Calculate the enthalpy change of a reaction using bond energies

Chemical Energetics: Exothermic & Endothermic Reactions 🔥❄️

What is Enthalpy Change?

Enthalpy change (ΔH) tells us how much heat is absorbed or released when a chemical reaction takes place. If ΔH is negative, the reaction gives off heat – it’s exothermic (think of a fire). If ΔH is positive, the reaction takes in heat – it’s endothermic (like a snow‑ball melting in your hand).

Bond Energies: The Energy Bank of Molecules

Every chemical bond has a certain amount of energy stored in it. Think of a bond as a rubber band that holds two atoms together. Breaking the band releases energy (like popping a balloon), while forming a new band stores energy (like tying a knot that holds a promise).

The enthalpy change of a reaction can be estimated from the bond energies of the bonds that are broken and formed:

$$\Delta H = \sum_{\text{bonds broken}} D_{\text{broken}} - \sum_{\text{bonds formed}} D_{\text{formed}}$$

Step‑by‑Step Calculation

  1. Write the balanced chemical equation.
  2. List every bond that is broken in the reactants.
  3. List every bond that is formed in the products.
  4. Look up the bond energies (kJ mol⁻¹) for each bond.
  5. Calculate the total energy for bonds broken and for bonds formed.
  6. Apply the formula above to find ΔH.
  7. Interpret the sign: negative = exothermic, positive = endothermic.

Example Problem

Calculate the enthalpy change for the combustion of methane:

$\ce{CH4 + 2O2 -> CO2 + 2H2O}$

Bond Energy (kJ mol⁻¹)
C–H 413
C–C 348
O=O 498
C=O 799
O–H 463

Bonds broken: 1 C–H (4 × 413) + 2 O=O (2 × 498) = 1652 kJ mol⁻¹ Bonds formed: 1 C=O (2 × 799) + 4 O–H (4 × 463) = 2604 kJ mol⁻¹

$$\Delta H = 1652 - 2604 = -952\ \text{kJ mol}^{-1}$$

The negative ΔH tells us the reaction is strongly exothermic – it releases a lot of heat (like a bright fire).

Exam Tips & Quick Reference

Always:

  • Write the balanced equation first.
  • List all bonds broken and formed.
  • Use the correct sign convention: broken = +, formed = –.
  • Check units – bond energies are in kJ mol⁻¹.
  • Round to the nearest 10 kJ mol⁻¹ unless otherwise specified.

💡 Remember: The more bonds you break, the more energy you need; the more bonds you form, the more energy you release.

Quick Reference Table of Common Bond Energies

Bond Energy (kJ mol⁻¹)
H–H 436
C–H 413
C–C 348
O=O 498
O–H 463
C=O 799

Good luck with your studies! Remember, chemistry is like a puzzle – each bond is a piece that fits together to give the full picture of energy flow. 🌟

Revision

Log in to practice.

15 views 0 suggestions