Angela's Tools · Chapter 14 · Energy in Chemical Reactions Worksheet 4 of 10

Boundaries, Bars and Arrows: Modeling Where the Energy Goes

Learning target: I can read a system-and-surroundings diagram and an energy-bookkeeping bar chart, then build the matching model for a cold pack and show the energy term in each equation.

Diagrams and modelsAbout 40 minIndividual MS-PS1-6 · practice visual modelingwriting
NameDatePeriod

Directions

Study each figure before answering the questions about it. Figure 1 shows where the boundary goes for a hand warmer. Figure 2 is an energy bookkeeping chart for the same reaction; its numbers are model units invented for teaching, not real measurements. Figure 3 traces a cold pack step by step. In Part D you will draw the cold pack's own bookkeeping chart and boundary, and in Part E you will place the energy term in two equations. Label everything you draw.

You will need: This worksheet, Pencil, Ruler, Colored pencils (optional, for the arrows)

Part A: The boundary around a hand warmer

What is inside and outside the boundary What is inside and outside the boundary Iron powder (reactant) Inside the boundary. Gray grains of iron that will combine with oxygen. Oxygen from the air that reaches the iron (reactant) Inside the boundary once it enters the pouch and reacts. Iron oxide (product) Inside the boundary. Red-brown grains made from the iron and oxygen atoms. Cloth pouch Outside the boundary. It warms up but does not react. Nitrogen in the air Outside the boundary. It enters the pouch but takes no part. Glove and hand Outside the boundary. They receive energy from the system. Energy arrow Points from inside the boundary to outside: the system releases thermal energy to the surroundings.
Figure 1. An iron-powder hand warmer resting in a gloved hand, with the boundary drawn around the reacting matter.
1.List the three items in Figure 1 that make up the system, and say in one sentence what the three have in common.
2.Nitrogen enters the pouch just as oxygen does, yet the figure places nitrogen outside the boundary and oxygen inside it. Explain the difference.
3.Which way does the energy arrow point, and what single word describes a reaction whose arrow points that way?

Part B: Energy bookkeeping for the hand warmer

Energy taken in and given out as iron reacts with oxygen (model units) Energy taken in and given out as iron reacts with oxygen (model units) simulated data 0 200 400 600 800 1000 Energy (model units) 500 Taken in to break bonds in the reactants 820 Given out as bonds form in iron oxide 320 Net energy released to the surroundings
Figure 2. Teaching model with invented energy units: how much energy the hand-warmer reaction takes in and gives out. Real bond energies are not shown.
4.Show the subtraction that gives the third bar from the first two.
5.The first bar is not zero. What does that tell you about what happens to bonds even in a reaction that gives off energy? Why does the first bar being smaller than the second make the reaction exothermic?
6.A classmate says, 'The 320 units were created when the iron caught the oxygen.' Rewrite the sentence so it is correct, using the words released and surroundings.

Part C: A cold pack, step by step

Inside an instant cold pack Inside an instant cold pack Squeeze bursts the inner water bubble Water reaches solid ammonium chloride Ammonium ions and chloride ions separate and spread through the water Separating the ions takes in more energy than water attaching to them gives out The solution's temperature drops to about 5 °C Energy flows from your warmer hand into the colder pack mixing begins dissolving energy balance system cools net flow inward
Figure 3. What happens from the squeeze to the cold feeling. In this figure the salt and the water it dissolves in are drawn as the system, and the bag, the air and your hand are the surroundings, so the thermometer in the water reads the system's temperature. (Labs 1 and 2 draw the water as surroundings instead; either boundary works if you use it consistently.)
7.Between which two boxes of Figure 3 does the pack actually become cold, and what is the energy balance that causes it?
8.Your hand is part of the surroundings. Use the last box to explain why the pack feels cold without any 'cold' moving into your hand.

Part D: Build the cold pack's model

Model numbers for your chart — Use these invented teaching values. Pulling the ammonium ions and chloride ions apart from one another takes in 700 units. Water molecules attaching to the separated ions gives out 640 units.
✎Draw two things, side by side. LEFT: a bar chart in the style of Figure 2 for the cold pack, with three labeled bars: energy taken in, energy given out, and the net energy. Label the third bar with its value and say whether it is absorbed from or released to the surroundings. RIGHT: a cold pack in a hand with a boundary drawn around the system, the surroundings labeled (hand, bag, air), and one energy arrow across the boundary pointing the correct way. Give your drawing a title that includes the word endothermic or exothermic.

Part E: Energy in the equation

9.Hand warmer: 4Fe + 3O₂ → 2Fe₂O₃. Rewrite the equation with the word energy on the correct side: __________________________________
10.Cold pack: NH₄Cl → NH₄⁺ + Cl⁻. Rewrite the equation with the word energy on the correct side: __________________________________
11.Count the iron atoms and the oxygen atoms on each side of your hand-warmer equation. Did adding the word energy change either count? Explain why not.

Two questions for every item

To decide whether something is inside the boundary, ask: 'Does it react, or was it made by the reaction?' If yes, it is system. For the bar charts, the rule is always the same: taken in (breaking apart) on the first bar, given out (joining) on the second, and the net is the difference. If the second bar is taller, the net is released and the arrow points out; if the first bar is taller, the net is absorbed and the arrow points in. For Part D, your first bar (700) is taller than your second (640), so decide which way the arrow goes before you draw it.

A reaction with a push to start

A match needs a scrape of energy before it burns, but burning is strongly exothermic. Draw a third bookkeeping chart for the match with a small extra bar labeled 'start-up energy from the scrape' before the two main bars, and explain in a few sentences why the start-up bar is not part of the net calculation once the match is burning. Then explain why the textbook's rule 'a reaction that needs a continuous supply of energy is endothermic' is safer to state as 'a reaction whose net transfer is inward is endothermic.'

Angela's Tools · Chapter 14 · Energy in Chemical ReactionsWorksheet 4 of 10