chapter 4 dunn

8/3/2019 Chapter 4 Dunn

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Chapter 4

Calculations and Chemical Equations

Mole Concept

Chemical Formulas

Chemical Equations

Balancing Reaction Equations

Mass Relationships2012 1

How chemical reactions are written

2012 2

C3H8 + O2 CO2 + O2

Reactantson the Left

Change (reaction) occurs

Products on the Left

5 3(g) (g) (g) (g)

How chemical reactions are written

2012 3

C3H8 + O2 CO2 + O25 3(g) (g) (g) (g)

States of Matter

(g) Gas(l) Liquid

(s) Solid

(aq) Aqueous solution: Means dissolved in water

Write chemical equations for thefollowing:

2012 4

1. Iron metal is heated with sulfur powder toproduce solid iron (III) sulfide

2. Zinc metal reacts with sulfuric acid to give

aqueous zinc sulfate and hydrogen gas

3. Aqueous solutions of sodium iodide andsilver nitrate yield silver iodide precipitateand aqueous sodium nitrate


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Balancing Chemical Equations1. All reactants and products must be written, with:2. Correct formulas for each.

Once the correct formula is written DO NOT change

the formula!

3. Changes in the coefficients are used to balance

reactant and product sides of the equation.

The process is simple accounting:

The number of atoms of each element must be

exactly equal on both sides of the reaction arrow

without changing the formula.

A good starting point is the first atom listed on the left-

hand side of the equation.

2012 5 2012 6

Balancing Chemical EquationsCH4(g) + O2(g) H2O(g) + CO2(g)

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

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

Always verify once balancing is complete

P4 + Cl2 PCl3

P4 + Cl2 4PCl3

P4 + 6Cl2 4PCl3

Balancing Equations Hints and

Tricks Keep track come up with a system

What to start with

Work left to right

Balance elements that appear only once on each side first

Polyatomic ions present? Balance the group instead of

individual atoms

Think of H2O as (H)(OH)

Its okay to use coefficients that are fractions (You can fix it

later by multiplying the equation)

CHECK YOUR WORK!!!!

2012 8

More Balancing of Reactions:

2012 9

SO3 + H2O H2SO4Sometimes reactions are balanced when written!

Phosphoric acid reacts with calcium hydroxide toproduce calcium phosphate and water.

H3PO4 + Ca(OH)2 H2O + Ca3(PO4)2

2H3PO4 + 3Ca(OH)2 6H2O + Ca3(PO4)2

Octane combusts forming normal hydrocarbon combustion products.C8H18 + O2 CO2 + H2O

C8H18 + 12.5O2 8CO2 + 9H2O

2C8H18 + 25O2 16CO2 + 18H2O

2x[ ]


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Types of Reactions:Single replacement/displacement

A + BC AC + B

2Na + 2H2O 2NaOH + H2

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Types of Reactions

Double Replacement/Precipitation reactions:

AB + CD AD + CB

2KI + Pb(NO3 )2 2 KNO3 + PbI2

The PbI2 is a solid precipitate

Acid/Base Reactions:

H3PO4 + Ca(OH)2 H2O + Ca3(PO4)2

2H3PO4 + 3Ca(OH)2 6H2O + Ca3(PO4)2

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Types of Reactions: Summary

Single replacement/displacement:

A + BC AC + B

Double Replacement/Precipitation reactions:

AB + CD AD + CB

Acid/Base Reactions:

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Equation Writing: Examples

Write a balanced chemical equation showing the

reaction between lithium chloride and lead (II)

acetate

Write a balanced chemical equation showing thereaction between aluminum and hydrochloric acid

2012 13
http://c/Users/Marie%20Dunn/PowerLecture%20GOB/Multi_media/General_Organic_and_Biochemistry_Power_Lecture/dswmedia/QuickTime_Movies/movie9_PbI2_full.movhttp://c/Users/Marie%20Dunn/PowerLecture%20GOB/Multi_media/General_Organic_and_Biochemistry_Power_Lecture/dswmedia/QuickTime_Movies/04M11VD1.MOVhttp://c/Users/Marie%20Dunn/PowerLecture%20GOB/Multi_media/General_Organic_and_Biochemistry_Power_Lecture/dswmedia/QuickTime_Movies/08m15vd2.mov


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Neutralization Reactions

Reaction Between an ACID and a BASE

HB + COHHOH + CB

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Aka

H2O*

* Reactions involving water (H2O ) are

often easier to understand if you think of

water as HOH

Neutralization Reactions: Examples

HCl + NaOH

Write the balanced equation for the reaction

between sulfuric acid and potassium hydroxide

Write the balanced equation for the reaction

between phosphoric acid and ammonium hydroxide

2012 15

For More Practice

2012 16

Formula Weight

Formula weight: sum of the atomic weights of all the

constituent elements of a compound.

FeSO4 (name?)

iron (II) sulfatemeans: 1Fe atom 1 S atom and 4 Oxygen atoms

1 Fe 55.847 amu/atom

1 S atom 32.066 amu/atom4 O atoms 4 x 15.9994 = 63.9976 amu/4atoms

151.9106

151.911 amu/formula unit

Recall: 1 amu = 1.6605 x 10-24 g

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The Mole

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Avogadros number (NA)

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Avogadros number (NA= )

Very much like the chemists dozen

1 dozen eggs = _________

1 dozen cookies = ________

1 dozen dimes = __________

1 mole of C atom = _______________

1 mole of NaCl (an ionic compound) =

______

1 mole of C6H12O6 molecules (a molecular compound)

= _________

1 mole of sand particles = __________________

6.022 X 1023

12 eggs

12 cookies

12 dimes

6.022 X 1023 C atoms

6.022 X 1023 NaCl formula units

6.022 X 1023 sand particles

6.022 X 1023 C6H12O6 molecules

Calculations involving Moles

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Mole

A

# of

particles*

of A

*Atom (element)* Molecules (molecular)

*Formula Units (ionic Compounds

6.022 X 1023

particles*A=1mole A

What is the Mass of a Mole?

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Mole

A Mass A(grams)_g A** = 1 mole A

**Use molar mass as a conversion

PERIODIC TABLE!!!!


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2012 22

Molecular Mass (MM)Molar Weight (M.W.)

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E.g What is the M.W of Al(H2PO4)3 ?

Al 1 x 26.98154 26.98154 g/mol

H 6 x 1.00794 6.04764 g/mol

P 3 x 30.97376 92.92128 g/mol

O 12 x 15.9994 191.9928 g/mol

317.94326 g/mol

317.9433 g/mole Al(H2PO4)3

Calculations involving Moles

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Mole

A Mass A(grams)

# of

particles*

of A



6.022 X 1023

particles*A=1moleA __g A** = 1 mole A


PERIODIC TABLE!!!!

Information M.W. gives If given gram quantity and name we can

answer:

How many moles do we have?

How many formula units or molecules do wehave?

How many total atoms and/or how many ofeach type of atom do we have?

What is the % by weight of each element.How many grams of each element are there?

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Answer all the above questions for 37.6g of

calcium nitrate


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Stoichiometry

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Mole

A Mass A(grams)

# of

particles*

of A



6.022 X 1023

particles*A=1moleA __g A** = 1 mole A


PERIODIC TABLE!!!!

Mole BMass B(grams)

# of

particles*

of B

6.022 X 1023

particles*B=1moleB __g B** = 1 mole B

_molA***

=__

molB

***

Coefficients in

balanced equationrepresent the MOLE

RATIO

Stoichiometry: Weight Relationships in

Chemical Reactions1. Write a balanced chemical equation

2. If given grams: Calculate the moles of the knownsubstance by dividing by molar weight.

3. Use the balanced chemical eqn. molar ratios and themoles of the known substance to determine themoles of those substances unknown.

4. Turn the moles of each unknown into grams bymultiplying by molar weight.

5. VERIFY using Law of Conservation of Mass, i.e., Left-side mass total = Right-side mass total

Reality Check: Theoretical vs. Actual Yield

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Example Stoichiometry Problems

1. Calculate the mass of aluminum necessary to react

with 1.4 grams of iron (II) oxide.

2. How many grams of zinc metal will react with

hydrochloric acid to give 0.500 grams of zincchloride?

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Limiting Reactants: ConceptConsider the following recipe for French toast

3 pieces of French toast

On a lazy Sunday morning, you want to make French

toast. In your kitchen you find that you have

12 pieces of bread 3 eggs

2 cups of milk

lots of cinnamon and/or vanilla

How much French toast can you make without going

to the store?2012 29

3 slices of bread + 1 egg +

c. milk + tsp. cinnamon +

tsp vanilla
http://../Tests/Periodic%20Table_Tests.pdfhttp://../Tests/Periodic%20Table_Tests.pdfhttp://../Tests/Periodic%20Table_Tests.pdfhttp://c/Users/Marie%20Dunn/Documents/CHEM121/Other%20instructor%20documents/Chem121Harold/Chem121PPT/ThermiteRxn.MOV


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Limiting and Excess Reagents In a real lab we often dont perform reactions

like the text book balanced equations

For some reactions we want to add more of

one reagent (excess) (e.g. more wood on the

fire, burns hotter)

The reactant that would be used up first

(leaving the excess reactant) is called the

limiting reagent

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Limiting Reactant Examples

How much molten iron is produced from the reaction of 25.0

grams of FeO with 25.0 g of Al?

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Percent Yield When we carry out a reaction we often get less product than we expect from

our calculations! S@#*!

Does this mean we are violating the Law of Conservation of Mass!?

No,

Some of our reactant may not react

Our lab technique may have been a touch sloppy

Reality Check: Theoretical Yield vs. Actual Yield

Actual Yield Mass of product formed

Theoretical Yield Mass of product that should form based on stoichiometryof the balanced equation

Percent Yield Actual yield divided by theoretical yield times 100

Percent Yield = Actual Yield X 100 =

Theoretical yield

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% Yield Calculations

A student calculates a theoretical yield of 60.0 grams

of acetic acid. In the laboratory, only 57.8 grams of

acetic acid are made. What is the % yield of acetic

acid.

Joe Chemgeek performs a chemical reaction. Prior

the reaction, he calculated a theoretical yield of

54.36 grams. However, the reaction yields a percent

yield of only 34.99%. What is the experimental

yield?

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Reduction/Oxidation RxnsOxidation:

1. The loss of electron(s) by a chemical species2. The gain of oxygen and/or loss of H by a chemical species3. The species being oxidized is the Reducing Agent

Reduction:1. The gain of electron(s) by a chemical species;2. The loss of oxygen and/or gain of H by a chemical species3. The species being reduced is the Oxidizing Agent

Basically a transfer of electrons between two species, so Both reduction and oxidation ALWAYS occur together.

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LEO says GER

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Loss of Electrons Oxidation

Gain of Electrons Reduction

A Redox Reaction Dissected

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Oxidization and Reduction

Zn (s) + Cu2+ (aq) Zn2+(aq) Cu (s)

Which substance is Oxidized? Which is

Reduced?

LEO says GER

Which Substance is the oxidizing agent?

Which is the Reducing Agent?

2012 37


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Oxidation-Reduction Reactions Batteries:

All batteries are electrochemical cells where theexchange of electrons is run through connectionsand the flow is used to power devices.

Lead/acid - car battery

2H2SO4(aq) + Pb(s) + PbO2(s) 2PbSO4(s) + 2H2O(l)

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0 4+ 2+

o The solid Pb loses two electrons to the Pb in PbO2, causing both to

become Pb2+ in PbSO4 solid.

o This reaction occurs spontaneously and produces electrical energy to

start the car. (Called discharging)

o To recharge the battery - the alternator supplies higher voltage

forcing the chemical reaction backwards

When Organic Compounds areInvolved

Using the second definition to define redox

reactions between covalent compounds

Oxidation is the gain of oxygen/loss of hydrogen

Reduction is the loss of oxygen/gain of hydrogen

Example: A redox reaction occurs during the

combustion of methane gas

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Categories of Oxidation-Reduction Reactions

Combustion: Burning reactions where compounds are

oxidized by oxygen

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

Respiration: The slowburn, oxygen oxidized carbon-

containing compounds in our cells

C6H12O6 + 6O2(g) 6CO2(g) + 6H2O(l)

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Categories of Oxidation-Reduction Reactions

Rusting:

4Fe(s) + 3O2(g) 2Fe2O3(s)

Bleaching: decolorizing NaOCl - household bleach

Most stains have metals involved or conjugatedhydrocarbons,

bleaches oxidize the metals or the double bonds and

either decolorize the stain or make it water soluble.

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http://redoxfe%20o2.mov/


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More Examples

Al(s) + Fe3+(aq) Al3+(aq) + Fe(s)

CH3OH + O2 HCOOH + H2O

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