Exothermic Reactions

Combustion reactions are chemical exothermic reactions that occur between a fuel and an oxidant. They generate heat that powers up our homes, our cars, planes, and even rockets. They also produce light. We often perform combustion reactions in our everyday lives ourselves. 

Wood is a good fuel, because it’s made up of organic, carbon-containing substances. Burning of wood is actually the oldest combustion reaction that humans have ever performed, and wood is the oldest fuel ever used.

When wood burns, its substances rapidly react with the oxygen from the air, releasing a lot of energy in the form of heat and light. The reaction mixture, together with the visible, UV, and IR radiation emitted is known as the flame. 

As the fire burns, it also releases smoke, which contains the products of the reaction: carbon dioxide, other oxides, water vapor, and solid particulates that are dispersed in the air. The particulates give the fire its characteristic red-orange color because they’re incandescent. 

But, what do we really mean when we say that an exothermic reaction generates heat, and what exactly is heat? To see if we can better understand this, let’s take a closer look at what happens when we put a kettle with water above our campfire.

Because we are looking at the energy changes during the combustion reaction, our system will be the reaction mixture and the products of the reaction. Everything else is the surroundings.

As you can see, our fire is an open system. There is no physical boundary that prevents energy and matter from flowing in or out into the surroundings. This is why the gaseous products of the reaction can escape as smoke.

As the combustion reaction is taking place, energy is “produced” in the reaction mixture. Because of this, the products are very high in energy, and they are moving around very rapidly in all directions of space. This is known as thermal motion. 

As the products of the reaction collide with molecules in the surroundings, for example the gases in the air or the kettle’s iron atoms, they transfer some of their energy to them. This new energy increases the speed of the thermal motion of the molecules in the surroundings, vaporizing the carbon-containing molecules in the fuel.

Water in the kettle boils and the temperature of the air close to the fire increases. Also, collisions with the molecules in the surroundings cause the speed of the thermal motion and the temperature to decrease. Smoke with particulates that are no longer incandescent leave the reaction mixture.

The energy produced during the combustion of wood was transferred from our system to the surroundings through thermal motion. This is what we really mean when we say that an exothermic reaction generates heat.

We know from the first law of thermodynamics that energy can’t be created or destroyed. It can only be transformed from one form to another. So, the energy “produced” during a chemical reaction has to come from somewhere. It must have been present in the system as some other kind of energy before the reaction took place. 

When atoms are on their own, they are a little unstable, and this means that they have higher energy. When they come together to form molecules, they become more stable and the excess energy is released. So, bond formation is an exothermic process.

If we want to break that bond, we have to add the same amount of energy that was released when the bond was formed. Bond breaking is an endothermic process. The amount of energy that is released when the bond is formed is the bond strength, or the potential energy.

When we burn natural gas, which is primarily methane, we have to first break the double bond in oxygen and the C-H bonds in methane. We have to add some energy by bringing a match close to the gas source. 

When the C=O and O-H bonds in the products, carbon dioxide and water are formed, and energy equal to the potential energy of these bonds is released. In the oxidation of methane, the energy released in forming the bonds of the products is greater than the energy absorbed in order to break the bonds of the reactants. 

Some of this extra energy is used to break the bonds in new molecules of the reactants that haven’t reacted yet. Some of it is also transferred to the surroundings through thermal motion as heat. 

Natural gas burns with a blue flame. The reason is that it normally combusts completely, producing only water and carbon dioxide. So, there are no incandescent particulates to color the flame yellow, orange, or red.

Heat is one way of transferring energy from the system to the surroundings. But, there is another way that energy can be transferred, and this is through work. If an exothermic reaction can release the energy stored in the chemical bonds of the reactants in the form of heat, then, under the right circumstances, this energy can also be transferred to the surroundings as work. 

Heat and work are very similar, in that they are related to the kinetic energy of the particles in the system. Kinetic energy is the energy associated with movement. 

By applying a force in a certain direction on the surroundings, this force can be used to move an object.  A rocket engine converts chemical energy into kinetic energy (and heat).

One of the most commonly used rocket fuels is a solid mixture of aluminum powder (the fuel) and ammonium perchlorate (which is both an oxidizer, and also a fuel), together with a small amount of iron (III) catalyst.

When aluminum and ammonium perchlorate react, the energy stored in their chemical bonds is released. This increases the thermal motion of the molecules in the reaction mixture, but they can’t transfer this excess energy as heat, so they keep moving very rapidly in all directions, hitting the walls of the combustion chamber, and increasing the pressure.

The reaction also produces a very large volume of combustion gases (water, nitrogen oxide), and particulates of aluminum chloride and aluminum oxide that are dispersed in the reaction chamber. This increases the pressure in the reaction chamber even more.

As the reaction proceeds, the temperature keeps increasing and the pressure keeps building up, but the gases can only escape in one direction, through the nozzle. Finally the exhaust gases escape through the nozzle very quickly, with great force, forming thick clouds of white powder and water vapor around the rocket at lift-off. 

According to Newton’s third law, the force with which the gases escape the combustion chamber will be accompanied by a force of equal strength acting in the opposite direction. This is the thrust that moves the rocket forward.

An explosion is a sudden, violent event where the system transfers energy to its surroundings, usually through a rapid increase in pressure and volume, by producing huge amounts of gas. This creates a region of increased pressure that spreads out into the surroundings, known as a shock wave, which causes the characteristic “bang” of the explosion.

During the exothermic bond formation in common rocket fuel, hydrogen and oxygen atoms come together to form the O-H bonds in water, and since one water molecule has 2 such bonds, energy equal to roughly twice the potential energy of the O-H bond is given off. 

Even though it generates a lot of heat, the combustion of hydrogen isn’t usually explosive. If we pop the balloon and ignite the hydrogen, it’ll burn, but it won’t explode. 

The reason for this is that when we ignite the hydrogen gas, the hydrogen molecules can only interact with the oxygen molecules that were near to the surface of the balloon.

It takes time for every hydrogen molecule to diffuse through the surrounding air to find 2 oxygen molecules to react with. The reaction is slower and the energy gradually dissipates in the environment during the course of the reaction.

Here we have another balloon where we’ve mixed hydrogen and oxygen in their stoichiometric ratio in water (2H:1O).When this balloon is popped, and the mixture is ignited, all of the hydrogen molecules can find an oxygen atom nearby, and they all react at the same time. 

The reaction happens very quickly, and all of the energy and the products are released at once. This is enough to create the sudden pressure increase in the reaction mixture required for a shock wave to form around it and spread out. 

Because all of the energy is released at once, the flame is much hotter, which makes it white, rather than yellow. In addition, more light is emitted in this reaction.

There is another class of exothermic reactions that are famous for being even more explosive than combustion reactions. Known as decomposition reactions, they occur when a compound is broken down into simpler compounds, or even into elements. This is the case with the famous molecule trinitrotoluene (better known as TNT). 

This molecule, and others like it, is so explosive because it contains both carbon and hydrogen, and oxygen and nitrogen within its molecular structure. So, in a way, the explosive decomposition of TNT can be considered as a more extreme case of combustion, where the oxidant (the nitro groups) is bound directly to the fuel (toluene).

When TNT decomposes, the reaction proceeds very rapidly because the participants in the reaction are already close and interacting with each other. Besides the speed of the reaction, the decomposition of TNT also generates a lot of energy and a large volume of gas. 

When one molecule of TNT decomposes, it gives off several molecules of nitrogen gas, carbon monoxide, and water vapor as well as solid particulates of carbon dispersed within the reaction mixture. 

TNT decomposition results in a very large increase in volume. Furthermore, the nitrogen molecule and carbon monoxide are both very stable, held together by strong triple bonds, which are much stronger than those in TNT. The formation of water molecules together with the formation of all these new bonds releases a lot of energy.

TNT will decompose very rapidly to produce a massive amount of gas and energy in the reaction mixture in a very short amount of time. This causes a sudden increase in pressure in the reaction zone, resulting in an explosive shock wave.

Fireworks are a good example of chemical reaction. The same mechanism of thrust caused by the accumulation of gaseous products that is the basis of the rocket engine, is what propels fireworks up in the sky as well. 

The aerial shell is a plastic case, filled with fuel, and has explosive stars arranged around it. The explosive stars give the points of light that we see falling in the sky when the fireworks explode.

The colors that we see come from salts that are added to the explosive stars because metal ions display nice colors when heated to very high temperatures. For example, the presence of copper ions results in different shades of blue. 

Before the aerial shell can explode in the sky, it first has to get there. For this purpose, a lift carriage filled with fuel and an oxidizing agent is attached to the shell, and then placed in a hollow tube, known as a mortar.

When the fuse on the lift carriage is lit, the fuel begins to react with the oxidizing agent, releasing a lot of heat and exhaust gases. This buildup of exhaust gases pushes the aerial shell flying high in the sky, similarly to a rocket engine. 

The time fuse lights up, and it causes the exploding firework display in the sky about 2 seconds later, when the aerial shell is high enough above the ground. The fuel determines how quickly and with what strength the fireworks will explode in the sky.

Common fuels are sulfur, charcoal, gunpowder, aluminum powder, magnesium powder, or a combination of these. Using magnesium powder gives the sparkling effect. Common oxidizers are potassium nitrate, potassium perchlorate, and strontium nitrate. 

The thermite reaction actually refers to a class of reactions during which a metal oxide, most commonly iron, is reduced by aluminum. In this reaction, aluminum, which is pretty high in the reactivity series, displaces a less reactive metal from its oxide. 

All single displacement reactions are exothermic. The reason for this is that the bond in the product is always more stable than the bond in the reactant, because a more reactive metal will always displace a less reactive one.

Probably the best known class of reactions, neutralization reactions happen when a strong acid and a strong base react in an aqueous solution to form water and salt. Bond formation is always an exothermic process, so the overall reaction is exothermic.

Let’s say we mix together dilute aqueous solutions of hydrochloric acid and sodium hydroxide. Because our reactants are a strong acid and a strong base, they will be almost entirely dissociated in the water.

Sodium hydroxide will be dissociated to sodium cations and hydroxyl anions. Hydrochloric acid will be dissociated to chlorine anions and protons. When these 2 solutions come together, the ions interact with each other, and a neutralization reaction takes place, giving sodium chloride and water.

Because we started with dilute solutions, sodium chloride will not precipitate out of the solution, so there is not much change in terms of the sodium and chlorine ions. But, the reaction has produced a molecule of water from a proton and a hydroxyl ion.