Updated March 13, 2018 By Allison Boley
You've conquered the naming of compounds and now you're ready to move onto balancing chemical equations. But the process involves more numbers, and already coefficients seem harder than subscripts. Subscripts in a chemical formula are constant for each compound. Sodium phosphate is always Na3PO4. Methane is always CH4. Even compounds that can be expressed in multiple ways (acetic acid: CH3COOH or C2H3O2) always contain the same number of their respective elements. Not so for coefficients. Methane may appear in a chemical equation as 3CH4, 4CH4 or even 18CH4. How can this number change without changing the compound? And what causes it to change? Please note that all numbers following chemical symbols should be subscripts.
The coefficient in a chemical formula is the number immediately preceding the compound. It appears full size, never as a subscript or superscript.
The coefficient in a chemical formula represents the amount of each chemical present. The amount of a substance is measured in moles.
The mole can be a tricky concept to master. The confusion usually surrounds the fact that it can be used to measure atoms, molecules or just about anything that involves an amount. Just remember that the mole measures the most basic unit of amount possible. If you're dealing with atoms of hydrogen, then a mole measures the amount of atoms present. If you're dealing with molecules of ethane (CH3CH3), then the molecule is the most basic unit, not the atom. A mole is 6.022x10^23 of the most basic unit. (A caret indicates superscript; 10^23 is 10 raised to the twenty-third power.) One mole of Hydrogen is 6.022x10^23 atoms of hydrogen. One mole of ethane is 6.022x10^23 molecules of ethane. A coefficient in a chemical formula indicates how many moles of that substance are present. 3CH4 means that 3 moles of CH4, and thus 1.8066x10^24 molecules of CH4, are present.
Coefficients are used in the process of balancing equations, known as stoichiometry. We add coefficients to compounds in chemical equations to assure that the number of moles of each element is the same on both sides of the equation. Examples: 3Na^(+) + PO4(3-) --> Na3PO4 3 moles Na, 1 mole PO4 --> 3 moles Na, 1 mole PO4 CH4 + 2O2 --> CO2 + 2H2O 1 mole C, 4 moles H, 4 moles O --> 1 mole C, 4 moles H, 4 moles O
We also use coefficients when determining the amount of a chemical to use in the laboratory. We can't weigh moles on our scales, so we must convert moles to grams. For this conversion, we use each element's molar mass, found on the periodic table. If, from our stoichiometric calculations, we know we need 5 moles of ice (H2O), then we simply use dimensional analysis to figure out how many grams of ice to add to the reaction: 10 mol H (1.00794 g/mol H) + 5 mol O (15.9994 g/mol O) = 90.0764 g ice Even though chemical compounds are broken up and new compounds are formed during a chemical reaction, atoms in the reactants do not disappear, nor do new atoms appear to form the products. In chemical reactions, atoms are never created or destroyed. The same atoms that were present in the reactants are present in the products—they are merely reorganized into different arrangements. In a complete chemical equation, the two sides of the equation must be present on the reactant and the product sides of the equation. There are two types of numbers that appear in chemical equations. There are subscripts, which are part of the chemical formulas of the reactants and products; and there are coefficients that are placed in front of the formulas to indicate how many molecules of that substance is used or produced. The subscripts are part of the formulas and once the formulas for the reactants and products are determined, the subscripts may not be changed. The coefficients indicate the number of each substance involved in the reaction and may be changed in order to balance the equation. The equation above indicates that one mole of solid copper is reacting with two moles of aqueous silver nitrate to produce one mole of aqueous copper (II) nitrate and two atoms of solid silver.
Because the identities of the reactants and products are fixed, the equation cannot be balanced by changing the subscripts of the reactants or the products. To do so would change the chemical identity of the species being described, as illustrated in Figure \(\PageIndex{1}\). Original molecule H2O: if the coefficient 2 is added in front, that makes 2 water molecules; but if the subscript 2 is added to make H2O2, that's hydrogen peroxide. The simplest and most generally useful method for balancing chemical equations is “inspection,” better known as trial and error. The following is an efficient approach to balancing a chemical equation using this method.
Balance the chemical equation for the combustion of Heptane (\(\ce{C_7H_{16}}\)). \[\ce{C_7H_{16} (l) + O_2 (g) → CO_2 (g) + H_2O (g) } \nonumber \]
Combustion of Isooctane (\(\ce{C_8H_{18}}\)) \[\ce{C8H18 (l) + O2 (g) -> CO_2 (g) + H_2O(g)} \nonumber \]
The assumption that the final balanced chemical equation contains only one molecule or formula unit of the most complex substance is not always valid, but it is a good place to start. The combustion of any hydrocarbon with oxygen produces carbon dioxide and water.
Aqueous solutions of lead (II) nitrate and sodium chloride are mixed. The products of the reaction are an aqueous solution of sodium nitrate and a solid precipitate of lead (II) chloride. Write the balanced chemical equation for this reaction.
Is each chemical equation balanced?
Balance the following chemical equations.
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