Video: Reversible and Irreversible Reactions

In this video, we will learn how to define reversibility in chemical reactions, identify examples of reversible and irreversible processes, and discuss how changing the conditions of a reaction can cause it to go in the reverse direction.

16:09

Video Transcript

We generally think of reactions occurring in one direction where the reactants combine to form the products. But we’ll see in this video how some reactions can go in the opposite direction where products can react to form reactants. In this case, the reaction is reversible. In this video, we will learn how to define reversibility in chemical reactions, identify examples of reversible and irreversible processes, and discuss how changing the conditions of a reaction can cause it to go in the reverse direction.

At the start of the 1800s, scientists thought reactions only went in one direction, which seems reasonable. After all, for a lot of processes, you can’t get things to react in the opposite direction. Imagine trying to uncombust a combustion reaction, like when methane burns in oxygen to form carbon dioxide and water. We’d have to force carbon dioxide and water to react to form methane and oxygen again. Impossible! But there was a French chemist named Claude Louis Berthollet that was studying a reaction where sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.

He took a trip to a salt lake where he made an interesting observation. There was sodium carbonate forming at the edge of the lake. He eventually realized that the large amount of salt in the salt lake or sodium chloride was reacting with the calcium carbonate that’s found in the shells of marine organisms. In other words, the reaction was going in the reverse direction of what he was studying in the lab. As it turns out, there’s many reactions that don’t just go in the forward direction where the reactants combine to form the products. There’s many reactions that can go in the reverse direction as well, where the products can combine to form the reactants.

When the reaction can only go in the forwards direction, like the combustion of methane, it’s called an irreversible reaction. But when the reaction goes both in the forward direction and the reverse direction, we call it a reversible reaction. We can indicate a reversible reaction in our chemical equation by using two single-headed arrows. So what makes it so reactions can sometimes go in the forward direction and other times go in the reverse direction like Berthollet saw with his reaction? Well, it turns out that changing the conditions that a reaction is reacting under can sometimes cause a reaction to reverse. To get a feel for this, let’s take a look at some classic examples of reversible reactions.

Anhydrous copper(II) sulfate is a white powder, but when it’s exposed to water, it forms a hydrated salt and turns blue. This makes it extremely useful as a test to identify if the substance is water. When you heat this blue hydrated copper(II) sulfate, it drives the water out of the salt, causing the reaction to go in the reverse direction, which gives us the white powder, the anhydrous copper(II) sulfate, once again.

NH4Cl or ammonium chloride is another white powder. When it’s heated, it decomposes, forming two colorless gases, ammonia and hydrogen chloride. If you perform this reaction in a stopper test tube, you’ll notice that as the reaction proceeds, a white powder will form at the cool end of the test tube. This white powder that’s forming is in fact ammonium chloride, the same substance we started off with. When the ammonia and hydrogen chloride gases cool down enough, they react with each other, causing the reaction to go in the reverse direction and form ammonium chloride again.

Now, this isn’t to suggest that all reactions go in the forward or reverse direction because we add or remove heat. There are plenty of other reversible reactions that occur due to changes in other kinds of conditions, like pH indicators. pH indicators are extremely useful in acid–base reaction experiments like titrations. We can add a few drops of indicator to our analyte in our titration experiment so that we can roughly monitor the pH of the solution. When we first add the pH indicator to our solution, it’s one color. But as we perform the titration experiment, in this case, adding a base to an acid, the pH will change. At a certain point when we’ve added enough of our base, the pH indicator will suddenly change color.

As it turns out, these pH indicators are large organic molecules that have two forms. In one form, the indicator has a hydrogen, and it’s one color. In the other form, the indicator has lost that hydrogen, and it’s another color. The color of the indicator changes depending on the concentration of hydrogen ions in the solution. When there’s more H+ ions in the solution, meaning that the solution is acidic, the indicator is one color. But as we add the base during this experiment, it will react with the H+ ions in the solution, causing the concentration of H+ ions to decrease, which will make the solution more basic, and the indicator will change color.

Since the pH is related to the concentration of hydrogen ions in the solution, we can use the color of the pH indicator to give us a rough estimate of the pH of the solution. The exact pH that this color change happens depends on the properties of the indicator. We have many different molecules that can act as indicators, meaning we have indicators that we can use for a variety of pH values.

Now, these examples shouldn’t suggest that for all reversible reactions, we have the forward reaction occurring in some conditions and the reversed reaction occurring in other conditions. Sometimes both the forward and reverse reactions are happening at the same time. This is common in reactions that involve weak acids and weak bases. For example, when we introduce the weak acid hydrofluoric acid to water, it will dissociate and form H+ ions and F− ions. But then the F− ions are capable of reacting with the H+ ions to form hydrofluoric acid. Then these newly formed molecules of hydrofluoric acid can dissociate to form H+ ions and F− ions. Well, some of the existing F− ions will again react with the hydrogen ions to form hydrofluoric acid.

In other words, the forward and the reverse reaction are occurring at the same time. When this happens, the forward and reverse reactions will continue to occur until the system reaches an equilibrium where the concentration of hydrofluoric acid, hydrogen ions, and F− ions are no longer changing. Now this equilibrium isn’t a static equilibrium where the rate of the forward reaction is zero and the rate of the backwards reaction is zero. Rather, both reactions are still occurring. Their rates are just equal to each other, so there’s no net change in the concentrations of our chemical species. This kind of equilibrium is called a dynamic equilibrium.

Now there’s one last topic to discuss before we practice what we’ve learned, and that’s the role of energy in reversible reactions. During the course of a chemical reaction, some or all of the bonds between the atoms in our reactants break apart so that the atoms can become rearranged to form the products. The process of breaking bonds requires energy. A process that must absorb energy is endothermic. Conversely, the process of making bonds is exothermic. It releases energy. When a reaction releases more energy than it absorbs, that reaction is exothermic, which corresponds to a negative change in enthalpy. But when a reaction absorbs more energy than it releases, the reaction is endothermic, which corresponds to a positive change in enthalpy.

Now what does this mean for reversible reactions? Well, let’s say that our forward reaction is endothermic, so the reaction would have a positive change in enthalpy. What would the reverse reaction be? When we look at our diagram, we can see that when we go from products to reactants, the reaction is endothermic because we have a positive change in enthalpy. But when we go the other way, forming the reactants from the products, the enthalpy change is negative, meaning that the reaction would be exothermic. This will always be true for reversible reactions. In one direction, the reaction will be endothermic, and in the other direction, the reaction will be exothermic.

However, when we look at the overall change in enthalpy for the reaction, that will be the same, only the sign will be different. In other words, if in the forward direction the enthalpy change is 100 joules, the enthalpy change will be equal to negative 100 joules in the reverse direction. So now we’ve discussed all we need to know about reversible and irreversible reactions, so let’s try some practice problems.

Which of the following statements about reversible reactions is true? (A) A reversible reaction is often a combustion reaction. (B) A reversible reaction is indicated by a single double-headed arrow in a chemical equation. (C) A reversible reaction is a chemical reaction that can proceed in both directions. (D) A reversible reaction always involves hydrated and anhydrous salts. (E) A reversible reaction is endothermic in both directions.

A reversible reaction is a reaction that not only goes in the forward direction where the reactants combine to form the products, but also the reverse direction where the products combine to form the reactants. We can indicate this in our chemical equation by using two single-headed arrows. With this information, answer choice (C) is clearly the correct answer. A reversible reaction is a chemical reaction that can proceed in both directions, the forward direction and the reverse direction. But let’s take a quick look through our other answer choices to see why they’re false.

Answer choice (A) says that a reversible reaction is often a combustion reaction. An example of a combustion reaction is the combustion of methane, where methane burns in oxygen to produce carbon dioxide and water. If this reaction were reversible, it would have to go in the reverse direction, where carbon dioxide reacts with water to form methane and oxygen. It’s hard to imagine this process happening. Since combustion reactions don’t typically go in the reverse direction, they’re not often reversible. Answer choice (B) says that reversible reactions are indicated by a single double-headed arrow. But we’ve seen how reversible reactions are indicated by two single-headed arrows in a chemical equation.

Answer choice (D) says that a reversible reaction always involves hydrated and anhydrous salts. There are multiple examples of reversible reactions that involve hydrated and anhydrous salts. For example, anhydrous copper(II) sulfate can react with water to form hydrated copper(II) sulfate. This reaction can be reversed when the hydrated copper(II) sulfate is heated. But there are plenty of examples of reversible reactions that don’t involve these hydrated salts, for instance, the reaction of any weak acid or base with water such as hydrofluoric acid. So reversible reactions sometimes involve hydrated and anhydrous salts, but not always.

Our final answer choice says that a reversible reaction is endothermic in both directions. Recall the endothermic means that the enthalpy change for the reaction is positive. This means that, overall, the reaction takes in energy. Looking at the sketch of an energy diagram, we can see that the forward reaction where A plus B reacts to form C plus D is endothermic since the change in enthalpy is positive. But what about the reverse direction where C plus D reacts to form A plus B? In this case, we would get a negative change in enthalpy, meaning that the reverse reaction is exothermic not endothermic. So a reversible reaction will be endothermic in one direction, but exothermic in the other direction.

So of the statements we looked at, the only one that was true about reversible reactions is that a reversible reaction can proceed in both directions.

What is the backwards reaction for the following reversible reaction? CoCl2 plus 6H2O in equilibrium with CoCl2 6H2O.

A reversible reaction is a reaction that can go in both directions. That means that not only can this reaction go in the forward direction, where the anhydrous cobalt(II) chloride reacts with water to form hydrated cobalt(II) chloride, but the reaction can also go backwards, which is what this question is asking us for. In the forward direction, like we’ve just seen, the reactants form the products. But in the backwards reaction, the products form the reactants. So the backwards reaction for this reaction would be our products, CoCl2 6H2O, reacting to form our reactants, the anhydrous cobalt(II) chloride and water.

Now let’s sum up everything we learned with the key points for this lesson. When a reaction goes in the forward direction, the reactants form the products. And when a reaction goes in the reverse direction, the products form the reactants. If a reaction can go in both the forward and reverse direction, it’s a reversible reaction, which we indicate with two single-headed arrows. But if a reaction can only go in the forward direction, it’s an irreversible reaction, which we can indicate in our chemical equation by using a single double-headed arrow. Changing the conditions that a reaction is occurring in can cause the reaction to reverse. Reversible reactions are always endothermic in one direction and exothermic the other direction.

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