Unit 7: Chemical Equations

HonorsChemistry

Evidence of a chemical reaction:

A reaction has occurred if the chemical and physical

properties of the reactants and products differ.

heatlight

soundgas emittedcolor change

odor

For a reaction to occur,particles of reactantsmust collide, and withsufficient energy

collision theory

activation energy:

Chemical reactions release or absorb energy.

energy needed to start a reaction

exothermic reactions

endothermic reactions

The reaction in an oxy-acetylene torch is exothermic.

Photosynthesis is anendothermic reaction.

catalyst: speeds up reaction wo/being consumed

-- it lowers the activation energy (AE)

AEAE

time time

without catalyst with catalyst

Examples:

-- enzymes catalyze biochemical reactions

-- catalytic converters convert CO into CO2

En

erg

y

Factors that influencethe rate of a reaction

To make reactionrate increase…

concentration of reactants

particle size

temperature

mechanical mixing

catalyst

nature of reactants

use one

N/A

In a reaction:

atoms are rearranged

AND mass

energy

charge

are conserved

Balancing Chemical Equations

=law of

conservationof mass

same # of atomsof each type on each

side of equation

Changing a ________ changes the substance.

To balance, modify only __________.

Right now, ___________ don’t enter intoour “balancing” picture.

subscript

coefficients

superscripts

The thermite reaction(2 Al + Fe2O3 2 Fe + Al2O3)releases enough heat that it

is used for underwater welding.

SiCl4 + H2O SiO2 + HCl

SiClClClCl HHO SiOO HCl

HHO HCl

HCl

HCl

2 4

Rule of Thumb A: Save the simplest substances for last (i.e., choose a complex substance on each side first).

Au2S3 + H2 Au + H2S

AuAuSSS HH Au HHS

HH HHSHHS

HH

Au

33 2

V2O5 + HCl VOCl3 + H2O

VVOOOOO HCl VOClClCl HHO

HCl HHO

HHOHCl

VOClClCl

36

HCl

HCl

HCl

2

Hg(OH)2 + H3PO4 Hg3(PO4)2 + H2O

HgOHOH HHH# HgHgHg## HHO

Rule of Thumb B: If polyatomic ions are the same on both sides of the equation, balance them first.

3 62

HHH#HgOHOH

HgOHOH

HHO

HHO

HHO

HHO

HHO

CaC2(s) + H2O(l) C2H2(g) + CaO(s)

CaSi2 + SbI3 Si + Sb + CaI2

Al + CH3OH Al(CH3O)3 + H2

3 2 26 3

3262 3

2 C2H2(g) + O2(g) CO2(g) + H2O(l) ** 5 4 22

C3H8 + O2 CO2 + H2O ** 5 3 4

C5H12 + O2 CO2 + H2O ** 8 5 6

** = complete combustion of a hydrocarbonyields CO2 and H2O

Reaction Conditions and Terminology

Certain symbols give more info about a reaction.

(s) = solid

(l) = liquid

(g) = gas

(aq) = aqueous(dissolved in H2O) NaCl(s) NaCl(aq)

More on aqueous…

-- synonyms: “soluble” or “in solution”

-- acids and bases are aqueous solutions

Other symbols…

means...

Temp. at which we perform rxn. might be given.

The catalyst used might be given.

means ______ is added to the reaction

MgCO3(s) MgO(s) + CO2(g)

C2H4(g) + H2(g) C2H6(g) Pt

C6H5Cl + NaOH C6H5OH + NaCl 400oC

“yields” or “produces”

heat

(i.e., clues about the reaction)

precipitate: a solid product that forms in an aqueous solution reaction

When ionic substances have “(aq)” written afterthem, the individual ions have dissociated fromthe ionic crystal and are floating around separately.

Na3PO4(aq) means… 3 Na+(aq) + PO43–(aq)

Sodium phosphate, Na3PO4,(sometimes called “sodium phosphate, tribasic”)

is a cleaning agent and food preservative.

Na2CO3 + Ca(NO3)2 CaCO3 +(aq) (aq)

Na+Ca2+CO3

2–

NO3–

2 (aq)(s)

Na+

NaNO3

NO3–

ppt

clearNa2CO3

solution

clearCa(NO3)2

solution

cloudy solutioncontaining CaCO3(s)

and NaNO3(aq)

“chunks”“sinkies”“floaties”

Word EquationsSolid iron reacts with oxygen gas to yield

solidiron(III) oxide.word equation:

balanced equation:

iron

Fe

+ oxygen iron(III) oxide

O2+

Fe3+ O2–

Fe2O324 3

2

Solid sodium reacts w/oxygento form solid sodium oxide.

O2(g) Na(s) Na2O(s)+

Na+ O2–

+

4

Aqueous aluminum sulfate reacts w/aqueous calciumchloride to form a white precipitate of calcium sulfate.The other compound remains in solution.

Al3+ SO42– Ca2+ Cl–

Al2(SO4)3(s)+ CaCl2 CaSO4 AlCl3(aq) (aq) (aq)+33 2

Write a balanced equation (w/rxn conditions)from the following word equations.

Methane gas (CH4) burns in oxygen.

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

Much of the methane that is liberated duringthe extraction of petroleum is simply burned

because the cost of transporting it is too high.

Write an equation for the combustion of C8H18.

C8H18 + O2 CO2 + H2O 25 8 92 16 18

Classifying Reactions

four types

synthesis: simpler substances combine to form more complex substances

sodium + chlorine gas sodium chloride

A + B AB AB + C ABC A + B + C ABC

Na Cl2+ NaCl2 2

2 2

decomposition: complex substances are broken down into simpler ones

lithium chlorate lithium chloride + oxygen

Li+ ClO3– Li+ Cl–

LiClO3 LiCl + O23

AB A + B ABC AB + C ABC A + B + C

single-replacement: one elementreplaces another

AB + C A + CB AB + C B + AC

? aluminum copper(II)sulfate +

Cu2+ SO42–

CuSO4 Al +

Al3+ SO42–

Al2(SO4)3 Cu+32 3

copper aluminumsulfate +

double-replacement:

AD + CBAB + CD

lead(IV)nitrate

+calciumoxide ?

Pb4+ O2–

Pb(NO3)4 +

Ca2+ NO3–

CaO Ca(NO3)2 PbO2 +2 2

lead(IV)oxide

+calciumnitrate

How do we know if a reaction will occur?

For single-replacement reactions, use Activity Series.In general, elements above replace elements below.

Ba + FeSO4

Mg + Cr(ClO3)3

Pb + Al2O3

NaBr + Cl2

FeCl3 + I2

CoBr2 + F2

Fe + BaSO4

Cr + Mg(ClO3)2

NR

NaCl + Br2

NR

CoF2 + Br2

3 2 2 3

2 2

(?)

For double-replacement reactions,reaction will occurif any product is: water a gas a precipitate

driving forces

Pb(NO3)2(aq) + KI(aq)

KOH(aq) + H2SO4(aq)

FeCl3(aq) + Cu(NO3)2(aq)

H+ OH–

Fe3+ Cl– NO3–

Pb2+ NO3– K+ I– Pb2+ NO3

– K+ I–

Check new combinations to decide.

(?)

K+ SO42–

Cu2+

H+ OH– K+ SO42– (?) (?)

Cl–

Cu2+ (?)

Fe3+ NO3– (?)

(ppt) (aq)

(aq) (water)

(aq) (aq)

NR

K2SO4(aq) + H2O(l)

PbI2(s) + KNO3(aq) 2 2

2 2

Quantitative Relationships in Chemical Equations

4 Na(s) + O2(g) 2 Na2O(s)

Particles 4 atoms 1 m’cule 2 m’cules

Moles 4 mol 1 mol 2 mol

Grams 4 g 1 g 2 g

** Coefficients of a balancedequation represent # ofparticles OR # of moles,but NOT # of grams.

Part.

Vol.

Mass

MOL

Mass

Vol.

Part.

MOL

SUBSTANCE “A” SUBSTANCE “B”

(known) (unknown)

Use coeff. from bal. eq. to gofrom “moles of A”

to “moles of B”

Na2O 4 Na(s) + O2(g) 2 Na2O(s)

How many moles oxygen will react with 16.8 moles sodium?

How many moles sodium oxide are produced from87.2 moles sodium?

How many moles sodium are requiredto produce 0.736 moles sodium oxide?

( )4 mol Na

1 mol O216.8 mol Na = 4.20 mol O2

87.2 mol Na = 43.6 mol Na2O

( )2 mol Na2O

4 mol Na0.736 mol Na2O = 1.47 mol Na

( )2 mol Na2O

4 mol Na

O2Na Na2ONa

0 0