chapter 8 chemical reactivity: chemicals in action

Chapter 8

Chemical

Reactivity:

Chemicals in

Action

Chapter Learning Objectives

By the end of the chapter, you will be able to a. Write a reaction equation with the initial

materials (reactants) on the left, followed by a reaction arrow pointing from left to right, and the final materials (products) on the right.

b. Write a complete description of a reaction (a balanced equation) with the same number and kinds of atoms on both sides of the equation.

Chapter Learning Objectives (cont)

c. Predict the relationship between the amounts of reactants and products (the stoichiometry) from a balanced reaction equation.

d. Calculate the number of moles of a material, given the mass.

e. Calculate the number of moles produced in a chemical reaction given a reactant’s mass.

f. Chemical reactions involve energy.

g. The first law of thermodynamics states that energy is conserved during chemical reactions, but it may be transformed from one type to another.

h. Reactions can be exothermic (releasing heat) or endothermic (absorbing heat).

i. Chemical reactions are spontaneous if the products are energetically downhill with respect to the reactants.

j. The second law of thermodynamics states that increasing molecular disorder (entropy) is favored.

k. The size of the energy barrier between reactants and products (the activation energy) dictates the rate of a chemical reaction (the kinetics).

l. A catalyst lowers the activation energy, thereby speeding up the rate of a chemical reaction.

Chapter Outline

• Balanced Reaction Equationsa. Writing a Chemical Reaction

1. The starting materials, the reactants, are written on the left.

2. The materials that are produced, the products, are written on the right.

3. Reactants are separated from products by a horizontal arrow pointing from left to right.

Na + Cl NaClReactants Product

Run the following web animations/movies. 8.1 H2 and O2 combustion

Chapter Outline

• Balanced Reaction Equations (cont)b. Balancing the Equation

1. The law of conservation of matter states that matter can neither be created nor destroyed in a chemical reaction.

2. The number and kind of atoms on the left-hand side of an equation must be equal to the number and kind of atoms on the right.

H2 + O2 H2O Incorrect

2 H2 + O2 2 H2O Correct

8.2 Balancing Chemical Equations Run the following web animations/movies.

Symbols Used in Equations

Symbols used in chemical

equations show

• the states of the reactants.

• the states of the products.

• the reaction conditions.

Chemical Equations are Balanced

In a balanced

chemical reaction

• atoms are not gained or lost.

• the number of reactant atoms is equal to the number of product atoms.

Chapter Outline

• Balanced Reaction Equations (cont)c. Stoichiometry

1. The stoichiometry of a chemical reaction is the relationship between the number of molecules of the reactants and products in the balanced reaction equation.

2. In the previous example, 2 molecules of H2 react with 1 molecule of O2, so the stoichiometry of H2 to O2 is 2:1.

3. The ratio of reactants can affect the products of a reaction.

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 of a balanced equation.

• numbers called coefficients are used in front of one or more formulas.

Al + S Al2S3 Not Balanced

2Al + 3S Al2S3 Balanced

2 Al = 2 Al

3 S = 3 S

Stoichiometry

Learning Check

State the number of atoms of each element on the

reactant side and the product side for each of the

following balanced equations.

A. P4(s) + 6Br2(l) 4 PBr3(g)

B. 2Al(s) + Fe2O3(s) 2Fe(s) + Al2O3(s)

Solution

A. P4(s) + 6Br2(l) 4 PBr3(g)

4 P 4 P

12 Br 12 Br

B. 2Al(s) + Fe2O3(s) 2Fe(s) + Al2O3(s)

2 Al 2 Al

2 Fe 2 Fe

3 O 3 O

Learning Check

Determine if each equation is balanced or not.

A. Na(s) + N2(g) Na3N(s)

B. C2H4(g) + H2O(l) C2H5OH(l)

Solution

Determine if each equation is balanced or not.

A. Na(s) + N2(g) Na3N(s)

No. 2 N on reactant side, 1 N on product side.

1 Na on reactant side, 3 Na on product side.

B. C2H4(g) + H2O(l) C2H5OH(l)

Yes. 2 C = 2 C

6 H = 6 H

1 O = 1 O

Guide to Balancing a Chemical Equation

To balance the following equation,

Fe3O4(s) + H2(g) Fe(s) + H2O(l)

• work on one element at a time.

• use only coefficients in front of formulas.

• do not change any subscripts.

Fe: Fe3O4(s) + H2(g) 3Fe(s) + H2O(l)

O: Fe3O4(s) + H2(g) 3Fe(s) + 4H2O(l)

H: Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

Steps in Balancing an Equation

Check the balance of atoms in the following.

Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

A. Number of H atoms in products.

1) 2 2) 4 3) 8

B. Number of O atoms in reactants.

1) 2 2) 4 3) 8

C. Number of Fe atoms in reactants.

1) 1 2) 3 3) 4

Learning Check

Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

A. Number of H atoms in products.

3) 8 (4H2O)

B. Number of O atoms in reactants.

2) 4 (Fe3O4)

C. Number of Fe atoms in reactants.

2) 3 (Fe3O4)

Solution

Balance each equation and list the coefficients in the balanced equation going from reactants to products:

A. __Mg(s) + __N2(g) __Mg3N2(s)

1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1

B. __Al(s) + __Cl2(g) __AlCl3(s)

1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2

Learning Check

A. 3) 3, 1, 1

3Mg(s) + 1N2(g) 1Mg3N2(s)

B. 3) 2, 3, 2

2Al(s) + 3Cl2(g) 2AlCl3(s)

Solution

Chapter Outline

• Molecular Mass a. The molecular mass is the mass of one molecule

of a compound.

b. The formula mass is the mass of one unit of an ionic compound.

c. The molecular/formula mass is calculated by adding the masses of the atoms that make up the compound.

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

A mole is a collection that contains

• the same number of particles as there are carbon atoms in 12.0 g of carbon.

• 6.02 x 1023 atoms of an element (Avogadro’s number).

1 mole element Number of Atoms

1 mole C = 6.02 x 1023 C atoms

1 mole Na = 6.02 x 1023 Na atoms

1 mole Au = 6.02 x 1023 Au atoms

A Mole of Atoms

Chapter Outline

• The Mole a. The mole is the SI unit of measure to describe

the amount of matter that is present. b. One mole is equal to 6.02 x 1023 particles

(Avogadro’s number).c. One mole of an element has a mass that is equal

to the atomic mass of that element in grams. d. One mole of a compound has a mass that is equal

to the molecular/formula mass of that compound in grams.

The Mole

Samples of One Mole Quantities

1 mole C = 6.02 x 1023 C atoms

1 mole Al = 6.02 x 1023 Al atoms

1 mole S = 6.02 x 1023 S atoms

1 mole H2O = 6.02 x 1023 H2O molecules

1 mole CCl4= 6.02 x 1023 CCl4 molecules

8.3 One mole of various substancesRun the following web animations/movies.

Subscripts and Moles

The subscripts in a formula show

• the relationship of atoms in the formula.

• the moles of each element in 1 mole of compound.

Glucose

C6H12O6

In 1 molecule: 6 atoms C 12 atoms H 6 atoms O

In 1 mole: 6 mole C 12 mole H 6 mole O

Molar Mass

The molar mass

• is the mass of one mole of an element or compound.

• is the atomic mass expressed in grams.

Molar Mass from Periodic Table

Molar mass is the atomic mass expressed in grams.

1 mole Ag 1 mole C 1 mole S = 107.9 g = 12.01 g = 32.07 g

Give the molar mass for each (to the tenths decimal place).

A. 1 mole K atoms = ________

B. 1 mol Sn atoms = ________

Learning Check

Give the molar mass for each (to the tenths decimal place).

A. 1 mole K atoms = 39.1 g

B. 1 mole Sn atoms = 118.7 g

Solution

Molar Mass of K3PO4

Calculate the molar mass of K3PO4.

Element Number of Moles

Atomic Mass Total Mass in K3PO4

K 3 39.1 g/mole 117.3 g

P 1 31.0 g/mole 31.0 g

O 4 16.0 g/mole 64.0 g

K3PO4 212.3 g

Prozac, C17H18F3NO, is an antidepressant that inhibits the uptake of serotonin by the brain. What is the molar mass of Prozac?

1) 40.1 g/mole2) 262 g/mole 3) 309 g/mole

Learning Check

Prozac, C17H18F3NO, is an antidepressant that inhibits the uptake of serotonin by the brain. What is the molar mass of Prozac?

3) 309 g/mole

17 C (12.0) + 18 H (1.01) + 3 F (19.0)

+ 1 N (14.0) + 1 O (16.0) =

204 + 18.2 + 57.0 + 14.0 + 16.0 = 309 g/mole

Solution

Chapter Outline

• Stoichiometry Calculationsa. The units of molar mass are grams/mole.

b. Moles x molar mass = mass. 1. Example: 2.0 mol CO2 x 44 g/mol = 88 g CO2

c. Mass/molar mass= moles. 1. Example: 132 g CO2 / 44 g/mol = 3.0 mol CO2

• Stoichiometry Calculationsa. The expected mass of a product or reactant can

be calculated for any reaction by using the balanced equation and the molar mass.

b. Example: how many grams of C6H12O6 are produced from 2 moles CO2 during photosynthesis?Reaction: 6 CO2 + 6 H2O C6H12O6 + 6 O2

2 moles CO2 x 1 mol C6H12O6 x 180 g = 60 g C6H12O6

6 mol CO2

Chapter Outline

Aluminum is often used to build lightweight bicycleframes. How many grams of Al are in 3.00 mole Al?

Molar mass equality: 1 mole Al = 27.0 g Al

Setup with molar mass as a factor:

3.00 mole Al x 27.0 g Al = 81.0 g Al1 mole Al

molar mass factor for Al

Moles to Grams

Learning Check

Allyl sulfide C6H10S is a compound that has the odor of garlic. How many moles of C6H10S are in 225 g?

Calculate the molar mass of C6H10S.

(6 x 12.0) + (10 x 1.01) + (1 x 32.1) = 114.2 g/mole

Set up the calculation using a mole factor.

225 g C6H10S x 1 mole C6H10S

114.2 g C6H10S

molar mass factor (inverted)

= 1.97 mole C6H10S

Solution

High pressurecan covert coal into a diamond

Chapter Outline

• Energy a. Measuring Energy

1. The two general types of energy are potential(stored energy) and kinetic (energy of motion).

2. Energy is most often measured in either joulesor calories.

Types of Energy

Chapter Outline

• Energy (cont)b. Energy and Temperature

1. Materials at higher temperatures contain more energy than the same amount of material at a lower temperature.

Thermal Energy

Molecules can be modeledas atoms held together by tiny springs

Chapter Outline

• Energy (cont)c. Energy and Chemical Reactions

1. In chemistry, having low energy means increased stability, which is favored.

2. Atoms and molecules undergo reactions to decrease their overall energies.

Reactions Go Energetically Downhill

Favorable reaction: energy putin is greater than energy released

8.4 Combination of H2 and O2 Run the following web animations/movies.

Chapter Outline

• Energy Changes During Chemical Reactions a. Energy Diagrams

1. Depict the relative energies of the reactants and products, as well as the energy barrier to reaction.

b. Equilibrium1. There is net conversion of reactants to products until

equilibrium, after which a reaction appears to stop.

Diamonds areof higher energy than graphite

Energy Diagrams

The larger theactivation energy, the slower the reaction

The carbons in diamond (left) and graphite (right)

Uncut diamonds

Credit: Masterfile

Chapter Outline

• The First Law of Thermodynamics• Energy is conserved during chemical reactions.a. Heat of Reaction

1. Heat transferred during a chemical reaction

b. Endothermic vs. Exothermic1. In endothermic reactions, heat can be considered to be

one of the reactants. Endothermic reactions often feel cold and are energetically uphill.

2. In exothermic reactions, heat can be considered to be one of the products. Exothermic reactions often feel warm and are energetically downhill.

Endothermic vs. Exothermic

An endothermicreaction is the basis for a cold pack

Examples of FamiliarExothermic and Endothermic Processes

Summary

Reaction Energy Heat

Type Change in Reaction

Endothermic Heat absorbed Reactant

Exothermic Heat released Product

Learning Check

Identify each reaction as

1) exothermic or 2) endothermic.

A. N2 + 3 H2 2 NH3 + 22 kcal

B. CaCO3 + 133 kcal CaO + CO2

C. 2 SO2 + O2 2 SO3 + heat

Solution

Identify each reaction as

1) exothermic or 2) endothermic.

1 A. N2 + 3 H2 2 NH3 + 22 kcal

2 B. CaCO3 + 133 kcal CaO + CO2

1 C. 2 SO2 + O2 2 SO3 + heat

A luciferin structureenables a firefly to glow

Credit: Getty Images

bioluminescence

light producedby fireflies

Diagramof an internal combustion engine

Chapter Outline

• The Second Law of Thermodynamics• No energy transformation can be absolutely efficient.

a. Entropy 1. Time’s arrow, molecular disorder

b. Examples of favorable entropy 1. Solids or liquids are converted to gases

2. More molecules of products than of reactants

3. A solid dissolves

Run the following web animations/movies. 8.8 Entropy and Enthalpy changes

Macroscopic eventsalways occur in a specific sequence

Credit: Courtesy of Julie Millard

Macroscopic eventsalways occur in a specific sequence

Molecular eventsoccur in specific sequences

Dissolving Solids Increases Entropy

Ice is a solid heldtogether by strong intermolecular forces

Chapter Outline

• Kineticsa. Activation Energy

1. The size of the energy barrier on the pathway from reactants to products determines how fast a reaction proceeds.

2. Slow reactions have relatively large energy barriers, while fast reactions have relatively small energy barriers.

Activation Energy

• The activation energy is the minimum energy needed for a reaction to take place.

• When a collision provides energy equal to or greater than the activation energy, product can form.

Illustration of activation energy

Low Energy Barrier = Fast Reaction

Chapter Outline

• Kinetics (cont)b. Collision Theory

1. Two chemical species must come together in the right orientation with sufficient energy to undergo reaction.

Collision Theory•Atoms, molecules, or ions must collide for a reaction to occur.•The collision between the atoms, molecules, or ions must be sufficiently energetic for reaction to occur.•In some reactions, collision are productive only if the reactants are properly oriented relative to each other upon collision.•Bonds are made or broken in most chemical reactions. Some reactions may involve only electron transfer, however.•Energy is absorbed during bond breakage and released during bond formation.

Collision Theory of Reactions

A chemical reaction occurs when

• collisions between molecules have sufficient energy to break the bonds in the reactants.

• bonds between atoms of the reactants (N2 and O2) are broken and new bonds (NO) can form.

Chapter Outline

• Kinetics (cont)c. Factors that Affect Reaction Rates

1. Geometry

2. Concentration of Reactants

3. Temperature

4. Catalysts

A purple atom must comebetween green atoms to react with a gray

Effect of Geometry

Effect of Temperature

Effect of Catalysts

A catalyst

• increases the rate of a reaction.

• lowers the energy of activation.

• is not used up during the reaction.

Catalysis of the reaction of gaseous ethylene (C2H4) with O2

Run the following web animations/movies. 8.5 Catalytic Decomposition of H2

Learning Check

State the effect of each on the rate of reaction as:

1) increases 2) decreases 3) no change

A. increasing the temperature.

B. removing some of the reactants.

C. adding a catalyst.

D. placing the reaction flask in ice.

E. increasing the concentration of one of the reactants.

Solution

State the effect of each on the rate of reaction as:

1) increases 2) decreases 3) no change

1 A. increasing the temperature.

2 B. removing some of the reactants.

1 C. adding a catalyst.

2 D. placing the reaction flask in ice.

1 E. increasing the concentration of one of the reactants.

Summary of Factors That Increase Reaction Rate

Key Words

• Potential energy

• Kinetic energy

• Joule (J)

• calorie (cal)

• Calorie (cal)

• kilocalorie (kcal)

• Thermal energy

• Chemical energy

• Activation energy

• Equilibrium

• Spontaneous• Thermodynamics• Kinetics• First law of

thermodynamics• Free energy• Exothermic• Endothermic• Second law of

thermodynamics• Entropy• Catalyst

Key Words

• Chemical reaction• Reactants• Products• Combustion• Law of conservation

of matter• Photosynthesis• Stoichiometry• Molecular mass• Formula mass• Mole

• Avogadro’s number

• Molar mass (grams/mole)

chapter 8 chemical reactivity: chemicals in action

chemical reactions

reaction arrow

reaction conditions

chemical reactivity

balancing chemical equations

reactants mass

chapter learning objectivesby

amounts of reactants

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