What they are? To understand limiting reagents consider this situation. Say you're making s'mores. To make adequately sized s'mores one needs one graham cracker, 1/2 bar of chocolate and a marshmallow per s'more. Marshmallows are sold in packages 15, and graham crackers are sold in packages of 12. So if you were to buy one package of each you could make only 12 s'mores, because the number of graham crackers in a package limit the number you could make. In this case the limiting reagent is the graham crackers. If you were to buy two packages of graham crackers, then you'd be able to make 15 s'mores, because you would run out of marshmallows. In this case the marshmallows are the limiting reagents.
Another example of a limiting reagent would be in gripping my tennis racquets. I use 4 racquets, but they sell grip in packages of three. Therefore if I buy one package I run out of grips without having gripped all my tennis racquets. In this case the grips are the limiting reagent. Whereas, if I were to buy two packages of grips, I'd have more grips than I needed and the tennis racquets would be the limiting reagent.
This applies in the same way to chemistry. In any chemical reaction one of the reactants limits the amount of substance you can have. Here's how you know which one.
How to find the limiting reagent of a chemical reaction
1 Balance the equation
2 Find the GFW of each reactant
3 Divide it by the number of grams of each reactant to find the number of moles of each reactant
4 Multiply the number of moles in each reactant by the coefficient of the other reactant as to compare the number of moles in proportion to the amount of the reactant
5 Compare the product of the moles in the reactant and the opposite coefficient and the lesser one is the limiting reagent because it's the one which you run out of first.
Another example of a limiting reagent would be in gripping my tennis racquets. I use 4 racquets, but they sell grip in packages of three. Therefore if I buy one package I run out of grips without having gripped all my tennis racquets. In this case the grips are the limiting reagent. Whereas, if I were to buy two packages of grips, I'd have more grips than I needed and the tennis racquets would be the limiting reagent.
This applies in the same way to chemistry. In any chemical reaction one of the reactants limits the amount of substance you can have. Here's how you know which one.
How to find the limiting reagent of a chemical reaction
1 Balance the equation
2 Find the GFW of each reactant
3 Divide it by the number of grams of each reactant to find the number of moles of each reactant
4 Multiply the number of moles in each reactant by the coefficient of the other reactant as to compare the number of moles in proportion to the amount of the reactant
5 Compare the product of the moles in the reactant and the opposite coefficient and the lesser one is the limiting reagent because it's the one which you run out of first.