Topic 6.1: Endothermic and Exothermic Processes

Endothermic or exothermic? The snow monkeys’ hot tub

This is Jigokudani, a valley up in the mountains of Nagano, in Japan, and its name means Hell Valley, because of its steep cliffs and the hot water that bursts out of the ground. In winter it can drop to minus ten degrees Celsius up here, and the snow piles up more than a meter deep, so this monkey has done what any of us would do, and gotten into the hot tub.

According to the park, it all started with one very young monkey, who one day wandered into the outdoor bath at an inn here in the valley. The other monkeys watched, and little by little they started doing it too, until the park built the monkeys a hot spring bath of their very own, so that monkeys and people wouldn’t be sharing the same water.

Now look at everything that’s going on in this one picture. Snowflakes land on the water and vanish, while warm water sits right underneath freezing air. And that white cloud drifting off the pool is made of millions of tiny drops of liquid water, because the water vapor itself is invisible. And when each of those drops formed, do you think it took energy from the freezing air, or gave energy to it?

So where is all of this energy really going, and how does it get there? By the end of this lesson, you’ll be able to look at this scene and say which way every bit of energy is flowing, and draw it the way a chemist would, and explain how the heat actually gets from this water into a snowflake, one molecular collision at a time.

So let’s start with the smallest thing in the picture, which is one single snowflake. Watch this one come all the way down and land on the water. Let’s draw a line around it. Chemists call whatever’s inside the line the system, and everything else, here the water and the air, is called the surroundings.

To melt, the snowflake needs energy, and it takes that energy from the warm water that’s touching it. When energy flows into the system like this, we call the process endothermic. In real life you can’t see energy at all, but you could notice the water right around the flake getting a tiny bit colder, and a change in temperature is the evidence that energy is moving.

Now let’s move the line and draw it around that little bit of water instead. The water gave some of its energy away, so for this system the energy flows out, and that makes it exothermic. Nothing about the event itself changed, because it’s the same snowflake melting into the same water, and all we did was move the line, which is why you always have to know what your system is before you call anything endothermic or exothermic, and when a question just names a change, like melting, the system is the stuff that’s changing.

Okay, now you try one. That mist is water vapor turning into tiny liquid drops right now, so set the arrow to show which way the energy goes for the drops.

Let’s put that on the board. Endothermic means energy flows into the system, and exothermic means it flows out to the surroundings, and in a lab, the evidence for either one is almost always a temperature change. The same idea covers heating and cooling and every phase change, which all come in pairs, because warming, melting and evaporating take energy in, while cooling, freezing and condensing give that same energy back out, so whenever you run a process backward, endothermic turns into exothermic, and exothermic turns into endothermic.

And this exact setup is a classic AP question. On the 2023 exam, a student mixed calcium carbonate with hydrochloric acid in a coffee-cup calorimeter and recorded the temperature every twenty seconds, and the question simply asked whether the reaction is endothermic or exothermic, and told you to justify your answer using the table. So let’s read the table. The solution starts at twenty-one point two zero degrees and climbs to twenty-one point nine zero, so its temperature went up by about seven tenths of a degree.

The solution is the surroundings here, and it got warmer, so the reaction must have released energy into it. That makes the reaction exothermic, and your answer has to point at what the table shows, the temperature going up, because the question told you to use it. Writing exothermic because it releases heat, without mentioning the temperature increase, skips what the question asked for, so say what the data shows, that the solution’s temperature increased, and then what that means.

Hand warmers: reactions that give energy away

Melting and condensing are physical changes, because the water stays water the whole time. Chemical reactions move energy around too, and you can carry one of them in your pocket. This is a disposable hand warmer, the kind they call a kairo in Japan. Inside it is mostly iron powder, along with some salt, a little water held in a crumbly mineral, and some charcoal.

It sits sealed in an airtight wrapper until you tear it open, and then oxygen from the air seeps in through the fabric and reaches the iron. What happens next is that the iron rusts, just much faster than a nail would, as the iron and the oxygen combine to make iron oxide.

The iron oxide ends up with less energy than the iron and oxygen had to begin with, and the difference flows out through the pouch, into your glove and into your hand. So rusting in a hand warmer is an exothermic reaction, and the energy the iron and oxygen lose is exactly the energy their surroundings gain.

That’s true of every reaction, because energy never just appears or vanishes, and it works the other way around too, so whatever a system gains, its surroundings lose. In the hand warmer, the energy leaves as heat. It can also leave as work, which means pushing on something, like the gas from an airbag’s reaction shoving the bag open.

Reactions can go the other way, too. When you pour baking soda into vinegar, besides all the fizzing, the cup gets noticeably colder, which tells you the reaction is taking energy from the water around it, so it’s endothermic.

And a reaction can also end up with the same energy it started with, so its energy change is zero, and there’s no net flow of energy in or out. Here’s one the way the AP exam likes to ask it. Two solutions, both at twenty-two degrees, get mixed in a foam cup, and the temperature drops to eighteen point five. Pick the statement that gets it right.

Hot packs and cold packs: why dissolving can go either way

Some packs work without any reaction at all, because all they do is dissolve something in water, and dissolving can go either way. Here are two white salts, calcium chloride on the left and ammonium nitrate on the right, each sealed in a pack with its own little bag of water, and both packs start out at exactly the same temperature. Dissolving means pulling the solid apart into separate particles, so before we squeeze them, what do you think happens to each pack’s temperature?

Let’s squeeze both packs and break the water bags. The calcium chloride pack heats up fast, while the ammonium nitrate pack goes cold, which is why packs like these get sold as instant hot packs and instant cold packs.

So how can dissolving do two opposite things? Let’s go into the hot pack and look. To dissolve, the ions have to come out of the crystal, and since opposite charges attract each other, pulling them apart costs energy.

The water molecules have to move apart to make room, and they attract each other too, so that costs energy as well. Then the water molecules swing around and crowd in close to each ion, with their oxygen side toward the calcium and their hydrogen side toward the chloride, and new attractions forming like that release energy.

For calcium chloride, the energy released by those new attractions is bigger than what it cost to pull everything apart, so there’s energy left over, it goes into the motion of the water, and the pack gets hot. The cold pack goes through the very same steps, with ammonium ions and nitrate ions coming out of their crystal.

Now you try the cold pack. The cost bar is fixed, so drag the release bar to wherever it has to be to make this pack colder.

So dissolving always comes down to one comparison. Pulling the solute apart and pushing the water apart costs energy, the new attractions between them release energy, and the only question is which one is bigger.

A lot of people think breaking the solid apart is what releases the energy, when in fact pulling attracting particles apart always costs energy, and energy only comes out when new attractions form. And sometimes the two come out so close that the energy barely changes at all, which is what happens with table salt, where the water’s temperature hardly moves.