AQA GCSE FOUNDATION Chemistry: Bond Energy Calculations

AQA GCSE Foundation Chemistry: Bond Energy Calculations

Bond energy is the amount of energy needed to break one mole of a specific bond in the gaseous state. It's a measure of the strength of a chemical bond.

Here's how you can calculate energy changes in chemical reactions using bond energies:

Step 1: Draw the displayed formula of the reactants and products.

This helps you visualize the bonds present in the molecules.

Step 2: Identify the bonds broken and the bonds formed.

• Bonds broken: These are the bonds present in the reactants that are broken during the reaction.
• Bonds formed: These are the new bonds that are formed in the products.

Step 3: Look up the bond energies of each bond.

You can find these values in a data booklet or online. Make sure you use the correct units (usually kJ/mol).

Step 4: Calculate the total energy change.

• Energy change = (sum of bond energies broken) - (sum of bond energies formed)

Remember:

• Breaking bonds requires energy (endothermic), so this value is positive.
• Forming bonds releases energy (exothermic), so this value is negative.

Example:

Let's calculate the energy change for the following reaction:

CH4(g) + 2O2(g) ? CO2(g) + 2H2O(g)

Step 1: Draw the displayed formulae:

      H
      |
H - C - H
      |
      H

   O = O
    ||
   O = O

   O = C = O

      O
      ||
     H - O - H

Step 2: Identify bonds broken and formed:

• Bonds broken: 4 C-H bonds, 2 O=O bonds
• Bonds formed: 2 C=O bonds, 4 O-H bonds

Step 3: Look up bond energies:

• C-H bond energy = 413 kJ/mol
• O=O bond energy = 498 kJ/mol
• C=O bond energy = 805 kJ/mol
• O-H bond energy = 464 kJ/mol

Step 4: Calculate the total energy change:

Energy change = (4 * 413 + 2 * 498) - (2 * 805 + 4 * 464) 
              = 1652 + 996 - 1610 - 1856
              = -818 kJ/mol

Conclusion:

The energy change for this reaction is -818 kJ/mol. This means that the reaction is exothermic, releasing energy into the surroundings.

Key points to remember:

• Bond energies are average values and can vary slightly depending on the molecule.
• Bond energy calculations are estimations and not always perfectly accurate.
• Bond energies are useful for understanding the energy changes that occur during chemical reactions.

Practice calculating bond energies for different reactions to solidify your understanding. Remember to follow the steps carefully and keep track of your calculations. Good luck!