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Systems and Scale | Lesson 4 - Investigating and Explaining Ethanol Burning

Lesson 4: Investigating and Explaining Ethanol Burning

Students investigate changes in mass and CO2 concentration for burning ethanol. Then they explain results using molecular models and chemical equations to answer the Three Questions.

Guiding Question

What happens to ethanol when it burns?

Activities in this Lesson

  • Activity 4.1: Predictions about Ethanol Burning (30 min)
  • Activity 4.2: Observing Ethanol Burning (30 min)
  • Activity 4.3: Evidence-based Arguments about Ethanol Burning (50 min)
  • Activity 4.4: Molecular Models for Ethanol Burning (50 min)
  • Activity 4.5: Explaining Ethanol Burning (40 min)

Objectives

  1. Measure mass changes in materials undergoing chemical change (ethanol burning)
  2. Detect changes in CO2 concentration in air samples (ethanol burning)
  3. Use evidence about changes in mass of fuels and CO2 concentration to defend claims about movements of atoms during chemical changes (ethanol burning)
  4. Find patterns in data collected by multiple groups about changes in mass and CO2 concentration (ethanol burning)
  5. Draw and explain movements of materials during a chemical change (burning ethanol)
  6. Explain chemical changes as atoms being rearranged into new molecules (ethanol burning)
  7. Identify forms of energy involved in combustion (chemical energy, light (motion), heat energy) and distinguish them from forms of matter (e.g., fuels) and phenomena (e.g., flames) (ethanol burning)
  8. Explain energy transformations during combustion: Chemical energy stored in C-C and C-H bonds of fuel molecules is transformed into heat and light (ethanol burning).

NGSS Performance Expectations

Middle School

  • Structures and Properties of Matter. MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
  • Chemical Reactions. MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
  • Chemical Reactions. MS-PS1-5. Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.

High School

  • Chemical Reactions. HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
  • Chemical Reactions. HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Background Information

This lesson will be particularly helpful for students struggling to identify that mass of a burning fuel is lost to the air. Students observe a fuel source losing mass inside a chamber. They also observe an increase in CO2 in the air using BTB. Students must explain where the carbon atoms in the CO2 came from.

In this lesson the students return to the guiding question for the unit about what happens when ethanol burns. We will consistently focus on the idea that understanding carbon-transforming processes involves answering the Three Questions:

  • The Matter Movement Question: Where are molecules moving? (How do molecules move to the location of the chemical change? How do molecules move away from the location of the chemical change?)
  • The Matter Change Question: How are atoms in molecules being rearranged into different? (What molecules are carbon atoms in before and after the chemical change? What other molecules are involved?)
  • The Energy Change Question: What is happening to energy? (What forms of energy are involved? What energy transformations take place during the chemical change?)

Matter (the Matter Movement and Matter Change Questions). We find that even students who have learned how to balance chemical equations do not appreciate the meaning of the procedure:

  • Conservation of atoms (the Matter Change Question): The numbers of atoms on the left and right side of a chemical equation have to be the same because they are THE SAME ATOMS! A chemical equation just shows how they are being rearranged into new molecules.
  • Conservation of mass (the Matter Movement Question): ALL the mass of any material is in its atoms (and none of the mass is in the bonds, which are just attractive forces between atoms). So the mass of the products is always the same as the mass of the reactants.

Energy (the Energy Change Question). Chemists, physicists, and biologists have many different conventions for describing and measuring chemical energy. We have a deeper explanation of the conventions used in Carbon TIME units and how they relate to conventions used in different scientific fields on the BSCS website in a document called “Carbon TIME Content Simplifications.” Here are some key points:

  • All bond energies are negative relative to individual atoms. So during a chemical reaction, it always takes energy (the activation energy) to break bonds. Then, energy is released when new bonds are formed.
  • Whether a chemical reaction releases energy or not depends on the total energy of the reactants, compared with the total energy of the products. So energy is released when the total bond energy of the products is lower (i.e., more negative relative to individual atoms) than the energy of the reactants.
  • Weak bonds (like C-C and C-H) generally have MORE chemical energy than strong bonds (like C=O). The energy of the stronger bonds is more negative relative to individual atoms.
  • In systems like our atmosphere, where excess oxygen is always present, the most abundant sources of chemical energy are substances that release energy when they are oxidized (e.g., substances with C-C and C-H bonds).

Our research has consistently showed that these ideas are extremely difficult for students who have not formally studied chemistry. We therefore use the convention of twist ties to identify bonds that release energy when they are oxidized.

The investigations in all units will make use of two essential tools:

  • Digital balances.  Students can detect movement of atoms (the Movement Question) by measuring differences in mass. This Activity introduces them to the balances, allows every student to weigh something, and compares results for different students.
  • Bromothymol blue (BTB) is an indicator that changes from blue to yellow in response to high levels of CO2. Thus changes in BTB can partially answer the Carbon Question by detecting whether carbon atoms are moving into or out of the air in the container.

Activity 4.1 is the Predictions Phase of the instructional model (beginning the climb up the triangle). During this phase, students record their predictions and express ideas about what happens to matter when ethanol burns. They use the Predictions Tool to do this.

Activity 4.2 is the Observations Phase of the instructional model (going up the triangle). During this phase, the students conduct the investigation for ethanol burning, record data, and try to identify patterns in their data and observations. The important practices students focus on in this activity are 1) making measurements and observations, 2) recording their data and evidence, and 3) reaching consensus about patterns in results. They use the Observations Worksheet and Class Results Poster to do this.

Activity 4.3 the Evidence-Based Arguments Phase of the instructional model (going up the triangle). During this phase, the students review the data and observations from their investigation of ethanol burning and develop arguments for what happened during the investigation. In this phase, they also identify unanswered questions: at this point they have collected data and observations about macroscopic scale changes (BTB color change and mass change), but they do not have an argument for what is happening at the atomic-molecular scale. They use the Evidence-Based Arguments Tool to record their arguments at this phase.

Activity 4.4 is the first part of the Explanations Phase of the instructional model (going down the triangle). Students construct molecular models of the chemical change they observed in the investigation to help them develop an atomic-molecular explanation for what happened.

Activity 4.5 the second part of the Explanations Phase of the instructional model (going down the triangle). Students use the Explanations Tool to construct final explanations of what happens when soda water fizzes. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale.

Key carbon-transforming processes: combustion

Lesson Map:

See the Systems and Scale Unit Read Me document for more details.

Unit Map for Lesson 1

Unit Map

Unit Map for Lesson 4

Talk and Writing

At this stage in the unit, students will complete the inquiry and application sequences for ethanol burning—they go both up and down the triangle. This means that they will go through the Predictions Phase, the Observations Phase, the Evidence-Based Arguments Phase, and the Explanations Phase in one lesson. The tables below shows specific talk and writing goals for these phases of the unit. 

Talk and Writing Goals