lecture 5. chemical reaction (part 1) (1)

CHEMICAL REACTIONS

GENERAL CHEMISTRY

LECTURE 5

The eye sees only what the mind is prepared to comprehend.

-Henri Bergson

5.1 Writing and Balancing Chemical Reactions

5.3 Calculating Amounts of Reactant and Product

5.4 Determining the Limiting Reactant

Lesson for Today

5.5 Determining the Theoretical, Actual and Percent Yield of Reactions

5.2 Identifying Different Types of Chemical Reactions

A three-level view of the chemical reaction in a flashbulb.

Chemical Reactions

A process by which a chemical change occurs.

A process in which one set of substances called reactants is converted to a new set of substances called products.

Chemical equations

Shorthand or symbolic way of representing a chemical reaction

Translate the statement

Balance the atoms

Specify states of

matter

Adjust the coefficients

Check the atom balance

Writing Chemical Equations

Nitrogen monoxide + oxygen nitrogen dioxide

NO + O2 NO2

2NO + O2 2NO2

2NO (g) + O2 (g) 2NO2 (g)

SOLUTION:

1. Within the cylinders of a cars engine, the hydrocarbon octane (C8H18), one of many components of gasoline, mixes with oxygen from the air and burns to form carbon dioxide and water vapor. Write a balanced equation for this reaction.

C8H18 + O2 CO2 + H2O

C8H18 + O2 CO2 + H2O 8 9

2C8H18 + 25O2 16 CO2 + 18H2O

2C8H18(l) + 25O2 (g) 16CO2 (g) + 18H2O (g)

25

2

Exercise 1 Balancing Chemical Equations

Exercise 2

a. __ Al2O3 (g) __Al(S) + __O2(g)

b. __ P4(s) + __O2( g) __P2O5(s)

c. __ C2H6(s) + __ O2( g) __ CO2(g) + __H2O(l)

d. __ Na(s) + __H2O (l) __ NaOH + __H2(g)

e. __C6H6(s) + __ O2(g) __ CO2(g) + __H2O(l)

Balancing Chemical Equations

2. Balance the following chemical reactions.

4 2 3

5 1 2

7 2 4 6

2 2 2 1

15 2 12 6

1. Combination Reaction

Types of Chemical Reactions

2. Decomposition Reaction

3. Single Displacement

4. Double Displacement/ Metathesis


1. Combination reaction

2. Decomposition reaction

A + B AB

2(s)g2s CaF FCa

5(s)g2s4 4PCl 10ClP

AB A + B

2(g)(s)MnO ,

s3 O32KCl 2KClO2

2(g)(l)s O2Hg 2HgO


3. Single Displacement

4. Double Displacement/Metathesis

A + BC AC + B

AB + CD AD + CB

(s)2(aq)aq3(s) Fe ZnClFeCl Zn

aq(s)aq2(g) HCl22Ag 2AgCl H

(aq)(l)2(aq)(aq) NaClOH NaOH HCl

3(aq)2(s)(aq)2(aq)3 2KNO +PbI2KI + )Pb(NO

3. Identify the type of the following chemical reaction

Exercise 3 Identifying Type of Chemical Reaction

3(g)g2g2 2SO O2SO

3(s)s2s CaSiO SiOCaO

4(l)23(g)s424 SOH2NH SO)(NH

2(g)2(aq)aq(s) H ZnClHCl Zn

(l)2(aq)3(aq)3 OH +COOKCHKOH + COOHCH aq

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

B. NH4Cl(aq) + H2O(l) NH3(g) + Cl-(aq) + H3 O

+(aq)

C. Cu(OH)2(s) CuO(s) + H2O(l) D. C12 H22O12(s) C12 H22O12(l)

4.From the following reactions, which one is a decomposition reaction?


A. Hydroiodic acid and zinc (II) nitrate

B. Aluminum bromide and potassium nitrate

C. Sodium acetate and calcium chloride

D. Copper (II) sulfate and sodium carbonate

**CuSO4(aq) + Na2CO3(aq) CuCO3(s) + Na2SO4(aq)

5. Which of the following aqueous solutions, when mixed, will form a precipitate?


The basis of all stoichiometry calculations is the RATIO between

the reactants and products, based on their STOICHIOMTERIC

COEFFICIENTS in the BALANCED chemical equation

Calculating Amount of Reactant and Product

Consider the reaction between ferric oxide and carbon monoxide forming iron metal and carbon dioxide

Relating reactants to each other:

Relating reactants to products:

2(g)(s)(g)3(s)2 CO 3 + Fe 2 3CO + OFe

CO moles 3

OFe mole 1 32

Fe moles 2

OFe mole 1 32


Fe moles 2

CO moles 3

2CO mole 1

CO mole 1

1. How many moles of CO are required to react with 25 mol of Fe2O3?


2(g)(s)(g)3(s)2 CO 3 + Fe 2 3CO + OFe

2. What mass of CO is required to react with 146 g of iron (III) oxide?

Divide by MW to get moles

Use Stoichiometric Factor

Multiply by MW


2(g)(s)(g)3(s)2 CO 3 + Fe 2 3CO + OFe

Summary of the mass-mole-number relationships in a chemical reaction.

6. Consider the reaction:

2 H2 (g) + O2 (g) 2 H2O (l)

a. How many moles of H2O are produced by burning 2.72 mol H2 in an excess of O2?

b. What mass of H2O is formed in the reaction of 4.16 g H2 with an excess of O2?

Exercise 6 Calculating Amount of Reactant and Product

7. In a lifetime, the average American uses 1750 lb (794 g) of copper in coins, plumbing, and wiring. Copper is obtained from sulfide ores, such as chalcocite, or copper(I) sulfide, by a multistep process. After an initial grinding, the first step is to roast the ore (heat it strongly with oxygen gas) to form powdered copper(I) oxide and gaseous sulfur dioxide.

(a) How many moles of oxygen are required to roast 10.0 mol of copper(I) sulfide?

(b) How many grams of sulfur dioxide are formed when 10.0 mol of copper(I) sulfide is roasted?

(c) How many kilograms of oxygen are required to form 2.86 kg of copper(I) oxide?


SOLUTION:

2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)

3 mol O2

2 mol Cu2S = 15.0 mol O2

= 641 g SO2

10.0 mol Cu2S x

64.07 g SO2

mol SO2

(a) How many moles of oxygen are required to roast 10.0 mol of copper(I) sulfide?

(b) How many grams of sulfur dioxide are formed when 10.0 mol of copper(I) sulfide is roasted?

x 10.0 mol Cu2S x 2 mol SO2

2 mol Cu2S


= 0.960 kg O2 1 kg O2

103 g O2

= 20.0 mol Cu2O

20.0 mol Cu2O x 3 mol O2

2 mol Cu2O

32.00 g O2

mol O2

2.86 kg Cu2O x 103 g Cu2O

kg Cu2O

mol Cu2O

143.10 g Cu2O

(c) How many kilograms of oxygen are required to form 2.86 kg of copper(I) oxide?

x

x x


SOLUTION:

2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)


8. An alloy used in aircraft structures consists of 93.7% Al and 6.3% Cu by mass. The alloy has a density of 2.85 g/cm3. A 0.691 cm3 piece of the alloy reacts with an excess of HCl (aq). If we assume that all the Al but none of the Cu reacts with HCl (aq), what is the mass of H2 obtained?

Answer: 0.207 g H2

The reactant that is completely consumed and determines the quantities of products that form.

The reactant that gives the SMALLEST AMOUNT OF PRODUCT upon calculation, and is COMPLETELY CONSUMED in the process.

The Limiting Reactant

Consider the case of the sandwich

We define the preparation of a HAM sandwich (or any other sandwich) as follows:

2 slices bread + 1 ham 1sandwich

2B + H B2H (BHB)

For example: There are seven pieces of ham and 14 pieces of bread, how many pieces of sandwich are formed?

# of sandwich = 14 B x (1 B2H/ 2B) = 7 pcs sandwich

# of sandwich = 7 H x (1 B2H/ 1 H) = 7 pcs sandwich

---------- NO EXCESS HAM or BREAD


Another ham sandwich problem: There are 7 pieces of ham and 10 pieces of slice bread. How

many pieces of sandwiches could be prepared?

Consuming all sliced bread: # of sandwich = 10 pcs of bread x (1 B2H/ 2 B) = 5 pcs sandwich Consuming all ham: # of sandwich = 7 pcs of ham x (1 B2H/ 1 H) = 7 pcs sandwich ---------- Only 5 pcs of sandwich could be prepared. ---------- Limiting Reactant: Bread ---------- Excess Reactant: Ham


9. A fuel mixture used in the early days of rocketry is composed of two liquids, hydrazine (N2H4) and dinitrogen tetraoxide (N2O4), which ignite on contact to form nitrogen gas and water vapor. A) How many grams of nitrogen gas form when 1.00 x 102 g of N2H4 and 2.00 x 10

2 g of N2O4 are mixed?

N2H4(l) + N2O4(l) N2(g) + H2O(l) 2 4 3

1.00 x 102 g N2H4 x mol N2H4

32.05 g N2H4

x 3 mol N2

2 mol N2H4

2.00 x 102 g N2O4 x mol N2O4

92.02 g N2O4

= 183 g N2

3 mol N2

mol N2O4

mol N2

28.02 g N2 = 131 g N2

x

x mol N2

28.02 g N2 x

Limiting Reactant: N2H4

Calculating Amounts of Reactant and Product in a

Limiting-Reactant Problem Exercise 9


Limiting-Reactant Problem Exercise 10

10. Phosphorus trichloride, PCl3, is a commercially important compound used in the manufacture of pesticides, gasoline additives, and a number of other products. It is made by the direct combination of phosphorus (P4) powder and chlorine gas. What mass of PCl3 (l) forms in the reaction of 125 g phosphorus with 323 g chlorine gas?

Answer: 417 g PCl3 Limiting reactant is Cl2

11. What mass of P4 remains in excess following the reaction in Exercise 10?

Determining the Quantity of Excess Reactant (s)

Remaining After a Reaction Exercise 11

Answer: 31 g P4 remaining

Percent yield indicates how much of the product is obtained from a reaction.

Theoretical yield is calculated by assuming that the reaction goes to completion.

Determined from the limiting reactant calculation.

Actual yield is the amount of a specified pure product made in a given reaction.

In the laboratory, this is the amount of product that is formed in your beaker, after it is purified and dried.

% yield = actual yield

theoretical yield 100%

Percent Yield

12. A 10.0 g sample of ethanol, C2H5OH, was boiled with excess acetic acid, CH3COOH. The reaction produced water and 14.8 g of ethyl acetate, CH3COOC2H5. What is the percent yield of ethyl acetate?

1. Determine the theoretical yield

2. Calculate the percent yield

CH3COOH (aq) + C2H5OH(s) CH3COOC2H5(l) + H2O(l)

10.0 C2H5OH x mol C2H5OH

46.0 g C2H5OH

x 1 mol CH3COOC2H5

1 mol C2H5OH 1 mol CH3COOC2H5

88.0 g CH3COOC2H5

= 19.1 g CH3COOC2H5

x

14.8 g CH3COOC2H5

19.1 g CH3COOC2H5 % yield = X 100 % = 77.5 %

Determining Percent Yield Exercise 12


13. When heated with sulfuric or phosphoric acid, cylcohexanol, C6H11OH is converted to cyclohexane, C6H10.

C6H11OH (l) C6H10 (l) + H2O (l)

If the percent yield of the reaction is 83%, what mass of cyclohexanol must we use to obtain 25 g of cyclohexane?

Answer: 37 g C6H11OH

14. A strip of zinc metal weighing 2.00 grams is placed in an aqueous solution containing 2.50 grams silver nitrate.

a. Write the balanced chemical reaction that will take place

b. How many grams of the solid product is formed?

d. Calculate the percent yield of the process if 1.20 grams of the solid product is obtained after the reaction.

Zn (s) + AgNO3(aq) Ag(s) + Zn(NO3)2(aq) 2 2

c. How much of the excess reagent remains?

2.00 g Zn x mol Zn

65.41 g Zn

x 2 mol Ag

1 mol Zn

2.50 g AgNO3 x mol AgNO3

169.91 g AgNO3

= 1.59 g Ag

2 mol Ag

2 mol AgNO3

mol Ag

107.9 g Ag = 6.60 g Ag

x

x mol Ag

107.9 g Ag x

Limiting Reactant: AgNO3


2.50 g AgNO3 x mol AgNO3

169.91 g AgNO3

= 0.48 g Zn

1 mol Zn

2 mol AgNO3

x mol Zn

65.41 g Zn x

14. A strip of zinc metal weighing 2.00 grams is placed in an aqueous solution containing 2.50 grams silver nitrate.

d. Calculate the percent yield of the process if 1.20 grams of the solid product is obtained after the reaction.

c. How much of the excess reagent remains?

Zn (s) + AgNO3(aq) Ag(s) + Zn(NO3)2(aq) 2 2

2.00 g Zn -0.48 g Zn =1.52g Zn

1.20 g Ag

1.59 g Ag % yield = X 100 % = 75.5 %


One of the steps in the commercial process for converting ammonia to nitric acid involves the conversion of NH3 to NO:

NH3(g) + O2(g) NO(g) + H2O(l)

In a certain experiment, 2.50 g NH3 reacts with 2.85 g O2. (a)Which is the limiting reagent? (b) How many molecules of NO gas is formed? (c) What mass of the reactants are left after the reaction is completed?

Quiz 5 June 28, 2013

A. What is the maximum mass of sulfur dioxide that can be produced by the reaction of 95.6 g of carbon disulfide with 110.0 g of oxygen?

2

2

2

2

2

2

222 SO g 147

SO mol 1

SO g 1.64

O mol 3

SO mol 2

O g 32.0

O mol 1O g 110SO g ?

2222 SO 2 CO O 3 CS

2

2

2

2

2222 SO g 161

SO mol 1

SO g 1.64

CS mol 1

SO mol 2

g 76.2

CS mol 1CS g 6.95 SO g ?


Limiting-Reactant Problem

B. How much of the reactants remain after the reaction?

Additional Exercises

lecture 5. chemical reaction (part 1) (1)

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