This statement is one example of a chemical equation, an abbreviated way of using symbols to represent a chemical change. The substances on the left side of the arrow are called reactants, and the substances on the right side of the arrow are called products. If we included phase labels for the reactants and products, under normal environmental conditions, the reaction would be as follows:. Chemical equations can also be used to describe physical changes.
We will see examples of this soon. This equation is still not complete because it does not satisfy the law of conservation of matter. Count the number of atoms of each element on each side of the arrow. On the reactant side, there are two H atoms and two O atoms; on the product side, there are two H atoms and only one oxygen atom. By counting the atoms of each element, we can see that the reaction is not balanced as written.
To make this chemical equation conform to the law of conservation of matter, we must revise the amounts of the reactants and the products as necessary to get the same number of atoms of a given element on each side. Because every substance has a characteristic chemical formula, we cannot change the chemical formulas of the individual substances.
For example, we cannot change the formula for elemental oxygen to O. However, we can assume that different numbers of reactant molecules or product molecules may be involved. For instance, perhaps two water molecules are produced, not just one:. The 2 preceding the formula for water is called a coefficient. It implies that two water molecules are formed.
There are now two oxygen atoms on each side of the equation. This point is so important that we should repeat it. You cannot change the formula of a chemical substance to balance a chemical reaction!
You must use the proper chemical formula of the substance. Unfortunately, by inserting the coefficient 2 in front of the formula for water, we have also changed the number of hydrogen atoms on the product side as well. As a result, we no longer have the same number of hydrogen atoms on each side. This can be easily fixed, however, by putting a coefficient of 2 in front of the diatomic hydrogen reactant:.
Now we have four hydrogen atoms and two oxygen atoms on each side of the equation. The law of conservation of matter is satisfied because we now have the same number of atoms of each element in the reactants and in the products. In most chemical reactions, molecular oxygen is reduced along the red and blue pathways highlighted in this redox scheme.
The selective reduction of oxygen to water in such biological systems is crucial, not only in order to maximize the energy produced for cellular metabolism but also because hydrogen peroxide is a powerful oxidant and cytotoxin, which harms living cells. Given the energetics presented above, there is a strong thermochemical bias for the production of water over hydrogen peroxide when H 2 and O 2 are reacted together.
For instance, when hydrogen gas is burned in the presence of oxygen, a large amount of energy is released and water is produced as the major product.
In cases where the reaction is more controlled, however, such as the consumption of hydrogen and oxygen in a fuel cell, the mechanism and kinetics of the O 2 reduction process can complicate issues greatly. For instance, the delivery of the protons and electrons derived from the ionization of hydrogen see redox half-reaction above to a molecule of oxygen has to be precisely controlled via a process know as proton-coupled electron transfer in order to ensure that the complete four-electron reduction of O 2 dominates.
Platinum metal is capable of serving as a catalyst that brandishes exquisite selectivity for the four-electron reduction of oxygen to water, and accordingly lies at the heart of fuel cell design and function.
Given that platinum is rare and extremely expensive, current research is aimed at the development of structural and functional models for oxygen activation and reduction to water via proton-coupled electron transfer.
Similar strategies are also being exploited to drive the energetically uphill reverse reaction, in which hydrogen is produced from water using solar energy. The success of both these areas of work may ultimately prove crucial to the development and sustainability of a global hydrogen economy.
Already a subscriber? Because hydrogen has a low activation energy only a small spark is needed to trigger a reaction with oxygen. Like all fuels, the reactants, in this case hydrogen and oxygen, are at a higher energy level than the products of the reaction. This results in the net release of energy from the reaction, and this is known as an exothermic reaction.
After one set of hydrogen and oxygen molecules have reacted, the energy released triggers molecules in the surrounding mixture to react, releasing more energy. The result is an explosive, rapid reaction that releases energy quickly in the form of heat, light and sound. On a submolecular level, the reason for the difference in energy levels between the reactants and products, lies with electronic configurations. Hydrogen atoms have one electron each. They combine into molecules of two so that they can share two electrons one each.
This is because the inner-most electron shell is at a lower energy state and therefore more stable when occupied by two electrons. Oxygen atoms have eight electrons each. They combine together in molecules of two by sharing four electrons so that their outer-most electron shells are fully occupied by eight electrons each.
However, a far more stable alignment of electrons arises when two hydrogen atoms share an electron with one oxygen atom. Only a small amount of energy is needed to "bump" the electrons of the reactants "out" of their orbits so that they can realign in the more energetically stable alignment, forming a new molecule, H2O. Following the electronic realignment between hydrogen and oxygen to create a new molecule, the product of the reaction is water and heat.
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