Worksheet: Predicting Energy Changes in Chemical Reactions

Worksheet: Predicting Energy Changes in Chemical Reactions
Learning Outcome: Students will predict whether a reaction will release or absorb energy based on bond energy data.

Instructions:

Use the provided bond energy table to calculate the total energy absorbed (to break bonds) and the total energy released (to form bonds) in the reactions below. Determine whether the reaction is exothermic (releases energy) or endothermic (absorbs energy).

Bond Energy Table

Reactions to Analyze

Reaction 1: Combustion of Methane

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

1. Identify Bonds Broken:

   • ___ C-H bonds in CH4
   • ___ O=O bonds in O2

2. Identify Bonds Formed:

   • ___ C=O bonds in CO2
   • ___ O-H bonds in H2O

3. Calculate Energy Absorbed (Breaking Bonds):
   (Number of bonds × Bond energy)
   Total Energy Absorbed = ___________ kJ

4. Calculate Energy Released (Forming Bonds):
   (Number of bonds × Bond energy)
   Total Energy Released = ___________ kJ

5. Net Energy Change:
   Net Energy = Total Energy Released - Total Energy Absorbed
   Net Energy = ___________ kJ

6. Prediction:
   Is the reaction exothermic or endothermic? Why?

Reaction 2: Formation of Hydrogen Chloride

H2 (g) + Cl2 (g) → 2 HCl (g)

1. Identify Bonds Broken:

   • ___ H-H bonds in H2
   • ___ Cl-Cl bonds in Cl2

2. Identify Bonds Formed:

   • ___ H-Cl bonds in HCl

3. Calculate Energy Absorbed (Breaking Bonds):
   Total Energy Absorbed = ___________ kJ

4. Calculate Energy Released (Forming Bonds):
   Total Energy Released = ___________ kJ

5. Net Energy Change:
   Net Energy = Total Energy Released - Total Energy Absorbed
   Net Energy = ___________ kJ

6. Prediction:
   Is the reaction exothermic or endothermic? Why?

Reaction 3: Decomposition of Water

2 H2O (g) → 2 H2 (g) + O2 (g)

1. Identify Bonds Broken:

   • ___ O-H bonds in H2O

2. Identify Bonds Formed:

   • ___ H-H bonds in H2
   • ___ O=O bonds in O2

3. Calculate Energy Absorbed (Breaking Bonds):
   Total Energy Absorbed = ___________ kJ

4. Calculate Energy Released (Forming Bonds):
   Total Energy Released = ___________ kJ

5. Net Energy Change:
   Net Energy = Total Energy Released - Total Energy Absorbed
   Net Energy = ___________ kJ

6. Prediction:
   Is the reaction exothermic or endothermic? Why?

Extension Activity:

Write your own balanced chemical equation and use the bond energy table to predict whether the reaction is exothermic or endothermic. Show your calculations and reasoning.