A chemical reaction describes a transformation of substances. Sometimes, the equation is incomplete, missing a starting material or a final substance. Understanding how to determine this unknown piece is a key exercise in chemistry. By following a systematic approach, one can deduce the identity of the missing component and complete the chemical description.
Initial Analysis of the Equation
The first step is a careful examination of the provided chemical equation. An equation displays reactants, the starting materials, on the left side of a reaction arrow (→) and products, the resulting substances, on the right. Recognizing which side your unknown substance is on—whether it is being consumed or created—is the initial piece of the puzzle.
Equations also contain other clues. Subscripts within a chemical formula, like the ‘2’ in H₂O, tell you the number of atoms in a molecule. You should also note the physical state of each substance, indicated by symbols like (s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution. Any symbols written above or below the reaction arrow, such as the Greek letter delta (Δ) for heat or a chemical formula like Pt for a platinum catalyst, specify the conditions required for the reaction to occur.
Identifying the Reaction Type
With an analysis of the given information, the next phase involves identifying the type of reaction. Many chemical reactions fall into common categories, each with a predictable pattern. Recognizing this pattern often provides a direct pathway to identifying the unknown substance by revealing the expected structure of the products or reactants.
A synthesis reaction, for example, involves two or more simpler substances combining to form a single, more complex product, following the pattern A + B → AB. If you see two elements as reactants and only one space for a product, you can predict they will combine. For instance, when solid magnesium, Mg(s), reacts with oxygen gas, O₂(g), they form magnesium oxide, MgO(s). The equation would be 2Mg(s) + O₂(g) → 2MgO(s).
A decomposition reaction is the opposite, where a single complex compound breaks down into two or more simpler substances (AB → A + B). If you start with one reactant like carbonic acid, H₂CO₃(aq), and see two empty product slots, you can predict it will decompose. In this case, it breaks down into water, H₂O(l), and carbon dioxide, CO₂(g).
A single displacement reaction is where one element takes the place of another in a compound (A + BC → AC + B). If zinc metal, Zn(s), is added to a copper(II) chloride solution, CuCl₂(aq), the zinc will displace the copper. This results in zinc chloride, ZnCl₂(aq), and solid copper, Cu(s).
Double displacement reactions involve two ionic compounds swapping ions in a pattern like AB + CD → AD + CB. When a solution of silver nitrate, AgNO₃(aq), is mixed with sodium chloride, NaCl(aq), the silver and sodium ions trade places. This forms solid silver chloride, AgCl(s), and sodium nitrate, NaNO₃(aq).
Combustion reactions occur when a substance, often a hydrocarbon, reacts with oxygen gas (O₂). This process releases energy and typically produces carbon dioxide (CO₂) and water (H₂O). The combustion of methane, CH₄(g), is represented as CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g).
Applying the Law of Conservation of Mass
After identifying the likely reaction type, the final step is to apply the law of conservation of mass. This law states that atoms are neither created nor destroyed during a chemical reaction. Therefore, the number and type of atoms on the reactant side of the equation must be equal to the number and type of atoms on the product side.
To use this law, count the atoms of each element present on the known side of the equation. Once you have a complete tally, compare it to the atoms present in the known substances on the other side. The discrepancy between these two counts will reveal the precise atomic composition of the missing component.
Consider the reaction where solid sodium (Na) combines with an unknown substance to produce sodium chloride (NaCl), written as 2Na + ? → 2NaCl. On the product side, the coefficient ‘2’ in front of 2NaCl indicates there are two sodium atoms (2 Na) and two chlorine atoms (2 Cl). Looking at the reactant side, we see two sodium atoms are already accounted for in 2Na. The atoms present on the product side but missing from the reactant side are the two chlorine atoms.
Since chlorine is a diatomic element, meaning it naturally exists as a two-atom molecule, the missing reactant must be Cl₂ gas. The final, balanced equation is 2Na(s) + Cl₂(g) → 2NaCl(s).