Chapter 8
Chemical Equations

Equations show the reactants (starting substances) on the left and the products (what you get) on the right, with an arrow in the middle, pointing to the products.

Equations can also tell us the physical states of each reactant and product. These are written in parentheses and usually as subscripts, after the molecular formula of each reactant or product. You can also write symbols above the arrow to show conditions such as heat, light, or the presence of a catalyst that are required for the reaction to occur. It is critical that you know the difference between numerical coefficients in a chemical equation and numerical subscripts in the molecular formulas of the reactants and products.

• Coefficients in a chemical equation show the number (actually the relative numbers, as it's the ratios that count) of atoms or molecules that participate in the chemical reaction. You can get mole-to-mole conversion factors from the coefficients of a balanced chemical equation.

• Subscripts in a molecular formula show the number of atoms of an element are in a molecule. You can get molecule-to-atom conversions for any chemical compound from the subscripts in a chemical formula.

Balancing Equations.

• If you know what the compounds are that react or are produced, then write them down without coefficients. This is the unbalanced chemical equation, and it doesn't tell us anything about the stoichiometry of the reaction (i.e., how much of each reactant or product participates).

• A balanced equation has the same number of atoms of each type on each side, the reactants and products. Sometimes, as in double displacement reactions, you can count the number of ions on each side (i.e., count the polyatomic ions, not the individual atoms). You can still count the atoms, just to check, if you want.

• When you want to balance a chemical equation, you change the coefficients in the equation, not the subscripts. Never change the subscripts! That would change the compounds involved and give you a different reaction.

• The best hint for balancing equations is to save the simple compounds (those with only one element, such as O₂) for last.

• Another hint: Watch for odd/even numbers of atoms such as H and O. If you have an even number on one side and an odd number on the other, you'll have to double the odd number eventually.

• Another hint (again): Count polyatomic ions, not atoms, if this is appropriate.

• Another hint: Sometimes simpler is better. If the numbers start getting too large, start over.

• Final hint: Remember to reduce to lowest integers. If every coefficient is divisible by 2, then divide by 2 for your final balanced equation. All coefficients must be integers.

Chemical equations tell us mole-to-mole ratios that we can use as conversion factors to determine how much of a product is produced or how much of a reactant reacts. If you want to go from grams-to-grams, you can't use the equation directly, you have to convert to moles first, then use the coefficients of the equation for the mole-to-mole ratio. Get it?

The rest of Chapter 8 was de-emphasized this semester. Make sure you know the basic types of reactions, such as combination, combustion, decomposition, single displacement, and double displacement.

We'll talk about heat in reactions later on, and you've seen it in lab. Be advised that you should read this section and know the difference between an endothermic and exothermic reaction. Look at the potential energy diagrams on page 163 and figure out what the hump is for. What is the "activation energy?"

Return to the CHEM 151 Syllabus.
Return to the Schedule.
Go back to the study guide for Chapter 7 or Chapter 9.
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