The Energy Cost of Chemical Change

Part 1: Engage

Think about two different types of gas:

  1. Natural Gas (Methane): When you burn it on a stove, it gets hot enough to cook food and warm a house.
  2. Hydrogen Gas: Scientists are excited about hydrogen because it doesn’t produce CO2 when burned. However, to get hydrogen gas from water (H2O), we have to use electricity to “split” the molecules.

The Puzzling Question: Why does burning methane give off so much heat, while splitting water takes so much energy? What is happening at the level of chemical bonds to cause this difference?

Write your “Need to Know” questions:

Part 2: Explore

Open the Bond Energy Changes Simulator.

Investigation A: Methane Combustion

  1. Select Combustion of Methane from the reaction list.
  2. Look at the Bond Energy Table. Record the energy required to break a single C-H bond and a single O=O bond.
  3. Click Break Bonds (Absorb Energy). Observe the animation and the “Energy Accounting” section.
    • How many bonds were broken?
    • What was the total “Energy Absorbed”?
  4. Click Form Bonds (Release Energy).
    • What new molecules were formed?
    • What was the total “Energy Released”?
  5. Look at the Net Energy Change (ΔH). Is this reaction exothermic or endothermic?

Investigation B: Water Splitting

  1. Select Water Splitting (Electrolysis).
  2. Repeat the steps to break the bonds of two water molecules and form hydrogen and oxygen gas.
    • Total Energy Absorbed: ______ kJ
    • Total Energy Released: ______ kJ
    • Net Energy Change (ΔH): ______ kJ

Part 3: Explain

  1. Analyze the Data: In the Methane Combustion reaction, was there more energy used to break the old bonds, or more energy released when the new bonds formed?
  2. Define Energy Balance: Based on your observations, what determines if a reaction will get hot (exothermic) or stay cold/require energy (endothermic)? Use the term “Total Bond Energy” in your answer.
  3. Molecular View: Look at the “Energy Profile” graph during both reactions. Describe the “Transition State” (the peak of the graph). What do the molecules look like at that moment?

Part 4: Elaborate & Evaluate

The Challenge: A friend says, “The energy from fire comes from the oxygen in the air, not the bonds in the fuel.”

Using the data you collected from the simulation, construct a scientific argument to refute this claim. In your argument, you must:

  1. Explain that energy is absorbed when bonds break.
  2. Explain that energy is released when bonds form.
  3. Use specific numbers from the Haber Process (Ammonia) reaction in the simulator to show how the difference between these two determines the net energy of the system.

(Space for scientific argument)