Bond Enthalpies

[106209106]

For the 4E problems, how do you know which bonds to exclude from adding all the enthalpies together, because for some of the answers, all of the bonds broken or formed aren't included.
Thank you!

[SandraChane_1B]

The best way to do this would be to break every single bond in the reactants and then form every single bond that is formed in the products so that you make sure that everything is included and no value is accidentally left out or double counted.

[MJLara]

Determining which bonds to include or exclude when calculating the enthalpy change of a reaction can depend on the level of detail needed for the calculation and the specific bonds involved. Here are some guidelines to help you decide which bonds to consider:

1. **Focus on Bonds Broken and Formed**: When calculating the enthalpy change (\( \Delta H \)) for a reaction, you typically focus on the bonds broken in the reactants and the bonds formed in the products. This is based on the principle that energy is required to break bonds, and energy is released when new bonds are formed.

2. **Consider Significant Bonds**: Include bonds that undergo significant changes during the reaction. These are usually the bonds directly involved in the breaking and forming of chemical bonds between atoms in the reactants and products.

3. **Exclude Internal Bonds**: You can often exclude bonds that remain unchanged or undergo minimal changes during the reaction. Internal bonds within a functional group or within a polyatomic ion, for example, may remain intact throughout the reaction and can be ignored when calculating \( \Delta H \).

4. **Look for Common Bonds**: Identify common bonds that appear on both sides of the reaction equation. These bonds will cancel out when you subtract the total energy of bonds broken from the total energy of bonds formed, simplifying the calculation.

5. **Use Standard Bond Energies**: If you're using bond enthalpies from tables, ensure consistency by using standard values for the bonds broken and formed. These values are averaged over a range of compounds and are useful for approximate calculations.

6. **Consider Reaction Mechanisms**: In some cases, particularly for complex reactions, the overall reaction may involve multiple steps with intermediate species. In such cases, you may need to consider the bonds broken and formed in each step separately.

7. **Practice and Experience**: With practice and experience, you'll develop an intuition for which bonds are most significant in a given reaction and which can be safely ignored for simplicity without significantly affecting the accuracy of the calculation.

In summary, focus on the bonds broken and formed in the reaction, consider significant bonds, and exclude internal or common bonds to simplify the calculation while ensuring accuracy in determining the enthalpy change of the reaction.