cmh 121 luca preziati chapter 4: chemical reactions + + 4 wood boards6 nails 1 fence panel + 4 dozen...

CMH 121 Luca Preziati

Chapter 4: Chemical Reactions

+

+4 Wood Boards 6 Nails 1 Fence Panel

+4 Dozen

Wood Boards6 Dozen

Nails1 Dozen

Fence Panels

For a 12 panels fence…



+

+4C 3H2 C4H6

+4 moles of

C3 moles of

H2

1 mole ofC4H6

For a mole of 1,3-Butadiene …



1 Dozen = 12 items

• The mole is defined (since 1960) as the amount of substance of a system that contains as many entities as there are atoms in 12 g of carbon-12.• Symbol: mol.• Coined by Wilhelm Ostwald in 1893

1 mol = 12 g of carbon-12

1 mole = 6.0221415×1023 items

# of Molecules = # of moles X 6.022×1023

• A mole of carbon contains 6.0221415×1023 atoms of carbon, but the same is true for anyother element or molecule; in general:

• 6.0221415×1023 is the Avogadro’s Number



m = number of nails X mass of 1 nailExample:500g = 100 nails X 5g

m = number of moles X mass of 1 mole of the substance

m = n X M.M.

Where:m = mass (g)n = number of moles (mol)M.M. = Molecular Mass (g/mol) = mass of 1 mole of the substance

Sometime the Molecular Mass is measured in Daltons (1Da = 1g/mol)



The molar mass of an element is numerically the same as the atomic weight of the element. They are not however the same; they have different units:• The atomic weight is defined as one twelfth of the mass of an isolatedatom of carbon-12 and is therefore dimensionless• The molar mass is measured in g/mol.

The molar mass of a compound is given by the sum of the atomic weights of the atoms which form the compound.

Example: molar mass of Ca(NO3)2

• Write a Solution Map for converting the units :

InformationGiven: 1.1 x 1022 Ag atomsFind: ? molesConv. Fact.: 1 mole = 6.022 x 1023

Example:A silver ring contains 1.1 x 1022 silver atoms. How many moles of silver are in the ring?

atoms Agatoms Ag moles Agmoles Ag

atoms Ag10022.6

Ag mole 123

• Check the Solution:

1.1 x 1022 Ag atoms = 1.8 x 10-2 moles Ag

The units of the answer, moles, are correct.The magnitude of the answer makes sense

since 1.1 x 1022 is less than 1 mole.

InformationGiven: 1.1 x 1022 Ag atomsFind: ? molesConv. Fact.: 1 mole = 6.022 x 1023

Sol’n Map: atoms mole

Example:A silver ring contains 1.1 x 1022 silver atoms. How many moles of silver are in the ring?

• Write a Solution Map for converting the units :

InformationGiven: 1.75 mol H2O

Find: ? g H2O

C F: 1 mole H2O = 18.02 g H2O

mol H2Omol H2O g H2Og H2O

OH mol 1

OH g 18.02

2

2

Example:Calculate the mass (in grams) of 1.75 mol of water


1.75 mol H2O = 31.5 g H2O

The units of the answer, g, are correct.The magnitude of the answer makes sense

since 31.5 g is more than 1 mole.

InformationGiven: 1.75 mol H2OFind: ? g H2OC F: 1 mole H2O = 18.02 g H2OSol’n Map: mol g

Example:Calculate the mass (in grams) of 1.75 mol of water

Chemical Equations

• CH4 and O2 are the reactants, and CO2 and H2O are the products• the (g) after the formulas tells us the state of the chemical• the number in front of each substance tells us the numbers of those molecules in the reaction

•called the coefficients

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

Reactants Products

Stateg=gasl=liquids=solid

Coefficients

Combustion of MethaneBalanced

• to show the reaction obeys the Law of Conservation of Mass it must be balanced

CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(g)

H

HC

H

H+

O

O

C +OO

OO

+

OH H

OH H

+

1 C + 4 H + 4 O 1 C + 4 H + 4 O

Tro's Introductory Chemistry, Chapter 7 12

Examples• when magnesium metal burns in air it

produces a white, powdery compound magnesium oxide

Mg(s) + O2(g) MgO(s)

1) count the number of atoms of on each side– count polyatomic groups as one “element” if on

both sides

Mg(s) + O2(g) MgO(s)

1 Mg 1 2 O 1


ExamplesMg(s) + O2(g) MgO(s)

2) pick an element to balance– avoid element in multiple compounds– do free elements last– since Mg already balanced, pick O

3) find least common multiple of both sides & multiply each side by factor so it equals LCM

Mg(s) + O2(g) MgO(s) 1 Mg 1

1 x 2 O 1 x 2


Examples

4) use factors as coefficients in front of compound containing the element

Mg(s) + O2(g) 2 MgO(s)

1 Mg 1 1 x 2 O 1 x 2


ExamplesMg(s) + O2(g) MgO(s)

5) Recount – Mg not balanced now – That’s OK!!

Mg(s) + O2(g) 2 MgO(s)

1 Mg 2 2 O 2

6) Repeat – attacking unbalanced element

2 Mg(s) + O2(g) 2 MgO(s)

2 x 1 Mg 2 2 O 2



The study of the numerical relationship between chemical quantities in a chemical reaction is called reaction

stoichiometry

Mole-to-Mole Conversions • the balanced equation is the “recipe” for a chemical

reaction• the equation 3 H2(g) + N2(g) 2 NH3(g) tells us that 3

molecules of H2 react with exactly 1 molecule of N2 and make exactly 2 molecules of NH3 or

3 molecules H2 1 molecule N2 2 molecules NH3 (in this reaction)

Example:• Sodium chloride, NaCl, forms by the following reaction

between sodium and chlorine. How many moles of NaCl result from the complete reaction of 3.4 mol of Cl2? Assume there is more than enough Na.

2 Na(s) + Cl2(g) 2 NaCl(s)


3.4 mol Cl2 6.8 mol NaClThe units of the answer, moles NaCl, are correct.

The magnitude of the answer makes sensesince the equation tells us you make twice as many

moles of NaCl as the moles of Cl2 .

InformationGiven: 3.4 mol Cl2

Find: ? moles NaClCF: 1 mol Cl2 2 mol NaCl

SM: mol Cl2 mol NaCl

Example:How many moles of NaCl result from the complete reaction of 3.4 mol of Cl2 in the reaction below?2 Na(s) + Cl2(g) 2 NaCl(s)

19

Mass-to-Mass Conversions• we know there is a relationship between the mass and

number of moles of a chemical1 mole = Molar Mass in grams

• the molar mass of the chemicals in the reaction and the balanced chemical equation allow us to convert from the amount of any chemical in the reaction to the amount of any other

Example:• In photosynthesis, plants convert carbon dioxide and

water into glucose, (C6H12O6), according to the following reaction. How many grams of glucose can be synthesized from 58.5 g of CO2? Assume there is more than enough water to react with all the CO2.

(aq)(g)(l)(g) 61262sunlight

22 OHC O 6 OH 6 CO 6

• Write a Solution Map:

gCO2

6126

6126

OHC mol 1

OHC g 80.21

2

2

CO g 4.014

CO mol 1

InformationGiven: 58.5 g CO2

Find: g C6H12O6

CF: 1 mol C6H12O6 = 180.2 g

1 mol CO2 = 44.01 g

1 mol C6H12O6 6 mol CO2

molCO2

molC6H12O6

gC6H12O6

2

6126

CO mol 6

OHC mol 1

Example:How many grams of glucose can be synthesized from 58.5 g of CO2 in the reaction?6 CO2(g) + 6 H2O(l) 6 O2(g) + C6H12O6(aq)

• Check the Solution:58.5 g CO2 = 39.9 g C6H12O6

The units of the answer, g C6H12O6 , are correct.It is hard to judge the magnitude.

InformationGiven: 58.5 g CO2

Find: g C6H12O6

CF: 1 mol C6H12O6 = 180.2 g 1 mol CO2 = 44.01 g 1 mol C6H12O6 6 mol CO2

SM: g CO2 mol CO2 mol C6H12O6 g C6H12O6

Example:How many grams of glucose can be synthesized from 58.5 g of CO2 in the reaction?6 CO2(g) + 6 H2O(l) 6 O2(g) + C6H12O6(aq)

Limiting reactant and YieldLimiting Reagent (or reactant): The reactant that is completely consumed in a chemical reaction.

Theoretical yield: The amount of product that can be madein a chemical reaction based on the amount of limiting reactant.

Actual yield: The amount of product actually produced by a chemical reaction.Because of both controllable and uncontrollable factors, the actual yield of product will always be less than the theoretical yield.

YieldPercent 100Yield lTheoretica

Yield Actual %


Example:• When 11.5 g of C are allowed to react with 114.5 g of

Cu2O in the reaction below, 87.4 g of Cu are obtained. Find the Limiting Reactant, Theoretical Yield and Percent Yield.

(g)(s)(s)(s) CO Cu 2 Cu OCu2


• Check the Solutions:

Limiting Reactant = Cu2OTheoretical Yield = 101.7 gPercent Yield = 85.9%

The Percent Yield makes sense as it is less than 100%.

Information

Given: 11.5 g C, 114.5 g Cu2O87.4 g Cu produced

Find: Lim. Rct., Theor. Yld., % Yld.

CF: 1 mol C = 12.01 g; 1 mol Cu = 63.54 g; 1 mol Cu2O = 143.08 g; 1 mol Cu2O 2 mol Cu; 1 mol C 2 mol Cu

Example:When 11.5 g of C reacts with 114.5 g of Cu2O, 87.4 g of Cu are obtained. Find the Limiting Reactant, Theoretical Yield and Percent Yield. Cu2O(s) + C(s) 2 Cu(s) + CO(g)


Dissociation• when ionic compounds dissolve

in water, the anions and cations are separated from each other - this is called dissociation– however not all ionic compounds

are soluble in water!

• when compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion


Precipitation Reactions

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


Ionic Equations• equations which describe the chemicals put into the water

and the product molecules are called molecular equations2 KOH(aq) + Mg(NO3)2(aq) 2 KNO3(aq) + Mg(OH)2(s)

• equations which describe the actual dissolved species are called ionic equations – aqueous electrolytes are written as ions

• soluble salts, strong acids, strong bases

– insoluble substances and nonelectrolytes written in molecule form

• solids, liquids and gases are not dissolved, therefore molecule form2K+1

(aq) + 2OH-1(aq) + Mg+2

(aq) + 2NO3-1

(aq) K+1(aq) + 2NO3

-1(aq) + Mg(OH)2(s)


Oxidation-Reduction Reactions

• We say that the element that loses electrons in the reaction is oxidized

• and the substance that gains electrons in the reaction is reduced

• you cannot have one without the other


Reactions of Metals with Nonmetals(Oxidation-Reduction)

• metals react with nonmetals to form ionic compounds– ionic compounds are solids at room temperature

• the metal loses electrons and becomes a cation– the metal undergoes oxidation

• the nonmetal gains electrons and becomes an anion– the nonmetal undergoes reduction

• In the reaction, electrons are transferred from the metal to the nonmetal

2 Na(s) + Cl2(g) NaCl(s)


Oxidation-Reduction Reactions• any reaction that has an element that is uncombined on

one side and combined on the other is a redox reaction– uncombined = free element– 2 CO + O2 2 CO2

– 2 N2O5 4 NO2 + O2

– 3 C + Fe2O3 3 CO + 2 Fe

– Mg + Cl2 MgCl2

• any reaction where a cation changes charge is redox– CuCl + FeCl3 FeCl2 + CuCl2

– SnCl2 + F2 SnCl2F2


Combustion Reactions• Reactions in which O2(g) is a

reactant are called Combustion Reactions

• Combustion reactions release lots of energy

• Combustion reactions are a subclass of Oxidation-Reduction reactions

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

cmh 121 luca preziati chapter 4: chemical reactions + + 4 wood boards6 nails 1 fence panel + 4 dozen...

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