Chemistry · 9th Grade

Active learning ideas

Hess's Law and Enthalpy of Formation

Active learning works well for Hess’s Law and enthalpy because students often struggle with abstract energy calculations. Having them manipulate equations and visualize energy changes through concrete activities builds the spatial and numerical reasoning needed for mastery.

• Common Core State Standards
• HS-PS1-4
• STD.CCSS.MATH.CONTENT.HSA.CED.A.1

• 20–35 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Inquiry Circle
• 30 min · Whole Class

Inquiry Circle: The Scent Race

Students stand in a line and record the time it takes to smell different substances (e.g., vanilla vs. peppermint) released at one end. They must work in groups to relate the 'speed of smell' to the molar mass of the molecules using Graham's Law.

Explain why enthalpy is a state function, allowing for the use of Hess's Law.

Facilitation Tip

During The Scent Race, circulate to ensure groups record distances traveled by each gas particle and relate them to molar mass using Graham’s Law.

What to look for

Present students with a simple thermochemical equation and ask them to identify the enthalpy change. Then, provide two related equations and ask them to write the steps they would take to calculate the enthalpy change of the target reaction using Hess's Law.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 02

• Think-Pair-Share
• 20 min · Pairs

Think-Pair-Share: Collecting Gas Over Water

Students are shown a diagram of gas collection and asked why the pressure inside the bottle isn't just the pressure of the gas they made. They discuss in pairs the role of 'vapor pressure' and how Dalton's Law helps them find the 'dry' gas pressure.

Construct calculations to determine the enthalpy change of a reaction using Hess's Law.

Facilitation Tip

For Collecting Gas Over Water, model the calculation on the board first, then ask students to check each other’s work using the 'Total = Gas + Water' checklist.

What to look for

Provide a chemical reaction and the standard enthalpies of formation for all reactants and products. Ask students to calculate the standard enthalpy change for the reaction and briefly explain why enthalpy is a state function.

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

Activity 03

• Simulation Game
• 35 min · Pairs

Simulation Game: Effusion in a Box

Using a digital simulation, students place two different gases in a container with a small hole. They observe which gas escapes (effuses) faster and use their data to verify the inverse relationship between mass and speed.

Calculate the standard enthalpy change of a reaction using standard enthalpies of formation.

Facilitation Tip

In Effusion in a Box, pause the simulation when students seem confused about particle movement to discuss kinetic energy distribution.

What to look for

Pose the question: 'If we know the standard enthalpies of formation for all substances involved in a reaction, why is Hess's Law still a useful tool?' Facilitate a discussion on the practical applications and conceptual understanding gained from both methods.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

A few notes on teaching this unit

Teach Hess’s Law by having students physically rearrange equation cards before calculating. Emphasize that enthalpy is a state function by showing how the same final state can be reached through different paths. Avoid focusing too much on memorizing formation values—instead, prioritize the logic of combining reactions.

Successful learning looks like students confidently applying Hess’s Law to rearrange equations, explaining why enthalpy is a state function, and correctly accounting for water vapor pressure when collecting gases over water. They should also articulate why particle speed depends on molar mass, not momentum.

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Watch Out for These Misconceptions

• During The Scent Race, watch for students who assume heavier gas particles travel farther because they have more 'momentum.'

  Pause the activity and ask students to calculate the speed of each particle using KE = 1/2 mv². Have them compare speeds for gases of different masses at the same temperature.

• During Collecting Gas Over Water, watch for students who forget to subtract water vapor pressure from the total pressure.

  Have students use the 'Total = Gas + Water' checklist on their lab sheets and require them to explain each step to a partner before calculating.

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Methods used in this brief

• Inquiry Circle
• Think-Pair-Share
• Simulation Game