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 and vaporization. These reactions feel cold to the touch and have a positive enthalpy change (ΔH). In contrast, exothermic reactions release thermal energy, causing molecules to slow down and form bonds, resulting in condensation and freezing. They feel warm and have a negative enthalpy change (ΔH). Energy diagrams illustrate these processes, showing reactants at higher energy for exothermic reactions and lower energy for endothermic reactions.

Endothermic & Exothermic Reactions

Now, endothermic reactions involve absorbing thermal energy by the system from the surroundings. As a result of absorbing this thermal energy, the molecules will start to speed up, and if they are given enough energy, they can use it to break their bonds. So, endothermic reactions are heat-absorbing, bond-breaking reactions. If we look at it in terms of phase changes, when heat is absorbed, it helps to spread molecules apart. Imagine having a solid, for instance, an ice cube; the solid absorbs enough heat, or thermal energy, and what happens over time? It melts. Now it's a liquid. Let's continue adding a little more heat to that liquid water. What happens to it eventually? It vaporizes into a gas, breaking the connections between water molecules as it transitions from solid to liquid to gas. Going from a solid to a liquid is called melting, or fusion in terms of thermochemistry. If you're going from a liquid to a gas, that's vaporization, and if you're going from a solid directly to a gas, that's called sublimation.

In real-world applications, if you are observing an endothermic reaction that's absorbing heat from the surroundings, and if you were to touch a container containing an endothermic reaction, it would absorb heat from your hand. Consequently, the substance absorbs heat from your hand, making it feel cold to you because you're losing heat to the container.

Finally, energy diagrams are a way of showing how a reaction progresses in terms of energy. You start off with reactants and proceed to your final product. In an endothermic reaction, your enthalpy value, which is Δh, is positive. At the beginning of this energy diagram, our reactants are represented, and here is where you end, which represents our products. Here, our y-axis is energy. In an endothermic reaction, our reactants start off at a lower energy level, and our products end at a higher energy level. This results in a positive enthalpy or Δh.

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 endothermic. 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 in 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. So 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 then 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 they drop down to become products over time. The products have less energy. Because we're releasing energy to go from reacting to product, our enthalpy delta h 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 have 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 down. You're breaking bonds. So, that would be indicative of an endothermic reaction, not an exothermic reaction. Remember, exothermic is 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. For 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 option 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. This results in phase changes like melting and vaporization. These reactions feel cold to the touch and have a positive enthalpy change (ΔH). In contrast, exothermic reactions release thermal energy to the surroundings, causing molecules to slow down and form bonds. This leads to phase changes like condensation and freezing. Exothermic reactions feel warm to the touch and have a negative enthalpy change (ΔH). Energy diagrams illustrate these processes, showing reactants at higher energy for exothermic reactions and lower energy for endothermic reactions.

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. As the system absorbs thermal energy, the molecules within it speed up and break bonds, leading to phase changes such as melting or vaporization. Because the reaction is taking in heat, if you were to touch the container of an endothermic reaction, it would feel cold to the touch. This is because the reaction is drawing heat away from your hand.

What are some real-world examples of exothermic reactions?

Real-world examples of exothermic reactions include combustion reactions, such as burning wood or gasoline, which release a significant amount of heat and light. Another example is the reaction between quicklime (calcium oxide) and water, which releases heat and forms calcium hydroxide. Additionally, many oxidation reactions, such as rusting of iron, are exothermic. In biological systems, cellular respiration is an exothermic process that releases energy stored in glucose to power cellular activities.

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

Enthalpy change (ΔH) is a measure of the total heat content in a system and indicates whether a reaction absorbs or releases heat. In endothermic reactions, ΔH is positive, meaning the system absorbs heat from the surroundings. In exothermic reactions, ΔH is negative, indicating the system releases heat to the surroundings. Understanding ΔH helps predict the energy changes during a reaction, which is crucial for applications in thermochemistry, engineering, and various industrial processes. It also aids in understanding reaction spontaneity and equilibrium.

How do energy diagrams differ for endothermic and exothermic reactions?

Energy diagrams visually represent the energy changes during a chemical reaction. For endothermic reactions, the diagram shows reactants starting at a lower energy level and products at a higher energy level, indicating a positive enthalpy change (ΔH). The energy absorbed is depicted as an upward slope. In contrast, exothermic reactions have reactants at a higher energy level and products at a lower energy level, showing a negative ΔH. The energy released is depicted as a downward slope. These diagrams help in understanding the energy flow and stability of reactants and products.