1 Introduction to Chemical Reactions A chemical change—a chemical reaction—converts one substance into another. Chemical reactions involve: breaking bonds

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Introduction to Chemical Reactions

A chemical change—a chemical reaction—converts one substance into another.

Chemical reactions involve:

• breaking bonds in the reactants (starting materials)

• forming new bonds in the products.

CH4 and O2 CO2 and H2O

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A chemical equation is an expression that useschemical formulas and other symbols to illustratewhat reactants constitute the starting materials in a reaction and what products are formed.

• The reactants are written on the left.

• The products are written on the right.

• Coefficients show the number of molecules of a given element or compound that react or are formed.

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• The law of conservation of mass states that atoms cannot be created or destroyed in a chemical reaction.

• Coefficients are used to balance an equation.

• A balanced equation has the same number of atoms of each element on both sides of the equation.

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Balancing Chemical EquationsHOW TO Balance a Chemical

EquationExampl

e Write a balanced chemical equation for the reaction of propane (C3H8) with oxygen (O2) to form carbon dioxide (CO2) and water (H2O).

Step [1]

Write the equation with the correct formulas.

C3H8 + O2 CO2 + H2O

• The subscripts in a formula can never be changed to balance an equation, because changing a subscript changes the identity of a compound.

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Balancing Chemical Equations

Step [2]

HOW TO Balance a Chemical Equation

Balance the equation with coefficients oneelement at a time.

• Balance the C’s first:

• Balance the H’s next:

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Step [2]


Balance the equation with coefficients oneelement at a time.

• Finally, balance the O’s:

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Step [3]

Check to make sure that the smallest setof whole numbers is used.


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Chemistry of an Automobile Airbag

• A severe car crash triggers the ignition of sodium azide (NaN3) converting it to sodium (Na) and nitrogen gas (N2).

• The nitrogen gas causes the bag to inflate fully in 40 milliseconds, protecting passengers from injury.

2 NaN3 2 Na + 3 N2

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The Mole and Avogadro’s Number

A mole is a quantity that contains 6.02 x 1023 items.

• 1 mole of C atoms = 6.02 x 1023 C atoms

• 1 mole of H2O molecules = 6.02 x 1023 H2O molecules

The number 6.02 x 1023 is Avogadro’s number.

It can be used as a conversion factor to relate thenumber of moles of a substance to the number ofatoms or molecules:

1 mol6.02 x 1023 atoms

or 6.02 x 1023 atoms1 mol

• 1 mole of vitamin C molecules = 6.02 x 1023 vitamin C molecules

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Sample Problem 5.6

How many molecules are contained in 5.0 moles of carbon dioxide (CO2)?

Step [1]

Identify the original quantity and the desired quantity.

5.0 mol of CO2

original quantity

? number of molecules of CO2

desired quantity

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Step [3]

Set up and solve the problem.

5.0 mol x 6.02 x 1023 molecules1 mol

=

3.0 x 1024 molecules CO2

Unwanted unit cancels.

Step [2]

Write out the conversion factors.

1 mol6.02 x 1023 molecules

or 6.02 x 1023 molecules1 mol

Choose this one to cancel mol.

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Mass to Mole Conversions• The formula weight/ molecular mass is the sum of the atomic weights of all the atoms in a compound, reported in atomic mass units (amu).

What is the formula weight of nicotine (C10H14N2)?

Determine the number of atoms of each element from the subscripts in the chemical formula.

• C10H14N2 contains 10 C atoms, 14 H atoms, and 2 N atoms.

Sample Problem 5.8

Step [1]

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Mass to Mole Conversions

Step [2]

Multiply the number of atoms of each element by the atomic weight and addthe results.

10 C atoms x 12.01 amu = 120.1 amu

14 H atoms x 1.08 amu = 14.11 amu

2 N atoms x 14.01 amu = 28.02 amu

Formula weight of C10H14N2 = 162.23 amu

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Mass to Mole ConversionsMolar Mass

• The molar mass is the mass of one mole of any substance, reported in grams.

• The value of the molar mass of a compound in grams equals the value of its formula weight in amu.

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Mass to Mole ConversionsRelating Grams to Moles

• The molar mass relates the number of moles to the number of grams of a substance.• In this way, molar mass can be used as a conversion factor.Sample Problem

5.10How many moles are present in 100. g of aspirin (C9H8O4, molar mass 180.2 g/mol)?

Step [1]

Identify the original quantity and thedesired quantity.

100. g of aspirinoriginal quantity

? mol of aspirindesired quantity

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Step [2]

Write out the conversion factors.

180.2 g aspirin1 mol

1 mol180.2 g aspirin

or

Choose this one to cancel g.

• The conversion factor is the molar mass, and it can be written in two ways. • Choose the one that places the unwanted unit, grams, in the denominator so that the units cancel:

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Step [3]


100. g aspirin x 1 mol180.2 g aspirin

= 0.555 mol aspirin


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Mole Calculations in Chemical Equations

A balanced chemical equation also tells us:

• the number of moles of each reactant that combine

• the number of moles of each product formed.

1 N2(g) + 1 O2(g) 2 NO(g)

1 mole of N2

1 molecule N2

1 mole of O2

1 molecule O2

2 moles of NO2 molecules NO

(The coefficient “1” has been written for emphasis.)

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Mole Calculations in Chemical EquationsCoefficients are used to form mole ratios, which canserve as conversion factors.

N2(g) + O2(g) 2 NO(g)

Mole ratios:1 mol N2

1 mol O2

1 mol N2

2 mol NO1 mol O2

2 mol NO

Use the mole ratios from the coefficients in the balanced equation to convert moles of one compound (A) into moles of another compound (B).

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Sample Problem 5.11

Using the balanced chemical equation, how many moles of CO are produced from 3.5 molesof C2H6?

Step [1] Identify the original and desired quantities.

3.5 mol C2H6

original quantity? mol CO

desired quantity

2 C2H6(g) + 5 O2(g) 4 CO(g) + 6 H2O(g)

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Step [2] Write out the conversion factors.

2 mol C2H6

4 mol COor 4 mol CO

2 mol C2H6

Choose this one to cancel mol C2H6.

Step [3]


3.5 mol C2H6 x 4 mol CO2 mol C2H6

= 7.0 mol CO


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Mass Calculations in Chemical EquationsHOW TO Convert Moles of Reactant to Grams of

Product

Example

Using the balanced equation, how manygrams of O3 are formed from 9.0 mol of O2?

3 O2(g) 2 O3(g)

Moles ofreactant

Moles ofreactant

Grams ofproduct

Grams ofproduct

mole–moleconversion

factor


factor

molar massconversion

factor


factor

Moles ofproduct

Moles ofproduct

[1] [2]

sunlight

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Mass Calculations in Chemical Equations

Step [1]

Convert the number of moles of reactantto the number of moles of product usinga mole–mole conversion factor.

HOW TO Convert Moles of Reactant to Grams of Product

3 mol O2

2 mol O3

or 2 mol O3

3 mol O2

Cancel mol O2

in step [1].

Step [2]

Convert the number of moles of productto the number of grams of product using the product’s molar mass.

1 mol O3

48.00 g O3

48.00 g O3

1 mol O3

or

Cancel mol O3

in step [2].

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Mass Calculations in Chemical EquationsHOW TO Convert Moles of Reactant to Grams of

Product

• Set up and solve the conversion.

9.0 mol O2x

2 mol O3

3 mol O2

Moles ofreactant

Moles ofreactant

Grams ofproduct

Grams ofproduct

x48.00 g O3

1 mol O3

= 290 g O3


factor


factor


factor


factor

Mol O2 cancel. Mol O3 cancel.

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Mass Calculations in Chemical EquationsHOW TO Convert Grams of Reactant to Grams of

ProductExampl

eEthanol (C2H6O, molar mass 46.07 g/mol) issynthesized by reacting ethylene (C2H4, molar mass 28.05 g/mol) with water.

How many grams of ethanol are formed from 14 g of ethylene?

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Product

Moles ofreactant

Moles ofreactant

Grams ofproduct

Grams ofproduct


factor


factor


factor


factor

Moles ofproduct

Moles ofproduct

Grams ofreactant

Grams ofreactant


factor


factor[1]

[2]

[3]

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Product

14 g C2H4 x1 mol C2H4

28.05 g C2H4

x1 mol C2H6O1 mol C2H4

x46.07 g C2H6O 1 mol C2H6O

Grams ofreactant

Grams ofreactant


factor


factor


factor


factor


factor


factor

= 23 g C2H6O Grams ofproduct

Grams ofproduct

Grams C2H4

cancel.Moles C2H4

cancel.Moles C2H6O

cancel.

Documents

1 Introduction to Chemical Reactions A chemical change—a chemical reaction—converts one substance into another. Chemical reactions involve: breaking bonds