Chemical Equations & Reactions

Chemical Equations

This equation means:

4 Al(s) + 3 O2(g) 2 Al2O3(s)

4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3

4 Al moles + 3 O2 moles yield 2 moles of Al2O3

or

4 g Al + 3 g O2 yield 2 g Al2O3

4 mol Al@27g/mol 3 mol O2@32g/mol 2 mol Al2O3@102g/mol

108 g + 96 g = 204 g

?

Visualizing a Chemical Reaction

Na + Cl2 NaCl

___ mole Cl2 ___ mole NaCl___ mole Na

2

10 5 10

2

10 5 10

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…

Practice: Balance and Classify1. Ca(OH)2 + HCl CaCl2 + H2O ________________________

2. C2H4 + O2 CO2 + H2O ________________________

3. N2 + O2 N2O

________________________

4. SrCO3 SrO + CO2 ________________________

5. NaI + Br2 NaBr + I2

________________________

6. C2H4O + O2 CO2 + H2O ________________________

7. MgBr2 + (NH4)2SO3 MgSO3 + NH4Br

________________________

8. AgClO3 + (NH4)2Cr2O7 Ag2Cr2O7 + NH4ClO3______________________

9. Cs + H2O CsOH + H2 ________________________

10. Fe + O2 Fe3O4 ________________________

Symbols Used in Chemical Equations

“Yields”; indicates result of reaction

Used to indicate a reversible reaction (equilibrium)

A reactant or product in the solid state; also used to indicate a precipitate

Alternative to (s), but used only to indicate a precipitate

A reactant or product in the liquid state

A reactant or product in an aqueous solution (dissolved in water)

A reactant or product in the gaseous state

(s)

(l)

(aq)

(g)

Additional Symbols Used in Chemical Equations

Alternative to (g), but used only to indicate a gaseous product

Reactants are heated

Pressure at which reaction is carried out, in this case 2 atm

Pressure at which reaction is carried out exceeds normalatmospheric pressure

Temperature at which reaction is carried out, in this case 0 oC

Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction

2 atm

pressure

0 oC

MnO2

D

Signs of Chemical Reactions

There 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

rele

ase

inpu

t

Combustion

O2

General form: CxHx + O2 CO2 H2O

+

+

carbon-hydrogen compound

carbondioxide

C4H10 +

oxygen water

CO2 H2O4 513/2

13 8 102

1. Write a word equation for the reaction

Write a balanced equation for the reaction between chlorineand solid sodium bromide to produce bromine and solid sodium chloride.

2. Write the correct formulas for all reactants and products, (with correct phases of matter)

3. Balance the resulting equation

chlorine + sodium bromide bromine + sodium chloride

Cl2(g) + NaBr(s) Br2(l) + NaCl(s)

Writing Equations Practice

2 2

1) Write a word equation for the reaction

2) Write the correct formulas for all reactants and products

3) Balance the resulting equation

aluminum sulfate + calcium chloride calcium sulfate

Write the balanced equation for the aqueous reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate.

+ aluminum chloride? ?

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

Oxidation-Reduction Reactions

• “Redox” reactions involve the transfer of electrons (e-)

•Reduction: gain e-

•Oxidation: lose e-

“LEO the lion says, ‘GER’”

“OIL RIG”

• Use oxidation states to keep track of the e-

Leo says Ger

“Lose electron oxidation” Zn 2e- + Zn2+

“Gain electron reduction”2e- + Cu2+ Cu

My name is Leo. Grr-rrrr…

Assigning Oxidation States

Specific rules for assigning Ox #’s • Usually the same charge assigned by the

PT• H is almost always +1 • O is almost always -2 • F is always -1 in compounds• For elements (H2, O2, F2, Ca, K, etc )

the oxidation state always = 0

Some exceptions do exist!

Assigning Oxidation Numbers

• Overall charge = sum of the oxidation states of all atoms in it

Neutral Compounds (e.g. H2O, CO2, CH4)• H2O :

The overall charge is 2(1) + -2 = 0• CO2: What is the oxidation state of C?

Since C + 2 (O) = 0…C + 2(-2) = 0, thus…

• CH4: Is C still +4?H is always +1 To remain neutral… 4(1) + C = 0 C must = - 4

H = +1 and O = -2

C = +4

Assigning Oxidation Numbers

• Charged compounds (e.g. NO3-, CO3

2-)NO3

- or (NO3)- : What is the oxidation # of N?

O is -2, and the overall charge is -1

So N + 3(O) = -1 or N + 3(-2) = -1

N = + 5

(CO3)2-: What is the oxidation # of C?

O is -2, and the overall charge is -2

So C + 3(O) = -2 or C + 3(-2) = -2

C = +4

The oxidation # of ions = charge of ions

Mn3+ has an oxidation # of +3

S2- has an oxidation # of -2

Assigning Oxidation # Practice

Assign oxidation numbers to each atomCl2

Fe2+

ClO3-

ClO4-

IO2-

CrO42-

Fe3(PO4)2

CoSO4

Cl: 0 (element)

Fe: 2+ (ion)

O: 2-, 3(2-) + Cl = 1-…Cl: 5+

O: 2-, 4(2-) + Cl = 1-…Cl: 7+

O: 2-, 2(2-) + I = 1-…I: 3+

O: 2-, 4(2-) + Cr = 2-…Cr: 6+

Fe: 2+ (ion) PO4:3- (ion)….O:2-, 4(2-) + P = 3-, P: 5+

Co: 2+ (ion) SO4:2- (ion)….O:2-, 4(2-) + S = 2-, S: 6+

Assigning Oxidation Numbers Review

• Try these…MnO4-, Cr2O7

2-, C2O42-

• (MnO4)-

O = -2, so [4(-2) + Mn = -1]

Mn = +7

• (Cr2O7)2-

O = -2, so [7(-2) + 2Cr = -2]

2Cr = 12, therefore…

• (C2O4)2-

O = -2, so [2C + 4(-2) = -2]2C = 6, therefore…

Cr = +6

C = +3

Oxidation-Reduction Reactions

• Two separate reactions occurring simultaneously• Oxidation: oxidation # of an atom increases

• e.g. Fe(s) → Fe3+(aq)

• Reduction: oxidation # of an atom is “reduced”• e.g. O2(g) → O2-(aq)

When occurring together…• Fe(s) + O2(g) → Fe3+(aq) + O2-(aq)

• This is the redox reaction responsible for rust!

But, how do we balance this?

(ox # goes from 0 → +3)

(oxidation # goes from 0 → -2)

Balancing by Half-Reactions*in acidic solution

1. Assign oxidation states for each element.

2. Write separate half-reactions for the reduction/oxidation reactions.

3. Balance all the atoms EXCEPT O and H.

4. Balance the oxygen with water (H2O).

5. Balance the hydrogen with hydrogen ions (H+).

6. Balance the charge with electrons.

7. Multiply each half-reaction by an appropriate number to make the electrons equal.

8. Combine both reactions into one and cancel the e -

Balancing by Half-Reactions*in acidic solution

CH3OH (aq) + Cr2O72-(aq) → CH2O(aq) + Cr3+(aq)

1. Assign oxidation states.

C-2H4+O2- + (Cr2

6+O72-)2- → C0H2

+O2- + Cr3+

2. Write separate half-reactions for the reduction and oxidation reactions. (only keep charges that are changing…)

Ox: C-2H4O → C0H2O (C is going from -2 to 0)

Red: (Cr26+O7)2- → Cr3+

(Cr is being reduced from +6 to +3)

3. For each half reaction, balance all atoms EXCEPT O and H.

4. Balance the oxygen by adding water (H2O).

5. Balance the hydrogen by adding hydrogen ions (H+)

6. Balance the charge by adding electrons.…use the oxidation state as a guide

7. Multiply each half-reaction by an appropriate number to make the electrons equal.

8. Add the reactions together and cancel e-/simplify.

Balancing the half reactions…

+ 2H+ + 2e-Ox: C2-H4O → C0H2ORed: (Cr2

6+O7)2- → Cr3+2 + 7H2O14H+ +6e- +

3 ( )

3CH4O → 3CH2O + 6H+ + 6e-

3 CH4O + + Cr2O72- → 3 CH2O + 2 Cr3+ + 7 H2O8 H+

Red: (Cr26+O7)2- → Cr3+

Practice Balancing Redox Reactions

Unbalanced reaction (in acid):

MnO4 + Fe2+ Mn2+ + Fe3+

Balanced Reduction half-reaction:

8H+ + MnO4 + 5e Mn2+ + 4H2O

Balanced Oxidation half-reaction:

Fe2+ Fe3+ + e

Balanced overall reaction:

8H+ + MnO4 + 5Fe2+ Mn2+ + 5Fe3+ + 4H2O

5( )

Balancing by Half-Reactions*in basic solution

1. Assign oxidation states.

2. Write separate half-reactions for the reduction/oxidation reactions.

3. Balance all the atoms EXCEPT O and H.

4. Balance the oxygen by adding water (H2O).

5. Balance the hydrogen by adding H+.

6. Balance the charge by adding electrons.

7. Multiply each half-reaction by an appropriate number to make the electrons equal.

8. Combine both reactions into one and cancel.

9. Add OH- to both sides to cancel out H+ and create H2O. Simplify further, if necessary.

Balancing by Half-Reactions(in basic solution)

Let’s balance a previous example in basic solution

Remember, it is all the same steps up to this point

3CH4O + 8H+ + Cr2O72- → 3CH2O + 2Cr3+ + 7H2O

3CH4O + + Cr2O72- → 3CH2O + 2Cr3+ + 7H2O + 8OH-

3CH4O + H2O + Cr2O72- → 3CH2O + 2Cr3+ + 8OH-

+ 8OH- + 8OH-

8H2O

Practice Balancing Basic Redox Rxns

Unbalanced reaction:ClO + Zn Cl- + Zn2+

Balanced Reduction half-reaction:2e- + 2H+ + ClO- Cl- + H2O

Balanced Oxidation half-reaction: Zn Zn2+ + 2e-

Balanced overall reaction (acidic):

2H+ + ClO + Zn Zn2+ + Cl- + H2O

Balanced overall reaction (basic):

H2O + ClO + Zn Zn2+ + Cl- + 2OH-

Ca

Activity Series

Foiled again:Aluminum is knocked out by Calcium

Element Reactivity

LiRbKBaCaNaMgAlMnZnCrFeNiSnPbH2

CuHgAgPtAu

Halogen Reactivity

F2

Cl2Br2

I2

PrintableVersion

ofActivitySeries

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, aluminum is less reactive than magnesium.

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.

More SR Reactions…

FeCl2 + Cu

MgBr2 + Cl2

“Magic blue-earth”

Zinc in nitric acid

2

A + BC AC + B

General Form

Zn(NO3)2 + H2

Can Fe replace Cu? Yes

LiRbKBaCaNaMgAlMnZnCrFeNiSnPbH2

CuHgAgPtAu

F2

Cl2Br2

I2

Can Zn replace H? Yes

Can Br replace Cl? No

NO REACTION

Fe + CuCl2

Zn + HNO3

MgCl2 + Br2

Activity Series

Double Replacement Reactions

K2CO3 (aq)Potassium carbonate

BaCl2 (aq)Barium chloride

2 KCl (aq)Potassium chloride

BaCO3 (s)Barium carbonate

+ +

Formation of a solid precipitate:

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

TABLE OF SOLUBILITIES IN WATER

aluminum ss s n s n i s s i s d

ammonium s s s s s s s s s s s

barium s s i s i s s s i i d

calcium s s i s s ss s s i ss d

copper (II) s s i s i i n s i s i

iron (II) s s i s n i s s i s i

iron (III) s s n s i i n s i ss d

lead s ss i ss i i ss s i i i

magnesium s s i s s i s s i s d

mercury (I) ss i i i ss n i s i ss i

mercury (II) s ss i s ss i i s i d i

potassium s s s s s s s s s s s

silver ss i i i ss n i s i ss i

sodium s s s s s s s s s s s

zinc s s i s s i s s i s i

acet

ate

brom

ide

carb

onat

e

chlo

ride

chro

mat

e

hydr

oxid

e

iodi

de

nitr

ate

pho

sph

ate

sulfa

te

sulfi

de

i = insolubless = slightly solubles = solubled = decomposesn = not isolated

SOLIDSOLIDAQUEOUS

Legend

Solubility Rules

1. Most nitrates are soluble.

2. Most salts containing Group I ion and ammonium ion, NH4+,

are soluble.

3. Most chloride, bromide, and iodide salts are soluble, except Ag+, Pb2+ and Hg2

2+.

Ohn-Sabatello, Morlan, Knoespel, Fast Track to a 5 Preparing for the AP Chemistry Examination 2006, page 91

4. Most sulfate salts are soluble, except BaSO4, PbSO4, Hg2SO4, and CaSO4.

5. Most hydroxides except Group 1 and Ba(OH)2, Sr(OH)2, and Ca(OH)2 are only slightly soluble.

6. Most sulfides, carbonates, chromates, and phosphates are only slightly soluble.

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

KNO3

Na+K+

potassium chloride + sodium nitrate

KCl(aq)

Using the SOLUBILITY TABLE:sodium chloride is soluble potassium nitrate is soluble

NaNO3 (aq)

K+ Na+ Cl - NO3- Cl- NO3

-

NaCl

sodium chloride potassium nitrate+

(aq) (aq)

Predict if a reaction will occur when you combine aqueous solutions of potassium chloride and sodium nitrate…

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

Notice that nothing has really changed because the ions are still dissolved in water!

NR

Pb2+NO3

–

Na+ CI–

Ions in Aqueous Solution Expt.

Pb(NO3)2(s) Pb(NO3)2(aq)

Pb2+(aq) + 2 NO3–(aq)

addwater

NaCI(s)

+ H2O(l)

Dissociation reactions: solids mixed with water dissociate into ions+ H2O(l)

Na+(aq) + CI–(aq)

• Mix them and get…• Balance to get complete ionic equation…• Cancel spectator ions to get net ionic equation…

NaCI(aq)

NO3–

Pb2+

NO3–

NO3–

in solution,aqueous, soluble,dissolved

Na+ CI–

ChemThink

Mix them and get…

Pb2+(aq) + 2 NO31–(aq) + 2 Na1+(aq) + 2 CI–(aq) PbCI2(s) + 2 NO3

1–(aq) + 2 Na1+(aq)

Pb2+(aq) + 2 CI–(aq) PbCI2(s)

Pb(NO3)2(aq) + NaCI(aq)

Balance to get complete ionic equation…separate anything (aq)

Cancel spectator ions to get net ionic equation…

SolubilityChart

Solid (precipitate)

in solution(aqueous)

Pb2+

NO3–

Na+ CI–

NO3–

Na+ CI–

Pb2+

NO3–

Na+

CI–

NO3–

Na+

CI–

PbCI2 + NaNO3(s) (aq) 2 2

Pre-lab:

1. What ions are present in the following solutions?

NaCl(aq) ____________________ AgNO3(aq) ____________________

2. When these solutions are mixed together, a precipitate is seen. What are the new combinations of ions that could have formed the precipitate?

____________________ and ____________________

3. Using the solubility table, which new combination will form a precipitate?

____________________

4. Which new combination will remain in solution?

____________________

5. Write the complete ionic equation for this reaction. Be sure to indicate the correct phase (reaction condition) for each reactant and each product.

6. Write the net ionic equation for this reaction by canceling out spectators. Again, include the phases (reaction conditions).

7. Explain why you would expect no reaction between solutions of KOH(aq) and NaOH(aq).

Na+(aq) Cl-(aq) Ag+(aq) NO3-(aq)

Na+(aq) Cl-(aq)Ag+(aq)NO3-(aq)

Cl-(aq)Ag+(aq)

Na+(aq) NO3-(aq)

AgCl(s)

Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) + Na+(aq) + NO3

-(aq)AgCl(s)

+ Cl-(aq)Ag+(aq) AgCl(s)

When the cations switch places they end with a hydroxide (no new combination is formed)

Ba2+

OH–

OH–

NO3–

NO3–

Mix together Zn(NO3)2(aq) and Ba(OH)2(aq):

Zn2+(aq) + 2 NO3–(aq) Ba2+(aq) + 2 OH–(aq)

Ba(OH)2(aq)Zn(NO3)2(aq)

Balance to get complete ionic equation…

Zn2+

Zn(NO3)2(aq) + Ba(OH)2(aq) Zn(OH)2(s) + Ba(NO3)2(aq)

Zn2+(aq) + 2 NO3–(aq) + Ba2+(aq) + 2OH–(aq) Zn(OH)2(s) + 2 NO3

–(aq) + Ba2+(aq)

Mix them and get…

Zn2+(aq) + 2 OH–(aq) Zn(OH)2(s)

Cancel spectator ions to get net ionic equation…

SolubilityChart

Separation of Cations

You have a solution containing Fe2+, Cu2+, Ba2+, Ag+

and K+ ions. By what means could you separate these ions from each other?

• In Chem I, we discussed various ways to separate things…• Distillation• Filtration• Centrifugation• Reactivity

Will any of these work to separate aqueous ions?

Separation of CationsFe2+, Cu2+, Ba2+, Ag+, K+ (aq)

+ Cl- (aq)

AgCl(s) Fe2+, Cu2+, Ba2+, K+ (aq)

BaSO4(s) Fe2+, Cu2+, K+ (aq)

CuCrO4(s) Fe2+, K+ (aq)

+ SO42- (aq)

+ CrO42- (aq)

FeS, Fe3(PO4)2 , Fe(OH)2 , or FeCO3(s) K+(aq)

+ S2-, PO43- OH- or

CO32- (aq)

Separation of Cations

Pb2+, Ca2+, Zn2+, NH4+ (aq)

+ CrO42-(aq)

PbCrO4 (s) Ca2+, Zn2+, NH4+ (aq)

CaSO4(s) Zn2+, NH4+ (aq)

NH4+ (aq)

+ SO42- (aq)

ZnCO3, Zn3(PO4)2 , Zn(OH)2, or ZnS(s)

+ CO32-, PO4

3- OH- or S2- (aq)

Try this example on your own…

Summary of Classes of Reactions

Chemical reactions

Double Replacementreactions

Acid-BaseReactions

Oxidation-ReductionReactions

Combustion Reactions

Single Replacementreactions

Synthesis and Decomposition

reactions