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# Stoichiometry and Chemical Equations Study Guide

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Updated on Sep 24, 2011

## Introduction

Reaction stoichiometry establishes the quantities of reactants (used) and products (obtained) based on a balanced chemical equation.

## Chemical Reactions

A chemical change or chemical reaction can be described by writing a chemical equation. A chemical equation uses chemical symbols to show what happens during a chemical reaction.

Let's look at the electrolysis of water, a decomposition reaction. When water is subjected to an electric current, hydrogen gas and oxygen gas are formed. A chemical equation can be written to show this reaction:

H2O → H2 + O2

The arrow (→) means yields. The reactants are a substance that undergoes a change in a chemical reaction (the left side of the arrow). The products are a substance that is formed as a result of a chemical reaction (the right side of the arrow). The chemical equation is like a mathematical equation where both sides must be equal. Recall that mass cannot be created or destroyed. Therefore, we must balance the equation. Balancing equations is a "trial-and-error" method of equalizing the elements and molecules on both sides of the equation. When balancing an equation, the following guidelines can help:

• Write the unbalanced equation, including the correct formulas, for all reactants and products.
• Compare the number of atoms on the reactants and product(s) sides.
• Balance the elements by changing the number of molecules or ions with coefficients. Do not change the molecules or ions. The coefficients represent the number of moles of a substance. Always balance the heavier atoms before trying to balance lighter ones such as hydrogen.
• If necessary, continue to rebalance and recheck. Consider reducing the coefficients so that the smallest possible whole numbers are used. Fractions can be used for oxygen gas.

For the electrolysis of water, there are 2 hydrogen and 1 oxygen on the reactant side and 2 hydrogen and 2 oxygen on the product side of the equation.

 H2O → H2 + O2
 #H 2 2 #O 1 2

A 2 can be added in front of the water to balance the oxygen:

 2H2O → H2 + O2
 #H 4 2 #O 2 2

However, this change unbalances the hydrogen, so a 2 can be added before the hydrogen gas in the product:

 2H2O → 2H2 + O2
 #H 4 4 #O 2 2

Because the electrolysis of water contains oxygen gas and fractions can be used, the equation could also be balanced:

 H2O → H2 + O2

#### Example:

Write the balanced equation for the combustion of methane and oxygen gas to yield carbon dioxide and water.

 1. Write the unbalanced chemical equation: CH4 + O2 → CO2 + H2O
 2. Identify the number of atoms: CH4 + O2 → CO2 + H2O
 #C 1 1 #O 2 3 #H 4 2
 3. Balance the oxygen: CH4 + 2O2 → CO2 + 2H2O
 #C 1 1 #O 4 4 #H 4 4
 4. Recheck (notice that the hydrogen is automatically balanced in this example): Answer: CH4 + 2O2 → CO2 + 2H2O

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