A chemical equation is a symbolic shorthand representing a chemical reaction, showing the starting substances (reactants) and the resulting substances (products). To accurately reflect the physical event, the equation must be verified as “balanced.” This verification confirms that all atoms are accounted for from start to finish by systematically counting every atomic unit on both sides of the reaction arrow.
Key Terminology for Chemical Equations
A chemical equation is divided into two sides by an arrow, which signifies the direction of the reaction. The substances on the left side are the reactants, which are the starting materials. The substances on the right side are the products, which are the new materials formed.
Two types of numbers are used in chemical notation. Subscripts are the small, lowered numbers appearing after an element’s symbol. They indicate the number of atoms of that specific element within a single molecule. For example, the “2” in \(\text{H}_2\text{O}\) means there are two hydrogen atoms in one water molecule.
Coefficients are the larger numbers placed directly in front of an entire chemical formula. They represent the number of molecules or units of that substance involved in the reaction. If no coefficient is written, a value of one is understood. A subscript is a fixed part of a molecule’s identity and cannot be altered, but the coefficient is the number adjusted to balance the equation.
The Step-by-Step Verification Process
The necessity for a balanced equation stems directly from the Law of Conservation of Mass. This law states that matter cannot be created or destroyed during a chemical reaction. Therefore, the total mass of the reactants must equal the total mass of the products. This is only true if the number of atoms of each element remains unchanged throughout the process, meaning atoms are merely rearranged, not lost or gained.
The first step in verification is to create a tally table. List every unique element symbol present in the equation. The equation’s arrow acts as a divider, requiring separate columns for the reactant side and the product side. The atom count for each element must then be systematically calculated for both sides.
To calculate the total number of atoms for an element, multiply the compound’s coefficient by the element’s subscript. For instance, in \(3\text{H}_2\text{O}\), the coefficient 3 is multiplied by the hydrogen subscript 2, yielding six hydrogen atoms. If an element appears in multiple compounds on the same side, the calculated atom counts from each compound must be added to find the total.
The final step is to compare the total atom count for each element on the reactant side with the total count on the product side. For the equation to be balanced, the final number for every element must be exactly the same on both sides. If even one element has a different count, the equation is unbalanced and does not satisfy the Law of Conservation of Mass.
Special Considerations for Counting Atoms
Counting atoms involves a special consideration for polyatomic ions, which are groups of atoms that act as a single unit and carry an overall charge (e.g., sulfate (\(\text{SO}_4\))). If a polyatomic ion appears on the reactant side and remains intact on the product side, it is most efficient to count it as a single unit rather than counting each element separately. For example, one would simply count the number of \(\text{SO}_4\) units on each side.
This technique simplifies verification, especially in equations involving many elements. Individual atoms must only be counted separately if the polyatomic ion breaks apart or changes its internal structure during the reaction.
State symbols are another feature included in a chemical equation, clarifying the physical form of the substance. These symbols are written in parentheses next to the formula: \(\text{(s)}\) for solid, \(\text{(l)}\) for liquid, \(\text{(g)}\) for gas, and \(\text{(aq)}\) for aqueous (dissolved in water). While important for complete notation, these symbols are disregarded when performing the numerical count of atoms to check for balance.
Applying the Verification Method
The verification method provides a definitive check on whether an equation is correctly written. Consider the formation of hydrogen chloride: \(\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}\). Counting the atoms shows two hydrogen atoms on the left and \(2 \times 1 = 2\) on the right. Similarly, there are two chlorine atoms on the left and \(2 \times 1 = 2\) on the right, confirming the equation is balanced.
In contrast, an unbalanced equation, like the formation of water, \(\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}\), demonstrates the tally method’s value. The reactant side has two hydrogen atoms and two oxygen atoms. The product side has two hydrogen atoms but only one oxygen atom.
Comparing the totals reveals that hydrogen atoms are balanced (two on each side), but oxygen atoms are not (two on the left, one on the right). Because the oxygen count is unequal, the entire equation is unbalanced. The verification process confirms that the equation does not satisfy the Law of Conservation of Mass as written, making the simple tally the final arbiter of an equation’s balance.