Quantitative Chemistry - Limiting Reactants

Why don't chemical reactions go on indefinitely?

A chemical reaction does not go on indefinitely and stops when one of the reagents is used up.

What is the reagent used up first in a reaction?

The reagent that is used up first is the limiting reactant, as it limits the duration of the reaction and hence the amount of product that a reaction can produce

What is the name of the reagent remaining after the reaction stops

The one that is remaining is the excess reactant

What is the limiting reactant?

The limiting reagent is the reactant which is not present in excess in a reaction

How does the amount of product available from the reaction relate to the limiting reactant?

The amount of product obtainable is therefore directly proportional to the amount of the limiting reagent added at the beginning of a reaction

If you use half the limiting reagent how much product will you get providing other reagents are present in excess?

if you use half of the limiting reagent then you will get half of the product

If you double the amount of limiting reagent how much product will you get?

If you double the amount of the limiting reagent then you obtain double the amount of product

How do you determine which reactant in the limiting reagent in the reaction?

consider the amounts of each reactant used and the molar ratio of the balanced chemical equation

Which reagent should you use when performing mass calculations? Why?

When performing reacting mass calculations, the limiting reagent is always the number that should be used, as it indicates the maximum possible amount of product that can form. Once all of a limiting reagent has been used up, the reaction cannot continue.

What are the steps to determine which is the limiting reactant?

1. Convert the mass of each reactant into moles by dividing by the molar masses

2. Write the balanced equation and determine the molar ratio

3. Look at the equation and compare the moles

How would a reactant be present in excess?

For a reactant to be present in excess, there only needs to be slightly more of it present than the other reactant, as determined from the molar ratio. In a two-reactant system, if one reactant is in excess then the other is by default the limiting reagent.

What is a common error made?

A common error is to determine the limiting reactant as the reactant with the least amount of moles in the molar ratio. This is incorrect as the masses of each reactant must also be considered.