Endothermic & Exothermic Reactions - Online Tutor, Practice Problems & Exam Prep

Endothermic reactions absorb thermal energy, causing molecules to speed up and break bonds, leading to phase changes like melting (fusion) and vaporization. These reactions feel cold to the touch due to heat absorption. In energy diagrams, reactants start at lower energy, and products end at higher energy, resulting in a positive enthalpy change (ΔH > 0). Conversely, exothermic reactions release heat, causing molecules to slow down and form bonds, leading to condensation and freezing. These reactions feel warm and have a negative enthalpy change (ΔH < 0), with reactants starting at higher energy and products at lower energy.

Endothermic & Exothermic Reactions

Now endothermic reactions involve absorbing thermal energy by the system from the surroundings. And we're going to say here, as a result of absorbing this thermal energy, the molecules will start to speed up, and if they're given enough energy, they can use it to break their bonds. So, endothermic reactions are heat absorbing, bond-breaking reactions. Here, if we take a look, in terms of phase changes, if you're absorbing heat it helps to spread molecules apart. Think about having a solid here; the solid absorbs enough heat, thermal energy; think of it as an ice cube. What happens to the ice cube over time? It melts. Now it's a liquid. Let's keep adding a little more heat to that liquid water. What happens to it eventually? It vaporizes into a gas. Here, we are breaking the connections between water molecules as it goes from solid, to liquid, to gas. Now if you're going from a solid to a liquid, that is melting or what we call fusion in terms of thermochemistry. If you're going from a liquid to a gas, that's vaporization, and then if you're going from a solid to a gas, that's called sublimation. Now, real-world applications. If you are an endothermic reaction, you're absorbing heat from the surroundings. Now, if I were to touch an endothermic reaction that's contained within a container, what would it do? Well, it would absorb heat from my hand. So, the substance is absorbing heat from my hand, so it would feel cold to me because I'm losing heat from my hand to the container. Finally, in terms of an energy diagram, energy diagrams are a way of showing how a reaction progresses in terms of energy. You start off as a reactant to get to your final product as your last stop. In an endothermic reaction, your enthalpy value which is delta h is positive. It's a positive sign. Here, the beginning part of this energy diagram is represented as our reactants, and then here's where you end. This represents our products. Here, our y-axis is energy. So in an endothermic reaction, our reactants start off at a lower energy and our products end at a higher energy. This gives us a positive enthalpy or delta h.

Endothermic & Exothermic Reactions Example 1

Here it says, which of the following processes represents an endothermic reaction? Remember, endothermic reactions are heat absorbing bond-breaking reactions. So here we have steam condensing. If you're condensing, you're going from a gas to a liquid. Here you're not breaking bonds, you're forming them. So, this is out. Next, molten lava solidifying. Solidifying means you're going from a liquid to a solid. Again, you're not breaking bonds. Water boiling. If you're boiling, that means you're vaporizing your liquid water into a gas. You're breaking bonds here, so this is endothermic. And then finally, we have water freezing. If you're freezing you're going from the liquid phase to the solid phase. Here, you're not breaking bonds, you're forming them. So this would not be an endothermic reaction. So out of all the options, only option C represents an endothermic reaction.

Endothermic & Exothermic Reactions

Exothermic reactions involve releasing thermal energy by the system to the surroundings. Now we're going to say as molecules in our system release heat, they slow down and with enough energy loss, they form bonds. So, exothermic reactions are heat-releasing bond-forming reactions. We're going to say in terms of phase changes, let's think of it as energetic gas molecules. As we release heat, both water molecules, they're going to come closer together. So think of liquid, gaseous water bouncing all over the place in a container. They slowly start to release their heat and they slow down because they're losing energy. If they slow down enough, they condense into a liquid. So this is condensation. If the liquid water that we've collected, we put it in the freezer, they will continue to lose heat and they will solidify. So liquid to solid is freezing. Some substances, under the right conditions, can skip the liquid phase altogether. One example is carbon dioxide. We call it dry ice. You could take it out of a very cold container where it's in its gaseous phase, and put it outside. It'll skip the liquid phase altogether. So if you're going from a gas to, if you're going from a gas to a solid though, where we're taking that gaseous carbon dioxide and putting it back into the container where it solidifies again, that's called deposition. Now let's think about it. Let's say a container has a liquid and that liquid is an exothermic reaction. It's exothermic, so it'll be releasing heat. So if I were to touch that container, I would feel the heat that it's releasing to my hand. Exothermic reactions feel warm to the touch. Now if we were to think of it in terms of an energy diagram, in an energy diagram our y-axis is our energy, and the x-axis is the progress of the reaction. Remember the whole point of a chemical reaction is to go from reactants to products. And what we need to realize is that in an exothermic reaction, the reactants have more energy. They release their excess energy and, over time, they drop down to become products. The products have less energy. Because we're releasing energy to go from reactant to product, our enthalpy delta ΔH would have a negative sign. So, just remember, this is the way we depict an exothermic reaction in terms of an energy diagram.

Endothermic & Exothermic Reactions Example 2

Here, we need to determine which of the following is an exothermic reaction. For option D, we're going to see steam condensing. So just make sure you see steam condensing there. Now it says, if we look at the options, we have CO₂ burning. Now, burning usually connotes that we're breaking something down. You know, you burn a piece of wood, it breaks it down. You're breaking bonds. So that would be indicative of an endothermic reaction, not an exothermic reaction. Remember, exothermic reactions are bond-forming, not bond-breaking.

Next, the reaction in a cold pack. Well, a cold pack feels cold to the touch, but we said earlier that exothermic reactions, which release heat, feel warm to the touch. So, a hot pack or heating pack would be an exothermic reaction. Dry ice subliming, remember, if you're subliming, you're going from a solid to a gas. But remember, we're talking about forming bonds, we want to do deposition, where we're going from a gas to a solid.

The only one that makes sense in terms of an exothermic process is steam condensing. We're going from a gas to a liquid, so we're forming bonds. So remember, exothermic reactions are bond-forming, heat-releasing reactions, meaning option D is the only one that makes the most sense.

Here’s what students ask on this topic:

What is the difference between endothermic and exothermic reactions?

Endothermic reactions absorb thermal energy from the surroundings, causing molecules to speed up and break bonds. These reactions feel cold to the touch and have a positive enthalpy change (ΔH > 0). Examples include melting (fusion) and vaporization. In contrast, exothermic reactions release thermal energy to the surroundings, causing molecules to slow down and form bonds. These reactions feel warm to the touch and have a negative enthalpy change (ΔH < 0). Examples include condensation and freezing.

How do endothermic reactions affect the temperature of their surroundings?

Endothermic reactions absorb heat from their surroundings, which causes the temperature of the surroundings to decrease. This is because the system takes in thermal energy, leading to a cooling effect. For example, when an ice pack is activated, it absorbs heat from the injured area, making it feel cold. This absorption of heat is a characteristic feature of endothermic reactions.

What are some real-world examples of exothermic reactions?

Real-world examples of exothermic reactions include combustion (burning of fuels), respiration (glucose reacting with oxygen in cells), and the setting of cement. In these reactions, energy is released in the form of heat, making the surroundings warmer. For instance, when wood burns in a fireplace, it releases heat, making the room warmer. Similarly, hand warmers use exothermic reactions to generate heat and keep hands warm in cold weather.

What is the significance of enthalpy change (ΔH) in chemical reactions?

Enthalpy change (ΔH) indicates the amount of heat absorbed or released during a chemical reaction. In endothermic reactions, ΔH is positive, meaning the system absorbs heat from the surroundings. In exothermic reactions, ΔH is negative, indicating that the system releases heat to the surroundings. Understanding ΔH helps predict the energy changes and temperature effects associated with chemical reactions, which is crucial for applications in thermochemistry and various industrial processes.

How can you identify an endothermic reaction using an energy diagram?

In an energy diagram, an endothermic reaction is identified by the reactants starting at a lower energy level and the products ending at a higher energy level. This upward slope indicates that energy is absorbed during the reaction. The enthalpy change (ΔH) for an endothermic reaction is positive, reflecting the energy absorbed from the surroundings. The y-axis of the diagram represents energy, while the x-axis represents the progress of the reaction.