Chemistry: Writing Complete Equations

Writing Complete Equations

As with recipes, all chemical equations include practical information for making our product. In the chili recipe, it mentioned that we should "simmer for at least two hours." In chemical equations, we use somewhat different terms.

The following are symbols that are commonly found in chemical equations to indicate the states of the products and reactants, as well as what reaction conditions are required.

Symbols of State

To indicate the states of the products and reactants in a reaction, we write the following symbols as subscripts after each chemical in the equation.

Symbol What It Means Example
(s) The chemical is a solid Fe(s)
(l) The chemical is a liquid H2O(l)
(g) The chemical is a gas N2(g)
(aq) The chemical is dissolved in water AgNO3(aq)

Sometimes it's easy to tell what symbols of state should be used, and sometimes it's not. For example, water is frequently in a liquid form. However, if we do a chemical reaction in which a large amount of heat is required, it may be a gas (steam).

Reaction Conditions

Frequently, symbols are written around the arrow in a chemical reaction to indicate to the reader what procedures need to be followed to make a chemical reaction occur. Here are some of the most common symbols:

SymbolWhat It Means
ΔAdd energy/heat to the reactants.
100º CHeat the reactants to the specified temperature.
2 atmThe reactants should be combined at the specified pressure.
refluxContinuously heat the reactants to boiling, and recondense the vapor.
chemical formulaThe specified chemical is needed for the reaction to proceed, or is the solvent.
3 hrsThe reaction should proceed for the specified quantity of time.


You've Got Problems

Problem 2: Write complete chemical equations for the following reactions:
(a) When dissolved lead(II) nitrate is added to an aqueous solution of potassium iodide,lead(II) iodide precipitates from the solution, and dissolved potassium nitrate is formed.
(b) When iron powder is heated in the presence of oxygen gas, iron(III) oxide powderis formed.

Sometimes you will observe arrows written immediately after the chemical formula of the products in a chemical reaction. An arrow pointing up (as in CO2↑) indicates that the product will form a gas that will bubble out of a solution. An arrow pointing down (as in PbI2↓) indicates that the product will spontaneously precipitate from the solution. ("Precipitate," in this sense, means the formation of a solid from the combination of two aqueous solutions.)

The symbols I've indicated in the previous table are by no means the only ones you'll find in chemical equations; depending on the type of reaction you're performing, there may be others. However, these are the most commonly seen in an introductory chemistry course.

As an example, let's add the appropriate symbols into the equation for the formation of water from hydrogen and oxygen. The reaction proceeds as follows: When energy is added to a mixture of hydrogen and oxygen gases, steam is formed. Using the symbols we discussed, the complete equation for this reaction is:

  • 2 H2(g) + O2(g)Δ 2 H2O2(g)
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Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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