Limiting Reagent

Did you know? Hydrogen is the most abundant element in the Universe…it makes up about 75% of it. So far, all of the examples and equations you have seen have dealt with complete reactions. There were never any of the reactants left at the end of the reaction. However, it often happens that there is an excess of one of the reactants and some of it is left over after the reaction.

Imagine This... Mix 1 teaspoon of baking soda and 1 teaspoon of vinegar in a glass. Observe the reaction. Allow the reaction to continue until no more bubbles are produced. Add 1 more teaspoon of baking soda. What happens? Repeat the experiment, but this time add 1 teaspoon of vinegar once the reaction has stopped. What happens? ◦ Which substance is the limiting reagent?

Limiting and Excess Reagent If one reactant is entirely used up before any of the other reactants, then that reactant limits how much product we get, or the maximum yield. Once it runs out, the reaction stops. This is called the limiting reagent or limiting reactant. The reactant that is left over after the other reactants have been used up is said to be in excess.

How can you tell? To find the amount of product that can be produced during a reaction, you must examine the amounts of all of the reactants. You have to calculate how much product can be produced from each of the reactants (assuming there is enough of all the other reactants). The substance that produces the least amount of the product is the limiting reagent—it is this reactant which governs how much product is formed and the amount of excess in the other reactants.

Example 1 Ammonia is produced from hydrogen and nitrogen according to: When 1 molecule of nitrogen reacts with 3 molecules of hydrogen, 2 molecules of ammonia are produced.

What would happen if 2 molecules of N2 reacted with 3 molecules of H2? According to the balanced equation, nitrogen and hydrogen react in a 1:3 molecule ratio. 1 molecule of N2 reacts with 3 molecules of H2 to produce 2 molecules of NH3. So, if you start with 2 molecules of N2 and 3 molecules of H2, 1 molecule of nitrogen will react with 3 molecules of hydrogen but 1 molecule of N2 will be left over at the end. In this reaction, only the hydrogen is completely used up, so it is the limiting reagent. The nitrogen is the excess reagent

Tip Limiting reagent recap 1. You can recognize a limiting reactant problem because there is more than one amount that is given in the question. 2. Convert all of the reactants to the SAME product (pick any product you choose). 3. The lowest answer corresponds to the substance that is the limiting reagent. 4. The other reactant(s) are in excess.

Example 2 Methane, CH4, burns in oxygen to give carbon dioxide and water according to the following equation: In an experiment, a mixture of 0.25 mol of methane is burned in 1.25 mol of oxygen in a sealed steel vessel. Find the limiting reactant. 1. You can recognize a limiting reactant problem because there is more than one amount that is given in the question. There are 2 values given in the question. 2. Convert all of the reactants to the SAME product (pick any product you choose.) 3. The lowest answer corresponds to the substance that is the limiting reagent. CH4 is the limiting reagent since it produces the least amount of CO2 4. The other reactant(s) are in excess. O2 is in excess; there is some left over in the end.

Tip: Balanced Chemical Reactions Use Mole Ratios The two examples above asked you to consider molecules and moles which directly match the numerical coefficients in the balanced chemical reaction. If you are given mass or volume of reactants, you must convert these measurements to moles to assess which is the limiting reactant.

Check Your Understanding 5.0 g of iron and 5.0 g of sulphur are heated together to form iron (II) sulphide. Which reagent is present in excess and what mass of product is formed?