# What is the balanced equation for heptane (C7H16) burning in oxygen to make carbon dioxide and water?

May 28, 2014

The balanced equation is $\text{C"_7"H"_16 + "11O"_2 → "7CO"_2 + "8H"_2} O$.

#### Explanation:

You follow a systematic procedure to balance the equation.

$\text{C"_7"H"_16 + "O"_2 → "CO"_2 + "H"_2"O}$

A method that often works is to balance everything other than $\text{O}$ and $\text{H}$ first, then balance $\text{O}$, and finally balance $\text{H}$.

Another useful procedure is to start with what looks like the most complicated formula.

The most complicated formula looks like ${\text{C"_7"H}}_{16}$. We put a $1$ in front of it to remind ourselves that the coefficient is now fixed.

$\textcolor{red}{1} \text{C"_7"H"_16 + "O"_2 → "CO"_2 + "H"_2"O}$

Balance $\text{C}$:

We have fixed 7 $\text{C}$ atoms on the left-hand side, so we need 7 $\text{C}$ atoms on the right-hand side. We put a $7$ in front of the ${\text{CO}}_{2}$.

$\textcolor{red}{1} \text{C"_7"H"_16 + "O"_2 → color(blue)(7)"CO"_2 + "H"_2"O}$

Balance $\text{O}$:

We can't balance $\text{O}$ yet because we have two formulas that contain $\text{O}$ and lack coefficients. So we balance $\text{H}$ instead.

Balance $\text{H}$:

We have fixed 16 $\text{H}$ atoms on the left-hand side, so we need 16 $\text{H}$ atoms on the right-hand side. We put an $8$ in front of the $\text{H"_2"O}$.

$\textcolor{red}{1} \text{C"_7"H"_16 + "O"_2 → color(blue)(7)"CO"_2 + color(green)(8)"H"_2"O}$

Now we can balance $\text{O}$:

We have fixed 22 $\text{O}$ atoms on the right-hand side: 14 from the ${\text{CO}}_{2}$ and 8 from the $\text{H"_2"O}$. We put an $11$ in front of the ${\text{O}}_{2}$.

$\textcolor{red}{1} \text{C"_7"H"_16 + color(teal)(11)"O"_2 → color(blue)(7)"CO"_2 + color(green)(8)"H"_2"O}$

Every formula now has a fixed coefficient. We should have a balanced equation.

Let’s check:

$\textcolor{w h i t e}{m} \text{Element"color(white)(m)"Left-hand side"color(white)(m)"Right-hand side}$
$\textcolor{w h i t e}{m m l l} \text{C} \textcolor{w h i t e}{m m m m m l} 7 \textcolor{w h i t e}{m m m m m m m m l l} 7$
$\textcolor{w h i t e}{m m l l} \text{H} \textcolor{w h i t e}{m m m m l l} 16 \textcolor{w h i t e}{m m m m m m m m} 16$
$\textcolor{w h i t e}{m m l l} \text{O} \textcolor{w h i t e}{m m m m l l} 22 \textcolor{w h i t e}{m m m m m m m m} 22$

All atoms balance. The balanced equation is

$\text{C"_7"H"_16 + 11"O"_2 → 7"CO"_2 + 8"H"_2"O}$