Chapter 12--Stoichiometry

12.1 The Arithmetic of Equationsinterpreting chemical equations and mass conservation in chemical reactions

12.2 Chemical Calculationsmole ratios (mole-mole; mass-mass; general dimensional anaylsis)

12.3 Limiting Reagent and Percent Yieldknow definitions of limiting and excess reagents only; percent yield

What is stoichiometry?

        It's not just a funny word--> Stoichiometry is  a very important, useful and mathy part of chemistry. Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. If you understand how it works, it really isn't that hard. Stoichiometry allows a chemist or scientist to know how much of an element or reactant to use, how much product is expected to come out of the reaction, and how to express different amounts of a substance in many units of measurement. So get ready, get set, and get excited to start learning about stoichiometry!

Guided Notes1. Write the key concept on p. 354.

2. Define stoichiometry.

3. Write the key concept on p. 356.

4. What does a chemical equation tell you about the following quantities (p. 356-357)?– number of atoms:– number of molecules:– moles:– mass:– volume:

5. Write the key concept AND the sentence following the key concept on p. 357.

6. Define mole ratio.

7. Write the equation on p. 359 and the mole ratios shown.

8. Write the key concept on p. 359.

9. Read and write sample problem 12.2 on p. 360.

10. (p. 360) Given , – write 6 mole ratios that can be derived from this equation:

– how many moles of aluminum are needed to form 3.7 mole Al2O3?

)(2)(3)(4 322 sOAlgOsAl

Guided Notes continued

11. Read and write sample problem 12.3 on p. 361.

12. (p. 361) Given

(acetylene gas and calcium carbide), how many grams of acetylene are produced by adding water to 5.00 g CaC2?

13. Read and write sample problem 12.4 on p. 364.

14. (p. 364) Given

how many molecules of oxygen are produced by the decomposition of 6.54 g of potassium chlorate?

15. Read and write sample problem 12.5 on p. 365.

16. (p. 365) Given

how many liters of oxygen are required to burn 3.86 L of carbon monoxide?

17. Define theoretical, actual, and percent yield.

18. Write the key concept on p. 373.

19. (p. 374) Given

what is the theoretical yield when 84.8 g of iron (III) oxide reacts with an excess of carbon monoxide?

)()()()(2)( 22222 aqOHCagHClOHsCaC

)(3)(2)(2 23 gOsKClsKClO

)(2)()(2 22 gCOgOgCO

)(3)(2)(3)( 232 gCOsFegCOsOFe

12.1 The Arithmetic of Equations

• More than 3000 cocoons are needed to produce enough silk to make just one elegant Japanese kimono. Like silk manufacturers, chemists must know how much reactant they need to make a certain amount of product. Determining the quantities of reactants and products in a reaction requires a balanced chemical equation.

12.1

Using Everyday Equations

An equation can represent the manufacturing of a single tricycle.

12.1

Using Balanced Chemical EquationsHow do chemists use balanced chemical equations?

12.1

Chemists use balanced chemical equations as a basis to calculate how much reactant is needed or product is formed in a reaction.

The calculation of quantities in chemical reactions is a subject of chemistry called stoichiometry.

Interpreting Chemical Equations

In terms of what quantities can you interpret a balanced chemical equation?

12.1

A balanced chemical equation can be interpreted in terms of different quantities, including numbers of atoms, molecules, or moles; mass; and volume.

We can describe in terms of number of atoms, molecules, or moles

We can also describe in terms of mass and volume

Mass Conservation in Chemical Reactions

What quantities are conserved in every chemical reaction?

12.1

mass and number of atoms

2 H2S + 3 O2 2 SO2 + 2 H2O

mass of reactants = mass of products

Interpret this equation terms of b. masses of reactants and products

Moles:

Volume:

Mass:

2 C2H2 (g) + 5 O2 (g) 4 CO2 (g) + 2 H2O (g)

Number of moles is not conserved

Volume is not conserved (7 moles vs. 6 moles of gas x 22.4 L/mol)

Mass is conserved

Atoms are conserved so the reactants 52 g + 160 g = products 176g + 36 g = 212 g

12.1 Section Quiz

1. A manufacturer of bicycles has 5350 wheels, 3023 frames, and 2655 handlebars. How many bicycles can be manufactured using these parts?

a) 2675 bicycles

b) 2655 bicycles

c) 3023 bicycles

d) 5350 bicycles

2. A reaction that produces iron metal from iron ore is shown below.

Fe2O3•H2O(s) + 3CO(g) 2Fe(s) + 3CO2(g) + H2O(g)

In this equation, the volume of gas at STP that reacts and the volume of gas at STP produced will be

a) 3 L and 4 L.

b) 67.2 L and 89.6 L.

c) 67.2 L and 67.2 L

d) 3 L and 3 L

12.1 Section Quiz

12.2

12.2 Chemical Calculations

The effectiveness of car’s air bags is based on the rapid conversion of a small mass of sodium azide into a large volume of gas. The entire reaction occurs in less than a second. You will learn how to use a balanced chemical equation to calculate the amount of product formed in a chemical reaction.

Writing and Using Mole RatiosMole-Mole Calculations

A mole ratio is a conversion factor derived from the coefficients of a balanced chemical equation interpreted in terms of moles.

12.2

Mole ratios are used to convert between moles of reactant and moles of product, between moles of reactants, or between moles of products.

Writing and Using Mole Ratios To determine the number of moles in a sample of a

compound, first measure the mass of the sample. Then use the molar mass to calculate the number of moles in that mass

12.2

N2 (g) + 3 H2 (g) 2 NH3 (g)

1 mol nitrogen reacts with 3 mol hydrogen to produce 2 mol ammonia

Writing and Using Mole RatiosMass-Mass Calculations

12.2

Other Stoichiometric CalculationsWhat is the general procedure for solving a

stoichiometric problem?

12.2

In a typical stoichiometric problem, the given quantity is first converted to moles. Then the mole ratio from the balanced equation is used to calculate the number of moles of the wanted substance. Finally, the moles are converted to any other unit of measurement related to the unit mole, as the problem requires.

Other Stoichiometric CalculationsSolution Diagram

12.2

Problem-Solving Approach

2mLO2.10L1

mL1000

2molO12LO4.22

3molSO22molO1

3LSO4.223molSO1

mL1000

L13mLSO4.20

12.2 Section Quiz.

1.How many moles of water are produced when 2.5 mol of O2 react according to the following equation?

C3H8 + 5O2 3CO2 + 4H2O

a) 2.0

b) 2.5

c) 3.0

d) 4.0

2. Nitrogen gas reacts with hydrogen gas to produce ammonia gas. N2(g) + 3H2(g) 2NH3(g). What volume of H2 is required to react with 3.00 L of N2, and what volume of NH3 is produced at 200°C?

a) volume of H2 = 9.00 L, volume of NH3 = 6.00 L

b) volume of H2 = 3.00 L, volume of NH3 = 3.00 L

c) volume of H2 = 3.00 L, volume of NH3 = 6.00 L

d) volume of H2 = 1.00 L, volume of NH3 = 1.50 L

12.2 Section Quiz

3. Automotive airbags inflate when sodium azide, NaN3, rapidly decomposes to its component elements via this reaction: 2NaN3 2Na + 3N2. How many grams of sodium azide are required to form 5.00 g of nitrogen gas?

a) 11.61 g

b) 17.41 g

c) 7.74 g

d) 1.36 g

12.2 Section Quiz

12.3 Limiting Reagent and Percent Yield

If a carpenter had two tabletops and seven table legs, he could only build one four-legged table. The number of table legs is the limiting factor in the construction of four-legged tables. Similarly, in chemistry, the amount of product made in a chemical reaction may be limited by the amount of one or more of the reactants.

12.3

Limiting and Excess Reagents How is the amount of product in a reaction affected by an

insufficient quantity of any of the reactants?

12.3

•In a chemical reaction, an insufficient quantity of any of the reactants will limit the amount of product that forms.

•The limiting reagent is the reagent that determines the amount of product that can be formed by a reaction.

•The reagent that is completely used up is the limiting reagent. The reagent that is not used up is called the excess reagent.

Percent YieldWhat does the percent yield of a reaction

measure?

12.3

The percent yield is a measure of the efficiency of a reaction carried out in the laboratory; it is the ratio of the actual yield to the theoretical yield expressed as a percent

(A batting average is actually a percent yield)

Percent YieldThe theoretical yield is the maximum amount of

product that could be formed from given amounts of reactants.

In contrast, the amount of product that actually forms when the reaction is carried out in the laboratory is called the actual yield.

12.3

12.3 Section Quiz1.In the reaction 3NO2 + H2O 2HNO3 + NO,

how many grams of HNO3 can form when 1.00 g of NO2 and 2.25 g of H2O are allowed to react?

a) 0.913 g

b) 0.667 g

c) 15.7 g

d) 1.37 g

Limiting reagent problem

12.3 Section Quiz.

2.How many grams of H2O can be formed from 24.0 g O2 and 6.00 g H2?

a) 30.0 g

b) 27.0 g

c) 54.0 g

d) 13.5 g

Limiting reagent problem

3. Octane burns according to the following equation.

2C8H18 + 25O2 16CO2 + 18H2O

What is the percent yield if 14.6 g of CO2 are produced when

5.00 g of C8H18 are burned?

a) 106%

b) 94.8%

c) 34.2%

d) 62.5%

12.3 Section Quiz.

Practice Problems

Write all possible mole ratios for the following chemical equation.

2 HgO (s) 2 Hg (l) + O2 (g)

2Hg

O,

2HgO

O,

HgO

Hg,

O

2Hg,

O

2HgO,

Hg

HgO 22

22

Practice Problems

Hydrogen gas can be produced through the following reaction.

Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g)

What mass of HCl is consumed by the reaction of 2.50 mol of magnesium?

HCl g 182HCl mol 1

HCl g 36.461

Mg mol 1

HCl mol 2

1

Mg mol 2.50

Molar Volume of a Gas Lab

Before you began, you determined the amount of zinc needed to produce a specific amount of hydrogen gas. However, the first calculation in the calculations section of the lab, is the theoretical value based on the amount of zinc you actually used.

Assume x = 0.30 g, then your theoretical yield is 103 mL. If your experimental value is 94 mL, then your

91.3% 100 x 103

94 100

ltheoretica

alexperiment yieldpercent x

2

2

2

22

H L 1

H mL 1000

H mol 1

H L 4.22

Znmol 1

H mol 1

Zn39.65

Znmol 1

1

Zngx

g

Ch. 12: What you need to know• 12.1• given a chemical equation, know the relationship of reactants and products in terms of atoms, mass, molecules, moles, and

volume• know definition of stoichiometry• know conversion factors such as molar mass, molar volume, Avogadro’s number, and mole ratio (carry over from ch. 10)• know that atoms and mass are conserved in a chemical equation (conservation of mass)•  • 12.2• know mole ratios• be able to use mole ratios in calculations (similar to ch. 10)• use ch. 12 worksheet as a guide•  • 12.3• know and be able to calculate theoretical yield, actual yield, and percent yield• Know limiting reagent and excess reagent•  • know Molar Volume of a Gas and Combustion of Mg Labs• know the chemical equations• know the calculations•  • review problems assigned from text