A chemical equation is a symbolic representation of a chemical reaction, showing the starting materials (reactants) transforming into new substances (products). The reactants are on the left side, and the products are on the right, separated by an arrow indicating the reaction direction. Balancing these symbolic representations is a foundational skill necessary for accurate chemical representation. This guide outlines the principles and methodology required to correctly balance any standard chemical equation.
Foundation of Balancing Chemical Equations
The necessity of balancing an equation stems directly from the Law of Conservation of Mass, a fundamental principle of physical science. This law dictates that matter can neither be created nor destroyed in a closed system, meaning the total mass of the reactants must exactly equal the total mass of the products. This translates to the requirement that the number of atoms for every single element must be identical on both sides of the reaction arrow.
To begin the process, create an inventory of all elements present. List each element and count the number of atoms for that element on both the reactant side and the product side. This initial count reveals which elements are currently unbalanced.
The only permissible way to adjust the number of atoms is by placing a large whole number, called a coefficient, in front of a chemical formula. This coefficient multiplies the number of all atoms within that compound. It is absolutely prohibited to change the small subscript numbers within a chemical formula, as doing so would fundamentally change the identity and properties of the substance itself.
Step-by-Step Balancing Method
Once the initial inventory is complete, balancing should proceed with a specific, systematic order to prevent undoing previous work. A general rule of thumb is to address the more complex elements first and save the simplest, most common elements for last.
Balancing Metals
The first step is to balance any metal atoms present in the reaction, such as iron, aluminum, or sodium. By placing a coefficient in front of the metal-containing compound, the atom count for that metal can be equalized across the equation. This coefficient must then be accounted for in the atom count of any other elements within that same compound.
Balancing Non-Metals
Next, attention should shift to balancing the non-metal atoms, specifically excluding hydrogen and oxygen. Elements like chlorine, sulfur, or nitrogen are typically addressed at this stage by adjusting coefficients to match the number of atoms on the reactant and product sides.
Balancing Hydrogen
After the metals and other non-metals are addressed, the focus moves to the hydrogen atoms. Balancing hydrogen atoms is accomplished by placing coefficients in front of the relevant compounds until the atom counts are equalized.
Balancing Oxygen
The final element to balance in nearly every reaction is oxygen. Oxygen is often the most challenging element to balance because it frequently appears in multiple compounds on both sides of the equation. By saving oxygen for last, all other coefficients have already been fixed, simplifying the final adjustment needed.
Once a coefficient has been placed to balance the oxygen atoms, perform a final, comprehensive check of the entire equation. Re-count the total number of atoms for every element on both the reactant and product sides. If the atom counts for all elements are now equal, the equation is considered correctly balanced, satisfying the Law of Conservation of Mass.
Advanced Techniques and Common Difficulties
For reactions involving complex ions, a helpful shortcut can simplify the balancing process. If a polyatomic ion, such as sulfate (SO4) or nitrate (NO3), remains chemically intact and unchanged on both the reactant and product sides, it should be treated as a single unit. Instead of counting the individual sulfur and oxygen atoms, the entire group is counted as one unit, which reduces the number of elements that need to be tracked.
Combustion reactions, which involve a hydrocarbon reacting with oxygen to produce carbon dioxide and water, benefit from a specific balancing order known as the “CHO” method. The atoms should be balanced in the sequence of Carbon (C) first, then Hydrogen (H) second, and finally Oxygen (O) last.
A common complication arises in combustion reactions when balancing carbon and hydrogen results in an odd number of oxygen atoms needed on the product side. Since oxygen gas (O2) on the reactant side must be added in pairs, it can be impossible to achieve a whole-number balance immediately. In this scenario, a fractional coefficient, such as 5/2 or 7/2, can be temporarily used in front of the O2 molecule to achieve the correct atom count.
To clear the temporary fraction and obtain the required simplest whole-number ratio, every single coefficient in the entire balanced equation must be multiplied by the denominator of the fraction. For instance, if the fraction used was 5/2, multiplying all coefficients by 2 will eliminate the fraction and result in the final, correctly balanced equation using only whole numbers.