Reactant
Definition and meaning of Reactant in chemistry.
A reactant is an essential starting ingredient in any chemical reaction. It is the substance that gets changed into something entirely new. Reactants are destroyed as their chemical bonds break apart to build new products.
In more detail
When you read a standard chemical equation, reactants sit on the left side of the arrow. The arrow points away from them to show that they are reacting. The unique traits of your starting reactants decide exactly what kind of reaction will happen.
Their starting amounts also control how fast the chemical process will actually run. As the reaction moves forward, the amount of each reactant slowly drops. Eventually, one specific reactant might completely run out before the others do.
Chemists call this the limiting reactant because it forcefully stops the whole process. This empty reactant puts a hard limit on how much final product you can possibly make. Any other starting materials left floating around are considered to be in excess.
A huge misconception is thinking that reactants simply vanish into thin air when they react. The law of conservation of mass proves they do not disappear at all. The atoms inside the reactants are just rearranged like plastic building blocks to form new shapes. The total starting weight of your reactants will always match the final weight of your products.
Key facts
| Field | General Chemistry |
|---|---|
| Chemical Role | Substance consumed in a chemical reaction |
| Location in Equation | Written on the left side of the yield arrow |
| Limiting Reactant | The first ingredient to completely run out |
| Excess Reactant | The leftover ingredients that did not react |
Consider the simple chemical reaction that creates plain liquid water. The balanced equation is written as 2H2 + O2 -> 2H2O. In this specific case, highly flammable hydrogen gas and reactive oxygen gas are the starting reactants. They sit clearly on the left side of the directional yield arrow. As the reaction sparks and burns, these two gas reactants are totally consumed. Their old chemical bonds are ripped apart to form brand new bonds. They change completely to become the final product, which is plain liquid water.
Frequently asked questions
What exactly is meant by the term limiting reactant?
The limiting reactant is the particular substance that is completely used up first. Once it is gone, the reaction stops and no more product can form.
Are all reactants always completely consumed?
No, often only the limiting reactant is completely used up in a chemical process. The other reactants will have some amount left over when the reaction finally stops.
Do the reactants weigh more than the products?
No, the total mass is always perfectly conserved during a chemical reaction. If you start with ten grams of reactants, you will end up with exactly ten grams of products.