Topic 6.2: Energy Diagrams

Energy diagrams: drawing where the energy went

The packs’ temperatures told us that something changed, so now let’s keep track of where the energy actually went, with a picture. Let’s put the hot pack’s starting point, the solid and the water, on one shelf, and the solution it turns into on another. The solution has less energy, so its shelf settles lower, and the energy in between is what went into the water’s motion.

Now you place one. Drag the cold pack’s solution shelf to wherever you think it belongs. The shelf went up, and the pack got colder, and both of those things are true at the same time. The energy it took to lift that shelf came out of the water’s own motion, so the water slowed down and its temperature dropped.

Chemists draw this as an energy diagram. Energy goes up the side, and across the bottom we just go from before to after, which isn’t a clock, just the direction the process goes. For the hot pack, we draw solid calcium chloride and water at the start, and the dissolved ions eighty-one point three kilojoules lower, for every mole of calcium chloride that dissolves.

To get the change, we always take the final level minus the initial level, and since the final level is lower, the change comes out negative, minus eighty-one point three kilojoules per mole, and a negative change means exothermic. When we measure this change as heat at constant pressure, the way these packs work out in the open air, chemists call it delta H, and you’ll calculate plenty of those in the next few lessons.

The cold pack is the one you just placed. Ammonium nitrate’s solution ends up twenty-five point seven kilojoules per mole higher, so the change is positive, and that means endothermic. This is the diagram that trips people up, because the line goes up while the pack feels cold. The height is stored energy, and the temperature is the water’s motion, which went down.

Energy diagrams make reverse processes easy, because you read the same two levels the other way. The hot pack’s change is minus eighty-one point three kilojoules per mole, so what’s the change for the exact reverse, with the ions coming back out of the water to form the solid?

Energy diagrams work for physical changes too. For our snowflake, the ice starts low and the liquid water ends up higher, because melting took energy in, even though, while it melts, its temperature stays right at zero degrees the whole time.

You might remember diagrams with a hump in the middle from reaction rates. The hump is about how fast something happens, and whether it’s endothermic or exothermic depends only on where the two ends sit. Now the mist over the pool. Which energy diagram matches the vapor turning into those tiny drops?