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Chemical Equations

by Ron Kurtus (revised 16 January 2004)

A chemical equation describes the amounts of chemical materials needed to form new substances. This type of equation is important is defining how many units of each substance must be mixed to get the desired result. It is similar to a cookbook recipe. The chemical equation also shows how many units there will be of each resulting substance. There is a parallel between chemical equations and algebraic equations.

Questions you may have include:

• How is a chemical equation similar to a cookbook recipe?
• Can the equations or recipes become complex?
• How do you balance chemical equations?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

Useful tools: Metric-English Conversion | Scientific Calculator.

Note: If you want to hear the text being read, click the Play button. It takes a few seconds for the sound to start. The voices are somewhat mechanical for computer use.

Time = 6 min. 25 sec.

Chemical cookbook

A chemical equation is similar to a cookbook recipe in that it shows how many units of each substance is required to give the desired result. It shows the combination of various elements and/or molecules and then the resulting elements and/or molecules.

Just like with an algebraic equation, the number of atoms on the left must equal the number of atoms on the right.

An example of a chemical recipe or equation is combining 2 units of Sodium (Na) with one molecule of Chlorine gas (Cl₂) to form 2 units of table salt:

2Na + Cl₂ → 2NaCl

As you recall in Chemical Formulas, the full-sized number in front of an element or molecule is how many units there are of that item. The small sub-number behind an element indicates how many atoms of that element there are in the molecule.

Also note that Chlorine gas is never a single atom. It is always a molecule (Cl₂). This is also true for Hydrogen gas (H₂) and Oxygen (O₂).

Yields symbol

The yields symbol ( → ) is used instead of the equal sign ( = ). The equation above is read, Sodium plus Chlorine yields Sodium Chloride. It means that this chemical reaction goes in one direction.

←→ symbol

There are chemical reactions where molecules may go back and forth or combine and separate. In those special cases, the ( ←→ ) symbol is used.

One example is when you mix salt in water, resulting in salty water, which is water containing Sodium and Chlorine ions. This chemcial reaction goes both ways.

NaCl + H₂O ←→ H₂O + Na⁺¹ + Cl^-1

Note that ions have a small superscript number indicating their excess charges. Na⁺¹ means the Sodium ion is missing an electron, thus its (+) charge.

Also note that ions are individual atoms, so when the solution is formed, an element like Cl does not need to be a molecule. It is only Cl₂ when existing as a gas.

Depending on the mixture and temperature, the water can be salty or the salt can precipitate out and collect on the bottom of the container.

(See Mixtures and Solutions for more about dissolving salt in water.)

Complex equations

Just as a cookbook recipe usually has a number of ingredients, so can chemical equations by complex. In some highly complex chemical reactions, you may even have a series of equations for chemical reactions that must be done in a particular order.

An example of a single-step chemical reaction involving several compounds is a method to create Chlorine gas by heating Manganese Dioxide mixed with Sodium Chloride and Sulfuric acid is seen in the following equation:

2NaCl + 2H₂SO₄ + MnO₂ → Na₂SO₄ + MnSO₄ + 2H₂O + Cl₂

You can see the importance of balancing such an equation.

Balancing equations

Sometimes you will see a chemical equation that must be balanced. For example, suppose you were going to burn some Propane gas (C₃H₈). Combining Propane with Oxygen results in Carbon Dioxide and water.

Does C₃H₈ + O₂ → CO₂ + H₂O ??

You can see that the number of Carbon (C), Oxygen (O) and Hydrogen (H) atoms on the left of the equation does not equal the number on the right side. There are 3 C, 8 H, and 2 O on the left and 1 C, 3 O, and 2 H on the right.

Use trial-and-error

So, to balance the equation, you must do some clever trial-and-error guesses. Sometimes the unbalanced equation is written with unknowns, similar to what you would do in Algebra:

wC₃H₈ + xO₂ → yCO₂ + zH₂O

where w, x, y and z are the unknown numbers from of each molecule in the equation.

Logical approach

One logical, trial-and-error approach to balancing this chemical equation is as follows:

1. Since there are 8 H on the left, perhaps there are 4 H₂O on the right.
2. Since there are 3 C on the left, perhaps there are 3 CO₂ on the right.
3. The resulting equation is then: C₃H₈ + O₂ → 3CO₂ + 4H₂O ??
4. The C's and H's balance, but there are 10 O on the right and only 2 on the left. So, let's try 5 O₂ on the left.

Now the equation balances out.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Count the number of Carbon atoms, Hydrogen atoms, and Oxygen atoms on the left and compare with the number on the right side of the equation.

Summary

Chemical equations are similar to algebraic equations, in that the total number of atoms of each element on the left side must equal the number for that element on the right side. You can have complex equations and series of equations for some chemical reactions. You usually can use a logical trial-and-error method to balance a chemical equation.

Answers to Readers' Questions

Resources

The following resources provide information on this subject:

Websites

Tutorial on Balancing Chemical Equations - from Ohio State University (good)

Balancing Chemical Equations - from Diamond Bar (CA) High School

Chemical Equations Quizzes - from Ohio State University (difficult)

Dave's Equation Balancer - interesting tool to automatically balance your equations

Chemistry Resources

Books

Top-rated books on Chemistry

Miscellaneous

Mini-quiz to check your understanding

1. Why is the ( → ) symbol used instead of ( = ) in a chemical equation?

2. What is the basic rule of a chemical equation?

3. If you put an electric current through water, you can break it into hydrogen and  oxygen gases. What is the equation for this reaction?

If you got all three correct, you are on your way to becoming a Champion in Chemistry. If you had problems, you had better look over the material again.

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