Balancing a chemical equation is a foundational process in chemistry that ensures the reaction adheres to the Law of Conservation of Mass. This principle means the total number of atoms for each element must be identical on both sides of the reaction arrow. To achieve this balance, whole numbers called coefficients are placed in front of the chemical formulas; these numbers multiply the atoms within the entire molecule. The small subscript numbers within a chemical formula must never be altered during the balancing process. This article will walk through the steps required to balance the combustion reaction involving propane (C3H8).
Setting Up the Unbalanced Equation
The reaction being examined is the combustion of propane, which involves the fuel (C3H8) reacting with oxygen gas (O2) to produce carbon dioxide (CO2) and water (H2O). Written in its initial, unbalanced form, the equation is C3H8 + O2 \(\rightarrow\) CO2 + H2O. Counting the atoms of each element on the reactant side reveals three carbon atoms, eight hydrogen atoms, and two oxygen atoms.
On the product side, the initial count is one carbon atom in CO2, two hydrogen atoms in H2O, and a total of three oxygen atoms (two from CO2 and one from H2O). Because the atom counts are unequal, the equation is unbalanced and does not satisfy the Conservation of Mass. The general strategy for balancing hydrocarbon combustion reactions is to address the carbon and hydrogen atoms first, leaving the oxygen atoms for the final step.
Balancing Carbon and Hydrogen Atoms
The first step in balancing is to equalize the carbon atoms. The propane molecule (C3H8) on the reactant side contains three carbon atoms, but the product side only shows one carbon atom within the carbon dioxide molecule (CO2). Placing a coefficient of 3 in front of the carbon dioxide molecule ensures that the carbon atoms are balanced. The equation is now partially balanced as C3H8 + O2 \(\rightarrow\) 3CO2 + H2O.
Next, the focus shifts to the hydrogen atoms. The C3H8 molecule has eight hydrogen atoms, while the water molecule (H2O) on the product side contains two hydrogen atoms. To achieve eight hydrogen atoms on the product side, a coefficient of 4 is required for the water molecule. The equation is now C3H8 + O2 \(\rightarrow\) 3CO2 + 4H2O, and both the carbon and hydrogen atoms are successfully balanced.
This approach of balancing carbon and hydrogen first is effective because those elements each appear in only one molecule on the product side, simplifying the coefficient assignment. The oxygen atoms, by contrast, appear in two different product molecules, which makes them much easier to balance last after the coefficients for the other products have been fixed.
Determining the Oxygen Coefficient
With the coefficients for carbon dioxide and water now fixed, the final step involves determining the correct coefficient for the molecular oxygen (O2) on the reactant side. Oxygen is the last element to be balanced because it appears in both of the product molecules. The number of oxygen atoms on the product side must be calculated by multiplying the new coefficients by the oxygen subscripts in both products.
The three molecules of carbon dioxide (3CO2) contribute a total of six oxygen atoms. The four molecules of water (4H2O) contribute an additional four oxygen atoms. Summing these two amounts reveals that a total of ten oxygen atoms are present on the product side of the equation.
Since the reactant side has diatomic oxygen (O2), the required coefficient must be a number that, when multiplied by 2, equals ten oxygen atoms. Dividing the total of ten by 2 yields the coefficient 5. The equation is now fully balanced: C3H8 + 5O2 \(\rightarrow\) 3CO2 + 4H2O.
Verifying the Final Balanced Equation
The final step is to verify that the entire equation is balanced by confirming that the total number of atoms for every element matches on both sides of the arrow. The reactants side, which is C3H8 + 5O2, contains three carbon atoms, eight hydrogen atoms, and ten oxygen atoms (\(5 \times 2\)).
The products side, 3CO2 + 4H2O, must be checked element by element using the assigned coefficients. The carbon count is three (\(3 \times 1\)), the hydrogen count is eight (\(4 \times 2\)), and the oxygen count is ten, derived from six in the carbon dioxide (\(3 \times 2\)) plus four in the water (\(4 \times 1\)). The balanced equation C3H8 + 5O2 \(\rightarrow\) 3CO2 + 4H2O accurately represents the chemical reaction.