Chemistry

Chapter 6 Chemical Reactions and Quantities
The Mole

General, Organic, and Biological Chemistry

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Collection Terms
A collection term states a specific number of items.
 1 dozen donuts

= 12 donuts

 1 ream of paper = 500 sheets  1 case = 24 cans

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A Mole of Atoms
A mole is a collection that contains  the same number of particles as there are carbon atoms in 12.0 g of carbon 12C  6.02 x 1023 atoms of an element (Avogadro’s number)
1 mole of Element 1 mole of C = 1 mole of Na = 1 mole of Au = Number of Atoms 6.02 x 1023 C atoms 6.02 x 1023 Na atoms 6.02 x 1023 Au atoms

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A Mole of a Compound
A mole  of a covalent compound has Avogadro’s number of molecules 1 mole of CO2 = 6.02 x 1023 CO2 molecules 1 mole of H2O = 6.02 x 1023 H2O molecules
 of an ionic compound contains Avogadro’s number of

formula units 1 mole of NaCl = 6.02 x 1023 NaCl formula units 1 mole of K2SO4 = 6.02 x 1023 K2SO4 formula units
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Avogadro’s Number
Define Avogadro's number, NA.


NA = 6.02 x 1023 particles/mole

Define the mole as


1 mole = 6.02 x 1023 atoms/molecules/ions

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Using Avogadro’s Number
Avogadro’s number is used to convert moles of a substance to particles.

How many Cu atoms are in 0.50 mole of Cu? Rearrange formula Number of atoms = Number of moles x NA = 0.5 x 6.02 x 1023 Cu atoms = 3.0 x 1023 Cu atoms
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Using Avogadro’s Number (continued)
Avogadro’s number is used to convert particles of a substance to moles. How many moles of CO2 are in 2.50 x 1024 molecules of CO2?

Number of moles =

2.5 x 1024 NA = 4.15 moles of CO2

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Subscripts and Moles
The subscripts in a formula give  the relationship of atoms in the formula  the moles of each element in 1 mole of a compound
Glucose C6H12O6
In 1 molecule: 6 atoms of C 12 atoms of H 6 atoms of O In 1 mole: 6 moles of C 12 moles of H 6 moles of O

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Subscripts State Atoms and Moles

1 mole of C9H8O4 = 9 moles of C 8 moles of H 4 moles of O
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Chemical Reactions and Quantities
Molar Mass

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Molar Mass
The molar mass is  the mass of one mole of a substance  the atomic mass of an element expressed in grams

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Molar Mass of CaCl2
 For a compound, the molar mass is the sum of the

molar masses of the elements in the formula. We calculate the molar mass of CaCl2 to the nearest 0.1 g as follows.
Element Number of Moles Atomic Mass Total Mass

Ca

1

40.1 g/mole

40.1 g

Cl
CaCl2

2
1

35.5 g/mole

71.0 g
111.1 g

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Molar Mass of K3PO4
Determine the molar mass of K3PO4 to 0.1 g.
Element Number of Moles Atomic Mass Total Mass in K3PO4

K P

3 1

39.1 g/mole 31.0 g/mole

117.3 g 31.0 g

O
K3PO4

4
1

16.0 g/mole

64.0 g
212.3 g

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One-Mole Quantities

32.1 g

55.9 g

58.5 g

294.2 g

342.3 g

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Guide to Calculating Molar Mass

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Moles and the molar mass

Example: Aluminium is often used for the structure of bicycle frames. How many grams are in 3.00 moles of Al?

Rearrange formula
Mass = moles x molar mass = 3 moles x 27g/mol = 81 g

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Guide to Calculations Using Molar Mass

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Percentage composition

Example: What is the percentage composition of ethanol (C2H60) Solution: Step 1: Determine molar mass of substance Mass of 2 carbon atoms Mass of 6 Hydrogen atoms Mass of 1 oxygen atom Mass of 1 C2H60 molecule = 2 x 12 g/mol = 24 g/mol = 6 x 1 g/mol = 6 g/mol = 1 x 16 g/mol = 16 g/mol = 46 g/mol
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Step 2: Now calculate individual percentages of all elements in substance
Step 2: Now calculate individual percentages of all elements in substance

Take note: percentages must add up to 100%
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Chemical Reactions and Quantities
Chemical Reactions

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Chemical Change
In a chemical change,  reacting substances form new substances with different compositions and properties  a chemical reaction takes place

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Writing equations for reactions
Reactants on LH-side of arrow Products on RH-side of arrow

Symbols used in chemical equations
+ → Δ (g) (l) (s) (aq) (↑) (↓) separates two or more formulas. reacts to form products. heat is supplied to the reaction mixture. gaseous state. liquid state. solid state. Aqueous. product is released as a gas. product is in the form of a precipitate
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Chemical Equations are Balanced
In a balanced chemical reaction,  atoms are not lost or gained  the number of atoms in the reactants is equal to the number of atoms in the products

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A Balanced Chemical Equation
In a balanced chemical equation,  there must be the same number of each type of atom on the reactant side and on the product side  numbers called coefficients are used in front of one or more formulas. Al + S Al2S3 Not balanced 2Al + 3S Al2S3 Balanced
2Al 3S = = 2Al 3S
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BALANCING CHEMICAL EQUATIONS
E.g.

P4 + Cl2  PCl4

- list all elements on LH and RH LH :4 P, 2 Cl RH 1 P, 4 Cl

- start with most complicated (PCl4) by adding more P (4 PCl4) LH :4 P, 2 Cl RH 4 P, 16 Cl

- adjust LH to compensate by adding Cl (8 Cl2) LH :4 P, 16 Cl RH 4 P, 16 Cl

Equation is now balanced

P4 + 8Cl2  4PCl4

Guide to Balancing Equations

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Equations with Polyatomic Ions

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Balancing with Polyatomic Ions
STEP 1 Write the equation with the correct formulas. Na3PO4(aq) + MgCl2(aq) NaCl(aq) + Mg3(PO4)2(s)
STEP 2 Determine if the equation is balanced. No, not all atoms are balanced. 3Na+ 1Na+ 1PO43 2PO43  1Mg2+ 3Mg2+ 2Cl 1Cl STEP 3 Balance with coefficients in front of formulas. Balance PO43 as a unit. 2Na3PO4(aq) Mg3(PO4)2(s)
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Balancing with Polyatomic Ions (continued)
STEP 3 (continued) Balance Mg 3MgCl2(aq) Balance Na and Cl 3MgCl2(aq) + 2Na3PO4(aq)
Mg3(PO4)2(s) 6NaCl(aq) + Mg3(PO4)2(s)

STEP 4 Check that atoms of each element are equal in reactants and products. 2PO43– = 2PO43– 3Mg2+ = 3Mg2+ 6Na+ = 6Na+ 6Cl = 6Cl
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Chemical Reactions and Quantities
Types of Reactions

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Types of Reactions combination reactions

Decomposition

single replacement reactions double replacement reactions
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Chemical Reactions and Quantities
Oxidation–Reduction Reactions

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Everyday Oxidation-Reduction Reactions
An oxidation–reduction reaction  provides us with energy from food  provides electrical energy in batteries  occurs when iron rusts 4Fe(s) + 3O2(g) 2Fe2O3(s)

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Rules for the assignment of oxidation numbers
1. The oxidation number of an element in the elemental state is zero 2. The oxidation number of a monatomic ion is the same as the charge of the ion 3. Metals have positive oxidation numbers in compounds 4. Most hydrogen compounds contain hydrogen with a +1 oxidation number 5. Most oxygen compounds contain oxygen with a -2 oxidation number 6. The oxidation number of fluorine in compounds is -1 7. The oxidation number of chlorine, bromine, and iodine in compounds is -1 except when combined with oxygen. 8. The algebraic sum of the oxidation numbers of atoms in a compound is zero 9. The algebraic sum of the oxidation numbers of atoms in a 34 polyatomic ion is the same as the charge of the ion

Assigning oxidation numbers
Example: 1. Assign oxidation numbers to elements in H2SO4
H = +1( rule 4) O = -2 ( rule 5) S = ?? Sulphur as per rule 8: 2x(+1) + S + 4x(-2) =0 2+ + ( -8) = 0 S = +6
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Transfer of Electrons
An oxidation–reduction reaction transfers electrons from one reactant to another Oxidation Is a Loss of electrons Increase in oxidation number Zn(s) Zn2+(aq) + 2e
Reduction Is a Gain of electrons Decrease in oxidation number Cu2+(aq) + 2e Cu(s)
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(OIL)

(RIG)

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Oxidizing Agent and Reducing Agent
Oxidizing Agent substance that is reduced in a redox reaction ( caution – must be a reactant) Reducing Agent substance that is oxidized in a redox reaction ( caution – must be a reactant)

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Example
3P + 5 HNO3
Step 1:Assign oxidation numbers

5 NO + 3 H2PO4

What is oxidized, reduced and identify OA and RA in the reaction

LH
P=0 H = +1 O = -2 N = +5

RH
N = +2 O = -2 H = +1 P = +6

P: increase in oxidation number N: decrease in oxidation number P is oxidised, Reducing Agent HNO3 is reduced, Oxidizing Agent

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Chapter 6 Chemical Reactions and Quantities
Mole Relationships in Chemical Equations

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Reading Equations with Moles
Consider the following equation: Fe + O2 Fe2O3, balancing equation  4Fe(s) + 3O2(g) 2Fe2O3(s)

An equation can be read in “moles” by placing the words “moles of” between each coefficient and formula. 4 moles of Fe + 3 moles of O2 2 moles of Fe2O3

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Learning Check
Consider the following equation: 3H2(g) + N2(g) 2NH3(g) 3 moles of H2 + 1 mole N2  1 mole H2 + mole N2  If I want 1mole NH3  If I have 3 moles N2  2 moles NH3 mole NH3

Moles H2 +

moles N2

Need 9 moles H2  6 moles NH3

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Guide to Using Mole–Mole Factors

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Chapter 6 Chemical Reactions and Quantities
Mass Calculations for Reactions

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Guide to Calculating the Masses of Reactants and Products

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Chapter 6 Chemical Reactions and Quantities
Percent Yield

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Theoretical, Actual, and Percent Yield
Theoretical yield:
 the maximum amount of product, which is calculated using the balanced equation.

Actual yield:
 the amount of product obtained when the reaction takes place

Percent yield:
 the ratio of actual yield to theoretical yield Percent yield = actual yield (g) x 100 theoretical yield (g)
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Guide to Calculations for Percent Yield

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Calculating Percent Yield
Suppose you have prepared cookie dough to make 5 dozen cookies. The phone rings and you answer. While you talk, a sheet of 12 cookies burns, and you have to throw them out. The rest of the cookies you make are okay. What is the percent yield of edible cookies? Theoretical yield: 60 cookies possible Actual yield: 48 cookies to eat Percent yield: 48 cookies x 100% = 80.% yield 60 cookies
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Chapter 6 Chemical Reactions and Quantities
Energy Changes in Chemical Reactions

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Reaction Conditions
Reaction conditions for a chemical reaction require  collisions between reacting molecules  collisions with sufficient energy to break the bonds in the reactants  the breaking of bonds between atoms of the reactants  the forming of new bonds to give products

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Chemical Reactions
In the reaction H2(g) + I2(g)  the reactants H2 and I2 collide  the bonds of H2 and I2 break  the bonds for HI form 2HI(g),

H2 + I2

collision

bonds break new bonds form
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HI
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Activation Energy
Activation energy  is the minimum energy required upon collision for a reaction to take place

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Heat of Reaction
The heat of reaction  is the amount of heat absorbed or released during a reaction
 is the difference in the energy of the reactants and the products

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Exothermic Reactions
In an exothermic reaction,  the energy of the products is less than the energy of the reactants
 heat of reaction is released

 heat is a product

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Endothermic Reactions
In an endothermic reaction,  heat is absorbed  the energy of the products is greater than the energy of the reactants  heat is a reactant

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Summary
Reaction Type Endothermic Exothermic Energy Change Heat absorbed Heat released Heat in Reaction Reactant side Product side

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Chapter 9

Chemical Equilibrium

Rates of Reactions

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Collision Theory of Reactions
A chemical reaction occurs when  collisions between molecules have sufficient energy to break the bonds in the reactants  molecules collide with the proper orientation  bonds between atoms of the reactants (N2 and O2) are broken and new bonds (NO) form

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Collision Theory of Reactions (continued)
A chemical reaction does not take place if the  collisions between molecules do not have sufficient energy to break the bonds in the reactants  molecules are not properly aligned

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Activation Energy
The activation energy  is the minimum energy needed for a reaction to take place upon proper collision of reactants

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Reaction Rate and Temperature
Reaction rate  is the speed at which reactant is used up
 is the speed at which product forms
 increases when temperature rises because reacting

molecules move faster, thereby providing more colliding molecules with energy of activation

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Reaction Rate and Concentration
Increasing the concentration of reactants  increases the number of collisions
 increases the reaction

rate

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Reaction Rate and Catalysts
A catalyst  speeds up the rate of a reaction  lowers the energy of activation  is not used up during the reaction

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Reaction rate and surface area
Increasing the reacting surface
 speeds up the reaction rate E.G. Granulated sugar dissolves faster than sugar cube Laundry that are hanged and spread would dry faster than when bundled up

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