Chemistry · 10th Grade

Active learning ideas

Hess's Law of Heat Summation

Hess's Law involves multi-step algebraic reasoning that can feel abstract to students until they manipulate equations themselves. Active learning lets them see how reversing or scaling reactions changes ΔH, turning invisible state functions into concrete, visual steps.

• Common Core State Standards
• STD.HS-PS1-4
• STD.CCSS.MATH.CONTENT.HSA.CED.A.2

• 20–45 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Jigsaw
• 45 min · Small Groups

Jigsaw: Manipulating Thermochemical Equations

Groups become expert in one type of equation manipulation: reversing an equation (and flipping ΔH sign), multiplying by a factor (and scaling ΔH), or combining two equations (and canceling intermediates). Each expert group teaches the other groups their manipulation, then the full class applies all three to solve a Hess's Law problem together.

Explain how Hess's Law illustrates the Law of Conservation of Energy.

Facilitation Tip

For the Think-Pair-Share, require pairs to write their final ΔH with a margin note explaining why the sign changed if they reversed a reaction, reinforcing the connection between direction and energy flow.

What to look for

Provide students with a target reaction and three related thermochemical equations. Ask them to write the steps they would take to manipulate the given equations (e.g., 'Reverse equation 2,' 'Multiply equation 1 by 3') before summing them to find the ΔH of the target reaction. Collect and review these steps for understanding of the manipulation process.

• Understand
• Analyze
• Evaluate
• Relationship Skills
• Self-Management

Activity 02

• Collaborative Problem-Solving
• 35 min · Pairs

Stepwise Problem-Solving Protocol

Students work through a Hess's Law problem using a four-step protocol card: (1) write the target equation, (2) identify which given equations to reverse or scale, (3) cancel intermediates and check that the result matches the target, (4) sum ΔH values. Each step is completed independently then checked with a partner before moving to the next.

Calculate the enthalpy change for a reaction using Hess's Law.

What to look for

Students work in pairs to solve a Hess's Law problem. After completing their calculation, they swap their work with another pair. Each pair reviews the other's work, checking for correct equation manipulation (reversing, scaling) and accurate summation of ΔH values. They provide written feedback on any errors found.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

Activity 03

• Think-Pair-Share
• 20 min · Pairs

Think-Pair-Share: The Dangerous Reaction Problem

Present the scenario: a chemist needs to know the ΔH of a highly explosive reaction that cannot be measured directly. Students individually describe a strategy using Hess's Law, then pairs formalize the plan using given thermochemical equations. The debrief connects the mathematical procedure to its practical justification as a safety and measurement tool.

Analyze how to find the energy of a reaction that is too dangerous to perform directly.

What to look for

On an index card, ask students to write a 2-3 sentence explanation of why Hess's Law is useful for calculating enthalpy changes of reactions that cannot be easily performed in a lab. Prompt them to mention at least one specific reason why a reaction might be difficult to perform directly.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

A few notes on teaching this unit

Experienced teachers approach Hess's Law by first anchoring the concept with concrete, relatable reactions—like combustion or formation—before moving to abstract algebra. They insist on two non-negotiables: labeling every step and checking units and signs at each turn. Research shows that students who verbalize their reasoning while solving (think aloud) make fewer sign and scaling errors than those who work silently.

Students will confidently manipulate thermochemical equations by reversing, scaling, and summing ΔH values to match a target reaction with at least 80% accuracy. They will explain why intermediates cancel and justify their choice of manipulation steps in writing or discussion.

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Watch Out for These Misconceptions

• During the Jigsaw: Manipulating Thermochemical Equations, watch for students who reverse a reaction but forget to change the sign of ΔH or keep the magnitude unchanged.

  After the jigsaw groups present, ask them to swap their manipulated equations and ΔH values with another group and verify each other’s sign changes and magnitude retention before moving to the next step.

• During the Stepwise Problem-Solving Protocol, watch for students who assume all species must cancel and incorrectly force equations together.

  Before students begin calculations, have them preview the target equation and highlight any species that also appear in the given equations, then predict which will cancel naturally before manipulating equations.

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Methods used in this brief

• Jigsaw
• Collaborative Problem-Solving
• Think-Pair-Share