Energy Interaction Model

Chapter 2.

Energy Interaction Model

false

Practice Question 1

Liquid water at 100°C boils to produce water vapor at 100°C. Choose the physical system to be all the water molecules.

Before doing an energy analysis, we need to choose a definite physical system whose energies we will follow. What should we choose as our physical system (and write at the top of our energy-interaction diagram)?

After choosing the physical system we need to define a beginning time and an ending time. What should we choose (and write on the timeline).

The situation refers to liquid water turning into steam. It is not usually useful to use “liquid water” as our physical system here because some of it “disappears”. On the other hand, what doesn’t disappear is are the water molecules so lets choose all of the water molecules as our physical system so that the system (some of them may make up the liquid and some make up the gas but the total number is constant). The interaction we are interested in is defined for us in the description of the situation. We start with purely liquid water at 100°C and end with purely gaseous water (steam) so those are our endpoints.

Practice Question 2

Liquid water at 100°C boils to produce water vapor at 100°C. Choose the physical system to be all the water molecules.

From the description of this interaction given above, decide whether bond energy is increasing, decreasing, or not changing. Show the direction of energy change by drawing the appropriate arrow where indicated on a separate sheet of paper. If there is no change then you can write Δ(Bond E) = 0. Click submit to see the Discussion.

In the liquid state the H2O molecules are stuck to each other (they are bound to their neighbors by hydrogen bonds). But in gas state the molecules are separated apart so bonds must have been broken in the process of boiling. You have to add energy (bond energy) to break bonds so bond energy must have been added to the water to boil it (i.e. bond energy went up).

Practice Question 3

Liquid water at 100°C boils to produce water vapor at 100°C. Choose the physical system to be all the water molecules.

From the description of this interaction given above, decide whether thermal energy is increasing, decreasing, or not changing. Show the direction of energy change by drawing the appropriate arrow where indicated on a separate sheet of paper. If there is no change then you can write Δ(Thermal E) = 0. Click submit to see the Discussion.

The situation explains that the initial temperature (T = 100° C) and the final temperature (T = 100° C) are the same. Since our model tells us that the indicator of thermal energy change is the temperature, the constant T tells us that the thermal energy did not change.

Practice Question 4

Liquid water at 100°C boils to produce water vapor at 100°C. Choose the physical system to be all the water molecules.

From the description of this interaction given above, decide whether energy was being added or removed from the system as heat. On a separate sheet of paper, show the direction of energy transfer with the appropriate direction of the arrow sketched above the word “Heat”. Click submit to see the Discussion.

You should understand at least two ways of thinking about this situation. i) You have probably have common knowledge that you have to add heat to something (e.g. turn on the “burner” on the range) to boil it. That would tell you that the arrow points in. ii) Your physics knowledge tells you that the bond energy went up (bonds were broken) and that the thermal energy did not change (no temperature change). From this you can say that the total internal energy (Eth + Ebond) increased and, according to our energy-interaction model, the total energy of a physical system can only increase if energy was added to it so heat must have been added. To check that we are consistent, note that the “Heat” arrow points inward and Bond E goes up.

Practice Question 5

Liquid water at 100°C boils to produce water vapor at 100°C. Choose the physical system to be all the water molecules.

From the description of this interaction given above, what can you say with absolute certainty about the relative magnitudes of the energy changes of any energy systems and the energy transfers and explain how this is shown on the energy-system diagram.

From previous questions, you have learned that the energy diagram looks something like the picture to the right. The only thing missing is the “conservation of energy” equation relating the energy changes with the energy transfers. The sum of the energy changes equals the energy transferred. ∆Eth +∆Eb=Q Note that ΔEth = 0 and ΔEbond > 0 so Q > 0 which tells us that heat was transferred in. In other words, the change (increase) in the bond energy is exactly equal to the heat added.

Practice Question 6

Photosynthesis in a plant happens according to the following:
sunlight + 6CO₂(g) + 6H₂O(l) = C₆H₁₂O₆(aq) + 6O₂(g).

Before doing an energy analysis, we need to choose a definite physical system whose energies we will follow. What should we choose as our physical system (and write at the top of our energy-interaction diagram)?

After choosing the physical system we need to define a beginning time and an ending time. What should we choose (and write on the timeline).

Molecules are disappearing and new molecules are appearing but the total number and types of atoms doesn’t change so a reasonable physical system could be “all the atoms involved in the interaction”. Note that sunlight is not “matter” and so is not a part of any physical system. For energy conservation situations you will usually choose a time before the relevant interactions as the starting time (i.e. before the sunlight hits the physical system) and an ending time after all the relevant interactions are done.

Practice Question 7

Photosynthesis in a plant happens according to the following:
sunlight + 6CO₂(g) + 6H₂O(l) = C₆H₁₂O₆(aq) + 6O₂(g).
Choose the physical system to be all the C, H, and O atoms (including the ones that are bound into molecules etc.). We are interested in which of the various energies changed and by how much.

Give a reason for the direction of the change, if any, of each of the two bond energy systems shown in the energy-system diagram. On a separate sheet of paper, show these arrows on the energy-system diagram. Click submit to see the Discussion.

For the purposes of bond energy changes, we can pretend that the reaction proceeds by first breaking up the reactants into atoms and, after that, forming the products from the atoms. Let’s think about the reactants first. Breaking bonds (without forming any new bonds yet) always requires that energy be added so the bond energy of the reactant had to increase (to break them up into atoms). Hence the upward arrow. Now the products. Forming bonds always requires that energy be removed (forming bonds is the opposite of breaking them) so the bond energy of the products must decrease. Hence the downward arrow.

Practice Question 8

Photosynthesis in a plant happens according to the following:
sunlight + 6CO₂(g) + 6H₂O(l) = C₆H₁₂O₆(aq) + 6O₂(g).
Choose the physical system to be all the C, H, and O atoms (including the ones that are bound into molecules etc.). We are interested in which of the various energies changed and by how much.

Give a reason for the direction of the change, if any, of the thermal energy system of all the atoms. On a separate sheet of paper, show the appropriate arrow on the energy-system diagram. Click submit to see the Discussion.

The story of this situation doesn’t tell us about temperature. However, thinking about the real situation should convince you that a leaf is not continually heating up and heating up as photosynthesis proceeds within it. Since our model tells us that the indicator of thermal energy change is the temperature, the constant T tells us that the thermal energy did not change (this is probably an approximation but it is very likely to be a good one). Hence, in the diagram to the right no change is shown.

Practice Question 9

Photosynthesis in a plant happens according to the following:
sunlight + 6CO₂(g) + 6H₂O(l) = C₆H₁₂O₆(aq) + 6O₂(g).
Choose the physical system to be all the C, H, and O atoms (including the ones that are bound into molecules etc.). We are interested in which of the various energies changed and by how much.

Is energy entering or leaving the system? On a separate sheet of paper, show the appropriate arrow on the energy-system diagram. Click submit to see the Discussion.

Sunlight (energy) absorption makes this reaction possible so we conclude that energy is entering the energy system from the outside (from the sun) so we draw the arrow pointing inward.

Practice Question 10

Photosynthesis in a plant happens according to the following:
sunlight + 6CO₂(g) + 6H₂O(l) = C₆H₁₂O₆(aq) + 6O₂(g).
Choose the physical system to be all the C, H, and O atoms (including the ones that are bound into molecules etc.). We are interested in which of the various energies changed and by how much.

a) What can you say about the relative sizes of any energy changes based on the completed energy-system diagram you just made?

b) Which group of molecules, i.e., the reactants or the products, has the bigger change in bond energy?

a) From previous questions, you have learned that the energy diagram looks something like the picture to the right. The only thing missing is the “conservation of energy” equation relating the energy changes with the energy transfers. The sum of the energy changes equals the energy transferred. We leave Eth out because ΔEth = 0 and get: ∆E_"b reactants" +∆E_(b products)=Q Note that Q represents the sunlight energy that was transferred in (so Q > 0). b) Sunlight energy was added so Q > 0 so (∆E_"b reactants" +∆E_"b products" )>0 and we can conclude that the increase in the bond energy of the reactants is larger than the decrease in the bond energy of the products.