Select your language

Suggested languages for you:
Log In Start studying!
StudySmarter - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
Free
|
|

All-in-one learning app

  • Flashcards
  • NotesNotes
  • ExplanationsExplanations
  • Study Planner
  • Textbook solutions
Start studying

Reversible Reaction

Save Save
Print Print
Edit Edit
Sign up to use all features for free. Sign up now
Reversible Reaction

Imagine a box filled with Lego bricks. You pick up a handful and start piecing them together to build a house. After a while, you stop and shake your head - building a chemical laboratory would be much cooler. You start taking your house apart, separating the bricks, and tossing them back into the pile. Soon, you are back where you started - faced with a big box of Lego, ready to be turned into another creation and used again and again. No matter what you build, you are always able to take the structure apart when you are finished and make something new. This is an example of a reversible reaction.

  • This article is about reversible reactions in chemistry.
  • We'll define reversible reactions and look at how you represent them.
  • We'll then explore some examples of reversible reactions.
  • After that, we'll learn about reversible reactions and equilibrium, and you'll be able to practice predicting the direction of a reversible reaction.

Reversible reaction definition

Some reactions only occur in one direction. The reactants react to form products, and that's it - the reaction is over. But in some cases, the products can react to form the reactants once again. We call these reactions reversible reactions.

A reversible reaction is a chemical reaction in which the reactants form products, which in turn can react to form the reactants once again.

You can look at a reversible reaction as being made up of two separate reactions:

  • The reaction in which the reactants react to make the products is known as the forward reaction.
  • The reaction in which the products react to make the reactants is down as the reverse reaction or the backward reaction.

Using our Lego analogy, combining the bricks together is like the forward reaction. We take the reactants, which are Lego bricks, and stack them up to build our product, the house. Taking the structure apart is like the backward reaction. We take the product, the house, and break it back down into the reactants, the Lego bricks. These two reactions combine to give one overall reversible reaction.

Reversible reaction symbol

So, we know that reversible reactions are made up of two separate reactions: the forward reaction and the backward reaction. Instead of writing both reactions out individually, we can combine them using two half-headed arrows to show a reversible reaction:

Here's an example. The reactants A and B react to form the product C. This is the forward reaction. C can then break down to give A and B again. This is the backward reaction. We can represent this reversible reaction using two separate equations, or we can combine them to give one overall equation:

Forward reaction:

Backward reaction:

Overall equation:

Reversible reaction example

Let's move on to look at some real-life examples of reversible reactions.

One visually interesting example is the hydration of cobalt(II) chloride. In its anhydrous form, it is blue. If you hydrate it, it turns pink. Evaporating the water off turns it back to its blue, anhydrous form once again. This makes cobalt(II) chloride a great test for water.

Forward reaction:

Backward reaction:

Overall equation:

Reversible Reaction hydrating cobalt chloride StudySmarterHydrating cobalt (II) chloride. Anna Brewer, StudySmarter Original

A biological example of a reversible reaction occurs in hemoglobin. Hemoglobin travels around in your blood, transporting oxygen from your lungs to your cells and waste carbon dioxide from your cells to your lungs. It does this by binding to the gas molecules with its four binding sites. You can think of the individual hemoglobin, oxygen and carbon dioxide molecules as the reactants and the bound hemoglobin molecule as the product. Once the bound hemoglobin molecule has reached its destination, it releases the gas molecules, breaking apart into the reactants once again.

Reversible reactions and equilibrium

If you leave the species involved in a reversible reaction in a closed system for a period of time, eventually, something particular will happen. If you start with high amounts of the reactants, initially you'll see lots of the forward reaction occurring. But as you produce more and more of the products, eventually the backward reaction starts to happen too. If you start with lots of the products, then the reverse happens - there'll initially be lots of the backward reaction, but as you produce more and more of the reactants, the forward reaction happens too. But no matter which side you start from, be it with lots of the reactants or lots of the products, eventually you'll reach a point of stability. Here, the rate of the forward reaction and the rate of the backward reaction is the same and the concentrations of reactants and products don't change. We call this a dynamic equilibrium.

You can learn more about dynamic equilibria in the self-titled article "Dynamic Equilibrium".

Under certain conditions, a dynamic equilibrium always has a certain ratio of reactants to products. It doesn't matter whether you start with lots of reactants or lots of products - provided you keep variables like temperature and concentration the same, you'll end up with the same equilibrium. We express the ratio of reactants to products in a system at equilibrium using the equilibrium constant, Keq.

There are many different types of equilibrium constant. For example, Kc is based on the ratio of concentrations at equilibrium whilst Kp measures partial pressure. You'll find out all about these constants in the article "Equilibrium Constant".

Predicting the direction of a reversible reaction

Finally, we can use our knowledge of reversible reactions and equilibria to predict the direction of a reversible reaction and its effect on the ratio of reactants to products.

  • If the rate of the forward reaction is greater than the rate of the backward reaction, then more of the reactants will be turned into products and there will be a net production of products.
  • If the rate of the backward reaction is greater than the rate of the forward reaction, then more of the products will be turned into reactants and there will be a net production of reactants.
  • If the rate of the forward reaction and the rate of the backward reaction are the same, then the system is at dynamic equilibrium and there is no net production of either products or reactants.

That's the end of this article. You should now feel confident defining reversible reactions and using the terms forward reaction and backward reaction. You should also be able to explain how a reversible reaction reaches equilibrium.

Reversible Reaction - Key takeaways

  • A reversible reaction is a chemical reaction in which the reactants form products, which in turn can react to form the reactants again.
  • Reversible reactions are made up of a forward reaction and a backward reaction and are represented by two half-headed arrows.
  • If you leave the species involved in a reversible reaction in a closed system, they'll eventually reach a dynamic equilibrium. Here, the rate of the forward reaction equals the rate of the backward reaction and the concentrations of reactants and products don't change.

Frequently Asked Questions about Reversible Reaction

An example of a reversible reaction is the evaporation of water from the surface of a sealed beaker. As some of the water evaporates, some of the water vapor in the beaker also condenses back into liquid form.

A reversible reaction is made up of two reactions: the forward reaction and the backward reaction. The forward reaction is the reaction that turns the reactants on the left into the products on the right of the equation. The backward reaction is the opposite reaction - it turns the products on the right to the reactants on the left.

To show a reversible reaction, we use two half-headed arrows: ⇌

A reversible reaction is made up from two separate reactions, with one being the reverse of the other. To write a reversible reaction, write out one of its constituent reactions as usual. However, instead of drawing a single arrow going from reactants to products, you show that it is reversible by drawing two half-headed arrows: ⇌ . 

For example, you might want to show the melting of ice. Liquid water can freeze back into solid ice, so this is a reversible reaction. You would show the reaction with the following equation: H2O(s) ⇌ H2O(l)

Yes - a reversible reaction is spontaneous in both directions. However, we can favor one reaction over the other by changing the conditions of the system. You'll learn more about this in the article Le Chatelier's Principle.

Final Reversible Reaction Quiz

Question

What is a reversible reaction?

Show answer

Answer

A reversible reaction is a chemical reaction in which the reactants form products, which in turn can react to form the reactants once again.

Show question

Question

Which of the following are reversible reactions?

Show answer

Answer

Dissolving a salt

Show question

Question

What is the name of the reaction that goes from reactants to products in a reversible reaction?

Show answer

Answer

The forward reaction

Show question

Question

What is the name of the reaction that goes from products to reactants in a reversible reaction?

Show answer

Answer

The backward reaction

Show question

Question

True or false? Reversible reactions always reach dynamic equilibrium.

Show answer

Answer

False

Show question

Question

Which of the following is true about a dynamic equilibrium?

Show answer

Answer

The rate of the forward reaction and the rate of the backward reaction is the same.

Show question

Question

Which of the following is true about a dynamic equilibrium?

Show answer

Answer

The concentrations of reactants and products stay the same.

Show question

Question

True or false? The ratio of reactants to products in a reversible reaction in a closed system under certain conditions is fixed. 

Show answer

Answer

True

Show question

Question

What is the equilibrium constant?

Show answer

Answer

A value expressing the ratio of reactants to products in a dynamic equilibrium.

Show question

Question

In a reversible reaction, there is a net production of products when ____.

Show answer

Answer

The rate of the forward reaction is greater than the rate of the backward reaction.

Show question

Question

In a reversible reaction, there is a net production of reactants when ____.

Show answer

Answer

The rate of the forward reaction is greater than the rate of the backward reaction.

Show question

Question

In a reversible reaction, there is no net production of either reactants or products when ____.

Show answer

Answer

The system is at dynamic equilibrium.

Show question

60%

of the users don't pass the Reversible Reaction quiz! Will you pass the quiz?

Start Quiz

Discover the right content for your subjects

No need to cheat if you have everything you need to succeed! Packed into one app!

Study Plan

Be perfectly prepared on time with an individual plan.

Quizzes

Test your knowledge with gamified quizzes.

Flashcards

Create and find flashcards in record time.

Notes

Create beautiful notes faster than ever before.

Study Sets

Have all your study materials in one place.

Documents

Upload unlimited documents and save them online.

Study Analytics

Identify your study strength and weaknesses.

Weekly Goals

Set individual study goals and earn points reaching them.

Smart Reminders

Stop procrastinating with our study reminders.

Rewards

Earn points, unlock badges and level up while studying.

Magic Marker

Create flashcards in notes completely automatically.

Smart Formatting

Create the most beautiful study materials using our templates.

Just Signed up?

Yes
No, I'll do it now

Sign up to highlight and take notes. It’s 100% free.