Reversible and Nonreversible Reactions — Overview

We're beginning to study equilibrium! I'm so excited! In college, I studied environmental chemistry. The environment has so many equilibrium reactions. Environmental chemists use equilibrium reactions to track pollutants, model climate change, and measure ocean acidification. Once you learn about equilibrium, the whole world opens up!

The Foundation: Reversible and Nonreversible Reactions

Some chemical reactions are reversible. A reversible chemical reaction is when the products transition back into reactants. How is that possible?! Let's consider collision theory. First, atoms or molecules need to collide. But, they need to hit with the correct orientation. Also, they must strike with enough energy. That's the activation energy.

Consider an energy diagram. Some chemical reactions have minuscule activation energies. So, there is a small hill between the reactants and products. But, there is plenty of energy at standard temperature and pressure. So, they can undergo a reverse reaction. Reactions with enormous activation energies are nonreversible.

Nonreversible Reactions

Combustion is the most common nonreversible reaction, also known as irreversible reactions. There are a few reasons it can't reverse. First, one of the products is carbon dioxide gas. Gases diffuse. As they diffuse, there are fewer collisions. So, it is unlikely for the reverse reaction to occur. Second, combustion is very exothermic. It releases energy by transferring heat to its surroundings. How would we create the reverse reaction? We'd have to put an equal amount of energy back into the system. Finally, combustion increases entropy. It produces more moles of product. So, the energy is more dispersed.

So, we've considered enthalpy and entropy. What about Gibb's free energy? We see that nonreversible reactions have significant, negative free energy changes. They are spontaneous.

Other examples of nonreversible reactions:

• Neutralization reaction between strong acids and bases.

• Formation of many ionic compounds.

• Ionic bonds are strong because of their electrostatic interactions. So, the bond energies are considerable. We see this from their enthalpies of formation.

• These could be single or double replacement reactions. But, ionic bonds must form.

• Some environmental redox reactions. For example, the rusting of iron metal.

Reversible Reactions

Reversible reactions are surprisingly common. Let me show you an acid-base reaction that happens in the environment.

$CO_{2(g)}+H_{2}O_{(l)} \rightleftharpoons HCO_{3(aq)}^{-} + H_{(aq)}^{+} \rightleftharpoons CO_{3(aq)}^{2-} + H_{(aq)}^{+}$.

Here is the reaction between carbon dioxide and water. You'll notice that it produces acid, $H^{+}$. It's the reason for ocean acidification. It's also why carbonated sodas are acidic and damage your teeth. By combusting carbon fuels, we increase the partial pressure of atmospheric carbon dioxide. Henry's law taught us that increasing the partial pressure increases solubility. So, we add more carbon dioxide to our oceans. Sadly, that creates more acid.

Notice each reaction has double arrows. That means there is a forward and reverse reaction. What if we could remove carbon dioxide from our atmosphere? It would reverse all the reactions. Our oceans would become less acidic! Removing carbon dioxide is carbon sequestration. Some companies have the technology to remove carbon dioxide from the air. Unfortunately, the scope is quite limited.

Other Examples of Reversible Reactions:

• Redox reactions in voltaic cells (batteries).

• Dissociations of weak acids and bases.

• Synthesis, single replacement, double replacement, and decomposition reactions of covalent molecules.

• Many biochemical reactions are reversible.

• One example is hemoglobin. It binds oxygen as a ligand. Then it deposits oxygen into your cells. Next, it returns to the lung. There it picks up more oxygen.

(Video) Reversible and Nonreversible Reactions

By Bozeman Science

In this video, Mr. Anderson explains that a nonreversible reaction as a reaction where reactants are converted to products but do not go back to reactants. On the other hand, a reaction in which reactants can form products while the products can break back down to reactants is called a reversible reaction. Mr. Anderson completes the lesson about reversible reactions with more examples from physical, chemical, biological, and environmental prospects.

Reversible and Nonreversible Reactions: Difference between reversible and nonreversible reactions

Be aware: Forward reaction corresponds to reactants converting to products. Backward reaction corresponds to products converting back to reactants. In a reversible reaction, both reactants and products are never fully consumed.