals.
- **\( \sigma_{2px^](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Ff7d651ae-bc59-4c10-bead-f2b72cbab32a%2Ff3732708-24c4-4320-b2a5-5c6a135386f4%2F95erxak_processed.jpeg&w=3840&q=75)
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