Chemical Reactions: An Introduction

Chemical Reactions: An Chemical Reactions: An IntroductionIntroduction

Ch. 8Ch. 8

Chemical EquationsChemical Equations

Chemical change involves a Chemical change involves a reorganization of the atoms in one or reorganization of the atoms in one or

more substances.more substances.

4 Evidences a Chemical Reaction 4 Evidences a Chemical Reaction Has OccurredHas Occurred

1.1. A color change is apparent.A color change is apparent.

2. A precipitate (solid) forms.2. A precipitate (solid) forms.

4 Evidences a Chemical Reaction 4 Evidences a Chemical Reaction Has OccurredHas Occurred

3. A gas (bubbles) is formed.3. A gas (bubbles) is formed.

4. Evolution/absorption of energy4. Evolution/absorption of energy

--heat, light, electrical energy--heat, light, electrical energy..

Reaction of Zinc and IodineReaction of Zinc and Iodine

Product: Zn IProduct: Zn I22Reactants: Zn + IReactants: Zn + I22

Chemical EquationChemical EquationA representation of a chemical reaction:A representation of a chemical reaction:

word equation -- qualitative significance onlyword equation -- qualitative significance only

ethanol + oxygen yields carbon dioxide + water vaporethanol + oxygen yields carbon dioxide + water vapor

formula equation -- qualitative & quantitativeformula equation -- qualitative & quantitative C C22HH55OHOH(l)(l) + 3O + 3O2(g)2(g)

2CO2CO2(g)2(g) + 3H + 3H22OO(g)(g)

reactants productsreactants products


Chemical equations give two important pieces Chemical equations give two important pieces of information:of information:

1.1. The identities of the reactants and products -- The identities of the reactants and products -- qualitativequalitative information. information.

2. The relative number of each --2. The relative number of each --quantitativequantitative information.information.

Physical StatesPhysical States

• solid (s)solid (s)

• liquid (l)liquid (l)

• gas (g)gas (g)

• aqueous (aq)aqueous (aq)

Important Equation SymbolsImportant Equation Symbols

• yields ------>yields ------>

cat.cat.

• catalyst ------->catalyst ------->

HH22SOSO44

• catalyst ------>catalyst ------>

• heat ------->

light

• light -------->

elect.

• electricity ------>

Chemical EquationChemical EquationQuantitative SignificanceQuantitative Significance

CC22HH55OH OH (l)(l) ++ 3O3O2(g)2(g) 2CO2CO22(g)(g) ++ 3H3H22OO(g)(g)

The equation is balanced.The equation is balanced.

1 mole of ethanol 1 mole of ethanol reacts with reacts with 3 moles of 3 moles of oxygenoxygen

to produceto produce

2 moles of carbon dioxide 2 moles of carbon dioxide and and 3 moles of 3 moles of waterwater

Chemical EquationsChemical EquationsQuantitative SignificanceQuantitative Significance

4 Al(s) + 3 O4 Al(s) + 3 O22(g) (g)

---> 2 Al---> 2 Al22OO33(s)(s)

This equation meansThis equation means

4 Al atoms + 3 O4 Al atoms + 3 O22 molecules molecules

---give--->---give--->

2 molecules of Al2 molecules of Al22OO33

4 moles of Al + 3 moles of O4 moles of Al + 3 moles of O22

---give--->---give--->

2 moles of Al2 moles of Al22OO33

Because of the principle of the Because of the principle of the conservation of matterconservation of matter, an , an equation must be equation must be balanced.balanced.

It must have the same number of It must have the same number of atoms of the same kind on both atoms of the same kind on both sides.sides.

The total mass for the reaction The total mass for the reaction cannot change.cannot change.


Lavoisier, 1788Lavoisier, 1788

Key WordsKey Words

Reactants are indicated by:Reactants are indicated by:

reacts with…..reacts with…..

combines with…..combines with…..

oxidizes……oxidizes……

burns in……burns in……

…….are required.are required

…….are needed.are needed

…….decomposes.decomposes

Products are indicated by:Products are indicated by:

…….is formed.is formed

…….is produced.is produced

…….is given off.is given off

…….is precipitated.is precipitated

yields….yields….

…….is prepared.is prepared

……..is synthesized..is synthesized

Coefficients, Subscripts, & Coefficients, Subscripts, & SuperscriptsSuperscripts

Coefficient SuperscriptCoefficient Superscript

SubscriptSubscript

22Hg2

Four Steps in Balancing EquationsFour Steps in Balancing Equations

1. Get the facts down.1. Get the facts down.

2. Check for diatomic molecules (subscripts).2. Check for diatomic molecules (subscripts).

3. Balance charges on compounds containing a metal, 3. Balance charges on compounds containing a metal, ammonium compounds, and acids (subscripts).ammonium compounds, and acids (subscripts).

4. Balance the number of atoms (coefficients).4. Balance the number of atoms (coefficients).

a. Balance most complicated molecule first.a. Balance most complicated molecule first.

b. Balance other elements.b. Balance other elements.

c. Balance hydrogen next to last.c. Balance hydrogen next to last.

d. Balance oxygen last.d. Balance oxygen last.

Balancing Equations CautionBalancing Equations Caution

The identities (formulas) of The identities (formulas) of the compounds must the compounds must nevernever be be

changed in balancing a changed in balancing a chemical equation!chemical equation!

Only coefficients can be used Only coefficients can be used to balance the equation-to balance the equation-

subscripts will subscripts will notnot change! change!

Combustion of MethaneCombustion of Methane

methane gas burns to produce carbon dioxide gas and liquid methane gas burns to produce carbon dioxide gas and liquid waterwater

• whenever something burns it combines with Owhenever something burns it combines with O22(g)(g)

CHCH44(g) + O(g) + O22(g) (g) CO CO22(g) + H(g) + H22O(O(ll))

H

HC

H

HOO+

O

O

C + OH H

1 C + 4 H + 2 O 1 C + 2 O + 2 H + O1 C + 2 H + 3 O

Combustion of MethaneCombustion of MethaneBalancedBalanced

The balanced reaction must obey the Law of The balanced reaction must obey the Law of Conservation of Mass:Conservation of Mass:

CHCH44(g) + (g) + 22 O O22(g) (g) CO CO22(g) + (g) + 22 H H22O(O(ll))

H

HC

H

H

OO

+

O

O

C +

OH H

OO

+O

H H

+

1 C + 4 H + 4 O 1 C + 4 H + 4 O

Balancing EquationsBalancing Equations___ Al___ Al(s)(s) + ___ Br + ___ Br2(l)2(l) ---> ___ Al ---> ___ Al22BrBr6(s)6(s)

Chemical ReactionsChemical Reactions

What evidence did What evidence did you observe that a you observe that a chemical reaction chemical reaction occurred?occurred?

1. Evolution of 1. Evolution of heat/lightheat/light

2. Color change2. Color change

Balancing EquationsBalancing Equations22 Al Al(s)(s) + + 33 Br Br2(l)2(l) ---> Al ---> Al22BrBr6(s)6(s)

Balancing EquationsBalancing Equations___C___C33HH8(g)8(g) + ___ O + ___ O2(g)2(g) ----> ____CO ----> ____CO2(g)2(g) + ____ H + ____ H22OO(g)(g)

___B___B44HH10(g)10(g) + ___ O + ___ O2(g)2(g) ----> ____ B ----> ____ B22OO3(g)3(g) + ____ H + ____ H22OO(g)(g)

Balancing EquationsBalancing Equations

CC33HH8(g)8(g) + + 55 O O2(g)2(g) ----> ----> 33COCO2(g)2(g) + + 4 4 HH22OO(g)(g)

2 2 BB44HH10(g)10(g) + + 1111 O O2(g)2(g) ----> ----> 4 4 BB22OO3(g)3(g) + + 1010 H H22OO(g)(g)

Final Equation Balancing CheckFinal Equation Balancing Check

SiOSiO2(s)2(s) + 4 HF + 4 HF(aq) (aq) -----> SiF -----> SiF4(g) 4(g) + 2 H+ 2 H22OO(l)(l)

Make a Make a gridgrid to do a final check to be sure the Law of to do a final check to be sure the Law of Conservation of Matter has been obeyed:Conservation of Matter has been obeyed:

Reactants ProductsReactants Products

1 Si1 Si 1 Si1 Si

2 O2 O 2 O2 O

4 H4 H 4 H4 H

4 F4 F 4 F4 F

Figure 6.5(a): The reaction of potassium with waterFigure 6.5(a): The reaction of potassium with water

Figure 6.5(b)&(c): The reaction of potassium with waterFigure 6.5(b)&(c): The reaction of potassium with water

What evidence is there for a chemical reaction?

Oxidation-Reduction ReactionsOxidation-Reduction Reactions(Redox Reactions)(Redox Reactions)

Redox reaction -- Redox reaction -- involves the transfer of involves the transfer of electronselectrons

• loss and gain of electrons must be loss and gain of electrons must be exactlyexactly equal. equal.

• loss and gain of electrons must be loss and gain of electrons must be simultaneous.simultaneous.

OxidationOxidation

• loss of electrons loss of electrons

• metal atoms -- Na, Ca, & Kmetal atoms -- Na, Ca, & K

• NaNaoo ---> Na ---> Na1+ 1+ + 1e+ 1e--

• CaCao o ---> Ca---> Ca2+ 2+ + 2e+ 2e--

• nonmetal ions -- Clnonmetal ions -- Cl--, S, S2-2-, & O, & O2-2-

• ClCl1-1- ---> Cl ---> Clo o ++ 1e-1e-

• SS2-2- ---> S ---> So o + 2+ 2 e-e-

ReductionReduction

• gain of electrons.gain of electrons.

• nonmetal atoms.nonmetal atoms.

• OOoo + 2e + 2e-- ---> O ---> O2-2-

• FFoo + 1e + 1e-- ---> F ---> F1-1-

• metal ions.metal ions.

• KK1+ 1+ + 1e+ 1e- - ---> K---> Koo

• BaBa2+2+ + 2e + 2e-- ---> Ba ---> Baoo

Figure 7.7: When powdered aluminum and iodine (shown in the Figure 7.7: When powdered aluminum and iodine (shown in the foreground) are mixed (and a little water added), they react foreground) are mixed (and a little water added), they react

vigorouslyvigorously

Oxidation & Reduction Half-Oxidation & Reduction Half-ReactionsReactions

• Always add electrons (negative) to the more Always add electrons (negative) to the more positive side of the equation.positive side of the equation.

• The charge on both sides of an equation The charge on both sides of an equation must be equal.must be equal.

• Oxidation -- NaOxidation -- Naoo ----> Na ----> Na1+ 1+ + 1 e-+ 1 e-

• Reduction -- ClReduction -- Cl2 2 + 2 e+ 2 e- - ----> 2 Cl----> 2 Cl- -

OIL RIGOIL RIG

Oxidation Is Loss.

Reduction Is Gain.

Redox ReactionsRedox Reactions

• Metal-nonmetal reactions are Metal-nonmetal reactions are alwaysalways redox redox reactions.reactions.

• Any reaction that has a free element (such Any reaction that has a free element (such as Oas O22) as a reactant or product are redox.) as a reactant or product are redox.

• All single replacement reactions are redox.All single replacement reactions are redox.

• All combustion reactions are redox.All combustion reactions are redox.

Synthesis I (Composition)Synthesis I (Composition)

A + X ----> AX A + X ----> AX ALWAYS REDOXALWAYS REDOX

Element + Element -----> Binary CompoundElement + Element -----> Binary Compound

FeFe(s)(s) + S + S(s)(s) ----> FeS ----> FeS(s)(s)

4 Al4 Al(s)(s) + 3 O + 3 O2(g)2(g) ----> 2 Al ----> 2 Al22OO3(s)3(s)

Synthesis II (Composition)Synthesis II (Composition)

A + X ----> AX A + X ----> AX NOT REDOXNOT REDOX

Compound + Compound ----> Compound (3 Compound + Compound ----> Compound (3 or more or more elements)elements)

Ammonia + Acid ----> Ammonium SaltAmmonia + Acid ----> Ammonium Salt

NHNH3(g)3(g) + HCl + HCl(g)(g) ----> NH ----> NH44ClCl(s)(s)

2 NH2 NH3(aq)3(aq) + H + H22SOSO4(aq)4(aq) ----> (NH ----> (NH44))22SOSO4(aq)4(aq)

Synthesis II (Composition)Synthesis II (Composition)ContinuedContinuedWater + An OxideWater + An Oxide

Rule # 1 --Rule # 1 --Water + Water + Metal OxideMetal Oxide ----> Metal ----> Metal Hydroxide (Base)Hydroxide (Base)

HOHHOH(l)(l) + CaO + CaO(s)(s) ----> Ca(OH) ----> Ca(OH)2(s)2(s)

HOHHOH(l)(l) + Na + Na22OO(s)(s) ----> 2 NaOH ----> 2 NaOH(aq)(aq)

Rule # 2 --Water + Nonmetal Oxide ----> Acid

HOH(l) + SO3(g) ----> H2SO4(aq)

HOH(l) + N2O5(g) ----> 2 HNO3(aq)

Single Replacement ISingle Replacement I

A + BX ----> AX + B ALWAYS REDOX

Element + Compound ----> Different Element + Different Compound

Metal Reactivities:

K > Na > Ca > Mg > Al > Zn > Cr > Fe > Cd > Co > Ni > Sn > Pb > H > Sb > Cu > Hg > Ag > Pt > Au

Single Replacement ISingle Replacement I(Continued)(Continued)A is a metal.

A is more reactive than B A + BX ----> AX + B

Cu(s) + 2 AgNO3(aq) ---->Cu(NO3)2(aq) + 2 Ag(s)

Zn(s) + 2 HCl(aq) ----> ZnCl2(aq) + H2(g)

Rule # 3 -- Active Metal + Water ----> Metal Hydroxide + Hydrogen

Ca(s) + 2 HOH(l) ----> Ca(OH)2(s) + H2(g)

Single Replacement ISingle Replacement I(Continued)(Continued)

A is a metal.

A is less reactive than B

A + BX ----> No Reaction (NR)

Cu(s) + HCl(aq) ----> NR

Single Replacement IISingle Replacement II

Y + BX ----> BY + X ALWAYS REDOX

Element + Compound ----> Different Element + Different

Compound

Nonmetal Reactivities:

F > O > Cl > Br > I

Single Replacement IISingle Replacement II(Continued)(Continued)

Y is a nonmetal.Y is more reactive than X Y + BX ----> BY + X

Cl2(g) + 2 KBr(aq) ----> 2 KCl(aq) + Br2(aq)

Y is less reactive than X Y + BX ----> No Reaction

Cl2(g) + KF(aq) ----> NR

Figure 7.6: The Figure 7.6: The thermite reaction thermite reaction

gives off so gives off so much heat that much heat that the iron formed the iron formed

is moltenis molten

Decomposition IDecomposition I

AX ----> A + X ALWAYS REDOX

AX is a binary compound.

Binary Compound ----> Element + Element

elect

2 NaCl(l) ----> 2 Na(l) + Cl2(g)

elect

2 HOH(l) ----> 2 H2(g) + O2(g)

Decomposition IIDecomposition IIAX -----> A + X MAY OR MAY NOT BE

REDOX

AX is a ternary compound.

Rule # 4 -- Metal Chlorates ----> Metal Chlorides + Oxygen

2 KClO3(s) ----> 2 KCl (s) + 3 O2(g) REDOX

Decomposition IIDecomposition II(Continued)(Continued)

Rule # 5 -- Metal Carbonates ----> Metal Oxides + Carbon

Dioxide

CuCO3(s) ----> CuO (s) + CO2(g) NOT REDOX


Rule # 6 -- Metal Hydroxides ----> Metal Oxides + Water

Ca(OH)2(s) ----> CaO (s) + HOH(g) NOT REDOX


Rule # 7 -- Acid ----> Nonmetal Oxide + Water

H2SO4(aq) ----> SO3(g) + HOH(g) NOT REDOX

CombustionCombustion

AX + OAX + O2 2 ALWAYS REDOXALWAYS REDOX

AX is a hydrocarbon.AX is a hydrocarbon.

CCxxHHyy + O + O2(g)2(g) ----> CO ----> CO2(g)2(g) + HOH + HOH(g)(g)

CHCH4(g)4(g) + 2 O + 2 O2(g)2(g) ----> CO ----> CO2(g)2(g) + 2 HOH + 2 HOH(g)(g)

2 C2 C66HH6(l) 6(l) + 15 O+ 15 O2(g) 2(g) ----> 12 CO----> 12 CO2(g) 2(g) + 6 HOH+ 6 HOH(g)(g)

Decomposition IDecomposition I

AX ----> A + X ALWAYS REDOX

AX is a binary compound.

Binary Compound ----> Element + Element

elect

2 NaCl(l) ----> 2 Na(l) + Cl2(g)

elect

2 HOH(l) ----> 2 H2(g) + O2(g)

Reaction of Potassium and Water

Write and balance the word and formula equation for potassium and water.

potassium(s) + water(l) potassium hydroxide(aq) + hydrogen(g)

2K(s) + 2HOH(l) 2KOH(aq) + H2(g)

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Chemical Reactions: An Introduction