Pentane gas (C5H12) combusts with oxygen gas (O2) to form water (H2O) and carbon dioxide (CO2). Write a balanced chemical equation for this reaction.

The balanced equation for when pentane gas (C5H12) combusts with oxygen gas (O2) to form water (H2O) and carbon dioxide (CO2) is C₅H₁₂ + 8O₂ --> 5CO₂ + 6H₂O.

Expert Answers

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Some of the rules that I usually teach students when manually balancing equations are as follows:

  • If there are carbons, balance them first
  • Balance any monatomic or diatomic molecules last

So, in this case, we do have carbon, so it should be balanced first. We also have oxygen molecules (O₂), so we should balance these last.

Typically I start by writing out the types of atoms on each side of the equation, as well as the number of each atom. I rewrite this after each balancing step.

Reactants: C=5, H=12, O=2

Products: C=1, H=2, O=3

One of the things we can immediately notice is that the product side has an odd number of oxygen molecules, but oxygens are only available to us in evenly numbered amounts (since O₂ is the only source). This means that the products will need to be balanced so that only even numbers of oxygens are tallied there. The only way to do this is to increase the amount of water. We should keep this in mind.

For the first step, we can immediately put a 5 in front of the CO₂ (to balance the carbons). Since we also need to account for 12 hydrogens, we should put a 6 in front of the waters. This will balance all of the atoms in the pentane molecule.

This gives us:

C₅H₁₂ + O₂  -->  5CO₂ + 6H₂O

The new tally is:

Reactants:  C=5, H=12, O=2

Products: C=5, H=12, O=16

So, the coefficient for water is even, as we required. All we have left to do is balance the oxygen molecules, which is easy because we saved the O₂ for last.

The balanced equation is;

C₅H₁₂ + 8O₂  -->  5CO₂ + 6H₂O

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Specifying the equation:

C5H12 + O2  --> H2O + CO2

So let's count up the number of atoms on each side:

5 carbons, 12 hydrogens, 2 oxygens for reactants, and

1 carbon, 2 hydrogens, 3 oxygens for products.

Now the number of atoms on each side must be equal, because atoms are neither created or destroyed in the reaction. If you have 5 carbons as reactants, you have to have 5 carbons as products, so:

C5H12 + O2  |  H2O + 5 CO2

There are 12 hydrogens as reactants, so you need 12 as products:

C5H12 + O2  |  6 H2O + 5 CO2

But 6 water molecules have 6 oxygen atoms on the product side, along with the 10 that are already there from the carbon dioxide, for a total of 16 oxygens.  To get 16 oxygens as products, you need 8 oxygen molecules as reactants:

C5H12 + 8 O2  -->  6 H2O + 5 CO2

...and your equation is balanced!

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