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Week -10: Lecture 19 & 20Chemical Chemical FormulasFormulas

• We know a Chemical SymbolChemical Symbol identifies an element.

• A FormulaFormula is a combination of symbols that identifies a compound, an ion, or a molecule of an element.Ex:formula of compounds = HClHCl, (NH(NH44))33POPO4 4 , HH22OOformula of ions = OHOH-- , (NH(NH44))

++

formula of molecule = HH22, NN22, OO22, FF22, ClCl22, BrBr22, II22, PP44, SS88, OO33

• A Formula also indicates the relative quantities of elements contained in the compound or ion and implies some kind of chemical bonding between the atoms

FormulaFormula ––Subscripts Subscripts vs. vs. CoefficientsCoefficients

• The subscripts tell you how many atoms of a particular element are in a compound.

• The coefficient tells you about the quantity, or number, of molecules of the compound.

Give the example of (NH4)3PO4 and some others. Ref: Read page-128 to 132 of Goldberg – fundamental of Chemistry

Chemical Reaction

A process in which at least one new substance is produced as a result of chemical change.

– Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken

– Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.

– Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction

– Chemical equations show the conversion of reactantsreactants (the molecules shown on the left of the arrow) into productsproducts (the molecules shown on the right of the arrow).

• + sign separates molecules on the same side

• The arrow is read as “yields”

• Example

C + O2 CO2

• This reads “carbon plus oxygen react to yield carbon dioxide”

Chemical Reaction Chemical Reaction EquationEquation

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Reading& writing A Chemical Equation

4 NH3 + 5 O2 4 NO + 6 H2O

Four molecules of NH3 react with five molecules O2

to produce four molecules NO and six molecules of H2O

or

Four moles NH3 react with 5 moles O2 to producefour moles NO and six moles H2O

Chemical symbols give a “before-and-after” picture of a chemical reaction

Reactants Products

MgO + C CO + Mg

magnesium oxide carbon-monoxide

reacts with carbon and magnesium to form

• Solid (s)

•Liquid (l)

•Gas (g)

•Aqueous solution (aq)

•Catalyst H2SO4

•Escaping gas ()

•Precipitate solid ( )

•Change of temperature ()

Ex:

4 Al(s) + 3 O4 Al(s) + 3 O22(g) (g) ------> 2 Al> 2 Al22OO33(s)(s)

Symbols Used in EquationsSymbols Used in Equations

A Balanced Chemical Equation

Same numbers of each type of atom on each side of the equation

Al + S Al2S3 Not Balanced

2Al + 3S Al2S3 Balanced

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 It must have the same number of atoms of the number of atoms of the

same kind on both sides.same kind on both sides. Lavoisier, 1788Lavoisier, 1788

Chemical EquationsChemical Equations

Balance Equations with Coefficients

Coefficients in front of formulas balance

each type of atom

4NH3 + 5O2 4NO + 6H2O

4 N = 4 N

12 H = 12 H

10 O = 10 O

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–When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but

you may notnot change the subscripts.

•Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Balancing EquationsBalancing EquationsThere are four basic steps to balancing a chemical equation.1. Write the correct formula for the reactants and the

products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!

2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if:

– The numbers of atoms on both sides of the equation are now balanced.

– The coefficients are in the lowest possible whole number ratios. (reduced)

Steps to Balancing EquationsSteps to Balancing Equations

Some Suggestions to Help YouSome Suggestions to Help You

Some of Helpful Hints for balancing equations:

• Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last.

• IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

Read from Page 207 to Page 213 of Goldberg- Fundamentals of Chemistry

Ex: Steps in Balancing An Equation

Fe3O4 + H2 Fe + H2O

Fe: Fe3O4 + H2 3 Fe + H2O

O: Fe3O4 + H2 3 Fe + 4 H2O

H: Fe3O4 + 4 H2 3 Fe + 4 H2O

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Learning Check E5

Balance each equation. The coefficients for each equation are read from left to right

A. Mg + N2 Mg3N2

1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1

B. Al + Cl2 AlCl3

1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2

C. Fe2O3 + C Fe + CO2

1) 2, 3, 2,3 2) 2, 3, 4, 3 3) 1, 1, 2, 3

D. Al + FeO Fe + Al2O3

1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1

E. Al + H2SO4 Al2(SO4)3 + H2

1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

A) ____A) ____CC33HH88(g) + _____ O(g) + _____ O22(g) (g) -------->>

_____CO_____CO22(g) + _____ H(g) + _____ H22O(g)O(g)

B) ____B) ____BB44HH1010(g) + _____ (g) + _____ OO22(g) (g) ------>>

___ B___ B22OO33(g) + (g) + _____H_____H22O(gO(g))

C) C) NaNa33POPO44 + Fe+ Fe22OO33 -------->>

NaNa22O + FePOO + FePO44

Types of Reactions

• There are five types of chemical reactions we will talk about:

1. Synthesis reactions

2. Decomposition reactions

3. Combustion reactions

4. Single displacement reactions

5. Double displacement reactions

• You need to be able to identify the type of reaction and predict the product(s)

1. Synthesis reactions

• Synthesis reactions occur when two substances (generally elements) combine and form a compound. (Sometimes these are called combination or addition reactions.)

reactant + reactant 1 product

• Basically: A + B AB• Example: 2H2 + O2 2H2O

• Example: C + O2 CO2

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Practice

• Predict the products. Write and balance the following synthesis reaction equations.

• Sodium metal reacts with chlorine gas

Na(s) + Cl2(g)

• Solid Magnesium reacts with fluorine gas

Mg(s) + F2(g)

• Aluminum metal reacts with fluorine gas

Al(s) + F2(g)

Combination (Synthesis) Reaction

oxidation of metals- rust, burning

burning of Mg metal

2Mg(s) + O2(g) 2MgO(s)

rusting of iron

4Fe(s) + 3O2(g) 2Fe2O3(s)

burning non-metals

C(s) + O2(g) CO2(g)

• A compound may combine with free element to from another compound

2CrCl2(s) + Cl2(g) 2CrCl3(s)

• Two compound may combine together to form a single compound.

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

CaO(s) + H2O(l) Ca(OH)2 (s)

2. Decomposition Reaction

a compound is broken down into simpler compounds or constituent elements

1 Reactant Product + Product• In general:

XY X + Y requires energyExample:

HYDROLYSIS - decompose waterwith electricity - hydrogen

fuel cells2H2O(l) 2H2(g) + O2(g)

decomposition of mercury(II) oxide2HgO(s) 2Hg(s) + O2(g)

electricity

Decomposition Exceptions

• Carbonates and chlorates are special case decomposition reactions that do not go to the elements.• Carbonates (CO3

2-) decompose to carbon dioxide and a metal oxide

• Example: CaCO3 CO2 + CaO

• Chlorates (ClO3-) decompose to oxygen gas

and a metal chloride• Example: 2 Al(ClO3)3 2 AlCl3 + 9 O2

2KClO3(s) 2KCl(s) + 3O2(g)MnO2

Heat

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Practice

Identify the type of reaction for each of the following synthesis or decomposition reactions, and write the balanced equation:

Solid Lead (IV) oxide decomposesPbO2(s)

Aluminum nitride decomposes

AlN(s)

N2(g) + O2(g)

BaCO3(s)

Co(s)+ S(s)

NH3(g) + H2CO3(aq)

NI3(s)

(make Co be +3)

Nitrogen monoxide

3. Combustion Reactions

• Combustion reactions occur when a hydrocarbon reacts with oxygen gas.

• This is also called burning!!! In order to burn something you need the 3 things in the “fire triangle”:1) A Fuel (hydrocarbon)2) Oxygen to burn it with3) Something to ignite the reaction (spark)

Combustion Reactions• In general:

CxHy + O2 CO2 + H2O

• Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide)

• Combustion is used to heat homes and run automobiles (octane, as in gasoline, is C8H18)

• Example

C5H12 + 8O2 5CO2 + 6H2O

• Write the products and balance the following combustion reaction:

C10H22 + O2

4. Single Replacement Reactions

• Single Replacement Reactions occur when one element replaces another in a compound.

• A metal can replace a metal (+) ORa nonmetal can replace a nonmetal (-).

• element + compound product + product

A + BC AC + B (if A is a metal) OR

A + BC BA + C (if A is a nonmetal)

Ex:

Zn(s) + CuSO4(aq) ZnSO4 (aq) + Cu(s)Zinc (metal) displaces copper (metal)

F2(g) + CuCl2(aq) CuF2(aq) + Cl2(g)

Fluorine (non metal) displaces chlorine (nonmetal)

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Single Replacement Reactions• Write and balance the following single

replacement reaction equation:

• Zinc metal reacts with aqueous hydrochloric acid

Zn(s) + HCl(aq) ZnCl2 + H2(g)

Note: Zinc replaces the hydrogen ion in the reaction

• Sodium chloride solid reacts with fluorine gas

NaCl(s) + F2(g) NaF(s) + Cl2(g)Note that fluorine replaces chlorine in the compound

• Aluminum metal reacts with aqueous copper (II) nitrate

Al(s)+ Cu(NO3)2(aq)

Practice Example 8.8: (from Goldberg- Page 218)

Rel

ativ

e re

acti

vity

of

un

com

bin

ed

el

emen

ts

Metals Nonmetals

Most Active

Least Active

LiK

BaCaNaMgAl

MnZnCrFeCoNiSnPbH2

CuHgAgPtAu

F2

O2

Cl2

Br2

I2

Most Active

Least Active

5. Double Replacement Reactions•• Reaction that has the interchanging of two ions from Reaction that has the interchanging of two ions from

two different compounds.two different compounds.

• Compound + compound product + product

•• general general form: form:

AB + CDAB + CD--------> AD + CB> AD + CB

•• ExampleExample::

PbPb(NO3)2 + 2 KI (NO3)2 + 2 KI --------> PbI2 + 2 KNO3> PbI2 + 2 KNO3

• During a reaction the cations (or anions) switch places.

• The products usually consist of a precipitate.

• Example:

AgNO3(aq) + NaCl(s) AgCl(s) + NaNO3(aq)

• Another example:

K2SO4(aq) + Ba(NO3)2(aq) KNO3(aq) + BaSO4(s)

Practice• Predict the products. Balance the equation

1. HCl(aq) + AgNO3(aq)

2. CaCl2(aq) + Na3PO4(aq)

3. Pb(NO3)2(aq) + BaCl2(aq)

4. FeCl3(aq) + NaOH(aq)

5. H2SO4(aq) + NaOH(aq)

6. KOH(aq) + CuSO4(aq)

• State the type, predict the products, and balance the following reactions:

1. BaCl2 + H2SO4

2. C6H12 + O2

3. Zn + CuSO4

4. Cs + Br2

5. FeCO3