Key Ideas: When substances react, bonds between atoms are broken, and new bonds are formed. Energy must be transferred from the surroundings to a reactant to break a bond. The average energy required to break a specific type of bond is called the bond energy. Conversely, when new bonds are made, the bond energy is transferred from the products to the surroundings. The heat transferred by a reaction is approximately equal to the difference in energy between the average bond energies of the reactants and the average bond energies of the products.
Key Terms: Potential energy, Total kinetic energy, Reaction enthalpy, Average bond enthalpy, Combustion
As a result of this lesson, students will be able to:
Focus on Understanding
What Takes Place: A key goal of the lesson is for students to understand how to use bond enthalpies to determine the enthalpy of a reaction. The focus is on combustion reactions of several molecules with oxygen.
The macroscopic focus is on three balloons filled with hydrogen, methane, and butane. These can be done as a demo or shown on video. The purpose is for students to "Feel" the more negative enthalpy per mole for butane when the balloons are popped with a spark (a flame). If the demo is shown at the beginning of this lesson, students can try to determine why the heat transferred changes. If the demo is shown at the end of this lesson, students should be asked to predict the enthalpies of the three combustion reactions, and test their predictions. A final video shows the combustion of a donut. This is then connected with food calories.
What Takes Place: A key goal of the lesson is for students to understand how to use bond enthalpies to determine the enthalpy of a reaction. The focus is on combustion reactions of several molecules with oxygen.
The atomic level focus is on a simulation that shows what happens as molecules of hydrogen and oxygen react to form water. A spark is required to initiate the reaction. The simulation shows that as new bonds form, the kinetic energy of the molecules increases. The increase in kinetic energy is due to a decrease in potential energy; the bonds in the product molecules are more stable. The increase in kinetic energy means the temperature increases. In order for the system to reach thermal equilibrium with the surroundings, heat is transferred to the surroundings. This is how you can use a chemical reaction to cook food.
The symbolic focus is on drawing and interpreting energy exchange diagrams and calculating the net energy exchange due to a chemical reaction.
Materials:
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