Limiting Reagents

Limiting Reagent -- the reactant used up first in the chemical reaction.

Excess Reagent(s) -- the reactant(s) present in quantities greater than necessary to react with the quantity of the limiting reagent.

Limiting Reagent

The concept of limiting reagent is analogous to the relationship between men and women in a dance at a club. If there are 14 men and only 9 women, then only 9 female/male pairs can compete. Five men will be left without partners. The number of women, thus limits the number of men that can dance in the contest, and there is an excess of men.

Example # 1Consider the formation of nitrogen dioxide (NO2) from nitric oxide (NO) and oxygen (O2)

2 NO(g) + O2(g) -----> 2 NO2(g)

Suppose initially we have 8 moles of NO and 7 moles of O2.

One way to determine the limiting reagent is to calculate the number of moles of NO2 obtained based on the initial quantities of NO and O2.

Remember the limiting reagent will yield the smaller amount of the product.

Calculate the number of moles of product obtained from each reactant. The one that yields the smallest amount of product is the limiting reagent.

2 NO(g) + O2(g) -----> 2 NO2(g)

Starting with 8 moles of NO, we find the number of moles of NO2 produced is

8 mol NO x 2 mol NO2 = 8 mole NO2

2 mol NO

Now, starting with 7 moles of O2, the number of moles of NO2 produced is

7 mol O2 x 2 mol NO2 = 14 mole NO2

1 mol O2

Because NO yields the smaller amount of NO2, it must be the limiting reagent.

Therefore, O2 is the excess reagent.

Example # 2

The depletion of ozone (O3) in the stratosphere has been a matter of great concern among scientists in recent years. It is believed that ozone can react with nitric oxide (NO) that is discharged from the high-altitude jet plane, the SST. The reaction is

O3 + NO -----> O2 + NO2

Example # 2 - continued

If 0.710 g of O3 reacts with 0.670 g of NO,

a) how many grams of NO2 will be produced?

b) Which compound is the limiting reagent?

c) Calculate the number of grams of the excess reagent remaining at the end of the reaction?

Example # 2 - continued

First we need to make sure the equation is balanced:

O3 + NO -----> O2 + NO2

This equation as it is written is balanced.

Example # 2 - continued

a) 0.710 g O3 1 mole O3 1 mol NO2 46.01 g NO2

48.00 g O3 1 mol O3 1 mole NO2

= 0.680 g NO2

0.670 g NO 1 mole NO 1 mol NO2 46.01 g NO2

30.01 g NO 1 mol NO 1 mol NO2

= 1.027 g NO2

Example # 2 - continued

b) Because O3 yields the smallest amount of NO2, it is the limiting reagent. Then, NO must be the excess reagent.

c) 1O3 + 1NO -----> 1O2 +1NO2

Because there is a 1 to 1 mole ratio between NO and NO2, then the amount of excess reagent left after the reaction is complete, assuming all of the O3 reacted, is:

0.710 g O3 1 mol O3 1 mol NO 30.01 g NO

48.00 g O3 1 mol O3 1 mol NO

= 0.444 g NO

Therefore, 0.670 g NO – 0.444 g NO = 0.226 g NO remaining

Practice Problem #1

1) Propane (C3H8) is a component of natural gas and is used in domestic cooking and heating.

(a) balance the following equation representing the combustion of propane in air:

C3 H8 + O2 -----> CO2 + H2O

b) How many grams of CO2 can be produced by burning 3.65 moles of C3 H8? Assume that O2 is the excess reagent in this reaction.

Practice Problem #2

2) Consider the reaction

MnO2 + 4HCl ----> MnCl2 + Cl2 + 2H2O

If 0.86 mole of MnO2 and 48.2 g of HCl react,

a) Which reagent will be used up first? Show your work.

b) How many grams of Cl2 will be produced?

Practice Problem #3

3) In the production of disulfur dichloride, molten sulfur reacts with chlorine gas according to the equation below:

S8 (l) + 4 Cl2(g) ----> 4SCl2(l)

If 200.0 g of S react with 100.0g of Cl2,

a) what mass of disulfur dichloride is produced?

b) What reactant is the limiting reagent? Show your work.

c) What reactant is the excess reagent? Show your work.

d) How much of the excess reagent is left in grams?

Practice Problem #4

4)The reaction between solid white phosphorus and oxygen produces solid tetraphosphorus decoxide (P4O10).

a) Write the equation and balance it. Hint use the info. below to determine the reagents.

b) Determine the mass of P4O10 formed if 25.0g of P4 and 50.0g of O2 are combined.

c) What is the limiting reagent? Show your work.

d) How much of the excess reactant remains after the reaction stops?

Practice Problem #5

5) The reaction between solid sodium and iron(III)oxide is one is a series of reactions that inflates an automobile airbag.

6Na(s) + Fe2O3(s) -----> 3Na2O(s) + 2 Fe(s)

If 100.0 g Na and 100.0 g Fe2O3 are used in this reaction, determine

a) The limiting reagent.

b) The reactant in excess.

c) The mass of solid iron produced.

d) The mass of excess reactant that remains after the reaction is complete.

Practice Problem #6

6) Photosynthesis reactions in green plants use carbon dioxide and water to produce glucose (C6H12O6) and oxygen.

a) Write the balanced equation.

b) If a plant had 88.0g of carbon dioxide and 64.0 g water available for photosynthesis, determine

i. The limiting reactant

ii. The excess reactant and the mass in excess

iii. The mass of glucose produced