Question

In a laboratory demonstration, a balloon filled with methane and oxygen was exposed to a flame. The result was a brief, large flame. The students were asked to formulate an equation for the reaction. One answer is below.
CH4 + O2 CO2

Is this equation correct? Explain your reasoning.

Answer 1

Answer:

The equation is not correct:

[CH4 + O2 → CO2 + 2H2O]

Explanation:

The chemical formulas for methane, oxygen, and carbon dioxide are correct, but the equation is not balanced, so we really don't know idf the overall reaction is written correctly.

Let's balance the reaction:

CH4 + O2 → CO2

We need to have the same numbers of atoms on both sides.  

Count the numbers of each atom:

Element    Reactants    Products    Difference

 C                         1              1                  0                  Balanced

 H                         4              0                -4                  XXX!!!!

 O                         2              2                 0                  Balanced

The hydrogens aren't showing up in the products.  In fact, there is a product missing entirely from the reaction, H2O, water.  The basic reaction is:

CH4 + O2 → CO2 + H2O

Now balance this reaction:

Element    Reactants    Products    Difference

 C                         1              1                  0                  Balanced

 H                         4              2                -2                  Hmmmm . . .

 O                         2              3                 1                  Uh = oh . . .

This reaction is not possible - there are either too many or too few element atoms (the "Difference").

We can adjust the coefficients for each of the molecules to balanced the equation.  This is a bit tricky, but it consists of trying some coefficients that would make things balance.  Only whole number coefficients are allowed in the final answer, although fractions might be used in the process to help find the whole numbers.

We need 2 H2O molecules to account for the 4 hydrogens on a single methane molecule

Step 1:  CH4 + O2 → CO2 + 2H2O

That leads to:

Element    Reactants    Products    Difference

 C                         1               1                 0                  Balanced

 H                         4              4                0                   Balanced

 O                         2              4                 2                  Darn

We need more oxygen atoms, so:

Step 2:  CH4 + 2O2 → CO2 + 2H2O

This leads to:

Element    Reactants    Products    Difference

 C                         1               1                  0                  Balanced

 H                         4              4                 0                   Balanced

 O                         4              4                 0                   Balanced

This is a balanced reaction, and the correct chemical equation.