Lesson 2: Thermochemical Equations
Curriculum Expectations
D2.2 write thermochemical equations, expressing the energy change as a ∆H value or as heat a heat term in the equation.
D3.2 compare the energy change from a reaction in which bonds are formed to one in which bonds are broken, and explain these changes in terms of endothermic and exothermic reactions.
D3.2 compare the energy change from a reaction in which bonds are formed to one in which bonds are broken, and explain these changes in terms of endothermic and exothermic reactions.
Lesson Hook: Mini-laboratory
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This laboratory opportunity gives students a hands-on lesson hook for the topic of thermochemical equations. Sodium acetate is a food additive that is also used to make reusable hand warmers. A supersaturated solution of sodium acetate can be disturbed by mixing or addition of a crystal (or in the case of hand warmers, a flexible metal disc). During crystallization, heat is released. Depending on the individual class and resources available, this can also be set up as a demonstration for students.
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Equipment
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Safety Precautions
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Procedure
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Pedagogical Approach
Making connections:
- Explain to students that this was in fact a supersaturated solution of sodium acetate. Facilitate a classroom discussion with students on the observations that students made before and after crystallization. How could we use a chemical reaction to describe what happened here? Link the information learned last day in terms of energy required for a temperature change to thermochemical equations. Ensure that students know the distinction between the two concepts.
- Link to STSE - What could be applications of chemistry such as the one we just saw? Hand warmers and ice packs are a good examples of this type of change.
- Discuss topics of enthalpy and enthalpy diagrams with students in a note.
Consolidation: Green-light, red-light
- Ask students to write one concept on a post-it note that they are confident with, and one that they are unsure of.