Chapter 4

Chemical Equations and Stoichiometry

 

 

Module Five Part I: Chemical Equations & Stoichiometry

                                 (Chap 2, 3 & 4)

A._____(04) Molecular Mass Calculation-Section 2.5, 3.5

B._____(04) Mole Calculations-Section 2.5

C._____(04) Percentage Composition Calculation-Section 3.6

D._____(04) Empirical Formula Calc. from % Comp & Analysis-Section 4.6

E._____(04) Basic Stoichiometric Definitions-Chapter 4 p139

F._____(05) Balancing Chemical Equations by Inspection-Section 4.2

G._____(05) Predicting Single Replacement Products . Section

H._____(05) Predict Double Replacement & Organic Combustion Products 5.2, 5.4

I.______(04) Mole-Mole Problems Lecture

J._____( 05) Mass-Mass Stoichiometric Problems-Section 4.3

K._____(05) Excess/Limiting Reagent Problems-Section 4.4

L._____(00) Impure reagents/% Yield Problem-Section 4.5

M._____(00) Multi step Synthesis/Mixture Problems-Section 4.6

N._____(10) Multiple Choice

______(60) Total = ______%

Chapter 4:  Chemical Equations and Stoichiometry

 

Chemical Puzzler (CD-Rom, Screen 4.1) – Baking Soda and Vinegar

 

4.1              Chemical Equations (p. 121)

 

(CD-Rom, Screen 4.2, Outline and Description)

A__________________________ is represented by a chemical equation.

 

2 Al(s)  +   3 Br2(l)  à  Al2Br6(s)   (See p. 81)

 

__________________  - Al and Br2

___________________ - Al2Br6

__________________ - yields /gives / produces

Physical states:  s - _________________, l - ______________, g - ________________, aq - ______________________

 

Phase Changes:  ↑____________________, ↓_______________________

 

What does ∆ denotes if written over the yields sign___________________

 

What is a catalyst? How do you demonstrate a catalyst in a chemical reaction?

 

 

___________________ are the numbers in front of the formulas.  They tell us how many of a particular substance are reacted are formed. Used to _______________ a chemical equation.

 

Why is it necessary to balance an equation? __________________________________

Mass of Reactants = Mass of Products

(CD-Rom, Screen 3.3, Outline, Description, Exercise 1)

 

____________________ refers to the relationship between the quantities of products and reactants.

 

4.2            Balancing Chemical Equations (p. 122)

(CD-Rom, Screen 3.4, Outline) You may check your work with ChemiCalc.

 

Equations MUST be balanced before performing any quantitative calculations.

 

Rules and Suggestions for Balancing Equations

1)      The same # and type of atom must be present on each side of the equation.

2)      Balancing is accomplished by adding coefficients.  NEVER change the subscripts.

3)      Coefficients must be in the smallest whole # ratio.

4)      Balancing is done by trial and error.

5)      Balance H’s and O’s last.

6)      Balance polyatomic ions as one unit.

 

CD-Rom, Screen 3.4, Description)

 

Forming Oxides

 

Nonmetal Oxides – Nonmetal + Oxygen – Nonmetal Oxides are ____________.

 

P4         +         O2          à              P4O10

 

 

 

           

 

            S8         +          O2        à            SO2

 

 

 

 

Metal Oxide – Metal + Oxygen – Metal Oxides are ______________________.

 

            Ca        +              O2          à          CaO

 

 

 

 

            Fe          +             O2           à          Fe2O3

 

 

 

Combustion Reactions – Burning a fuel in oxygen – produces heat and light.  

Example:   Alkane +    O2   à CO2    +     H2O   (products of complete combustion.)

 

            C3H8            +       O2             à          CO2          +            H2O

 

 

           

           

 

            C4H10            +       O2             à        CO2          +            H2O

 

 

 

 

 

 

 

 

 

 

 

Balancing Ionic Equations

 

CaCrO4            +             NaCl             à                Na2CrO4              +            CaCl2

 

 

 

 

 

Na2SO4           +               AlP              à               Na3P                  +              Al2(SO4)3

 

 

 

 

4.3 Mass Relationships in Chemical Reactions:   Stoichiometry (p. 125)

 (CD-Rom, Screen 3.5 and 3.6, Outline, Description)

 

Balance the following equation.

Show how mass is balanced.

 

 

P4              +                 Cl2                  à                   PCl3

 

 

 

 

 

 

 

 

Mass of A            à           Moles of A            à       Moles of B       à       Mass of B  

         (1 mole = Molar Mass in g)           (mole ratio)           (1 mole = Molar Mass in g)          

 

 

Exercise 4.3, p. 127

 

What mass of oxygen, O2, is required to completely combust 454 g of propane, C3H8?  What masses of CO2 and H2O are produced?

 

C3H8            +       5 O2             à       3 CO2          +            4 H2O

 

 

 

 

 

 

 

 

 

4.4 Reactions in Which One Reactant Is Present in Limited Supply   (Limiting Reactants)   (p. 127)

(CD-Rom, Screen 3.7 and 3.8, Outline and Description)

 

When a chemist carries out a reaction in the real world, stoichiometric amounts are rarely used.  Usually one reactant is limiting and the other is in excess.

 

______________________ - reactant which is used up completely.

______________________ - some of this reactant will be left over after the reaction has taken place.

 

(NH4)2PtCl4    +        2 NH3         à    2 NH4Cl     +        Pt(NH3)2Cl2

$100/gram               pennies/gram                                       cisplatin – cancer drug

 

 

Which reactant is used in excess? _______________________

Which reactant is limiting? _____________________________

Why? _________________________________________________________________

 

How many cheeseburgers (1 piece of meat, 1 piece of cheese, and 1 bun) could you make if you had 5 buns, 7 pieces of meat and 4 pieces of cheese?

What is limiting?   What is in excess?

 

 

 

 

 

How do you decide which reactant is limiting?

 

3 A      +        1 B         à         C

 

Compare stoichiometric ratio to ratio based on amounts given in problem.

 

Stoichiometric Ratio is 3 A / 1 B

 

Use masses from problem for A and B to calculate moles of A and B.

 

Compare ratios.

 

If the ratio is larger than the stoichiometric ratio, then A is in excess and B is limiting.

If the ratio is smaller than the stoichiometric ratio, then A is limiting and Bis in excess.

 

****ALWAYS calculate the mass of the product based on the limiting reactant.****

**** If you are given masses for both reactants, you have a limiting reactant problem.***

 

 

 

 

 

Exercise 4.5, p. 131

Pure silicon, required for computer chips and solar cells, is made by the reaction

 

            SiCl4        +         2 Mg         à          Si         +          2 MgCl2

 

If you begin with 225 g of each reactant, which is the limiting reactant in this reaction?  What quantity in mass of Si can be produced?

What quantity in moles and mass of the reactant in excess remains after the reaction is over?

 

 

 

4.4  Percent Yield (p. 132)

 

(CD-Rom, Screen 4.9, Description)

 

Percent Yield  =       Actual Yield___   X 100%

                              Theoretical Yield

 

______________________ is the expected yield assuming all reactants were converted into products.

______________ is the amount of product that was actually collected or obtained in the laboratory.

 

Exercise 4.6, p. 133

Methanol, CH3OH, can be burned in oxygen to provide energy, or it can be decomposed to form hydrogen gas, which can be used as a fuel.

CH3OH (l)   à   2 H2(g)   +   CO(g)

If 125 g of methanol is decomposed, what is the theoretical yield of hydrogen?  If only 13.6 g of hydrogen is obtained, what is the percent yield of the gas?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4.5  Chemical Equations and Chemical Analysis

 

(CD-Rom, Screen 4.10, Outline and Description)

 

Quantitative chemical analyses generally depend on one or two basic ideas:

1)      A substance present in an unknown amount can be allowed to react with a known quantity of another substance.  If the stoichiometric ratio for their reaction is known, the unknown amount can be determined.

2)      A material of unknown composition can be converted to one or more substances of known composition.  Those substances can be identified, their amounts determined, and these amounts related to the amount of the original, unknown substance.

 

See Figure 4.5, p. 135 for analysis of Sulfur in a sample

 

Exercise 4.7, p 136

One method for determining the purity of a sample of titanium(IV) oxide, TiO2, an important industrial chemical, is to combine the sample with bromine trifluoride.

 

3 TiO2(s)   +  4 BrF3(l)  à  3 TiF4(s)    +   2 Br2(l)   +   3 O2(g)

 

Oxygen gas is evolved quantitatively.  The gas can be captured readily, and its mass can be determined.  Suppose 2.367 g of TiO2-containing sample evolves 0.143 g of  O2.   What is the mass percent of TiO2 in the sample?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Determining the Formula of a Compound by Combustion

 

Burning a sample in oxygen produces CO2 and H2O.  The empirical formula of a compound can be calculated based on the masses of CO2 and H2O collected.

 

See Figure 4.6, p. 138 or CD-Rom Screen 4.10, Description, p. 2

 

Exercise 4.8, p. 139

A 0.523 g sample of the unknown compound CxHy is burned in air to give 1.612 g of CO2 and 0.7425 g of H2O.  A separate experiment gave a molar mass of CxHy of 114 g/mol.  Determine the empirical formula and molecular formulas of the hydrocarbon.

 

 

 

 

 

 

 

 

 

 

 

 

Problem

Vitamin C is composed of C, H, and O.  Determine the empirical formula of vitamin C from the following data:  burning 0.400 g of solid vitamin C in pure oxygen gives 0.600 g of CO2 and0.163 g of H2O.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chemical Puzzler, CD-Rom, Screen 4.11