Basic Properties of Chemical Reactions

What Are Chemical Reactions?

Think back to childhood. Didn't glo sticks and firecrackers just seem like magic? Glo sticks somehow produce light without any fire and firecrackers went from those tiny little paper tubes to dazzling lights and sound in the sky. Although these products may seem to defy nature, they actually can be explained by chemical reactions.

A chemical reaction is a process in which molecules interact to form new substances. Now, to make it easier to talk about chemical reactions, scientists will use symbols. Here's a basic representation of a chemical reaction: A + B -- > C + D.

The actual glo stick and firecracker reactions are kind of complicated, so let's just consider a simple example to begin with. Let's talk about the neutralization of acids and bases.

Reactants interact and recombine to form new molecules

Neutralizing Acids and Bases

So let's consider the neutralization between hydrochloric acid (HCl) and sodium hydroxide (NaOH). Remember that hydrochloric acid and sodium hydroxide are a strong acid and a strong base. So, what does that mean? So what that means is that both of them are going to dissociate completely. HCl, because it's a strong acid, is going to donate hydrogen ions into solution. That means it's also going to produce chlorine ions. Sodium hydroxide is going to produce hydroxide ions and sodium ions.

Now, these things to the left of the arrow generically appear as A and B, and down here as these four ions, are called the reactants (H+ + Cl- + Na+ + OH-). They are the things that are going to interact with each other and recombine in different ways. The arrow signifies that a reaction is taking place. That means that whatever's on the right side of the arrow are the products of the chemical reaction.

All right, so what are the products of this chemical reaction? How do you think that these reactants can recombine? Well, do these two ions (Cl- and Na+) look familiar to you? That could form NaCl, otherwise known as table salt.

So that just leaves two ions that are left (H+ and OH-) that could potentially interact with each other. What could those two ions form? Well, if you add H+ and OH- together, you actually end up with water (H20). So this reaction is considered a neutralization reaction because the H+ ions and the OH- ions have consumed each other to produce water. That means that there are less H+ ions in solution, which raises the pH and brings it closer to seven.

Exergonic reactions occur spontaneously without having to input energy

Endergonic and Exergonic Reactions

Ok, great. But doesn't that just raise the question of why reactions happen? I mean, why not the opposite? Why can't I make hydrochloric acid and sodium hydroxide from table salt and water?

Let's consider a boulder on a hill. The height of the hill is a measure of the free energy in the system. The boulder has potential energy. The amount of energy that the boulder has is dependent on how high the hill is. Now, if the boulder is just placed here at the edge of the hill, it's just going to naturally roll down the hill, releasing the potential energy that it had stored up.

If we think about this situation in terms of a chemical reaction, it is a spontaneous reaction which will occur without any extra input of energy. The glo stick is an example of an exergonic reaction. As soon as you crack that inner compartment, the chemicals mix and the glo stick spontaneously produces light.

On the other hand, if the boulder was at the bottom of the hill, you would actually have to expend energy to push it up to the top of the hill. If free energy has to be added for a chemical reaction to proceed, the reaction is called endergonic. This means that the reaction will not occur spontaneously.

Endergonic reactions require an outside energy contribution before they can take place

A firecracker is a good example of an endergonic reaction. Nothing's going to happen to that firecracker until you light the fuse. Unless you provide a spark, the firecracker isn't going to explode.

Now, the hydrochloric acid and sodium hydroxide example is simple because the reaction is strongly driven to produce salt and water. But what happens if it's not so clear cut?

Chemical Equilibrium

Most biological reactions exist in a state of chemical equilibrium, where both reactants and products exist simultaneously because both forward and reverse reactions are competing. All right, so that sounded complicated, but consider this simple example. I can have one reaction where AB + C produces new products AC and B. But at the same time, I could also write a chemical reaction where AC + B produces AB and C.

These seem to be in competition with each other. And, in fact, I could rewrite these two equations into one simple equation (AB + C <--> AC + B), where the reactants on one side are actually the products for the other side. Now, to represent this idea that the chemical reaction can proceed in both directions, both to the left and to the right, we redraw the arrows so that the arrows are pointing in both directions.

On the surface, incomplete reactions seem like a super inefficient thing, right? But our bodies actually wouldn't function properly if all reactions were irreversible. Consider how the body transports carbon dioxide. Carbon dioxide (CO2) plus water (H2O) can produce carbonic acid (H2CO3). And carbonic acid can break down into an H+ ion and an HCO3- ion, also known as a bicarbonate ion.

Chemical equilibrium is when chemical reactions can occur in both directions

A bicarbonate ion is easier for the body to transport. Remember that ions dissolve in water easily. Recall how sodium chloride was a solid crystal until it's placed in water. Once it's placed in water, those salt crystals quickly dissolve and turn into ions which are dissolved in solution.

Similarly, the body can dissolve the bicarbonate ion in blood and then transport it to the lungs where it can be retransformed back into carbon dioxide gas, which we can exhale.

Lesson Summary

In summary, a chemical reaction is a process in which molecules interact to form new substances. Chemical reactions can be written as equations.

However, most chemical reactions in biology work in opposition to their counterpart reaction. This is called chemical equilibrium. Although reactants and products exist simultaneously in chemical equilibrium, altering the characteristics of the environment can change the proportions at which they exist.

An exergonic reaction is a chemical reaction that will occur spontaneously. An endergonic reaction is a chemical reaction that will not occur spontaneously.