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

Endothermic reactions are characterized by the absorption of thermal energy, resulting in molecules gaining enough energy to break bonds. This can be observed in phase changes such as melting (fusion), vaporization, and sublimation, where heat causes molecules to spread apart from solid to liquid to gas, respectively. These reactions feel cold to the touch due to the heat absorption from the surroundings. In contrast, exothermic reactions release thermal energy, causing molecules to slow down and form bonds, observable in condensation and freezing, or deposition in the case of substances like dry ice. Exothermic reactions feel warm as they release heat to the surroundings. On an energy diagram, endothermic reactions show a positive change in enthalpy (ΔH), with products at a higher energy level than reactants, while exothermic reactions depict a negative ΔH, indicating products with less energy than reactants.

$\Delta H$

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, if we look at it in terms of phase changes, if you're absorbing heat, it helps to spread molecules apart. Think about you have 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 bit more heat to that liquid water. What happens to it? Eventually it vaporizes into a gas. Here we're breaking the connections between water molecules as it goes from solid, 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 thermal chemistry, 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 ΔH, 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. 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 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 well. Actually, you're going from, yes, the liquid phase to the solid phase. Here you're not breaking bonds, you're forming them. So this would not be endothermic. 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 lost 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 those 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 right conditions, they 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 gashes 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, its exothermic, so it will 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 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, their products have less energy because we're releasing energy to go from reacting to product. Our enthalpy 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 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 CL₂ 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 is bond forming, not bond breaking.

Next 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. So remember, if you're subliming, you're going from a, let's see, you're going from a solid to a gas. But remember, we're talking about forming bonds. We want to go 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 D is the only option that makes the most sense.

Here’s what students ask on this topic:

What is an endothermic reaction?

An endothermic reaction is a chemical process in which the system absorbs energy from its surroundings in the form of heat. This means that the total enthalpy, or heat content, of the reacting system increases. During an endothermic reaction, the energy required to break the bonds in the reactants is greater than the energy released when new bonds form in the products. As a result, the reaction vessel or environment often feels cold to the touch, as heat is being taken in rather than given off.

A common example of an endothermic process is the reaction of barium hydroxide octahydrate with ammonium thiocyanate. When mixed, these chemicals absorb heat from their surroundings and become much colder, demonstrating the endothermic nature of the reaction. Photosynthesis in plants is another example of an endothermic reaction, where plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

What is an exothermic reaction?

An exothermic reaction is a chemical process that releases energy in the form of heat or light. In such reactions, the total energy of the products is less than the total energy of the reactants. This difference in energy is released to the surroundings. The energy is often transferred as heat, making the surroundings warmer, which is why exothermic reactions are commonly associated with warmth or even flames in cases like combustion. Examples of exothermic reactions include the burning of fuels, such as wood or gasoline, rusting of iron, and many metabolic processes that occur in living organisms. These reactions can be represented in chemical equations with heat as a product. For instance, when natural gas (methane) burns in a furnace, it combines with oxygen to produce carbon dioxide, water, and heat, which warms your house.

How is enthalpy used to predict whether a reaction is endothermic or exothermic?

Enthalpy, represented as $H$, is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.

A reaction is endothermic if it absorbs heat from its surroundings, which results in a positive change in enthalpy ($\mathrm{\Delta H}>0$). In an endothermic reaction, the enthalpy of the products is greater than the enthalpy of the reactants, indicating that energy is taken in during the reaction.

Conversely, a reaction is exothermic if it releases heat to its surroundings, leading to a negative change in enthalpy ($\mathrm{\Delta H}<0$). In an exothermic reaction, the enthalpy of the products is less than the enthalpy of the reactants, showing that energy is given off during the reaction.

To predict whether a reaction is endothermic or exothermic, you can calculate the change in enthalpy ($\mathrm{\Delta H}$) by subtracting the enthalpy of the reactants from the enthalpy of the products. The sign of $\mathrm{\Delta H}$ will indicate the nature of the reaction.

How can you tell if a reaction is endothermic or exothermic?

To determine if a reaction is endothermic or exothermic, you need to consider the flow of energy during the reaction. An exothermic reaction releases energy to the surroundings, usually in the form of heat, and can often be identified by a temperature increase in the reaction mixture or surroundings. Common signs of an exothermic reaction include the reaction container feeling warm or hot to the touch and the reaction emitting light or sound.

On the other hand, an endothermic reaction absorbs energy from the surroundings, which typically results in a temperature decrease in the reaction mixture or surroundings. This type of reaction might feel cold to the touch as it proceeds. Additionally, you can use a calorimeter to measure the heat absorbed or released by a reaction, which can provide a quantitative indication of whether the process is endothermic or exothermic.

Chemical equations can also give you clues: if the energy term is written on the products' side, it's exothermic; if it's on the reactants' side, it's endothermic. However, the most accurate method is to measure the heat change directly using calorimetry.