Share on Facebook Share on Twitter Email
Answers.com

chemical equation

 
Britannica Concise Encyclopedia: chemical equation
Home > Library > Miscellaneous > Britannica Concise Encyclopedia

Method of writing the essential features of a chemical reaction using chemical symbols (or other agreed-upon abbreviations). By convention, reactants (present at the start) are on the left, products (present at the end) on the right. A single arrow between them denotes an irreversible reaction, a double arrow a reversible reaction. The law of conservation of matter (see conservation law) requires that every atom on the left appear on the right (the equation must balance); only their arrangements and combinations change. For example, one oxygen molecule combining with two hydrogen molecules to form two water molecules is written 2H2 + O2 ® 2H2O. The dissociation of salt into sodium and chloride ions is written NaCl ® Na+ + Cl-. See also stoichiometry.

For more information on chemical equation, visit Britannica.com.

Search unanswered questions...

Enter a question here...
Search: All sources Community Q&A Reference topics

 
Columbia Encyclopedia: chemical equation
Top
Home > Library > Miscellaneous > Columbia Encyclopedia
chemical equation, group of symbols representing a chemical reaction.

Basic Notation Used in Equations

The chemical equation 2H2+O2→2H2O represents the reaction of hydrogen and oxygen to form water. The arrow points in the direction of the reaction-from the reactants (substances that react) toward the product or products. In this case the reactants are hydrogen (written H2 because each molecule consists of two atoms of hydrogen) and oxygen (written O2 because each molecule consists of two atoms of oxygen) and the product is water. The coefficient 2 before the H2 indicates that two molecules of hydrogen take part in the reaction, and the 2 before the H2O indicates that two molecules of water are produced. When no number is written, as in front of the O2, a one is assumed; one molecule of oxygen takes part in the reaction. The equation shows that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water. Because of the relationship between molecules and the mole, the equation also shows that two moles of hydrogen react with one mole of oxygen to form two moles of water. The same sort of relationship holds with the gram-formula weight.

Methodology for Writing an Equation

There are three steps involved in writing a chemical equation. The first step is to decide which substances are the reactants and which are the products. For example, natural gas (cooking gas) burns in air, providing heat and producing no visible products. The natural gas is principally methane, and the portion of the air that reacts (supports combustion) is oxygen. These are the reactants. Products of the reaction are heat and two invisible gases, carbon dioxide and water vapor. We can now write the word equation methane + oxygen → carbon dioxide + water vapor + heat. The next step is to determine the correct formula for each substance and substitute it for the name. The equation now becomes CH4+O2→CO2+H2O. (A notation for heat is often omitted.)

The final step is to balance this equation. As the equation is now written, three oxygen atoms are produced from two, and four hydrogen atoms become only two. This cannot occur, since atoms are not created or destroyed in chemical reactions. The equation is already balanced for carbon, since there is one carbon atom on the reactant side and one carbon atom on the product side. There are four hydrogen atoms in the methane molecule on the reactant side, so there must be four hydrogen atoms in water molecules on the product side (since water is the only product containing hydrogen); thus there must be two water molecules, each containing two hydrogen atoms. The equation can now be written CH4+O2→CO2+2H2O. It is not yet balanced, since there are only two oxygen atoms shown as reactants and four as products. The equation is completely balanced by showing two oxygen molecules (four atoms) as reactants: CH4+2O2→CO2+2H2O.

Additional Symbols Used in Chemical Equations

There are a number of other symbols used in chemical equations. A symbol written above or below the reaction arrow indicates special reaction conditions. For example, when mercuric oxide is heated it decomposes into mercury metal and oxygen gas; this reaction is shown by the equation 2HgO →Δ 2Hg + O2↑. The Greek letter delta under the arrow represents the heating. The upward-pointing arrow after the O2 indicates that this product is gaseous and escapes. When a precipitate is formed by a reaction, the substance that precipitates is often followed by a downward-pointing arrow, e.g., AgNO3 + NaCl AgCl↓ + NaNO3. The H2O above the arrow shows that the reaction takes place in the presence of water-in this case, in water solution. The formulas AgNO3, NaCl, and NaNO3 do not represent molecules, since these substances are almost completely ionized in water solution (see ion).

When chemical equilibrium occurs in a reaction, the double arrow is used instead of the single arrow. For example, liquid water dissociates to form hydronium ions (H3O+) and hydroxide ions (OH−). These ions exist in equilibrium with water molecules. The equation is 2H2O H3O+ + OH−. The sign = is sometimes used in place of the double arrow.

Bibliography

See J. B. Dence, Mathematical Techniques in Chemistry (1975).

Wikipedia: Chemical equation
Top
Home > Library > Miscellaneous > Wikipedia

A chemical equation may be described as a chemical reaction[1] or a means of writing out and describing such a phenomenon. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers. The first chemical equation was diagrammed by Jean Beguin in 1615.

Contents

Form

A chemical equation consists of the chemical formulas of the substances that are mixed (called the reactants), and the chemical formula of the substances that result from the chemical reaction (called the products). The two are separated by an → —usually read as "yields"— and each chemical formula is separated from others by a plus sign. Sometimes a triangle is drawn over the arrow symbol to denote energy must be added to the substances for the reaction to begin.

As an example, a reaction where hydrogen combines with oxygen to produce water can be denoted

H2 + O → H2O

This can be read as "H two plus O, yields H two O". Occasionally, rather than reading the letter and its subscript, the chemical formulas are read using compositional nomenclature (such as dihydrogen monoxide for H2O). However, atomic oxygen is extremely rare in nature, oxygen is more frequently found in O2 compounds. But making H2 and O2 react does not produce H2O since that would be an unbalanced reaction (see "Balancing chemical equations" below).

For this reason, each chemical formula may be preceded by a scalar coefficient indicating the proportion of that substance necessary to produce the reaction in formula. For instance, the formula for the burning of methane

CH4 + 2 O2 → CO2 + 2H2O

indicates that twice as much O2 as CH4 is needed, and when they react will produce twice as much H2O as CO2. This is because during the reaction,each atom of carbon needs exactly two atoms of oxygen to combine with, to produce the CO2, and every two atoms of hydrogen need an atom of oxygen to combine with to produce the H2O. If the proportions of the reactants are not respected, when they are forced to react either not all of the substance used will participate in the reaction, or the reaction that will take place will be different from the one noted in the equation.

Balancing chemical equations

The law of conservation of mass dictates the quantity of each element does not change in a chemical reaction. Thus, each side of the chemical equation must represent the same quantity of any particular element. Similarly, the charge is conserved in a chemical reaction. Therefore, the same charge must be present on both sides of the balanced equation.

One balances a chemical equation by changing the scalar number for each molecular formula. Simple chemical equations can be balanced by inspection, that is, by trial and error. Another technique involves solving a system of linear equations.

Example #1: Na + O2 → Na2O

In order for this equation to be balanced, there must be an equal amount of Na on the left hand side as on the right hand side. As it stands now, there is 1 Na on the left but 2 Nas on the right. This is solved by putting a 2 in front of the Na on the left hand side:

2Na + O2 → Na2O

In this there are 2 Na atoms on the left and 2 Na atoms on the right. In the next step the oxygen atoms are balanced as well. On the left hand side there are 2 O atoms and the right hand side only has one. This is still an unbalanced equation. To fix this a 2 is added in front of the Na2O on the right hand side. Now the equation reads:

2Na + O2 → 2Na2O

Notice that the 2 on the right hand side is "distributed" to both the Na2 and the O. Currently the left hand side of the equation has 2 Na atoms and 2 O atoms. The right hand side has 4 Na total and 2 O. Again, this is a problem, there must be an equal amount of each chemical on both sides. To fix this 2 more Na are added on the left side. The equation will now look like this:

4Na + O2 → 2Na2O

This equation is a balanced equation because there is an equal number of atoms of each element on the left and right hand sides of the equation.

Example #2:

P4 + O2 → 2P2O5

This equation is not balanced because there is an unequal amount of O on both sides of the equation. The left hand side has 4 P and the right hand side has 4 P. So the P atoms are balanced. The left hand side has 2 O and the right hand side has 10 O.

To fix this unbalanced equation a 5 in front of the O2 on the left hand side is added to make 10 O on both sides resulting in

P4 + 5O2 → 2P2O5

The equation is now balanced because there is an equal amount of substances on the left and the right hand side of the equation.

Ionic equations

An ionic equation is a chemical equation in which electrolytes are written as dissociated ions. Ionic equations are used for single and double displacement reactions which occur in aqueous solutions. For example in the following precipitation reaction:

CaCl2(aq) + 2AgNO3(aq) → Ca(NO3)2(aq) + 2AgCl(s)

the full ionic equation would be:

Ca2+ + 2Cl- + 2Ag+ + 2NO3- → Ca2+ + 2NO3- + 2AgCl(s)

and the net ionic equation would be:

2Cl-(aq) + 2Ag+(aq) → 2AgCl(s)

or, in reduced balanced form,

Ag+ + Cl- → AgCl(s)

In this aqueous reaction the Ca2+ and the NO3- ions remain in solution and are not part of the reaction. They are termed spectator ions and do not participate directly in the reaction, as they exist with the same oxidation state on both the reactant and product side of the chemical equation. They are only needed for charge balance of the original reagents.

In a neutralization or acid/base reaction, the net ionic equation will usually be:

H+ + OH- → H2O

There are a few acid/base reactions that produce a precipitate in addition to the water molecule shown above. An example would be the reaction of barium hydroxide with phosphoric acid because the insoluble salt barium phosphate is produced in addition to water.

Double displacement reactions that feature a carbonate reacting with an acid have the net ionic equation:

2 H+ + CO32- → H2O + CO2

If every ion is a "spectator ion", then there was no reaction, and the net ionic equation is null.

References

  1. ^ IUPAC Compendium of Chemical Terminology

External links


This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Shopping: chemical equation
Top
Chemical Equation For Cod Liver Oil
 
 

 

Copyrights:

Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Chemical equation".  Read more