The Strength of Modeling in Science

The Strength of Modeling in Science Focus: Energy in temperature changes and changes of state

The Modeling Approach Applied to Energy…

Energy is conserved. • We have a very clear understanding of what this statement means, and how it applies to scientific thinking and problem solving. • Students don’t always understand it as clearly.

The “go to” Lab • Ice is put into a beaker and the beaker is put on a warming plate. • A temperature probe is inserted into the ice in the beaker. • Vernier software LoggerPro begins to record temperature changes over time – students understand that energy is constantly being added to the system from the hot plate.

Heating Curve • Curve generated from data:

Observations • Label each plateau: Liquid only Solid to liquid Liquid to vapor (gas)

Pose the obvious question: Where did all of that energy go? • It increased the temperature. • It “melted it”. • Energy wasn’t really being added yet.

“Aha” Moment • It all becomes clear when you get responses like these that there are misconceptions about energy transfer and its role at the particle level. • What to do next?

Energy “modeling” style • Compare energy to money – you cannot have money in your savings account and your checking account at the same time. • Energy cannot be causing two different changes at once… it either causes a phase change or a temperature change.

Your turn! On your group’s white board, Draw particle representations for each portion of your graph.

Remember the plateaus? Solid to liquid Liquid to vapor (gas)

Phase Energy • When the temperature doesn’t change, but you understand energy has been added to the system, maybe it caused a change of phase. • We know it didn’t cause a change in temperature because we can read it from the graph. • There are two places on the graph that show change of phase because there are two temperature plateaus.

Temperature Changes Temperature change

What happens in a temperature change? • As energy is added, the temperature increases. • This means that, on average, the particles are moving faster and faster. • The energy that is coming into the system is going into the “thermal” account and speeding up the motion of the particles, but they stay in the same state of matter.

Where do we go from here? • The heating curve for water is used to show change in every chemistry text book, and it is a great graphical representation of the energy changes. • The modeling program has a second approach to graphically showing changes in energy –” Energy Bar Charts”.

Constructing an Energy Bar Chart A cup of hot coffee cools as it sits on the table. • 1. Determine what is in the system • Everything else makes up the surroundings cup coffee

Decide whether Ech is involved • In this case, you start with coffee and end with coffee; particles are not rearranged to form new substances • So, ignore Ech for now.

Assign values to Eph • Due to interactions between particles, the energy stored due to the arrangement of particles is ranked:solids < liquids < gases • We choose to represent these phases by using: • Solids = 1 bar • Liquids = 2 bars • Gases = 4 bars

cup coffee Assign values to Eph • Use two Eph bars before and after

cup coffee Choose bars for Eth depending on temperature • Use 4 bars for hot coffee and 2 bars for room temp coffee • Other values might also work; try to be consistent in your representations

Now show energy transfer • Final situation has 2 less bars of E than initial; 2 bars had to leave the system

Now, consider phase change • A tray of ice cubes (-8 ˚C) is placed on the counter and becomes water at room temperature • What do we know about the situation? • The system is the tray of ice cubes. • The solid water turns to liquid water - no change in Ech • The Eph increases (solidliquid) • The Eth increases (temp rises) • Now represent these changes in bar graph.

Initial & Final States • Choice of bars for Eth arbitrary, but consistent. • We used 2 bars for room temp and 1 bar for cold soda before. • Temp < 0˚C should be < 1 bar.

Account for Energy • Energy must flow into system via heating

Breaking It Down • In the beginning if students struggle with the energy bar diagrams. I break them apart into separate ones that are part of a sequence. • After they have the sequence, we summarize by going from the first picture directly to the last and showing the overall change.

Adding the quantitative portion… DH = mHfv DH = mCDT Use the equation with temperature change b/c temperature is changing DH = mHf

Tying it all together. • When students are asked to solve thermal energy problems, they are required to show a heating curve and an energy bar diagram. • Let’s look at the white boards…

In Conclusion - What is Modeling? • Modeling is not a set curriculum • Follows the Modeling Cycle Model Development Stage occurring in this lab • Students see a phenomenon • Students plan experiment • Students analyze data • Students present results to class • Results are generalized • Uses multiple representations of content to develop a conceptual model

Modeling Success in Literature FCI % Modeler Modeler Figure 1 shows that traditional high school instruction (lecture, demonstration, and standard laboratory activities) has little impact on student beliefs, with an average FCI posttest score of about 42%, still much below the Newtonian threshold[1] [2]. This failure of traditional instruction is largely independent of the instructor’s knowledge, experience and teaching style.

For more information about Modeling… http://modeling.asu.edu/ http://modelinginstruction.org/

The Strength of Modeling in Science