Chemical Reactions

Chemical ReactionsChemical Reactions

Chapter 10Chapter 10

Part I: Counting AtomsPart I: Counting Atoms

How Many Atoms in a Molecule?How Many Atoms in a Molecule?

Counting AtomsCounting Atoms Most substances that we encounter are compounds, not

elements. • A chemical compound is a pure substance formed

from the combination of two or more different elements. The properties of the compound may be completely unlike those

of the elements that form it. • The formula for a compound lists the symbols of the

individual elements followed by subscripts which indicate the number of atoms of that element. (If no subscript is given, it is understood to be “1.”) E.g., NaCl,

H2O, C12H22O11.

Counting AtomsCounting Atoms A molecular formula gives the actual number of

atoms of each element in a molecule of a compound. Hydrogen peroxide H2O2 Water H2O Glucose C6H12O6

A structural formula uses lines to represent covalent bonds, and shows how the atoms in a molecule are joined together: H—O—O—H H—O—H O=C=O

Counting AtomsCounting Atoms

Example: How Many Atoms?Example: How Many Atoms?

2 H’s and 2 O’s = 4 atoms2 H’s and 2 O’s = 4 atomsHH22OO22

2 C’s+ 5 H’s + 1 O’s + 1 H = 9 atoms2 C’s+ 5 H’s + 1 O’s + 1 H = 9 atomsCC22HH55OHOH

3 K’s+1 P’s + 4 O’s = 8 atoms3 K’s+1 P’s + 4 O’s = 8 atomsKK33POPO44

6 C’s + 12 H’s + 6 O’s = 24 atoms6 C’s + 12 H’s + 6 O’s = 24 atomsCC66HH1212OO66

Counting Atoms with Polyatomic Counting Atoms with Polyatomic Ions - AlIons - Al22(SO(SO44))3 3

Al

Al

SO

O

OO

SO

O

OO

SO

O

OO

=17 ATOMS

Counting Atoms with Polyatomic Counting Atoms with Polyatomic IonsIons

When counting atoms with polyatomic ions;When counting atoms with polyatomic ions; Count number of atoms in one polyatomic ionCount number of atoms in one polyatomic ion

o Ions inside the parenthesesIons inside the parentheses

Multiply by number of polyatomic groups in the Multiply by number of polyatomic groups in the molecule (number outside the parenthesis)molecule (number outside the parenthesis)

Examples:Examples: AlAl22(SO(SO44))3 3 - 2 Al’s + 3(1 S + 4 O’s) = 2 + 3(5) = 2+15 = - 2 Al’s + 3(1 S + 4 O’s) = 2 + 3(5) = 2+15 =

17 atoms17 atoms Mg(NOMg(NO33))22 – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 = – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 =

9 atoms9 atoms

Hydrates Hydrates are ionic compounds which also

contain a specific number of water molecules associated with each formula unit. The water molecules are called waters of hydration. The formula for the ionic compound is followed by a

raised dot and #H2O Example: MgSO4•7H2O.

They are named as ionic compounds, followed by a counting prefix and the word “hydrate” CuSO4•5H2O copper(II) sulfate pentahydrate BaCl2•6H2O barium chloride hexahydrate MgSO4•7H2O magnesium sulfate heptahydrate

(Epsom salts)

How Many Atoms in a Hydrate?How Many Atoms in a Hydrate?When counting atoms in the hydrate, When counting atoms in the hydrate,

count the water atoms also.count the water atoms also.Example:Example:

CuSO4•5H2Oo 1 Cu + 1 S + 4 O’s + 5(2 H’s + 1 O’s)o =1+1+4+5(3)o = 6+15 = 21 atoms

BaCl2•6H2Oo 1 Ba + 2 Cl + 6(2 H’s + 1 O’s)1 Ba + 2 Cl + 6(2 H’s + 1 O’s)o = 1+2+6(3)= 1+2+6(3)o =3 + 18 = 21 Atoms=3 + 18 = 21 Atoms

Part II: Conservation of MassPart II: Conservation of Mass

Conservation of MassConservation of Mass In a normal chemical reaction, the mass of In a normal chemical reaction, the mass of

substances in a closed system will remain substances in a closed system will remain constant, no matter what processes are acting constant, no matter what processes are acting inside the system. inside the system. How ever many atoms a reaction starts with, ends How ever many atoms a reaction starts with, ends

with the same number.with the same number. Atoms don’t change their identity in a chemical Atoms don’t change their identity in a chemical

reactionreaction Number of atoms for EACH ELEMENT Number of atoms for EACH ELEMENT STAYS THE STAYS THE

SAMESAME in a chemical reaction in a chemical reaction The elements just rearrange their organizationThe elements just rearrange their organization The beginning MASS of the reaction EQUALS the The beginning MASS of the reaction EQUALS the

ending MASS of the reactionending MASS of the reaction

Conservation of MassConservation of Mass

Total MassTotal Mass stays the same in a chemical stays the same in a chemical reactionreaction

2g H2 + 16g O2 yields 18g H2O Number and Identity of AtomsNumber and Identity of Atoms stays the same in a chemical stays the same in a chemical

reactionreaction 2 2 H2 + 1 O2 yields 2 H2O

H

H

H

H

O

O HH

O

HH

O

Part III: Writing ReactionsPart III: Writing Reactions

How Do You Write a Chemical How Do You Write a Chemical Reaction?Reaction?

III. Chemical ReactionsIII. Chemical Reactions

Definition – process by which the atoms of Definition – process by which the atoms of one or more substances are rearrangedone or more substances are rearranged KEY: new substances are formedKEY: new substances are formed KEY: No Atoms are Gained or LostKEY: No Atoms are Gained or Lost

A chemical reaction is the process by A chemical reaction is the process by which atoms of one or more substances which atoms of one or more substances are rearranged into new substancesare rearranged into new substances Chemical change occursChemical change occurs How do you know?How do you know?

III. Evidences of a Chemical III. Evidences of a Chemical ReactionReaction

1) gas production1) gas production

2) light production2) light production

3) temperature change (endo/exothermic)3) temperature change (endo/exothermic)

4) precipitate formed (solid from 2 liquids)4) precipitate formed (solid from 2 liquids)

5) permanent color change5) permanent color change

Energy is stored in compounds as Energy is stored in compounds as chemical potential energy chemical potential energy due to specific arrangements of atoms. due to specific arrangements of atoms.

A chemical reaction changes the potential A chemical reaction changes the potential energy present. energy present.

III. Energy ChangesIII. Energy Changes

•When energy is gained; heat is added for When energy is gained; heat is added for a reaction to occur. These are called a reaction to occur. These are called ______________________, ______________________,

Energy ChangesEnergy Changes

exothermic reaction

endothermic reactions

These reactions get hotter.These reactions get hotter.

•When energy is lost as heat, it is called an When energy is lost as heat, it is called an __________________. __________________.

These reactions get colder.These reactions get colder.

•Energy in a reaction is shown with:•ΔH (heat)•kJ•Joules

•Heat•energy

III. Chemical ReactionsIII. Chemical Reactions

Representing Chemical Reactions:Representing Chemical Reactions: ReactantsReactants – the ‘stuff’ you start with – the ‘stuff’ you start with An An ‘arrow’‘arrow’ which means ‘yields’, or ‘becomes’ which means ‘yields’, or ‘becomes’ ProductsProducts – the ‘stuff’ you end up with – the ‘stuff’ you end up with

Principle of “Conservation of Mass” Principle of “Conservation of Mass” applies to chemical reactions. applies to chemical reactions. Why?Why?

III. Chemical ReactionsIII. Chemical Reactions

Word Equations:Word Equations: Reactant-A + Reactant-B Reactant-A + Reactant-B yieldsyields Product-AB Product-AB Example:Example:

o Sodium(s) + Chlorine(g) → Sodium Chloride(s)Sodium(s) + Chlorine(g) → Sodium Chloride(s)o The small letters in paretheses () indicate the state The small letters in paretheses () indicate the state

of the reactant or product (solid, liquid, gas, or of the reactant or product (solid, liquid, gas, or aqueous solution)aqueous solution)

(s) = solid(s) = solid (l) = liquid(l) = liquid (g) = gas(g) = gas (aq) = aqueous = dissolved in water(aq) = aqueous = dissolved in water

Part IV: Balancing EquationsPart IV: Balancing Equations

Applying Conservation of Mass to Applying Conservation of Mass to EquationsEquations

VI. Chemical EquationsVI. Chemical Equations

Step 1: Write a Skeleton EquationStep 1: Write a Skeleton EquationSkeleton EquationSkeleton Equation uses chemical formulas uses chemical formulas

and symbols instead of words:and symbols instead of words: Words: Sodium + Chlorine gas yields Sodium Words: Sodium + Chlorine gas yields Sodium

Chloride Chloride Symbols: Na(s) + ClSymbols: Na(s) + Cl22(g) → NaCl(g) → NaCl Skeleton Equations are not complete Skeleton Equations are not complete

equations, but are the first step in writing a equations, but are the first step in writing a complete equationcomplete equation

IV. Chemical EquationsIV. Chemical Equations

Chemical Equation is BALANCEDChemical Equation is BALANCED Balanced means that “conservation of mass” Balanced means that “conservation of mass”

is upheldis upheld All atoms in reactants are also in productsAll atoms in reactants are also in products

o No more, no lessNo more, no lesso Just rearrangedJust rearranged

IV. Chemical EquationsIV. Chemical Equations

Balancing EquationsBalancing Equations Use a number before the compound/element symbol Use a number before the compound/element symbol

to indicate how many of them are neededto indicate how many of them are neededo Called a COEFFICIENTCalled a COEFFICIENTo Written in front of the atom/compoundWritten in front of the atom/compoundo KEY: Coefficient is a MULTIPLIERKEY: Coefficient is a MULTIPLIER

Number of atoms per molecule is SUBSCRIPTNumber of atoms per molecule is SUBSCRIPT Change Change ONLYONLY the COEFFICIENTS to balance the the COEFFICIENTS to balance the

equationequation

IV. Chemical ReactionsIV. Chemical Reactions Steps to Balance EquationsSteps to Balance Equations

1.1. Write the skeleton equationWrite the skeleton equation

2.2. Count the atoms of EACH element in the reactantsCount the atoms of EACH element in the reactants

3.3. Count the atoms of EACH element in the productsCount the atoms of EACH element in the products

4.4. Change the coefficients to make the number of atoms Change the coefficients to make the number of atoms of each element equal on both sides of the equationof each element equal on both sides of the equation

5.5. Write the coefficients in the lowest possible ratioWrite the coefficients in the lowest possible ratio

6.6. Check your workCheck your work NEVER CHANGE A SUBSCRIPTNEVER CHANGE A SUBSCRIPT

IV. Chemical EquationsIV. Chemical Equations1.Write the skeleton equation:

Al + O2 → Al2O3 2. Count Number of atoms for each element on both sides

This is not balanced because the numbers don’t match

3. Multiply coefficients until they match – multiply the entire units

2 Al + O2 → Al2O3

Go to 6 Oxygens

IV. Balancing EquationsIV. Balancing Equations

Al + O22 Al2O3

4 3Al + 2 Al2O3 O2

Multiply each atom by 2

IV. Balancing Equations 2IV. Balancing Equations 2The work of balancing a chemical equation is in many ways a series of trials and errors.

Consider the equation given below. Does this represent a balanced chemical equation?

N2 + H2 NH3

IV. Balancing Equations 3IV. Balancing Equations 3To balance this reaction, it is best to choose one kind of atom to balance initially. Let's choose nitrogen in this case.

2 Nitrogen Atoms in Reactants requires 2 Ammonia molecules in Product to balance the nitrogen

2NH3H2+N2

IV. Balancing Equations 2IV. Balancing Equations 2• Once we know what the molecules are (N2, H2, and NH3 in this

case) we cannot change them (only how many of them there are).

• The nitrogen atoms are now balanced, but there are 6 atoms of hydrogen on the product side

• only 2 of them on the reactant side. • The next step requires multiplying the number of reactant

hydrogen molecules by three to give:

N2

2NH3H2+ 3H2

IV. Don’t Forget: Diatomic ElementsIV. Don’t Forget: Diatomic Elements

Definition – 7 elements that NEVER occur Definition – 7 elements that NEVER occur as as singular atoms (singular atoms (always paired with an always paired with an

the same or different elementthe same or different element))

HH22 O O22 F F22 Br Br22 I I22 N N22 Cl Cl22

Ex: 2 HCl + 2K 2 KCl + H2

IV. Balancing Equations 3IV. Balancing Equations 3

1. Start with an unbalanced equation

2. Draw boxes around the compounds so you don’t mess with the groups

Don’t be threatened by how complex it looks!

IV. Balancing Equations 2IV. Balancing Equations 2

3. Make an element inventory – count number of atoms for each element on each side of the equation

IV. Balancing Equations 3IV. Balancing Equations 34. Write coefficients in front of each of the boxes until the inventory for each element is the same both before and after the reaction

•Save Oxygen and Hydrogen for last, Treat Polyatomic like an atom.•Let’s start with Sodium

NN

YY

YY

NN

•We have 2 in products, so I need 2 in reactants

Multiply reactant with sodium by 2 and recount atoms

223 3

1111

1111

2211

HH

SOSO44

OO

NaNa

ProductProductReactantReactantElementElement Balanced?Balanced?

1 21 2 YY

1 21 2

3 4 3 4

NN

IV. Balancing Equations 3IV. Balancing Equations 3•Inventory Shows:

•Reactant side has FOUR hydrogen atoms •Product side has TWO hydrogen atoms

•Using your amazing powers of mathematics •two hydrogen multiplied two becomes four hydrogen

NN

YY

NN

YY

2244

1111

1122

2222

HH

SOSO44

OO

NaNa

ProductProductReactantReactantElementElement Balanced?Balanced?

1 21 2 YY

2 4 2 4 YY

Helpful HintsHelpful Hints Balance hydrogen and oxygen lastBalance hydrogen and oxygen last

Balance polyatomic ions as a group Balance polyatomic ions as a group if present on if present on both reactants and productsboth reactants and products You can consider a polyatomic ion as a single You can consider a polyatomic ion as a single

elementelement

If the balancing starts to get very complex:If the balancing starts to get very complex: StopStop Start overStart over Select a different atom to balance first.Select a different atom to balance first.

Example Using PolyAtomicsExample Using PolyAtomicsBeforeBefore

MgCl2 + NaOH Mg(OH)2 + NaCl

1 Mg 1 Mg

2 Cl 1 Cl1 Na 1 Na

1 OH 2 OH

MgCl2 + 2 NaOH Mg(OH)2 + 2 NaCl

1 Mg 1 Mg

2 Cl 2 Cl2 Na 2 Na

2 (OH) 2 (OH)

AfterAfter

Types of Chemical Types of Chemical ReactionsReactions

Part VPart V

Classifying Chemical ReactionsClassifying Chemical Reactions

SynthesisSynthesisDecompositionDecompositionSingle replacementSingle replacementDouble ReplacementDouble ReplacementCombustionCombustion

SynthesisSynthesis

Definition – two or more substances Definition – two or more substances react to form ONE productreact to form ONE product Product is usually bigger or more Product is usually bigger or more

complex than either reactantcomplex than either reactant

A + B A + B AB AB

Hey baby let’s get jiggy.

SynthesisSynthesis

reaction of two elementsreaction of two elements

___Al + ___Cl2 ___AlCl3

Al3+ Cl1-

22 2233

DecompositionDecomposition

definition – one substance breaks definition – one substance breaks down down into two or more simpler into two or more simpler productsproducts

AB AB A + B A + B

Break yoself fool!

DecompositionDecomposition

Example reaction:Example reaction:

__ NaN3 (s) ___ Na (s) + ___ N2 (g)2 2 3

__ CaO (s) ___ Ca (s) + ___ O2 (g)2 2 1

Single Replacement ReactionsSingle Replacement Reactions

Definition – one element replaces Definition – one element replaces another element in a compound to another element in a compound to form new compoundform new compound

A + BX A + BX AX + BAX + B

I’m gon’ dance with yo’ lady

Double ReplacementDouble Replacement

Defn – exchange of Defn – exchange of cationscations between two between two ionic compoundsionic compounds

A B + C D AD + CBswitch

3 possible products of double 3 possible products of double replacement reactionsreplacement reactions

PrecipitatePrecipitateGasGasWaterWater

Reactivity Series (or Activity Series)Reactivity Series (or Activity Series)

More active will replace less activeMore active will replace less active Less active will NOT replace more activeLess active will NOT replace more active

metalsmetals

Li K Ca Na Mg Al Mn Zn Fe Ni Sn Pb Cu Ag Au

most active least active

F Cl Br IF Cl Br I

most active least active

halogenshalogens

examplesexamples

aluminum + iron (III) oxidealuminum + iron (III) oxide

Al Fe2O3+

Stronger?

Al3+ O2-

Al2O3Fe +

Fe3+ O2-

2 21 1

examplesexamples

silver + copper (I) nitratesilver + copper (I) nitrate

Ag CuNO3+

Stronger?

Cu1+ NO31-

NO RXN

examplesexamples

fluorine gas + sodium bromidefluorine gas + sodium bromide

F2 NaBr+

Stronger?

Br2NaF +

Na1+ F1-

221 1

examplesexamples

chlorine gas + hydrogen fluoridechlorine gas + hydrogen fluoride

Cl2 HF+

Stronger?

NO RXN

Example ProblemExample Problem

lithium iodide and aqueous silver nitrate lithium iodide and aqueous silver nitrate reactreact

Li1+ I1- Ag1+ NO31-

Li I AgNO3 LiNO3Ag I (s)+ +

CombustionCombustion

definition – compound reacts with Odefinition – compound reacts with O22

HydrocarbonHydrocarbon – compound w/ only – compound w/ only carbon and hydrogencarbon and hydrogen

CombustionCombustion

Combustion of hydrocarbonsCombustion of hydrocarbons

ALWAYS produces ALWAYS produces COCO22 and and HH22OO

CxHy + O2 CO2 + H2O

Ex problemEx problem

show combustion of propane (Cshow combustion of propane (C33HH88) gas) gas

C3H8 O2 CO2 H2O+ +1 5 3 4

I sell propane and propane accessories!