Chemical Reaction Equations: Coefficients

Coefficients:

In most chemical changes, the relative amounts of each of the reactants and products are not one to one. Information about the relative number of moles of each type of substance is indicated by the coefficients. These are numbers written in front of the chemical formulae. In the previous examples, there weren’t any numbers written in front of the chemical formulae, so the relative mole ratios were one to one to one.

In this reaction equation, the relative number of moles is two moles of hydrogen, one mole of oxygen, and two moles of water: 2H₂ + O₂ à 2H₂O. As before, the number and kinds of atoms on each side of the reaction arrow is the same (four hydrogen atoms and two oxygen atoms).

Coefficients are necessary because reactions obey the Law of Conservation of Matter which states that matter is neither created nor destroyed during a chemical change. In the previous example, water has only one oxygen atom yet oxygen element is diatomic. Two water molecules have to be produced to use both oxygen atoms; since the extra atom can't be destroyed, it has to be used. If two water molecules are formed, four hydrogen atoms must be used; since atoms can't be created, we need to have four hydrogen atoms to start with. Four hydrogen atoms is the number in two diatomic hydrogen molecules, hence the coefficient in front of the hydrogen reactant.

The process of assigning coefficients to a chemical reaction equation is called balancing the equation. This process will be covered in the next section of the outline.

Translating Chemical Reaction Equations:

A great deal of information is imbedded in any reaction equation. We can write a single line of chemical symbols to represent a paragraph of words. For example, this reaction equation is translated in the next paragraph.

CaCO_3(s)+ 2HNO_3(aq) à Ca(NO₃)_2(aq)+ CO_2(g)+ H₂O_(l)

“Calcium carbonate and nitric acid react to form calcium nitrate, carbon dioxide, and water. The relative proportions are one mole of calcium carbonate to two moles of nitric acid reactants form one mole each of the products calcium nitrate, carbon dioxide, and water. Calcium carbonate is an ionic compound; it consists of one calcium two positive ion for every one carbonate two negative ion (the carbonate ion has one carbon atom and three oxygen atoms). Nitric acid is an acid; it consists of one hydrogen one positive ion for every one nitrate one negative ion (the nitrate ion has one nitrogen atoms and three oxygen atoms). Calcium nitrate is an ionic compound; it consists of one calcium two positive ion for every two nitrate one negative ions. Carbon dioxide is a molecular compound; it consists of molecules of two oxygen atoms bonded to one carbon atom. Water is a molecular compound; it consists of two hydrogen atoms bonded to one oxygen atom. In this particular reaction, solid calcium carbonate is reacted with aqueous nitric acid; the calcium nitrate that forms is an aqueous solution; the carbon dioxide is a gas; and the water is a liquid.”

The information that this reaction equation conveys is

The identities of the substances involved.
Which of the substances are the reactants and which of the substances are the products.
The types of substances involved in the reaction (element versus compounds, and ionic, molecular, etc) as well as the number and kinds of atoms/ions which make up these substances.
The states of matter for the substances in this particular reaction.

The single line of symbols in a chemical reaction equation is a lot shorter!

Concept Check: Translate this reaction equation into words. Include information about the products and reactants as well as their names.

2Fe(NO₃)_3(aq) + 3Na₂S_(aq) à Fe₂S_3(ppt)+ 6NaNO_3(aq)

Answer: Iron (III) nitrate reacts with sodium sulfide to form iron (III) sulfide and sodium nitrate. All of these substances are ionic compounds. The iron (III) nitrate has one iron +3 charge ion for every three nitrate -1 ions. The sodium sulfide has two sodium +1 ions for every one sulfide -2 ions. The iron (III) sulfide has two iron +3 ions for every three sulfide -2 ions. The sodium nitrate has one sodium +1 ion for every one nitrate -1 ion. The iron (III) ion, sodium ion, and the sulfide ion are all monatomic ions. Nitrate is a polyatomic ion. It has one nitrogen atom and three oxygen atoms. The relative amounts of products and reactants for this reaction are two moles of iron (II) nitrate and three moles of sodium sulfide reactants form one mole iron (III) sulfide and six moles sodium nitrate products. The iron (III) nitrate, sodium sulfide and sodium nitrate are aqueous solutions. The iron (III) sulfide is a precipitate.

This site was last updated 09/14/06