STC_Writing and Balancing Equations Lesson

Writing and Balancing Equations

Chemical equations are shorthand ways of writing the quantitative relationships between compounds and elements in a chemical reaction. You can think of equations like recipes. The reactants are the ingredients and the products are what the recipe will make.

Recall from the atomic theory unit that chemical equations must obey the Law of Conservation of Matter. This means that the mass of chemicals you start with (the reactants) must equal the mass of chemicals you end with (the products). The reactants can be rearranged, but the same number of each chemical must be present before and after the reaction. The way that we show this in a chemical equation is by using coefficients, numbers placed in front of a chemical, to indicate the numbers of molecules, atoms, or formula units of a substance. These coefficients must be written so that the equation is balanced, meaning same number of individual reactants as there are individual products.

Watch the video below to learn how to write and balance equations.

The most important thing you need to remember is that when balancing equations, do NOT change the formulas themselves. You may only change the coefficients. Refer to the rules you learned on page 2 for writing chemical formulas and then only change the coefficients to balance the equation.

The best way to become comfortable with balancing equations is to practice!

Balanced Equations and Mole Ratios

Once an equation is balanced, the coefficients tell us the molar relationship between the chemicals. This is known as molar ratio.  

Diatomic Molecules

As you have been practicing balancing equations, have you noticed that some elements are followed by the subscript 2 even when they are not in a compound or molecule with another element?  Very few elements exist in nature as single atoms. Some elements combine two of their own atoms to form diatomic molecules. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine are the seven elements that occur naturally as diatomic molecules. It is super easy to remember these because they form the #7 on the periodic table! Just don't forget to include hydrogen!

These seven elements will never be written as single atom formulas; they do not exist that way.  

Hydrates

There is another group of chemicals that you will come across when writing balanced equations. These are called hydrates. Hydrates are a group of compounds that contain water within their crystal structure. The water molecules on a hydrate are not held very tightly, and can even be removed with the addition of heat, but must be accounted for in the chemical formula.

Plaster is an example of a hydrate. The chemical formula for plaster is:

$\text{CaSo}_{4}\cdot\text{2H}_2\text{O}$

The dot between the formula for calcium sulfate and water indicates that this is a hydrate, and although the water is part of the chemical formula, it is not held too strongly.

Watch for both hydrates and diatomic molecules as you write and balance chemical equations. Writing these incorrectly is a common mistake!

Remember to work on the module practice problems as you complete each section of content.

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