Learning target: I can use evidence from a passage to explain how energy crosses the boundary between a reaction and its surroundings, why it never simply appears, and how engineers use that flow in a device.
Read the passage once without stopping. Then read it again with a pencil, numbering the paragraphs 1 to 7 and underlining every sentence that tells you which direction energy is moving. Answer in complete sentences. When a question asks for evidence, quote the passage or paraphrase it closely and give the paragraph number. The hand-warmer test numbers in paragraph 6 are an invented example, not measurements from a real company.
You will need: This worksheet, Pencil
[1]An accountant does not need to watch money change hands to know where it went. She draws a line around the business and records each time money crosses it. Chemists track energy the same way, drawing an imaginary boundary around the matter that is reacting, the reactants and the products they become, and calling that matter the system. Everything outside the boundary that can trade energy with it, such as the container, the air or a hand, is the surroundings. The boundary is a tool for thinking, not a real wall.
[2]The surroundings are where the evidence shows up. If the air next to a reaction grows warmer, or light and sound burst out of it, energy has crossed the boundary outward, and the change is exothermic. Open a pouch of iron powder to the air and the iron slowly combines with oxygen; over several hours the pouch warms a pair of gloves. A heater packet in some field rations is a louder example. It holds a powdered mixture of magnesium and iron; add water and the mixture reacts, releasing enough energy to heat a sealed food pouch to eating temperature in about ten to fifteen minutes, with no flame at all.
[3]If instead the surroundings grow cooler, energy has crossed the boundary inward, and the change is endothermic. Squeezing an instant cold pack bursts an inner bubble of water so that it reaches a solid such as ammonium chloride. As the solid dissolves, its particles, ammonium ions and chloride ions, separate and spread through the water, and separating them takes in more energy than is given out when water molecules attach to them. The solution chills, and energy then flows from your warmer hand into the colder pack. Nothing called cold ever moves into your hand; your hand is simply losing energy to the pack.
[4]Why does one change give energy out and another take it in? Think of bonds as attractions that must be paid for. Pulling bonded atoms apart always costs energy, and forming a new bond always pays energy back. When forming the new bonds pays back more than breaking the old ones cost, the extra flows out to the surroundings and the reaction is exothermic. When breaking costs more than forming pays, the difference must come in from the surroundings, and the reaction is endothermic. This is why a match must be struck: the scrape pays to break the first bonds, and after that the reaction pays for itself and far more. Needing a push to start does not make a reaction endothermic. Only the net transfer, the overall balance once the whole reaction is counted, decides.
[5]Chemists write this balance into their equations. Energy is not made of atoms, so it never changes the atom count, but its position shows the direction of flow. For the iron pouch, energy belongs on the right, with the products: 4Fe + 3O₂ → 2Fe₂O₃ + energy. For the cold pack, energy belongs on the left, because it enters the system: NH₄Cl + energy → NH₄⁺ + Cl⁻. Read this way, an equation shows at a glance whether a process warms or cools its surroundings.
[6]Engineers turn that direction into a device and judge it by numbers. Suppose a team designing a pocket warmer for a ski patrol decides it must reach at least 42 °C and stay above 38 °C for three hours, and builds a first prototype: 20 g of iron powder in a thin cloth pouch. In a room at 22 °C it climbs to 47 °C in half an hour, but it drops below 38 °C after only 90 minutes. The team changes one thing: a thicker pouch that lets air in more slowly. The second prototype peaks lower, at 44 °C, but holds above 38 °C for four hours. Slowing the reaction met the time criterion, because the same amount of iron releases the same total energy whether it does so quickly or slowly.
[7]Energy is never created inside a hand warmer or destroyed inside a cold pack. It is moved and stored in new arrangements of atoms, and a careful accountant can always find it. The books always balance.
As you reread, draw a small arrow in the margin next to each example in the passage: an arrow pointing out of a circle for energy leaving a system, an arrow pointing in for energy entering. You should end up with arrows beside the iron pouch, the ration heater, the cold pack and the match. Questions 2, 3 and 8 can be answered by finding the right arrow and rereading the sentences around it.
Paragraph 1 calls the boundary a tool for thinking, so you can place it wherever it helps. Redraw the cold-pack story with the boundary around the whole pack, water and all, and then with the boundary around your entire hand plus the pack. For each choice, say which matter is now the system, which direction energy crosses the new boundary (if it crosses at all), and whether the word endothermic still applies. Write three or four sentences explaining why, with the second boundary, the energy moving from hand to pack no longer crosses the boundary at all (only energy from the air and the rest of your body does), and why that is still consistent with the passage's last paragraph.