Section 9-3:Section 9-3:Limiting Reactants and Limiting Reactants and

Percent YieldPercent Yield

Coach KelsoeCoach Kelsoe

ChemistryChemistry

Pages 312–318Pages 312–318

Section 9-3 ObjectivesSection 9-3 Objectives

Describe a method for determining which of Describe a method for determining which of

two reactants is a limiting reactant.two reactants is a limiting reactant. Calculate the amount in moles or mass in Calculate the amount in moles or mass in

grams of a product, given the amounts in moles grams of a product, given the amounts in moles

or masses in grams of two reactants, one of or masses in grams of two reactants, one of

which is in excess.which is in excess. Distinguish between theoretical yield, actual Distinguish between theoretical yield, actual

yield, and percent yield.yield, and percent yield. Calculate percent yield, given the actual yield, Calculate percent yield, given the actual yield,

and quantity of a reactant.and quantity of a reactant.

Limiting Reactants and Percent YieldLimiting Reactants and Percent Yield

In the lab, a reaction is In the lab, a reaction is rarely carried out with rarely carried out with exactly the required exactly the required amounts of each of the amounts of each of the reactants. In most cases, reactants. In most cases, one or more reactants is one or more reactants is present in excess.present in excess.

Think of it like cooking hot Think of it like cooking hot dogs. Hot dog buns come dogs. Hot dog buns come in packs of 8, while the in packs of 8, while the actual hot dogs are actual hot dogs are packaged in tens.packaged in tens.

Limiting Reactants and Percent YieldLimiting Reactants and Percent Yield

Once one of the reactants is used up, no Once one of the reactants is used up, no more product can be formed. The more product can be formed. The substance that is completely used up is substance that is completely used up is called the limiting reactant.called the limiting reactant.

The The limiting reactantlimiting reactant is the reactant that is the reactant that limits the amounts of the other reactants limits the amounts of the other reactants that can combine and the amount of that can combine and the amount of product that can form in a chemical product that can form in a chemical reaction.reaction.

The substance that is not completely used The substance that is not completely used up in a reaction is sometimes called the up in a reaction is sometimes called the excess reactantexcess reactant..

Limiting Reactants and Percent YieldLimiting Reactants and Percent Yield

Limiting reactant may Limiting reactant may also be referred to as a also be referred to as a limiting reagent.limiting reagent.

Your book compares Your book compares limiting reactants and limiting reactants and excess reactants to excess reactants to passengers on a plane. passengers on a plane. If you have 400 people If you have 400 people wanting to travel and a wanting to travel and a plane that can carry plane that can carry 350, what is the limiting 350, what is the limiting reactant?reactant?

Limiting Reactants and Percent YieldLimiting Reactants and Percent Yield

The same reasoning can be applied to The same reasoning can be applied to chemical reactions. Consider the reaction chemical reactions. Consider the reaction between carbon and oxygen to form carbon between carbon and oxygen to form carbon dioxide:dioxide:

C(s) + O C(s) + O22(g) (g) CO CO22(g)(g) According to the equation, one mole of According to the equation, one mole of

carbon reacts with one mole of oxygen to carbon reacts with one mole of oxygen to form one mole of carbon dioxide. However, form one mole of carbon dioxide. However, if you tried to react 5 mol of C with 10 mol if you tried to react 5 mol of C with 10 mol of O, you would only get 5 mol of COof O, you would only get 5 mol of CO22..

Sample Problem 9-6Sample Problem 9-6

Silicon dioxide (also known as quartz) is Silicon dioxide (also known as quartz) is usually unreactive, but reacts readily usually unreactive, but reacts readily with hydrogen fluoride according to the with hydrogen fluoride according to the following equation:following equation:

SiOSiO22(s) + 4HF(g) (s) + 4HF(g) SiF SiF44(g) + 2H(g) + 2H22O(l)O(l) If 2 mol of HF are exposed to 4.5 mol of If 2 mol of HF are exposed to 4.5 mol of

SiOSiO22, which is the limiting reactant?, which is the limiting reactant? Given: amount of HF = 2.0 mol, amount Given: amount of HF = 2.0 mol, amount

of SiOof SiO22 = 4.5 mol = 4.5 mol Unknown: limiting reactantUnknown: limiting reactant

Sample Problem 9-6Sample Problem 9-6

To solve this problem, we’ll have to use a To solve this problem, we’ll have to use a mole ratio, but which amount do we need?mole ratio, but which amount do we need?

Let’s try one:Let’s try one: 2.0 mol HF x 1 mol SiO2.0 mol HF x 1 mol SiO22/4 mol HF = 0.50 mol /4 mol HF = 0.50 mol

SiOSiO22

Under ideal conditions, the 2.0 mol of HF Under ideal conditions, the 2.0 mol of HF will require 0.5 mol of SiOwill require 0.5 mol of SiO22 for complete for complete reaction. Because the amount of SiOreaction. Because the amount of SiO22 available (4.5 mol) is more than the available (4.5 mol) is more than the amount required (0.5 mol), the limiting amount required (0.5 mol), the limiting reactant is HF.reactant is HF.

Sample ProblemSample Problem

Some rocket engines use a mixture of Some rocket engines use a mixture of hydrazine, Nhydrazine, N22HH44, and hydrogen peroxide, , and hydrogen peroxide, HH22OO22, as the propellant. The reaction is , as the propellant. The reaction is given by the following equation:given by the following equation: N N22HH44(l) + 2H(l) + 2H22OO22(l) (l) N N22(g) + 4H(g) + 4H22O(g)O(g)

a)a) Which is the limiting reactant in this Which is the limiting reactant in this reaction when 0.750 mol of Nreaction when 0.750 mol of N22HH44 is mixed is mixed with 0.500 mol of Hwith 0.500 mol of H22OO22??

b)b) How much of the excess reactant, in moles, How much of the excess reactant, in moles, remains unchanged?remains unchanged?

Sample ProblemSample Problem

Given: amount of NGiven: amount of N22HH44: 0.750 mol, : 0.750 mol, amount of Hamount of H22OO22: 0.500 mol: 0.500 mol

Unknown: limiting reactant, how much Unknown: limiting reactant, how much remains unchanged, how much of each remains unchanged, how much of each is formedis formed

0.750 mol N0.750 mol N22HH44 x 2 mol H x 2 mol H22OO22/1 mol N/1 mol N22HH44= = 1.50 mol1.50 mol

a)a) HH22OO22 is our limiting reactant. is our limiting reactant.

b)b) L.R. (0.500) x mol excess (1)/mol limit (2) = L.R. (0.500) x mol excess (1)/mol limit (2) = 0.5000.500

Sample Problem 9-7Sample Problem 9-7

The black oxide of iron, FeThe black oxide of iron, Fe33OO44, occurs in nature , occurs in nature as the mineral magnetite. This substance can as the mineral magnetite. This substance can also be made in the laboratory by the reaction also be made in the laboratory by the reaction between red-hot iron and steam according to between red-hot iron and steam according to the following equation:the following equation:

3Fe(s) + 4H 3Fe(s) + 4H22O(g) O(g) Fe Fe33OO44(s) + 4H(s) + 4H22(g)(g)

a)a) When 36.0 g of HWhen 36.0 g of H22O is mixed with 167 g of Fe, which O is mixed with 167 g of Fe, which is the limiting reactant?is the limiting reactant?

b)b) What mass in grams of black iron oxide is produced?What mass in grams of black iron oxide is produced?c)c) What mass in grams of excess reactant remains What mass in grams of excess reactant remains

when the reaction is completed?when the reaction is completed?

Sample Problem 9-7Sample Problem 9-7

Given: mass of HGiven: mass of H22O: 36 g, mass of Fe: 167 gO: 36 g, mass of Fe: 167 g Unknown: limiting reactant, mass of FeUnknown: limiting reactant, mass of Fe33OO44 in in

grams, mass of excess reactant remaininggrams, mass of excess reactant remaining

a)a) 36.0 g H36.0 g H22O x 1 mol HO x 1 mol H22O/18.02 g = 2.00 molO/18.02 g = 2.00 mol167 g Fe x 1 mol Fe/55.85 g = 2.99 mol167 g Fe x 1 mol Fe/55.85 g = 2.99 mol2.99 mol Fe x 4 mol H2.99 mol Fe x 4 mol H22O/3 mol Fe = 3.99 mol; O/3 mol Fe = 3.99 mol; HH22O is limiting reactantO is limiting reactant

b)b) 2.00 mol x 1 mol Fe2.00 mol x 1 mol Fe33OO44/4 mol H/4 mol H22O x O x 231.55 g/1 mol Fe231.55 g/1 mol Fe33OO44= 116 g Fe= 116 g Fe33OO44

c)c) 2 mol H2 mol H22O x 3 mol Fe/4 mol HO x 3 mol Fe/4 mol H22O x 55.85 g/1 mol O x 55.85 g/1 mol Fe = 83.2 g Fe consumed Fe = 83.2 g Fe consumed 167 g originally 167 g originally present – 83.8 g consumed = 83.2 g Fe leftpresent – 83.8 g consumed = 83.2 g Fe left

Percent YieldPercent Yield

The amount of products calculated in The amount of products calculated in the stoichiometric problems in this the stoichiometric problems in this chapter so far represent theoretical chapter so far represent theoretical yields.yields.

The The theoretical yieldtheoretical yield is the maximum is the maximum amount of product that can be produced amount of product that can be produced from a given amount of reactant.from a given amount of reactant.

In most chemical reactions, the amount In most chemical reactions, the amount of product obtained is less than the of product obtained is less than the theoretical yield.theoretical yield.

Percent YieldPercent Yield

There are several reasons we don’t get a There are several reasons we don’t get a theoretical yield when we perform a theoretical yield when we perform a reaction:reaction: Some of the reactant may be used in competing Some of the reactant may be used in competing

side reactions that reduce the amount of the side reactions that reduce the amount of the desired product.desired product.

Once a product is formed, it often is usually Once a product is formed, it often is usually collected in impure form, and some of the collected in impure form, and some of the product is often lost during the purification product is often lost during the purification process.process.

The measured amount of a product The measured amount of a product obtained from a reaction is called the obtained from a reaction is called the actual yieldactual yield of that product. of that product.

Percent YieldPercent Yield

Chemists are usually interested in the Chemists are usually interested in the efficiency of a reaction. The efficiency efficiency of a reaction. The efficiency is expressed by comparing the actual is expressed by comparing the actual and theoretical yields.and theoretical yields.

The The percent yieldpercent yield is the ratio of the is the ratio of the actual yield to the theoretical yield, actual yield to the theoretical yield, multiplied by 100.multiplied by 100.

Percent yield = actual yield x 100Percent yield = actual yield x 100theoretical yieldtheoretical yield

Sample Problem 9-8Sample Problem 9-8

Chlorobenzene, CChlorobenzene, C66HH55Cl, is used in the Cl, is used in the production of many important chemicals, production of many important chemicals, such as aspirin, dyes, and disinfectants. One such as aspirin, dyes, and disinfectants. One industrial method of preparing chlorobenzene industrial method of preparing chlorobenzene is to react benzene, Cis to react benzene, C66HH66, with chlorine, as , with chlorine, as represented by the following equation:represented by the following equation: C C66HH66(l) + Cl(l) + Cl22(g) (g) C C66HH55Cl(s) + HCl(g)Cl(s) + HCl(g)

When 36.8 g of CWhen 36.8 g of C66HH66 react with an excess of react with an excess of ClCl22, the actual yield of C, the actual yield of C66HH55Cl is 38.8 g. What Cl is 38.8 g. What is the percent yield of Cis the percent yield of C66HH55Cl?Cl?

Sample Problem 9-8Sample Problem 9-8

Given: mass of CGiven: mass of C66HH66= 36.8 g, mass of Cl= 36.8 g, mass of Cl22 = excess, actual yield of C= excess, actual yield of C66HH55Cl = 38.8 gCl = 38.8 g

Unknown: percent yield of CUnknown: percent yield of C66HH55ClCl Find theoretical yield of CFind theoretical yield of C66HH55ClCl

36.8 g C36.8 g C66HH66 xx 1 mol C 1 mol C66HH66/78.12g x /78.12g x 1 mol C1 mol C66HH55Cl/1 mol CCl/1 mol C66HH66 xx 112.56 g/1 mol 112.56 g/1 mol CC66HH55Cl = 53.0 g CCl = 53.0 g C66HH55Cl (theoreticalCl (theoretical

Find percent yieldFind percent yield % yield = actual/theor. X 100 % yield = actual/theor. X 100 38.8/53.0 x 38.8/53.0 x

100 = 73.2%100 = 73.2%

VocabularyVocabulary

Actual yieldActual yield Excess reactantExcess reactant Limiting reactantLimiting reactant Percent yieldPercent yield Theoretical yieldTheoretical yield