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CHEMISTRY - BROWN 14E

CH.3 - CHEMICAL REACTIONS AND REACTION STOICHIOMETRY

CONCEPT: EMPIRICAL FORMULA

The empirical formula is also known as the __________________________.

• It represents the _______________________________ ratio of moles

of each element in the compound.

The molecular formula is also known as the __________________________.

• It represents the _______________________________ ratio of moles of each element in the compound.

EXAMPLE: What is the empirical formula of dimethylhydrazine, C2H8N2, a colorless liquid used as a rocket fuel?

EXAMPLE: Elemental analysis of a sample of an ionic compound showed 2.82 g of Na, 4.35 g of Cl, and 7.83 g of O. What

is the empirical formula and name of the compound?

EXAMPLE: After a workout session, lactic acid (M = 90.08 g/mol) forms in muscle tissue and is responsible for muscle

soreness. Elemental analysis shows that this compound contains 40% C, 6.7% H and 53.3% O. Determine the molecular

formula.

C6H12O6 =



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CONCEPT: COMBUSTION ANALYSIS

Under a combustion reaction a compound made of _______________ or _______________ reacts with ______________ .

• The products formed will be ________________ and ________________ .

EXAMPLE: A 0.2500 g sample contains carbon, hydrogen and oxygen and undergoes complete combustion to produce

0.3664 g of CO2 and 0.1500 g of H2O. What is the empirical formula of the compound?



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CONCEPT: CHEMICAL COMPOSITION

The mass per mole of an element is called its _____________________________ (M).

The mass per mole of a compound is called its ____________________________ (M).

• They both have the units of _______________.

1. Elements. To find mass of an element just look up its atomic mass in the periodic table.

EXAMPLE: What is the total mass of each of the following elements?

a. Sodium b. Gold c. Mercury 2. Compounds. The mass of a compound is the sum of the individual masses of the elements in the chemical formula.

EXAMPLE: What is the total mass of each of the following compounds?

a. N2O5 b. C12H22O11 c. (NH4)3PO4



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CONCEPT: MASS PERCENT

Mass or weight percent is the percentage of a given element in a compound.

EXAMPLE: What is the percentage of carbon in sodium hydrogen carbonate, NaHCO3?

EXAMPLE: A sample of toothpaste contains tin (II), SnF2. Analysis of a 5.25 g sample contains 8.77 x 10-3 g of F. What is

the percentage of tin (II) fluoride in the sample?

PRACTICE: Hemoglobin contains 0.33% iron and has a molecular weight of 68 kg. How many iron atoms are in each

molecule of hemoglobin?

Mass Percent (%) =

MassComponent)(TotalMass)(

•100



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CONCEPT: BALANCING CHEMICAL REACTIONS

When balancing an equation always make sure the ________ and ________ of atoms on both sides of the arrow are equal.

EXAMPLE: Write balanced equations for each of the following by inserting the correct coefficients in the blanks:

a. ____ Al (s) + ____ Cl2 (g) ____ AlCl3 (s)

b. ____ Ba3(PO4)2 (s) + ____ KOH (aq) ____ K3PO4 (aq) + ____ Ba(OH)2 (aq)

c. ____ C4H10 (aq) + ____ O2 (g) ____ CO2 (g) + ____ H2O (l)



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CONCEPT: STOICHIOMETRIC REACTIONS

2 H2 (g) + 1 O2 (g) 2 H2O (g)

In the above equation the numbers that are in bold are called _______________________.

• They tell us the number of ______________ of each compound that reacts.

• This numerical relationship between compounds in a balanced equation is called __________________________.

STOICHIOMETRIC CHART

Before we get into solving stoichiometric reactions lets work out a plan of attack.

Entities means ______________________ , ______________________ or ______________________.

Entities of Given

Grams of Given

Entities of Unknown

Grams of Unknown

Moles of Given Moles of Unknown

Use this chart when given a chemical equation with the ____________ quantity of a compound or element and asked to find

the ____________ quantity of another compound or element.

EXAMPLE: How many grams of H2O are produced when 12.3 g H2 reacts?

2 H2 (g) + 1 O2 (g) 2 H2O (g)



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PRACTICE: STOICHIOMETRIC REACTIONS

EXAMPLE 1: The oxidation of chromium solid is represented by the following equation:

4 Cr (s) + 3 O2 (g) 2 Cr2O3 (s)

a. How many moles of chromium (III) oxide are produced when 34.69 g Cr reacts with excess oxygen gas?

b. How many grams of O2 were needed to produce 4.28 x 103 molecules Cr2O3?

EXAMPLE 2: If the density of ethanol, CH3CH2OH, is 0.789 g/mL, how many milliliters of ethanol are needed to produce 4.8

g of H2O in the following reaction?

CH3CH2OH (l) + 3 O2 2 CO2 (g) + 3 H2O (l)

PRACTICE: Dinitrogen monoxide gas decomposes to form nitrogen gas and oxygen gas. How many molecules of oxygen

are formed when 8.00 g of dinitrogen monoxide decomposes?



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CONCEPT: LIMITING REAGENT

In a chemical reaction the reactant that is consumed when a reaction occurs and determines the maximum amount of

product formed is called the _________________________________.

• The amount of product it forms is called the _________________________ yield.

The reactant that remains after the completion of the chemical reaction is called the ____________________ reactant.

EXAMPLE: Chromium (III) oxide reacts with hydrogen sulfide (H2S) gas to form chromium (III) sulfide and water:

Cr2O3 (s) + 3 H2S (g) Cr2S3 (s) + 3 H2O (l) [Balanced]

a. What is the mass of chromium (III) sulfide formed when 14.20 g Cr2O3 reacts with 12.80 g H2S?

b. Identify the limiting reactant, excess reactant and theoretical yield.

c. What mass of excess reactant remains?



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CONCEPT: PERCENT YIELD

The percent yield of a reaction is used to determine how effective the chemist was in creating their desired products.

• A high percent yield would signify that the reaction is _______________________________________.

EXAMPLE 1: A scientist performs an experiment in the laboratory and obtains 13.27 g Cr2S3. If his calculations on scratch

paper give him a theoretical yield of 18.23 g what is the percent yield?

PRACTICE: Consider the following balanced chemical reaction:

2 C6H6 (l) + 15 O2 (g) 12 CO2 (g) + 6 H2O (l)

a. If a 2.6 g sample of C6H6 reacted with excess O2 to produce 1.25 g of water, what is the percent yield of water?

b. If the above reaction only went to 75% completion, how many moles of CO2 would be produced if 1.57 x 10-5 molecules of C6H6 were reacted with excess oxygen?



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CONCEPT: GROUP REACTIVITY

The central principle of Organic Chemistry is based on the _________________________________. • The reactivity of an organic compound is all based on which type is present.

C

CC

C

CC

CO

C

C

CC

CC

CH3

H3C

H3C

OH

Tetrahydrocannabinol (THC)

CH2CH2CH2CH2CH3

Alkane

Alkene

Alkyne

Alcohol

Amine

Aldehyde

Ketone

Carboxylic Acid

Ester



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CONCEPT: GROUP REACTIVITY (PRACTICE)

EXAMPLE: In each of the following molecules, identify the type(s) of functional groups present.

a. b.

O

OH

PRACTICE: In each of the following molecules, identify the type(s) of functional groups present.

a. b.

O

O



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CH.3 - CHEMICAL REACTIONS AND REACTION STOICHIOMETRYlightcat-files.s3.amazonaws.com/packets/admin... · soreness. Elemental analysis shows that this compound contains 40% C, 6.7%