Chemical reaction

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"Chemical reactions" redirects here. For the 2007 television episode, see Chemical Reactions (Men in Trees).

 

A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another.^[1] Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds, although the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles (such as illustrated by Feynman diagrams), as well as nuclear reactions.

The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which graphically present the starting materials, end products, and sometimes intermediate products and reaction conditions.

Different chemical reactions are used in combination in chemical synthesis in order to obtain a desired product. In biochemistry, series of chemical reactions catalyzed by enzymes form metabolic pathways, by which syntheses and decompositions impossible under ordinary conditions are performed within a cell.

Chemical Reactions

by Anthony Carpi, Ph.D.

The reaction of two or more elements together results in the formation of a chemical bond between atoms and the formation of a chemical compound (see our Chemical Bonding module). But why do chemicals react together? The reason has to do with the participating atoms' electron configurations (see our The Periodic Table of Elements module).

In the late 1890s, the Scottish chemist Sir William Ramsay discovered the elements helium, neon, argon, krypton, and xenon. These elements, along with radon, were placed in group VIIIA of the periodic table and nicknamed inert (or noble) gases because of their tendency not to react with other elements (see our Periodic Table page). The tendency of the noble gases to not react with other elements has to do with their electron configurations. All of the noble gases have full valence shells; this configuration is a stable configuration and one that other elements try to achieve by reacting together. In other words, the reason atoms react with each other is to reach a state in which their valence shell is filled.

Let's look at the reaction of sodium with chlorine. In their atomic states, sodium has one valence electron and chlorine has seven.

Sodium	Chlorine

Chlorine, with seven valence electrons, needs one additional electron to complete its valence shell with eight electrons. Sodium is a little bit trickier. At first it appears that sodium needs seven additional electrons to complete its valence shell. But this would give sodium a -7 electrical charge and make it highly imbalanced in terms of the number of electrons (negative charges) relative to the number of protons (positive charges). As it turns out, it is much easier for sodium to give up its one valence electron and become a +1 ion. In doing so, the sodium atom empties its third electron shell and now the outermost shell that contains electrons, its second shell, is filled - agreeing with our earlier statement that atoms react because they are trying to fill their valence shell.

Sodium Chloride

This trait, the tendency to lose electrons when entering into chemical reactions, is common to all metals. The number of electrons metal atoms will lose (and the charge they will take on) is equal to the number of electrons in the atom's valence shell. For all of the elements in group A of the periodic table, the number of valence electrons is equal to the group number (see our Periodic Table page).

Nonmetals, by comparison, tend to gain electrons (or share them) to complete their valence shells. For all of the nonmetals, except hydrogen and helium, their valence shell is complete with eight electrons. Therefore, nonmetals gain electrons corresponding to the formula = 8 - (group #). Chlorine, in group 7, will gain 8 - 7 = 1 electron and form a -1 ion.

Hydrogen and helium only have electrons in their first electron shell.  The capacity of this shell is two.  Thus helium, with two electrons, already has a full valence shell and falls into the group of elements that tend not to react with others, the noble gases.  Hydrogen, with one valence electron, will gain one electron when forming a negative ion.  However, hydrogen and the elements on the periodic table labeled metalloids, can actually form either positive or negative ions corresponding to the number of valence electrons they have.  Thus hydrogen will form a +1 ion when it loses its one electron and a -1 ion when it gains one electron. 

Reaction energy

All chemical reactions are accompanied by a change in energy. Some reactions release energy to their surroundings (usually in the form of heat) and are called exothermic. For example, sodium and chlorine react so violently that flames can be seen as the exothermic reaction gives off heat. On the other hand, some reactions need to absorb heat from their surroundings to proceed. These reactions are called endothermic. A good example of an endothermic reaction is that which takes place inside of an instant '"cold pack." Commercial cold packs usually consist of two compounds - urea and ammonium chloride in separate containers within a plastic bag. When the bag is bent and the inside containers are broken, the two compounds mix together and begin to react. Because the reaction is endothermic, it absorbs heat from the surrounding environment and the bag gets cold.

Reactions that proceed immediately when two substances are mixed together (such as the reaction of sodium with chlorine or urea with ammonium chloride) are called spontaneous reactions. Not all reactions proceed spontaneously. For example, think of a match. When you strike a match you are causing a reaction between the chemicals in the match head and oxygen in the air. The match won't light spontaneously, though. You first need to input energy, which is called the activation energy of the reaction. In the case of the match, you supply activation energy in the form of heat by striking the match on the matchbook; after the activation energy is absorbed and the reaction begins, the reaction continues until you either extinguish the flame or you run out of material to react.

Atoms, Molekul, and Ions

We need to start with a little chemistry because living organisms are made of and use chemicals.

Individual substances are called elements, substance which cannot be broken down or subdivided by ordinary chemical means. We recognize about 105 or 106 elements. About 92 are natural and the rest are man-made. These are things like oxygen, sulfur, carbon, copper, etc. Each element has a symbol made of the first letter or two of its name. Some are from the old Latin names: sodium = natrium, iron = ferrum, potassium = kalium. Of the 92 naturally-occurring, four of these make up about 96% of all living matter. These are carbon, oxygen, hydrogen, and nitrogen, and the name COHN can help you remember these. Another 21 are needed in smaller amounts in order to live and stay healthy.

One piece, one particle of an element is an atom a unit of matter or the smallest possible amount of an element. Two or more atoms can bond together to form a molecule. Often the compound thus formed has properties quite different from the elements in it. For example, sodium (Na), an extremely reactive, nearly explosive metal, and chlorine (Cl), a toxic gas combine to form sodium chloride (NaCl), which is common table salt.

Atoms are made up of even smaller things called subatomic particles. There are three main types: proton (which has a very small positive electrical charge), neutron (which is neutral), and electron (which has a very small, negative electrical charge). You may see these referred to as p⁺, e^–, and n^o. The protons and neutrons form the nucleus of the atom (not to be confused with the nucleus of a cell), while the electrons are zipping around them somewhere and are traveling at about the speed of light.

The number of protons is important: this determines what element something is. Each element has a different number of protons, and if the number of protons in an atom changes, then it what element it is. The number of protons in an atom is called its atomic number. This is written as a subscript to the left: ₈O, ₆C, etc.

Within limits, the number of neutrons and electrons in an atom can vary. Isotopes are atoms of the same element with different numbers of neutrons. Protons and neutrons weigh about the same as each other, but electrons are so much smaller, their weight is negligible by comparison (like carrying a feather when you weigh yourself on the bathroom scale). Thus, when we want to know how much an atom of something weighs, we can just add up the number of protons and neutrons. The number of protons plus the number of neutrons in an atom = its atomic weight. These are written as superscripts to the left: ¹²C, ¹³C. Atomic weight minus atomic number equals the number of neutrons. This means that Carbon-12 has 6 neutrons, Carbon-13 has 7, and Carbon-14 has 8. In Carbon-14, the ratio of neutrons to protons (8 to 6) is far enough off that the nucleus of the atom is not stable, but undergoes radioactive decay, in which it turns into some other chemical (beyond the scope of this course, but if you are interested, the actual radioactive reaction, called -emission, is ₆¹⁴C  ₇¹⁴N + e^–, and the electron that is “kicked out” is called a beta [] particle). The half life of a radioactive chemical is the amount of time it takes for half of the starting quantity to undergo radioactive decay. Thus, if you started with 1 gm of a substance with a half life of 1 year, after one year, you would have 0.5 gm left, after two years you would have only 0.25 gm, after three years, 0.125, etc.

Molecular weight equals the sum of the atomic weights of the atoms in the molecule. For NaCl, the atomic weight of sodium is 23, of chlorine is 35 and a molecule contains one sodium and one chlorine, so 23 + 35 = 58, the molecular weight of NaCl. The formula for glucose ( a very common sugar) is C₆H₁₂O₆. The subscripts to the right mean that it contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen. The atomic weight for carbon is 12, for hydrogen is 1, and for oxygen is 16, so the molecular weight of glucose can be calculated thus:

Element	Atomic  Weight	No. of  Atoms	Total  Weight
C	12	6	6	×	12	= 72
H	1	12	12	×	1	= 12
O	16	6	6	×	16	= 96
Total = Molecular Weight			180

Use the Periodic Table below to help you with this molecular weight practice problem. (You’ll get a different problem each time you click this link.)

The numbers we will see listed on the periodic table are averages, For example, for carbon the number given is an average atomic weight of all the Carbon-12, Carbon-13, and Carbon-14 in the world. For our purposes, it’s OK if you round to the nearest whole number (C = 12, O = 16).

Normally, the number of protons and electrons match so the charge is balanced out. Sometimes, however, the number of electrons can vary. Ions are atoms with electrons added or removed resulting in an overall positive or negative charge. Generally, the charge on an ion is indicated to the upper right of its symbol. For example, a calcium ion with a +2 charge would be indicated as Ca⁺⁺ or Ca⁺². Electrons are moving rapidly all around the atom, and can possess certain discrete quantities of energy (like making change where you might have either a penny or a nickel or a dime, but not a 3.5 ¢ coin). These quantities are referred to as energy shells or orbitals. These are not orbits like the planets around the sun, but an attempt to show the energy quantities as pictures.

Each energy quantity/level/shell can only have a certain number of electrons with that much energy, kind of like if you would go to a movie where there are 1, 5, and 10¢ seats. The first energy level can have two electrons (there are two 1¢ seats). If there are more electrons, they must have the next amount of energy (they have to buy 5¢ seats), or as a chemist would say, they’re filling the second energy shell. Eight electrons can have the second level of energy (eight 5¢ seats). If there are still more electrons in the atoms of a particular element, then they must go into the third energy shell (buy 10¢ seats) where there are another eight spots available. Beyond that, things get too complicated for biology, so we’ll leave that to the chemists.

Chemists, however, prefer not to think in terms of 1, 5, and 10¢ seats at a movie. Often, the energy orbitals/shells are pictured as circles around the center of the atom. Again, the electrons do NOT travel around these circles like planets. Rather, this is just a way of showing how much energy they have. Our movie seats would convert to “standard” orbitals something like this.

This is how a chemist would diagram the energy levels of this atom. Notice that the electrons come in pairs. That idea is important in showing how atoms bond with each other to form molecules. Thus, the first energy level can have one pair of electrons, the second level can have four pairs, etc.

As atoms of different elements gain increasing numbers of electrons, one is put into each pair in a level before the second “half” of each pair is filled. Thus, for carbon, with six protons and six electrons, two of the electrons fill the available pair in the first energy shell. The remaining four electrons distribute themselves, one in each of the four pairs in the second level.

Electrons like to have the least amount of energy possible, so these levels will fill “from the bottom up” (they all fill the cheap seats first), thus not all atoms have electrons in all shells. The number of electrons in each of the shells depends on how many total electrons they have. This number is generally the same as the number of protons because the electrical charge of an electron is equal but opposite to that of a proton. Thus, hydrogen has one electron in the first level, helium has two in the first level, lithium has two in the first and one in the second level, etc. Note the arrangement of the periodic table: elements are organized into columns by how many electrons they have in their outermost energy levels (for example, H, Li, and Na each have one electron in whichever energy level is “outermost”), and organized into rows by which energy level (1^st, 2^nd, etc.) they’re filling (for example, Li, C, and N all have their first energy level full and are in various stages/numbers of electrons of filling their second level).

Periodic Table

For any element, the electrons in the outermost energy level/shell are the most important. These determine an element’s chemical properties – how it will react in a chemical reaction. These important electrons are known as the valence electrons. Know how many valence electrons carbon, oxygen, hydrogen, nitrogen, sodium, and chlorine have.

All elements are most stable with a full outer energy level, whatever that level is, so they will gain or give up electrons to make whatever is the outermost level be full even if it doesn’t match with the number of protons in that atom. This would be an ion, and if the atom gained electrons to form an ion (there are more electrons than protons), then its overall electrical charge would be negative by however many “extra” electrons it has. If an atom gives up electrons to form an ion (there are more protons than electrons), then its overall charge is positive.

Consider sodium and chlorine. Sodium is in column I so it has one valence electron. If it could get rid of that one, lonely electron from level 3, then its outer level would be level 2 which is nice and full. Chlorine is in column VII, so it has seven valence electrons, one short of a full outer shell. Thus, it would be more stable if it could grab an electron from somewhere to fill up that one, last spot.

Thus, when sodium and chlorine come together, in an explosive reaction, chlorine grabs sodium’s “unwanted” electron. This forms sodium ions with a +1 electrical charge (extra proton because it lost an electron) and chloride ions with a –1 charge. Chemists would write this as Na + Cl Na⁺ + Cl^–. These positive and negative ions are still strongly attracted to each other, forming ionic bonds, bonds in which one atom grabs electrons from another. When compounds with ionic bonds are put into water, the ions come apart and dissolve in the water.

Because carbon has four valence electrons, one in each of the four pairs, it is more willing to share electrons with another atom (rather than grabbing them or giving them up). For example, methane is one atom of carbon bonded to four atoms of hydrogen. This would have the chemical formula, CH₄. When atoms share electrons as they bond together, this is called a covalent bond. Many compounds with covalent (co- = with, together; valent = strength) bonds are not water soluble. Carbon can also form covalent bonds with other atoms of carbon, thus making long, stable chains possible. These are very important to living organisms.

The shape of a molecule of methane is a tetrahedron. The hydrogen nuclei (one proton each) are all “trying” to get as close as possible to all the electrons around the carbon, yet keep as far away as possible from each other (like + and – poles on a magnet). In a tetrahedron, there are four sides, all of which are triangles (in a pyramid, the bottom is square and there are five sides). The hydrogen protons are equally spaced in three dimensions around the carbon.

As a review, we have discussed the placement of four numbers around an element’s symbol (atomic number, atomic weight, charge, and number of atoms in a molecule), but all four are rarely used simultaneously. Just an an illustration, suppose a molecule of sodium carbonate is made with radioactive sodium-24 and then dissolved in water to form sodium ions (recall that sodium’s atomic number is 11). To indicate that there are two atoms of that sodium in the sodium carbonate, we would write “Na₂CO₃,” and if we wanted to indicate that they are sodium-24, we could rewrite this as “²⁴Na₂CO₃,” or maybe even “₁₁²⁴Na₂CO₃.” Once the compound is dissolved in water, forming sodium ions, we would use “Na⁺” or maybe, in a rare situation, “²⁴Na⁺” or “₁₁²⁴Na⁺” to indicate those ions, but since they are floating in the water and not attached to the carbonate (CO₃^–2) ion, now we don’t use the “2” after the “Na”. Note: if I’m using WordPerfect, I can put the superscripts and subscripts directly over/under each other (for example ), but HTML doesn’t do that.

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass ("go") between electrodes in a solution, when an electric field is applied. It is from Greek ιον, meaning "going".

An ion consisting of a single atom is an atomic or monatomic ion; if it consists of two or more atoms, it is a molecular or polyatomic ion.

Hydrogen atom (center) contains a single proton and a single electron. Removal of the electron gives a cation (left), whereas addition of an electron gives an anion (right). The hydrogen anion, with its loosely held two-electron cloud, has a larger radius than the neutral atom, which in turn is much larger than the bare proton of the cation. Hydrogen forms the only cation that has no electrons, but even cations that (unlike hydrogen) still retain one or more electrons, are still smaller than the neutral atoms or molecules from which they are derived.

Tabel Periodik