Chemical Reactions

So far, we have been building molecules by putting together individual atoms. Atoms chemically bond and form molecules because, in general, they are unstable on their own. But this means that most atoms are already locked into molecules. So how do any new molecules ever get built?

A chemical reaction is a process where molecules get pulled apart, and their atoms reused to build new molecules. When hydrogen gas is burned in the presence of oxygen gas, a reaction occurs and water is created. Basically, the chemical bond between hydrogen atoms in diatomic hydrogen (H2) molecules and the chemical bond between oxygen atoms in diatomic oxygen (O2) molecules are broken, and then the hydrogen and oxygen atoms are reassembled into water (H2O) molecules and new chemical bonds are formed.

water synthesis reaction

In this reaction, two substances (hydrogen and oxygen gas) are transformed into a completely new substance (water). The hydrogen and oxygen gas are called the reactants, and the water is called the product. All chemical reactions involve the transformation of one substance into another by the breaking of existing chemical bonds and the formation of new ones.

Breaking chemical bonds and activation energy

Breaking the chemical bonds that hold atoms together in a molecule requires energy. Some chemical bonds are fairly weak and are easily broken. One example of a weak bond is the covalent bond holding the hydrogen atom to an oxygen atom in a nitrous acid molecule. This bond is so weak that it breaks spontaneously, forming separate hydrogen and nitrite ions.

nitrous acid equilibrium

Even though this covalent bond breaks spontaneously, breaking the bond still requires energy. It is just that the thermal energy of the system (the kinetic energy from molecular motion) is enough to break the bond without any additional energy source. The nitrous acid molecule literally collides hard enough with a second molecule to knock the hydrogen atom loose. Except for reactions that involve catalysts (you will learn more about catalysts later in this unit), chemical bonds are almost always broken by the collision of molecules.

H+ + NO2-
reaction energy (HNO2)

In this reaction, a nitrous acid molecule collides with another molecule. The energy from the collision breaks the covalent bond holding the hydrogen atom to one of the oxygen atoms. The energy needed to break the chemical bond is the reaction’s activation energy. Once the chemical bond is broken, the reaction enters an unstable and higher energy transition state where the hydrogen ion and the nitrite ion are momentarily separate. Then the hydrogen ion and the nitrite ion attract each other, and an ionic bond is formed. This drops the molecule into a metastable state.

Exothermic and endothermic reactions

Some bonds are much stronger and harder to break. At room temperature, the collisions between H2 and O2 molecules are too slow to break the covalent bond in either molecule. (Actually, by now, you should recognize that this is not true. Some of the molecules will be moving fast enough to break a covalent bond in a collision. But the number of those molecules is so low that, on a macroscopic level, you do not notice it.) However, increase the temperature to 500 °C, and the molecules will now be moving fast enough for random collisions to spontaneously break chemical bonds.

2H2 + O2
transition state
reaction energy (H2O)

Notice how much higher the activation energy… the energy needed to break the chemical bonds and get things started… is in this reaction. But while it takes a lot of energy to get this reaction started, once you reach the transition state, even more energy gets released as chemical bonds form between hydrogen and oxygen atoms, and stable H2O molecules are created. This is known as an exothermic reaction. Energy is released in an exothermic reaction because the energy state of the reactants (2H2 + O2) is higher than the energy state of the products (2H2O). So the amount of energy that is released, ΔE, is the difference in energy between the two states.

Burning hydrogen gas in the presence of oxygen releases 4 × 10-19 J of energy for each molecule of water created. That may not seem like much, but 16 g of H2 will produce an explosion equivalent to one pound of TNT. If you ignite hydrogen gas in the presence of oxygen with a 700 °C spark, the gas will explode. The spark is enough energy to activate the chemical reaction for a small handful of hydrogen and oxygen molecules. And the energy released by those reactions will then activate the chemical reaction for many more hydrogen and oxygen molecules… and a chain reaction will occur.

Many (but not all) chemical reactions can go in both directions. It is possible, through a process called electrolysis, to break the covalent bonds between hydrogen and oxygen atoms in water (H2O) molecules, and then to reassemble those atoms into hydrogen (H2) and oxygen (O2) molecules by forming new chemical bonds. This would make water the reactant, and hydrogen and oxygen gas the products of the reaction.

transition state
2H2 + O2
reaction energy (H2O)

When the reaction moves in this direction, the activation energy needed to get the reaction started is much greater. This is because the water molecule is incredibly stable and it takes a lot of energy to pull it apart. And instead of releasing energy, ΔE is the amount of energy that must be absorbed to make the reaction happen. When energy is absorbed because the energy state of the reactants (2H2O) is lower than the energy state of the products (2H2 + O2), the reaction is endothermic.

Reaction mechanisms and chemical equilibrium
water synthesis reaction

A reaction mechanism is the step-by-step sequence of individual reactions that enable an overall reaction to occur. It is extremely unlikely that two H2 molecules and one O2 molecule will collide and break every covalent bond simultaneously, freeing the four hydrogen atoms and two oxygen atoms to form new molecules. Covalent bonds are simply too strong for that to happen. Instead, the reaction actually occurs through a series of smaller reactions.

water synthesis step 1
An H2 molecule collides with an O2 molecule. The covalent bond between the two hydrogen atoms is broken, and one of the hydrogen atoms chemically bonds with the O2 molecule. Both the hydrogen atom and the HO2 fragment are unstable.
water synthesis step 2
When the unstable HO2 fragment runs into an H2 molecule, a double replacement reaction occurs. The oxygen atom with the unfilled valence shell switches places with a hydrogen atom. This gives you a stable water molecule and an unstable OH fragment.
water synthesis step 3
The unstable OH fragment then runs into a hydrogen atom from the first reaction, and they chemically bond to form a second stable water molecule.

In the first step of the reaction, when the H2 molecule collides with the O2 molecule, the molecules briefly enter a transition state. In this transition state, one of the hydrogen atoms in the H2 molecule has bonded to one of the oxygen atoms in the O2 molecule. The dotted lines in the structural formula below represent partial chemical bonds. These partial bonds are extremely weak (you can tell from the Lewis structures that they do not represent full pairs of shared electrons), and one or more of the partial bonds will break spontaneously.

water synthesis transition

At this point in time, the reaction is at its peak in the energy diagram and can go in one of two ways. Depending on which partial bond breaks first, the reaction can either go forward to form a hydrogen atom and a HO2 fragment, or go backwards and re-form a H2 molecule and an O2 molecule.

Reactions that can go in both directions are reversible and can be represented by double arrows in a chemical equation. There are many factors that can affect the rate of a reaction (how fast the reaction occurs), including the temperature and the concentration of the reactants. The more reactants there are, the more frequently the reaction will occur. A system will be in chemical equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. This is similar to the dynamic equilibrium between evaporation and condensation.

Balancing chemical equations

The chemical equation for the combustion (burning) of hydrogen gas in oxygen is:  2H2 + O2 → 2H2O. The numbers in front of the molecules are called coefficients (the same as the numbers in front of variables in algebraic equations). They tell you how many molecules are involved in the chemical reaction. So, two H2 molecules react with one O2 molecule to form two H2O molecules.

The reason why the equation for this chemical reaction is not simply  H2 + O2 → H2O  is because that equation would be unbalanced. On the left side of the equation, there are two hydrogen atoms and two oxygen atoms. On the right side of the equation, there are two hydrogen atoms but only one oxygen atom. We cannot just lose one oxygen atom. (Remember the conservation of matter!) You could try to write the equation like this:  H2 + O2 → H2O + O. However, that does not work because the equation is not complete… the free oxygen atom on the right side of the equation is not stable, and burning hydrogen gas does not leave lots of free oxygen atoms lying around. On the other hand, the equation  2H2 + O2 → 2H2O  is both complete and balanced. There are four hydrogen atoms and two oxygen atoms on both sides of the equation. (You could also write  H2 + ½O2 → H2O  if you do not mind fractions.)

Balancing an equation is a bit like solving a puzzle. The key thing to keep in mind is that you need to end up with exactly the same number and type of atoms you started with (unless some kind of nuclear reaction is occurring).