# How do we represent the complete combustion of "octane", C_8H_18(l)?

Mar 4, 2016

${C}_{8} {H}_{18} \left(l\right) + \frac{25}{2} {O}_{2} \left(g\right) \rightarrow 8 C {O}_{2} \left(g\right) + 9 {H}_{2} O \left(l\right)$

#### Explanation:

The above equation is stoichiometrically balanced: garbage out equals garbage in. If you like you can double the equation to remove the non-integral coefficient. I have never seen the need to do so, inasmuch the arithmetic is easier when you use this form with the 1 equivalent of hydrocarbon reactant.

So 2 questions with regard to this reaction: (i) How does energy transfer in this reaction; (ii) does this represent an oxidation reduction reaction?

Mar 4, 2016

We need to have the same number of moles of each substance on each side. The balanced equation is:

${C}_{8} {H}_{18} + \frac{25}{2}$ ${O}_{2} = 8$ $C {O}_{2} + 9$ ${H}_{2} O$ or $2$ ${C}_{8} {H}_{18} + 25$ ${O}_{2} = 16$ $C {O}_{2} + 18$ ${H}_{2} O$

#### Explanation:

We start out with the unbalanced equation:

$a$ ${C}_{8} {H}_{18} + b$ ${O}_{2} = c$ $C {O}_{2} + d$ ${H}_{2} O$

We need to find the values of the coefficients $a , b , c$ and $d$ to balance the equation.

For the moment, let's leave $a$ as 1. There are 8 moles of C on the left, so to balance we need 8 moles of C on the right, so let's make $c = 8$, because each $C {O}_{2}$ contains 1.

${C}_{8} {H}_{18} + b$ ${O}_{2} = 8$ $C {O}_{2} + d$ ${H}_{2} O$

There are 18 moles of H on the left so we need 18 on the right, but each ${H}_{2} O$ contains 2, so we make $d = 9$.

${C}_{8} {H}_{18} + b$ ${O}_{2} = 8$ $C {O}_{2} + 9$ ${H}_{2} O$

Now we turn our attention to the right side. There are two O in each of 8 $C {O}_{2}$ for a total of 16 in the carbon dioxide and one in each of 9 ${H}_{2} O$ for a total of 9 in the water, so we need 25 O all together.

On the left, each ${O}_{2}$ contains two O, so one way to balance the equation is to take $\frac{25}{2}$ of them:

${C}_{8} {H}_{18} +$ $\frac{25}{2} {O}_{2} =$ $8$ $C {O}_{2} + 9$ ${H}_{2} O$

If the fraction makes you uncomfortable, another way is to multiply all the coefficients by two:

$2$ ${C}_{8} {H}_{18} + 25$ ${O}_{2} = 16$ $C {O}_{2} + 18$ ${H}_{2} O$