Unit 7: Chemical Equations

Chemistry

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

CH4 + 2 O2 CO2 + 2 H2O

Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms

EX. solid iron reacts with oxygen gas to yieldsolid iron (III) oxide

If all coefficients are 1…

If we change subscripts…

Fe3+ O2– ___Fe(s) + ___O2(g) ___Fe2O3(s)

+

___Fe(s) + ___O2(g) ___Fe2O3(s) 11 1

___Fe2(s) + ___O3(g) ___Fe2O3(s) 11 1

+

But we can’t!!!

Changing a ___________ changes the substance.

To balance, modify only _____________. Right now, _______________ don’t enter intoour “balancing” picture.

subscript

coefficientssuperscripts

___Fe(s) + ___O2(g) ___Fe2O3(s)

+

23

Hint: Start with most complicated substances firstand leave simplest substances for last.

4

2

solid sodium reacts w/oxygento form solid sodium oxide

___O2(g)___Na(s) ___Na2O(s)+

Na+ O2–

+

14

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)+1 33 2

1

1

Methane gas (CH4) reacts withoxygen to form carbon dioxidegas and water vapor.

_ CH4(g) + _ O2(g) _ CO2(g) _ H2O(g)+2 1 2

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

_ CaSi2 + _ SbI3 _ Si + _ Sb + _ CaI2

_ Al + _ CH3OH _ Al(CH3O)3 + _ H2

1 1 1

3 2 26 3

3262 13

Furnaces burnprimarily methane.

2_ C2H2(g) + _ O2(g) _ CO2(g) + _ H2O(l) ** 5 4 21 2 1

1_ C3H8 + _ O2 _ CO2 + _ H2O ** 5 3 4

1_ C5H12 + _ O2 _ CO2 + _ H2O ** 8 5 6

** = complete combustion (+ O2) of a hydrocarbon (CxHy) yields CO2 and H2O

Write equations for the combustion of C7H16 and C8H18

1_ C7H16 + _ O2 _ CO2 + _ H2O 11 7 8

1_ C8H18 + _ O2 _ CO2 + _ H2O 25 8 92 16 18

Odd # Oxy

Odd # Oxy

Signs of Chemical ReactionsThere are five main signs that indicate a chemical

reaction has taken place:

change in color change in odor production of newgases or vapor

input or releaseof energy

difficult to reverse

releaseinput

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.

Endothermic ReactionEnergy + Reactants Products

Reaction progress

Ene

rgy

Reactants

ProductsActivation Energy

Exothermic ReactionReactants Products + Energy

10 energy = 8 energy + 2 energy

Reactants

Products

Ene

rgy

Energy of reactants

Energy of products

Reaction Progress

Decomposition of Nitrogen Triiodide

2 NI3(s) N2(g) + 3 I2(g)

NI3 I2

N2

catalyst: speeds up reaction w/o being consumed … it lowers the activation energy (Ea)

AEAE

time time

without catalyst with catalyst

Examples: enzymes catalyze biochemical reactions catalytic converters convert CO into CO2

Ener

gy

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…

“soluble” or “in solution” indicates that a substance is dissolved in water (usually) all acids 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)

Factors that influencethe rate of a reaction

To make reactionrate increase…

concentration of reactants

particle size

temperature

mechanical mixing

pressure

catalyst use onenature of reactants N/A

Types of Chemical Reactions

Synthesis (combination) reaction

Decomposition reaction

ASingle-replacement reaction

BDouble-replacement reaction

Combustion reaction (of a hydrocarbon)

A + B AB

AB A + B

A + BC AC + B

AB + CD AD + CB

CxHy + O2 CO2 + H2O

Ause activity series to predict products/reactivityBuse solubility chart to predict products/reactivity

element compound elementcompound

All compounds…

Balance and classify the following reactions:

_ F2 _ MgI21 _ I2 + _ MgF2 +1 1 1

Single replacement

_ C3H5N3O94 _ N2 + _ CO26 12 + _ H2O10 + _ O21

Decomposition

1_ C5H12 + _ O2 _ CO2 + _ H2O 8 5 6

Double Replacement?

Combustion

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.

Single and Double Replacement Reactions

Double-replacement reaction

CaCO3 + 2 HCl CaCl2 + H2CO3

General form: AB + CD AD + CB

Single-replacement reaction

Mg + CuSO4 MgSO4 + Cu

General form: A + BC AC + B

single-replacement: one elementreplaces another

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

chlorine sodiumbromide

sodiumchloride bromine + +

? aluminum copper (II)sulfate +

_ Cl2 Na+ Br– _ NaBr1 _ Br2 +

Na+ Cl– _ NaCl +2 2 1

Cu2+ SO42–

_ CuSO4_ Al +Al3+ SO4

2– _ Al2(SO4)31 _ Cu+32 3

copper aluminumsulfate +

Ca

Activity Series

Foiled again:Aluminum is knocked out by Calcium

Element Reactivity

LiRbKBaCaNaMgAlMnZnCrFeNiSnPbH2

CuHgAgPtAu

Halogen Reactivity

F2

Cl2Br2

I2

PrintableVersion

ofActivitySeries

Mg + AlCl3

Al + MgCl2

Predict if these reactions will occur

Al + MgCl2

Can magnesium replace aluminum?• Activity Series YES, magnesium is more reactive than aluminum.

2 23 3

Can aluminum replace magnesium?NO, magnesium is less reactive than aluminum.

Therefore, no reaction will occur.

NR (No Reaction)

MgCl2 + Al No reaction

We must determine if the lone element is more reactive than the bonded one… metals replace metals or non-metals replace nonmetals

Order of reactants DOES NOT

determine how they react.

1

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

1 1 1

3 2 2 3

2 1

1 1

2 1

1 1

double-replacement:

AD + CBAB + CD

iron (III)chloride + potassium

hydroxide?

Fe3+ Cl– _ FeCl3 +1

K+ OH– _ KOH

Fe3+ OH– K+ Cl– _ KCl_ Fe(OH)3 +3 1 3

iron (III)hydroxide+ potassium

chloride

lead (IV)nitrate + calcium

oxide ?

Pb4+ O2– _ Pb(NO3)4 +1

Ca2+ NO3–

_ CaO _ Ca(NO3)2_ PbO2 +2 1 2Pb4+ O2– Ca2+ NO3

–

lead (IV)oxide + calcium

nitrate

Formation of a solid: AgCl

AgNO3(aq) + KCl(aq) KNO3 (aq) + AgCl(s)

Double Replacement Reactions

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.

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

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”

FeCO3

Na+Fe2+

iron (II) chloride + sodium carbonate

Cl2

Using the SOLUBILITY TABLE:sodium chloride is soluble

iron (II) carbonate is insoluble

CO3

Fe2+

Fe

Na+

Na2

Cl - CO32- Cl- CO3

2-

NaCl

sodium chloride iron (II) carbonate+

(aq) (s)

2FeCl2 Na2CO3 NaCl FeCO3(aq) (s)+ +

Predict if a reaction will occur when you combine aqueous solutions of iron (II) chloride and sodium carbonate…

If the reaction does occur, write a balanced chemical equation showing it (be sure to include phase notation).

(aq) (aq)

Balanced chemical equation

(?)

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) 1 2 1 2

2 1 1 2

Ions in Aqueous Solution Pb(NO3)2(s) Pb(NO3)2(aq)

Pb2+(aq) + 2 NO3–(aq)

addwater

dissociation:

Pb2+NO3

–

NO3–

Pb2+

NO3–

NO3–

“splitting into ions”

NaI(s) NaI(aq) Na+(aq) + I–(aq)

Na+ I– Na+ I–add

water

ChemThink

Mix them and get the overall ionic equation…

__Pb2+(aq) + __NO3–(aq) + __Na+(aq) + __I–(aq)

__PbI2(s) + __NO3–(aq) + __Na+(aq) yields

reactants

products

Cancel spectator ions to get net ionic equation… __PbI2(s)1

1 2 2 2

221

Pb2+

NO3–

NO3– Na+ I–

NO3–

NO3–

Na+ I–

Na+

Na+

__Pb2+(aq) + __I–(aq) 1 2

Pb2+I–

I–

Pb2+I–

I–

Pb2+I–

I–

Polymers and Monomers

polymer: a large molecule (often a chain) made of many smaller molecules called monomers

Polymers can be made more rigid if the chains arelinked together by way of a cross-linking agent.

Monomer Polymer

amino acids…………

nucleotides (w/N-bases A,G,C,T/U)…..

styrene………………

PVA………………….

proteins

nucleic acids

polystyrene

“slime”polyvinylalcohol

H–N–C–C–O–HH H O

R