A reactant is the starting material in any chemical process, a substance that undergoes a transformation to yield a different substance. Reactants interact, causing their molecular structures to break apart and rearrange. Their primary role is to be consumed during the process, driving the chemical change forward to its conclusion. Reactants are necessary ingredients for any chemical reaction to occur.
Reactants and the Structure of a Chemical Equation
The way reactants are represented in chemistry is highly standardized, using a symbolic shorthand known as a chemical equation. In this notation, the reactant substances are always positioned on the left-hand side of the reaction arrow. This placement instantly identifies them as the materials present before the transformation takes place.
When more than one reactant is involved in the transformation, a plus sign (+) is used to separate the different chemical formulas. For example, in a simple reaction where substance A reacts with substance B to form product C, the reactants A and B would be written as A + B. The arrow (->) functions as a symbol of change, indicating the direction of the reaction from the starting materials to the newly formed substances.
Each reactant is represented by its specific chemical formula, which describes its atomic composition, and may be preceded by a number called a stoichiometric coefficient. This coefficient indicates the precise ratio of molecules or moles of that reactant required to satisfy the reaction. Additionally, the physical state of the reactant—solid (s), liquid (l), gas (g), or aqueous solution (aq)—is sometimes noted in parentheses next to its formula.
Distinguishing Reactants from Products
The defining difference between a reactant and a product lies in their role during the chemical process: reactants are the substances consumed, and products are the new substances created. Reactants are chemically changed as their atoms break existing bonds and form new ones, resulting in molecules with entirely different properties from the starting materials.
This transformation is governed by the Law of Conservation of Mass, which dictates that matter cannot be created or destroyed in a chemical reaction. This means the total mass of all reactants must exactly equal the total mass of all products. The atoms are simply rearranged into new molecular combinations.
Consequently, every atom present in the reactant molecules must be accounted for in the product molecules. For example, if a reactant contains three carbon atoms, the products collectively must also contain exactly three carbon atoms, even though they are bonded differently. This balanced atomic inventory is the foundational principle for balancing chemical equations.
Conditions That Affect Reactant Transformation
The speed at which reactants transform into products, known as the reaction rate, is sensitive to the surrounding physical conditions. One significant factor is temperature, as an increase generally speeds up the reaction. Higher temperatures increase the kinetic energy of reactant molecules, causing them to move faster and collide more frequently and forcefully.
More energetic collisions mean a larger proportion of molecules possess the minimum energy required, called the activation energy, to effectively break old bonds and form new ones. Another important condition is the concentration of the reactants, which refers to the number of reactant molecules packed into a given volume. Increasing the concentration leads to a greater frequency of molecular collisions, increasing the chances of a successful reaction occurring.
A catalyst also plays an accelerating role in reactant transformation without being consumed in the process. Catalysts function by providing an alternative reaction pathway that has a lower activation energy than the uncatalyzed reaction. By lowering the energy hurdle, the catalyst makes it easier for reactant molecules to convert into products, accelerating the rate of the overall reaction.