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SECTION
Limiting and Excess Reactants
Consider again the turkey sandwich example in Figure 7.1. The recipe for a turkey
sandwich can be written as an equation:
2 toast slices + 2 turkey slices + 1 lettuce leaf + 1 tomato slice → 1 turkey sandwich
A certain number of each ingredient is necessary to make a turkey sandwich according
to the recipe. However, there is always the possibility that there will be a short supply
of at least one ingredient. When an ingredient runs out, the production of turkey
sandwiches by the recipe stops. In other words, the ingredient that runs out first limits
the quantity of sandwiches that can be produced. For example, suppose that you have
four toast slices, six turkey slices, three lettuce leaves, and three tomato slices, as shown
in Figure 7.6. Because each sandwich requires two slices of toast, you can make only
two sandwiches, even though you have enough of the other ingredients to make three
turkey sandwiches. The number of slices of toast limits the number of sandwiches you
can make. The other ingredients are in excess and are left over.
+ + +
+ + +
4 toast slices
6 turkey slices
3 lettuce leaves
3 tomato slices
2 turkey sandwiches + 2 turkey slices + 1 lettuce leaf + 1 tomato slice
Figure 7.6 Once you make two turkey sandwiches, there is no more toast to make additional sandwiches. Toast is the “limiting ingredient.” The other ingredients are in excess and are left over.
Limiting and Excess Reactants in Chemical Reactions
If the reactants in a chemical reaction are present in amounts that correspond exactly
to the mole ratios from the balanced chemical equation, they are said to be present in
stoichiometric amounts. If the reactants are present in stoichiometric amounts, ideally
the reaction stops when no trace of the reactants are left. However, in actual chemical
reactions, one reactant is usually in shorter supply than the other reactants. In other
words, it is rare for the reactants in a chemical reaction to be present in amounts that
correspond exactly to the mole ratios from the balanced chemical equation. In most
reactions, there is at least one leftover reactant when the chemical reaction stops.
Consider a candle burning in a room. There is an unlimited amount of oxygen in
the air, so the reaction proceeds until the wax is gone. However, if a candlesnuffer, like
the one in Figure 7.7, is put over the burning candle, the amount of oxygen is limited.
The combustion reaction stops when the supply of oxygen is gone, even though there
is wax available. The reactant that limits or stops a reaction, such as the oxygen in
Figure 7.7, is called the limiting reactant or limiting reagent. The limiting reactant
determines the amount of product that is formed. The reactants that are left over, such
as the candle wax, are called excess reactants.
stoichiometric amount the exact molar amount of a reactant or product, as predicted by a balanced chemical equation limiting reactant a reactant that is completely consumed during a chemical reaction, limiting the amount of product that is produced excess reactant a reactant that remains after a reaction is over
Key Terms
stoichiometric amount limiting reactant excess reactant
306 MHR • Unit 3 Quantities in Chemical Reactions
The Limiting Reactant Forms Less Product
The limiting reactant is not necessarily the reactant that is present in the smaller
amount. It is the reactant that forms the smaller amount of product. For example,
consider the chemical reaction that produces water:
2 H2(g) + O2(g) → 2 H 2 O(g)
If oxygen is present in excess and 2 mol of hydrogen is available, then 2 mol of water
is produced. However, if hydrogen is in excess and 2 mol of oxygen is available, then
4 mol of water is produced. In each situation, the limiting reactant is the reactant
that forms the smaller amount of product. Similarly, the limiting reactant is not
necessarily the reactant that has the lower mass. It is the reactant that produces the
lower mass of product.
Identifying the Limiting Reactant in a Chemical Reaction
It is important to identify the limiting and excess reactants. This is because the amount
of limiting reactant that is available for a chemical reaction determines the amount of
product that is formed and the amount of excess reactant that is left over. To identify
the limiting reactant, you need to determine which reactant yields the smaller amount
of product in a chemical reaction. The activity below demonstrates the importance of
considering the limiting reactant.
Suggested Investigation Inquiry Investigation 7-A, Limiting and Excess Reactants
Figure 7.7 There was excess oxygen available for the burning candle until a candlesnuffer was used to limit the oxygen.
Suppose that you have been hired by a furniture company. Your job is to put together kits for making kitchen chairs. Each kit contains all the parts that are needed to assemble one kitchen chair. The equation for one kitchen chair is given below.
Procedure
1. Assume that you have 36 frames, 128 legs, 256 leg braces, 100 hardware packages, and 1000 assembly manuals. How many complete chair kits can you make? 2. Determine the item that will limit the number of complete chair kits you can make. 3. Determine the items you have in excess amounts. 4. Calculate how much of each excess item remains after you make the chair kits.
Questions
1. You have 36 chair frames. Why are the chair frames not the limiting item, even though they are present in the smallest quantity? 2. Does an item that is available in excess affect the quantity of complete chair kits that you can make? Explain your answer.
Activity (^) 7.2 Identify the Limiting Item
3 leg braces
1 frame (^) + 4 legs + + 2 hardware + = 1 kitchen chair packages
1 assembly manual
Chapter 7 Chemical Reactions and Stoichiometry • MHR 307
31. Hydrogen fluoride, HF(g), is a highly toxic gas. It is produced according to the following balanced chemical equation: CaF2 (s) + H2SO 4 (aq) → 2HF(g) + CaS O 4 (s) Determine the limiting reactant when 1.00 g of calcium fluoride, Ca F 2 (s), reacts with 15.5 g of sulfuric acid, H2SO4(aq). 32. An ester is an organic compound that forms when a carboxylic acid reacts with an alcohol. Esters often are used as essences or scents. One such ester is methyl salicylate, C 8 H8O3(aq), which is oil of wintergreen. It is formed by the reaction of salicylic acid, C 7 H6O3(aq), and methanol, CH 3 OH(aq), as shown below: C7H6O3 (aq) + C H3OH(aq) → C (^) 8H 8 O3(aq) + H 2 O() If 100.11 g of salicylic acid and 90.4 g of methanol are used to produce oil of wintergreen, which is the limiting reactant? 33. Acetylene, C 2 H2(g), is used in welding. It forms when calcium carbide, CaC 2 (s), reacts with water, as shown below: CaC 2 (s) + 2 H 2 O() → Ca(OH ) 2 (aq) + C 2 H (^) 2(g) If 5.50 mol of calcium carbide reacts with 3.75 mol of water, which is the limiting reactant? 34. Nickel(II) chloride, Ni Cl2(aq), reacts with sodium phosphate, Na 3 PO 4 (aq), according to the following balanced chemical equation: 3NiCl 2 (aq) + 2Na (^) 3P O 4 (aq) → Ni 3 (PO (^) 4)2(s) + 6NaCl(aq) If 10.0 g of each reactant is used, which is the limiting reactant? 35. Copper metal reacts with nitric acid, HNO (^) 3(aq), as follows: 3Cu(s) + 8HNO 3 (aq) → 3Cu(NO 3 )2(aq) + 2NO(g) + 4 H2O() If 2.5 g of copper reacts with 25.0 g of nitric acid, which reactant is in excess? 36. Lithium reacts with oxygen to form lithium oxide, Li2O(s). 4Li(s) + O 2 (g) → 2Li2O(s) When 20.0 g of lithium metal reacts with 30.0 g of oxygen gas, which reactant is limiting and which reactant is in excess? 37. Chlorine gas is used in the textile industry to bleach fabric. Excess chlorine is removed by a reaction with sodium thiosulfate, Na 2 S 2 O3(aq), as shown below: Na2S2O3(aq) + 4Cl 2 (g) + 5H (^) 2O() → 2NaHS O4(aq) + 8HCl(aq) If 42.5 g of sodium thiosulfate and 175 g of chlorine gas react with excess water, which is the limiting reactant? 38. Acrylonitrile, C 3 H3N(g), is prepared by the reaction of propylene, C3H6(g), with nitric oxide, NO(g). 4 C3H6(g) + 6NO(g) → 4 C (^) 3H (^) 3N(g) + 6 H2 O(g) + N 2 (g) If 126 g of propylene reacts with 175 g of nitric oxide, which is the limiting reactant? 39. Insoluble silver carbonate, Ag (^) 2CO3 (s), forms in the following balanced chemical reaction: 2AgNO 3 (aq) + K 2 CO 3 (aq) → Ag 2 CO 3 (s) + 2KNO 3 (aq) What mass of silver nitrate, AgNO 3 (aq), reacts with 25.0 g of potassium carbonate, K2CO3(aq), if there is at least 5.5 g of silver nitrate in excess?
Practice Problems
7. Explain what is meant by the term “stoichiometric amount.” 8. Identify the limiting reactant and excess reactant in each situation. Describe any assumptions you made. a. A pilot flame flickers in a gas fireplace. b. Vinegar is used to remove deposits in a kettle. c. A peeled potato turns brown while sitting on a kitchen counter. 9. Four slices of toast, four slices of turkey, two lettuce leaves, and one slice of tomato are available to make
turkey sandwiches. Based on Figure 7.1 , which ingredient is the limiting ingredient?
10. Is the limiting reactant always the compound that is present in the smaller amount? Explain your answer. 11. Why are reactants present in excess amounts not considered when determining the product yield by stoichiometric calculations? 12. When a small quantity of phosphorus, P4(s), reacts with oxygen gas in open air, which reactant do you think is in excess? Explain your answer.
Learning Check
Chapter 7 Chemical Reactions and Stoichiometry • MHR 309
Using the Limiting Reactant to Find the Amount of Product
Identifying the limiting reactant is crucial for predicting the amount of product that is
formed in a chemical reaction, as shown in the following Sample Problem.
Stoichiometry Using a Limiting Reactant
Problem The thermite reaction, shown on the right, is a reaction of powdered aluminum with iron(III) oxide, Fe 2 O3(s). This reaction produces so much heat that the iron formed is actually molten (liquid). The balanced chemical equation is 2Al(s) + Fe2O3(s) → Al (^) 2O3(s) + 2Fe() If 113.00 g of aluminum powder is mixed with 279.50 g of iron(III) oxide, what mass of molten iron forms?
What Is Required? You need to find the mass of molten iron that forms.
What Is Given? You know the balanced chemical equation: 2Al(s) + Fe (^) 2O3(s) → Al (^) 2O3(s) + 2Fe() You know the mass of aluminum powder: 113.00 g You know the mass of iron(III) oxide: 279.50 g
Plan Your Strategy Act on Your Strategy Calculate the molar masses, M, of aluminum, iron(III) oxide, and iron.
MAl = 26.98 g/mol MFe 2 O 3 = 2 MFe + 3 MO = 2(55.85 g/mol) + 3(16.00 g/mol) = 159.70 g/mol MFe = 55.85 g/mol Convert the masses of aluminum and iron(III) oxide into amounts (in moles) using the equation
n = _ Mm.
nAl = _ MmAl
Al
= _26.98 g/mol113.00 g = 4.18829 mol Al
nFe 2 O 3 = _m MFeFe^2 O^3
2 O 3
= _159.70 g/mol279.50 g = 1.75016 mol Fe 2 O
Calculate the amount of iron that forms by the given amount of aluminum and the given amount of iron(III) oxide.
nFe = 4.18829 mol Al × _2 mol Al2 mol Fe = 4.18829 mol Fe(s)
nFe = 1.75016 mol Fe 2O3 × _1 mol Fe2 mol Fe 2 O 3 = 3.50032 mol Fe(s)
Compare the amounts of iron that form by aluminum and iron(III) oxide to determine the limiting reactant.
Iron(III) oxide produces less iron than aluminum does. Therefore, the limiting reactant is iron(III) oxide.
Determine the mass of iron that forms using m = n × M and the amount of iron that the limiting reactant forms.
m = n × M = 3.50 mol × 55.85 g/mol = 195.49 g Fe Therefore, 195.49 g of molten iron is formed.
Check Your Solution The amount in moles of iron(III) oxide is less than half the amount of aluminum. The mole ratio is 2 mol of aluminum to 1 mol of iron(III) oxide. Iron(III) oxide is the limiting reactant.
Sample Problem
Thermite Reaction
310 MHR • Unit 3 Quantities in Chemical Reactions
Applications of Stoichiometry and Limiting Reactants
Stoichiometry, chemical reactions, and limiting reactants are not topics that are
relevant only in chemistry class or in the laboratory. You benefit from stoichiometry in
your daily life, although you might not realize it. The photographs in Figure 7.8 show
some examples of quantitative chemistry in everyday applications. The activity below
also highlights the importance of quantities in chemical reactions in our society.
Processes in the home and in the workplace often involve the use of chemical quantities and calculations. When you mix cleaning solutions in your home, you often mix them in required proportions according to instructions on the product labels. These proportions are determined by chemists and are designed to give the best results. Pharmacists and other medical professionals must mix medication doses, such as chemotherapy, using stoichiometry to obtain a mixture that achieves the desired results without harming patients. Gardeners, farmers, and nursery workers often mix fungicides, pesticides, herbicides, and fertilizers with water or other ingredients to achieve the correct proportions. If these products are not mixed correctly, the plants might die and damage to the surrounding environment can occur.
Procedure
1. Choose one of the following products that involve the use of stoichiometry and limiting and excess reactants. - a specific type of pesticide, herbicide, or fungicide (organic or synthetic) that is used in home gardens or in commercial operations
- a pharmaceutical product (either an over-the-counter medication or a prescribed medication)
- a household cleaning solution (organic or synthetic)
- a consumer product, such as hair colouring, hair relaxer, or permanent wave mixture
- a product of your choice (approved by your teacher) 2. Use the Internet, product instruction sheets, pharmaceutical inserts, or other reliable resources to research information about your topic.
Questions
1. Answer the following questions as you complete your research: a. What is the purpose of your product? b. What chemical quantities or calculations are required to ensure safe use of your product? c. What are the possible consequences if your chosen product is not mixed correctly? d. When your product is being applied or used, what are the limiting and excess reactants? 2. Prepare a short presentation of your findings to share with the class. Consider questions that your classmates might ask about the topic as you prepare your presentation.
Activity (^) 7.3 Stoichiometric Applications
Limiting Reactants and Product Formed
In this section, you were introduced to the concept of the limiting reactant in a chemical
reaction. You learned that once you have identified the limiting reactant, you can use
stoichiometric calculations to predict how much product will be formed in a chemical
reaction. However, sometimes the amount of product that is actually formed is quite
different from the amount you predicted. In the next section, you will learn why.
Figure 7.8 Many processes in the home, workplace, and environment involve the use of chemical quantities and calculations. For example, concentrated herbicides (A) must be properly diluted before they are applied to invasive species. Hairdressers (B) mix chemicals to colour, straighten, and curl hair.
A B
312 MHR • Unit 3 Quantities in Chemical Reactions
Section Summary
Section 7.2 REVIEW
Review Questions
1. K/U Calcium reacts with water. If a small piece is dropped into a beaker of water, which reactant is in excess during this reaction? 2. T/I If 47.2 mol of lead(II) oxide, PbO(s), reacts with 6 mol of oxygen according to the equation below, which is the limiting reactant? 6PbO(s) + O (^) 2(g) → 2 Pb3O4(g) 3. T/I If 72.15 g of pentane, C5H12(g), reacts with 6.9 mol of oxygen, according to the equation below, which is the limiting reactant? C5H 12 (g) + 8 O2(g) → 5CO 2 (g) + 6 H (^) 2O(g) 4. T/I In an experiment, 57.4 g of iron(III) chloride in solution reacts with 45.3 g of sodium hydroxide in solution, as shown below. FeCl3(aq) + 3NaOH(aq) → Fe( OH) (^) 3(s) + 3NaCl(aq) a. Which reactant is the limiting reactant? b. How much of the excess reactant remains after the reaction? c. How much of each product forms? 5. T/I Solid sodium metal reacts with chlorine gas to form table salt. 2Na(s) + Cl (^) 2(g) → 2NaCl(s) What is the minimum mass of chlorine gas that is required to consume 2.25 g of solid sodium? 6. K/U Define the term “limiting reactant,” and give a specific example from your own experience. 7. C Using a numbered list of steps, explain the process of identifying the limiting reactant in a chemical reaction. 8. C A candle is burning at the bottom of a container. The container is then covered until the candle is almost extinguished. When the cover is removed, the candle flame recovers and burns normally again. Use stoichiometry and the concept of the limiting reactant to explain your observations. 9. T/I Copper reacts with nitric acid, HNO 3 (aq), as follows: 3Cu(s) + 8HNO 3 (aq) → 3Cu(NO 3 )2(aq) + 2NO(g) + 4H 2 O() What mass of nitrogen monoxide, NO(g), is produced when 50.0 g of copper reacts with 150.0 g of nitric acid? 10. T/I The following chemical equation represents the reaction of silver nitrate, AgNO (^) 3(aq), with sodium chloride: AgNO 3 (aq) + NaCl(aq) → AgCl(s) + NaNO (^) 3(aq) Different amounts of silver nitrate are added to a fixed amount of sodium chloride. For each mass of silver nitrate, the mass of sliver chloride, AgCl(s), precipitate is determined and plotted on the graph below.
Mass of AgCl(s) (grams)
25
20
15
10
5
(^0 5 10 15 20 25 30 )
Mass of Silver Chloride vs. Mass of Silver Nitrate
Mass of AgNO3(s) (grams)
a. Why does the graph level off after 25 g of silver nitrate is added to the sodium chloride? b. What amount in moles of sodium chloride is the fi xed amount?
11. A You pour household vinegar on mineral deposits on a kitchen faucet. Some of the deposits wash away. You pour more vinegar on the deposits and they all wash way. Explain what occurred using the terms “limiting reactant” and “excess reactant.” - A limiting reactant is a reactant that is completely consumed during a chemical reaction, and therefore limits or stops the reaction. Reactants that remain after the reaction stops are called excess reactants. - To identify the limiting reactant, the amount of product that is produced from each reactant is determined. Then the amounts from both reactants are compared to determine which reactant produces the smaller amount of product. - Determining the limiting reactant is necessary for all stoichiometric calculations that are used to determine the amount of product that forms. - In chemical processes, limiting and excess reactants must be managed to ensure that the reactants produce the products safely and efficiently.
Chapter 7 Chemical Reactions and Stoichiometry • MHR 313
