# Bond Energy Calculations & Enthalpy Change Problems, Basic Introduction, Chemistry | Summary and Q&A

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October 24, 2017
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The Organic Chemistry Tutor
Bond Energy Calculations & Enthalpy Change Problems, Basic Introduction, Chemistry

## TL;DR

Learn how to estimate the enthalpy of a reaction by calculating the sum of bond dissociation energies of reactants and subtracting the sum of bond energies of products.

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### Q: How do you estimate the enthalpy of formation of hydrochloric acid?

To estimate the enthalpy of formation of hydrochloric acid, write a balanced chemical equation, calculate the sum of bond dissociation energies of reactants (hydrogen and chlorine) multiplied by their coefficients, and subtract the bond dissociation energy of hydrochloric acid.

### Q: Why is breaking a bond considered an endothermic process?

Breaking a bond requires energy because it involves separating atoms within a molecule. This energy is absorbed, making the process endothermic.

### Q: How is the enthalpy of combustion of methane estimated?

Start by writing the balanced combustion equation for methane with carbon dioxide and water as products. Calculate the sum of bond dissociation energies for all bonds involved, both in reactants and products, and subtract the product energies from the reactant energies.

### Q: What is the enthalpy of the reaction between methane and chlorine gas to produce carbon tetrachloride and hydrochloric acid?

Write the balanced equation and calculate the sum of bond dissociation energies of reactants (methane and chlorine) and products (carbon tetrachloride and hydrochloric acid). Subtract the product energies from the reactant energies to find the enthalpy of the reaction.

## Summary & Key Takeaways

• To estimate the enthalpy of a reaction, write a balanced chemical equation and calculate the sum of bond dissociation energies of reactants and products.

• Breaking bonds requires energy (endothermic process), while bond formation releases energy (exothermic process).

• The enthalpy of reaction is calculated by subtracting the sum of bond dissociation energies of products from that of the reactants.