Chapter 7 Chemical Chapter 7 Chemical ReactionsReactions

Integrated Chemistry and Physics

Chemical EquationsChemical Equations

The substances involved in a chemical reaction before the change occurs are called the reactants

The new substances formed after the change are called the products

We say that reactants yield products

productsreactants yield

7.1

Using Equations to Represent Using Equations to Represent

ReactionsReactions

Reactants Products C(s) + O2(g) CO2(g)

1 carbon reacts with 1 oxygen molecule yields 1 molecule of carbon dioxide

The letters behind the chemical indicates whether the substance is solid (s) or (cr), liquid (l), aqueous solution (aq), or gas (g).

7.1

Writing Chemical EquationsWriting Chemical Equations

A chemical equation is a shorthand for the expression of the reaction.

For example: Hydrogen gas reacts with Oxygen gas to produce Dihydrogen Oxide

: can be written in shorthand as:

H2 + O2 H2O

7.1

Conservation ObeyedConservation Obeyed

In any chemical reaction, the conservation of mass is observed

Principle of the conservation of mass states: matter can not be created or destroyed in a chemical reaction The mass of the reactants and the mass

products must be the same

7.1

CoefficientsCoefficients

A number in front of compounds or elements in a chemical equation represent the number of particles in the reaction

The coefficients are used to balance the equation to obey the law of conservation of mass

H2 + O2 H2O

7.1

2 2

When balancing a chemical reaction you may add coefficients in front of the compounds to

balance the reaction, but you may notnot change the subscripts.

Changing the subscripts changes the compound.

Balancing EquationsBalancing Equations

There are four basic steps to balancing a chemical equation.1. Write the correct formula 2. Find the number of atoms for each element on

each side.3. Determine where to place coefficients.4. Check your answer to see if:

The numbers of atoms on both sides of the equation are now balanced.

The coefficients are in the lowest possible whole number ratios. (reduced)

Steps to Balancing EquationsSteps to Balancing Equations

Balancing EquationsBalancing Equations

Keep track of the elements in a table, adjusting the amount of atoms as you put in

coefficients You must go back and forth putting in

coefficients until all of the atoms’ numbers are equal

7.1

Fe(s) + O2(g) Fe2O3(s)

Focus on one element at a time.

Fe(s) + O2(g) Fe2O3(s)

There are 2 O’s on the left and 3 on the right.

You cannot change subscripts. Fe(s) + 3O2(g) 2Fe2O3(s)

Balancing EquationsBalancing Equations7.1

Fe 1

O 2

Fe 2

O 3X 6 X 6

X 4X 4

Fe is now wrong. 4Fe(s) + 3O2(g) 2Fe2O3(s)

Fe + O2 Fe2O3

Balancing Equations Balancing Equations

Try to balance H2 + N2 NH3

7.1

23

H 2

N 2

H 3

N 1

X 6

X 2

X 6

Na 2

S 9(1+8)

O 3

Na 2

S 2

O 3

Balancing Equations Balancing Equations

Try to balance Na2SO3 + S8 Na2S2O3

7.1

88

X 16

X 16

X 24

X 16

XXXX16

X 24

Balancing Equations Balancing Equations

Try to balance CH4 + O2 CO2 + H2O

CH4 + 2O2 CO2 + 2H2O

Counting with MolesCounting with Moles

12 eggs is a eggs? 144 pencils is a of pencils? 500 sheets of paper is a ? 6.02 x 1023 of anything is a

?

Mole

7.1

UNITSUNITS

Not this type of Mole

7.1

Or this type of Mole

7.1

A Mole is a unit.A Mole is a unit.

6.02 x 1023 is a mole -602,000,000,000,000,000,000,000

If you counted paper at the rate of one sheet per second it would take you 19,089,294,774,226,281 years to count a mole of paper.

It is a big number because atoms are small.

7.1

The Mole The Mole

A mole of sugar weighs about one half pound (between 180 and 342 g’s) but contains how many molecules of sugar?

602,000,000,000,000,000,000,000

6.02 x 1023 molecules.

7.1

Avogadro's Number Avogadro's Number

How do we get Avogadro’s Number?

Relative Mass in grams = 6.02 X 1023

Actual Mass in AMU’s

1 mole = ?? grams = 6.02 X 1023 molecules

7.1

Molar Masses Molar Masses

A chemical reaction is like a recipe. A specific number of atoms or molecules

react to form a specific product. Consider:

1C(s) + 1O2(g) 1CO2(g) This recipe calls for 1 carbon atom and 1

oxygen molecule to make 1 molecule of CO2 , carbon dioxide.

7.1

Molar Masses Molar Masses

Because atoms and molecules are too small to see you can’t count them out.

To make CO2 we need to weigh atoms or molecules.

To get the right proportions, we need to know the relative masses.

We do! The atomic masses = the relative masses.

7.1

Molar MassesMolar Masses

On the periodic table the atomic mass is usually placed under the symbol for the atom.

One mole of Carbon is equal to 12.0 grams of carbon.

7.1

C6

12.001Atomic Mass = protons + neutrons

Atomic Number = protons

C6

12.001Atomic Mass = protons + neutrons

Atomic Number = protons

Molar MassesMolar Masses

C(s) + O2(g) CO2(g)

The atomic mass of C is 12.0. The atomic mass of one oxygen atom is 16.0. The oxygen molecule contains two oxygen atoms

and has a relative mass of (2 x 16) or 32. Therefore 12.0 g of carbon will react with 32.0 g

of O2.

To form 44 grams of CO2.

7.1

Chemical CalculationsChemical Calculations

Take a look at the formation of water.

7.1

C 5

H 12

O 3

C 1

H 2

O 3

Balancing Equations Balancing Equations

Try to balance # 8

C5H12O + O2 CO2 + H2O

X 24

XX 32

7.1

102

X 10

X 21

X 10

XX 24

X 4

15 12

X 32

End Section 1End Section 1

Types of Reactions 7.2

Types of ReactionsTypes of Reactions• There are six types of chemical

reactions we will talk about:1. Synthesis reactions2. Decomposition reactions3. Single displacement reactions4. Double displacement reactions5. Combustion reactions6. Oxidation-Reduction reactions

• You need to be able to identify the type of reaction

7.2

1. Synthesis reactions1. Synthesis reactions

• Synthesis reactions occur when two substances react and form a single compound.

• reactant + reactant 1 product• Basically: A + B AB

• Example: 2H2 + O2 2H2O

• Example: C + O2 CO2

7.2

Synthesis ReactionsSynthesis Reactions

• Here is another example of a synthesis reaction

7.2

2. Decomposition 2. Decomposition ReactionsReactions

• Decomposition reactions occur when a compound breaks up into the elements or in a few to simpler compounds

• 1 Reactant Product + Product • In general: AB A + B• Example: 2 H2O 2H2 + O2

7.2

Decomposition Decomposition ReactionsReactions

• Another view of a decomposition reaction:

7.2

3. Single Replacement 3. Single Replacement

• Single Replacement Reactions occur when one element replaces another in a compound.

• A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-).

• element + compound product + product A + BC AC + B (if A is a metal) ORA + BC BA + C (if A is a nonmetal)

(remember the cation always goes first!)

7.2

Single Replacement Single Replacement ReactionsReactions

7.2

Single Replacement Single Replacement ReactionsReactions

• Sodium chloride solid reacts with fluorine gas

NaCl(s) + F2(g) NaF(s) + Cl2(g)

Note that fluorine replaces chlorine in the compound

• Aluminum metal reacts with aqueous copper (II) nitrate

Al(s)+ Cu(NO3)2(aq) Cu(s) + Al(NO3)3(aq)

2 2

2 3 3 2

7.2

4. Double Replacement 4. Double Replacement ReactionsReactions

• Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound

• Compound + compound product + product

• AB + CD AD + CB

7.2

Double Replacement Double Replacement ReactionsReactions

• Think about it like “foil”ing in algebra, first and last ions go together + inside ions go together

• Example: AgNO3(aq) + NaCl(s) AgCl(s) + NaNO3(aq)

• Another example:K2SO4(aq) + Ba(NO3)2(aq) KNO3(aq) + BaSO4(s)

2

7.2

5. Combustion 5. Combustion ReactionsReactions

• Combustion reactions occur when a hydrocarbon reacts with oxygen gas.

• This is also called burning!!! In order to burn something you need the 3 things in the “fire triangle”:1) A Fuel (hydrocarbon)2) Oxygen to burn it with3) Something to ignite the reaction (spark)

7.2

Combustion ReactionsCombustion Reactions• In general:

CxHy + O2 CO2 + H2O

• Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide)

• Combustion is used to heat homes and run automobiles (octane, as in gasoline, is C8H18)

7.2

CombustionCombustion

• Example• C5H12 + O2 CO2 + H2O

• Write the products and balance the following combustion reaction:• C10H22 + O2 CO2 + H2O

5 68

2 31

20

22

7.2

Oxidation-ReductionOxidation-Reduction

Many of these 5 types of reactions are also oxidation-reduction reactions

Oxidation: Loss of electrons Reduction: Gain of electrons

7.2

You can’t have one… without the You can’t have one… without the other!other!

Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.

Reduction has to occur at the cost of oxidation

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lleeccttrroonnss

xxiiddaattiioonn

aaiinn

lleeccttrroonnss

eedduuccttiioonn

GER!GER!

7.2

Another way to Another way to rememberremember

OIL RIGxxiiddaattiioonn

ss oossee

eedduuccttiioonn

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7.2

Mixed PracticeMixed Practice• State the type and balance the

following reactions:1. BaCl2 + H2SO4

2. C6H12 + O2

3. Zn + CuSO4

4. Cs + Br2

5. FeCO3

BaSO4 + HCl2

CO2 + H2O6 69ZnSO4 + Cu

CsBr22

FeO + CO2

7.2

Signs of Chemical Signs of Chemical ChangeChange

Energy change Exothermic Endothermic

From 7.3

Why do reactions take place?Why do reactions take place?

Reactions take place is because of energy Energy is somehow involved in all

reactions Some reactions will gain energy from the

surroundings Some reactions will give off energy

7.3

Chemical Bonds and Chemical Bonds and EnergyEnergy

Chemical reactions involve the breaking and making of chemical bonds.

It takes energy to break bonds. Energy is released when we make

bonds.

∆∆H: H: How much energy is transferred?How much energy is transferred?

Energy is needed to break chemical bonds Energy is given out when bonds are made ∆H is the difference between the energy

needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

Energy Level DiagramsEnergy Level Diagrams Endothermic ReactionsEndothermic Reactions

energy

course of reaction

energy taken in

∆H is positivereactants

productsIt gets cold!!!

7.3

Exothermic Energy Level Exothermic Energy Level DiagramsDiagrams

products

reactantsenergy

course of reaction

energy given out

∆H is negative

It gets HOTHOT!!!

7.3

Bond energiesBond energiesThe energy needed to break a chemical bond

Different chemical bonds have different bond energies

Chemical bondBond energy,

kJ/mole

H―H 436

O=O 498

C―C 347

O―H 464

C-H C-C O=O C=O O-H 414 347 498 803 464 X 8 x 2 x 5 x 6 x 8 3312 694 2490 4818 3712

6496 8530 ∆H 2034 net energy

Working out ∆HWorking out ∆H

energy

course of reaction

2H2 + O2

2H2O

Working out ∆HWorking out ∆H Show all the bonds in the products

energy

course of reaction

H―HH―H

O=O+

H HO

H HO

Working out ∆HWorking out ∆H Show the bond energies for all the bonds

energy

course of reaction

436436

O=O+

H HO

H HO

Working out ∆HWorking out ∆HShow the bond energies for all the bonds

energy

course of reaction

436436

498+

H HO

H HO

Working out ∆HWorking out ∆H Show the bond energies for all the bonds

energy

course of reaction

436436

498+

H HO

464 464+

Working out ∆HWorking out ∆H

energy

course of reaction

436436

498+

464 464+

464 464+

= 1370

= 1856

1370-1856-486

Exothermic

EnergyReleased

Working out ∆HWorking out ∆H

• The energy values have units of kJ/mole

∆H is energy in – energy out

• Energy goes in to break bonds

• Energy goes out when bonds are made

Reaction ratesReaction rates

Defined as how quickly a reaction occurs.

Collision theory = The more collisions that occur at the atomic level the faster the reaction will go

Factors Affecting Factors Affecting Reaction RateReaction Rate

1) Temperature2) Surface Area3) Stirring4) Concentration5) Catalysts

TEMPERATURETEMPERATURE

Generally, the higher the temperature of the reactants the faster the rate of reaction.

SURFACE AREASURFACE AREA

The greater the surface area, the faster the rate of reaction.

STIRRINGSTIRRING

Speeds up the molecules thus increasing the number of collisions

CONCENTRATIONCONCENTRATION

Defined as the number of particles in a given unit of volume.

More particles more reactions. Example: 12 M HCl 6 M HCl 1 M HCl

CATALYSTSCATALYSTS

A substance that speeds up a reaction without being used up or directly involved in the reaction.

Chemical EquilibriumChemical Equilibrium

The object of the game is to get as many balls in your opponents court as possible.

A B

7.5

Chemical Equilibrium 2Chemical Equilibrium 2

At the start of the game A is winning.

A B

7.5

Chemical Equilibrium 3Chemical Equilibrium 3

After a few minutes A has enough balls to start throwing them back.

A B

7.5

Chemical Equilibrium Chemical Equilibrium

After a few more minutes there are just as many balls in A’s court as B.

A B

7.5

Chemical Equilibrium Chemical Equilibrium

Now both A and B are throwing balls back and forth without any change in the number of balls on each side.

A B

7.5

Chemical Equilibrium Chemical Equilibrium

This is an equilibrium condition.

A B

7.5

Chemical Equilibrium Chemical Equilibrium

If B were bigger and stronger, the equilibrium would lie to the left.

A B

7.5

Chemical Equilibrium Chemical Equilibrium

The same sort of equilibrium occurs with chemical reactions.

The equilibrium can be affected by temperature and pressure.

7.5

Balance and Bond Balance and Bond EnergyEnergy

N2 + H2 NH323

H 2

N 2

H 3

N 1

X 6

X 2

X 6

Working out ∆HWorking out ∆H

946436

+

389 389+

389 389+

= 2254

= 2334

2254-2334-80

Exothermic

EnergyReleased

389+

389+

436436

2NH3

Chemical bond

Bond energy

N―N 946

H―H 436

H―N 389N2 + 3H2

N N H―HH―HH―H

H HN

H

H HN

H

2NH3

Chemical bond

Bond energy

N―N 946

H―H 436

H―N 389

N2 + 3H2

+N NH―HH―HH―H

H H

N

H

H H

N

H

NOW!NOW!

EOC page 223-224 Questions 12, 14, 15, 19, 20, 23,

24, 28, Make it happen!