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Balancing Scales

Balancing Chemical Equations


Balance puzzles play an important role in chemistry. Molecules are created from specific numbers of specific types of atoms. In addition, the amounts of each type of atom reacting in a chemical equation must balance with what is being produced.

Chemical formulas use letters and numbers to indicate the proportions of atoms of each type. In other words, these formulas indicate the numbers of each type of atom in a molecule.

For example, a water molecule with the notation of \(\ce{H2O}\) has two hydrogen atoms and one oxygen atom.

A glucose, or sugar, molecule must be created from exactly 6 carbon, 12 hydrogen, and 6 oxygen atoms.

How many carbon atoms will be present in 5 glucose molecules?

Do one hydrogen molecule and one oxygen molecule combine to create exactly one water molecule, as shown on the balance below? (Note that hydrogen and oxygen do not exist as singular atoms, but rather as diatomic molecules where two of them are bonded together.)

Chemical reactions must be balanced. In other words, the number of a certain type of atoms that go into a reaction must also come out. If 10 carbon atoms are involved in the reaction, then 10 carbon atoms must also be part of the products.

Photosynthesis is the chemical reaction by which plants use water, carbon dioxide, and the sun's energy to create energy, or glucose, which is a type of sugar.

Written in words, the equation for photosynthesis looks like this: \[\text{water + carbon dioxide } \rightarrow \text{ glucose + oxygen.}\] Coverting those words to their chemical formulas, we have \[\ce{H2O + CO2 -> C6H12O6 + O2}.\] Is this chemical equation balanced?

Given how many carbon, hydrogen, and oxygen atoms are required to create one glucose molecule, we must use more than one carbon dioxide molecule and more than one water molecule to create one glucose molecule. Because glucose has 6 carbon atoms and 12 hydrogen atoms, let's start by thinking that we might need 6 carbon dioxide molecules and 6 water molecules.

In order for this chemical equation to be balanced, how many oxygen molecules are needed? In other words, determine the number that belongs in place of the \(\square\): \[\ce{6CO2 + 6H2O \rightarrow C6H12O6 + \square O2}.\]

The process that is almost the exact opposite of photosynthesis is called cellular respiration. In cellular respiration, the glucose molecules are broken down to release energy: \[\text{glucose + oxygen } \rightarrow \text{ carbon dioxide + water + energy.}\] What numbers belong in the boxes to balance this chemical equation: \[\ce{6O2 + C6H12O6 -> \square CO2 + \square H2O}?\]

When methane combusts, it produces carbon dioxide and water.

How many oxygen molecules are required to combust with one methane, \(\ce{CH4},\) molecule? \[\ce{CH4 + \square O2 -> CO2 + 2H2O}\]

The balanced equation for photosynthesis is shown below.

With 50 carbon dioxide molecules and 60 water molecules, what is the maximum number of glucose molecules that could be produced? \[\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}\]


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