Chemistry 103

Lecture 16

Outline

I. Stoichiometry - Limiting Reactant - Percent Yield

III. Classification of Reaction Types

EXAM II - Thursday, July 3rd

Stoichiometry

Chemical Stoichiometry: using mass and quantity relationships among reactants and products in a chemical reaction to make predictions about how much product will be made.

Stoichiometry Problems continued…The complete combustion of octane, C8H18, a

component of gasoline, proceeds as follows

C8H18 + O2 ------> CO2 + H2O

(unbalanced)

Stoichiometry Problems continued…The complete combustion of octane, C8H18, a component

of gasoline, proceeds as follows

2C8H18 + 25O2 ------> 16CO2 + 18H2O

Stoichiometry Problems continued…The complete combustion of octane, C8H18, a component

of gasoline, proceeds as follows

2C8H18 + 25O2 ------> 16CO2 + 18H2O

How many moles of O2 are needed to burn 1.25 moles

of C8H18?

Stoichiometry Problems continued…The complete combustion of octane, C8H18, a component

of gasoline, proceeds as follows

2C8H18 + 25O2 ------> 16CO2 + 18H2O

How many grams of O2 are needed to burn 10.0 grams

of C8H18?

Chemical Reactions

Limiting Reactant

&

Percent Yield

Copyright © 2008 by Pearson Education, Inc. publishing as Benjamin Cummings

Limiting Reactant

What is the limiting reactant when 2.00g of Na and 2.00g of Cl2 combine as follows:

2Na + Cl2 -----> 2NaCl

Problem continued….

How many grams of the remaining reactant would be left over once the reaction has run to completion? 2Na + Cl2 ----> 2NaCl

Theoretical, Actual, and Percent Yield

Theoretical yield

• The maximum amount of product calculated using the balanced equation (Solving the Limiting Reactant Problem).

Actual yield

• The amount of product obtained when the reaction takes place.

Percent yield

• The ratio of actual yield to theoretical yield.

percent yield = actual yield (g) x 100 theoretical yield (g)

Back to our NaCl Problem

If the actual yield of NaCl is 2.29g, what is the percent yield?

Limiting Reactant

If 4.80 grams Ca are mixed with 2.00 grams N2, how many grams of the remaining reactant are left over after the reaction has run to completion?

3Ca(s) + N2(g) Ca3N2(s)

Limiting Reactant

If 4.80 grams Ca are mixed with 2.00 grams N2, how many grams of the remaining reactant are left over after the reaction has run to completion?

3Ca(s) + N2(g) Ca3N2(s)

Limiting Reactant

If 4.80 grams Ca are mixed with 2.00 grams N2 and the reaction has a 69.2% percent yield, what is the actual yield (in grams)?

3Ca(s) + N2(g) Ca3N2(s)

More LR problems

Methanol (CH3OH) is used as a fuel in racing cars. It burns in the engine according to the equation:

2CH3OH + 3O2 ----> 2CO2 + 4H2O

If 7.82 x 1023 molecules of methanol are mixed with 46.0g of O2, what is the mass of CO2 produced?

a). 57.2g b). 42.2g c). 63.3g

Learning CheckWithout proper ventilation and limited oxygen, the reaction of carbon and oxygen produces carbon monoxide.

2C(g) + O2(g) 2CO(g)

What is the percent yield if 40.0 g CO are produced when 30.0 g O2 are used?

A) 25.0%

B) 75.0%

C) 76.2%

Chemical Reactions

Types of Reactions

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Combination In a combination reaction,• Two or more elements form one product• Or simple compounds combine to form one product

+

2Mg(s) + O2(g) 2MgO(s)

2Na(s) + Cl2(g) 2NaCl(s)

SO3(g) + H2O(l) H2SO4(aq)

A B A B

Formation of MgO

Decomposition

In a decomposition reaction, • One substance splits into two or more simpler substances

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

2KClO3(s) 2KCl(s) + 3 O2(g)

Decomposition of HgO

Single Replacement

In a single replacement reaction, • One element takes the place of a different element in a

reacting compound

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)

Zn and HCl Is a Single Replacement Reaction

Double Replacement

In a double replacement, • Two elements in the reactants exchange places

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g)

Example of a Double Replacement

In a combustion reaction,

• A compound such as carbon reacts with oxygen, O2

C(s) + O2(g) CO2(g)

CH4(g) + 2O2(g) CO2 (g) + 2H2O(g)

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

• Fuels burned in oxygen produce CO2, H2O, and energy.

Combustion

Example of a Combustion Reaction

Learning Check

Balance the combustion equation.

___C5H12 + ___O2 ___CO2 + ___H2O

Summary of Reaction Types

Learning Check

Identify each reaction as: 1) combination 2) decomposition 3) combustion4) single replacement 5) double replacement

A. 3Ba(s) + N2(g) Ba3N2(s)

B. 2Ag(s) + H2S(aq) Ag2S(s) + H2(g)

C. SiO2(s) + 4HF(aq) SiF4(s) + 2H2O(l)

D. PbCl2(aq) + K2SO4(aq) 2KCl(aq) + PbSO4(s)

E. K2CO3(s) K2O(aq) + CO2(g)

F. C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(g)

Limiting Reactant

Ethylene burns in air according to the following unbalanced equation:

C2H4(g) + O2(g) -------> CO2(g) + H2O(l)

a. How many grams of CO2 will be formed when a mixture of 2.93g of C2H4 and 4.29g of O2 combine?

Limiting Reactant

Ethylene burns in air according to the following unbalanced equation:

C2H4(g) + 3O2(g) -------> 2CO2(g) + 2H2O(l)

a. How many grams of CO2 will be formed when a mixture of 2.93g of C2H4 and 4.29g of O2 combine?

Problem continued….

b. How many grams of each reactant would be left over once the reaction has run to completion?