1. 12/2/2008
Formula –
Week -10: Lecture 19 & 20
Subscripts vs. Coefficients
Chemical Formulas
• The subscripts
• We know a Chemical Symbol identifies an tell you how
element. many atoms of
• A Formula is a combination of symbols that a particular
identifies a compound, an ion, or a molecule of element are in
an element. a compound.
Ex:
• The
formula of compounds = HCl (NH4)3PO4 , H2O
HCl, coefficient
formula of ions = OH- , (NH4)+ tells you about
formula of molecule = H2, N2, O2, F2, Cl2, the quantity,
Br2, I2, P4, S8, O3 or number, of
• A Formula also indicates the relative quantities molecules of
of elements contained in the compound or ion
and implies some kind of chemical bonding the
between the atoms compound.
Give the example of (NH4)3PO4 and some others. Ref: Read page-128 to 132 of
Goldberg – fundamental of Chemistry
Chemical Reaction Equation
Chemical Reaction
– Chemical equations show the conversion
A process in which at least one new of reactants (the molecules shown on the
substance is produced as a result of left of the arrow) into products (the
chemical change. molecules shown on the right of the
arrow).
– Chemical reactions occur when bonds
• + sign separates molecules on the same
between the outermost parts of atoms are
side
formed or broken
• The arrow is read as “yields”
– Chemical reactions involve changes in matter,
the making of new materials with new • Example
properties, and energy changes. C + O2  CO2
– Symbols represent elements, formulas • This reads “carbon plus oxygen react to
describe compounds, chemical equations yield carbon dioxide”
describe a chemical reaction
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2. 12/2/2008
Reading& writing A Chemical Equation
4 NH3 + 5 O2 4 NO + 6 H2O
Symbols Used in Equations
Four molecules of NH3 react with five molecules O2
to produce four molecules NO and six molecules of • Solid (s)
H2O • Liquid (l)
or
• Gas (g)
Four moles NH3 react with 5 moles O2 to produce
four moles NO and six moles H2O • Aqueous solution (aq)
H SO
Chemical symbols give a “before-and-after” • Catalyst 2 4
picture of a chemical reaction
• Escaping gas ()
Reactants Products
• Precipitate solid ( )
MgO + C CO + Mg
• Change of temperature ()
magnesium oxide carbon-monoxide Ex:
to form
reacts with carbon and magnesium 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)
--->
A Balanced Chemical Equation Balance Equations with
Same numbers of each type of atom on each Coefficients
side of the equation
Coefficients in front of formulas balance
Al + S Al2S3 Not Balanced each type of atom
2Al + 3S Al2S3 Balanced
4NH3 + 5O2 4NO + 6H2O
Chemical Equations 4N = 4N
Because of the principle of the 12 H = 12 H
conservation of matter,
10 O = 10 O
an equation must be
balanced.
It must have the same
number of atoms of the
same kind on both sides. Lavoisier, 1788
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3. 12/2/2008
Steps to Balancing Equations
Balancing Equations
There are four basic steps to balancing a chemical
– When balancing a chemical reaction equation.
1. Write the correct formula for the reactants and the
you may add coefficients in front of the products. DO NOT TRY TO BALANCE IT YET!
compounds to balance the reaction, but You must write the correct formulas first. And most
importantly, once you write them correctly DO NOT
CHANGE THE FORMULAS!
you may not change the subscripts. 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.
• Changing the subscripts changes the 3. Determine where to place coefficients in front of
compound. Subscripts are formulas so that the left side has the same number of
atoms as the right side for EACH element in order to
determined by the valence electrons balance the equation.
(charges for ionic or sharing for 4. Check your answer to see if:
– The numbers of atoms on both sides of the
covalent) equation are now balanced.
– The coefficients are in the lowest possible
whole number ratios. (reduced)
Some Suggestions to Help You Ex: Steps in Balancing An
Some of Helpful Hints for balancing equations: Equation
• Take one element at a time, working left to
right except for H and O. Save H for next Fe3O4 + H2 Fe + H2O
to last, and O until last.
• IF everything balances except for O, and Fe: Fe3O4 + H2 3 Fe + H2O
there is no way to balance O with a whole
number, double all the coefficients and try
again. (Because O is diatomic as an O: Fe3O4 + H2 3 Fe + 4 H2O
element)
• (Shortcut) Polyatomic ions that appear on H: Fe3O4 + 4 H2 3 Fe + 4 H2O
both sides of the equation should be
balanced as independent units
Read from Page 207 to Page 213 of Goldberg- Fundamentals
of Chemistry
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4. 12/2/2008
Learning Check E5 Balancing
Balance each equation. The coefficients for
Equations
each equation are read from left to right A) ____C3H8(g) + _____ O2(g) ---->
____C ---->
A. Mg + N2 Mg3N2
1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1 _____CO2(g) + _____ H2O(g)
B. Al + Cl2 AlCl3
B) ____B4H10(g) + _____ O2(g) --->
____B --->
1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2
C. Fe2O3 + C Fe + CO2 ___ B2O3(g) + _____H2O(g)
O(g)
1) 2, 3, 2,3 2) 2, 3, 4, 3 3) 1, 1, 2, 3
D. Al + FeO Fe + Al2O3
C) Na3PO4 + Fe2O3 ---->
---->
1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1
E. Al + H2SO4 Al2(SO4)3 + H2
Na2O + FePO4
1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3
1. Synthesis reactions
Types of Reactions
• There are five types of chemical • Synthesis reactions occur when two
reactions we will talk about: substances (generally elements)
1. Synthesis reactions combine and form a compound.
2. Decomposition reactions (Sometimes these are called
3. Combustion reactions combination or addition reactions.)
4. Single displacement reactions
reactant + reactant  1 product
5. Double displacement reactions
• Basically: A + B  AB
• You need to be able to identify the type
of reaction and predict the product(s) • Example: 2H2 + O2  2H2O
• Example: C + O2  CO2
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5. 12/2/2008
Combination (Synthesis) Reaction
Practice oxidation of metals- rust, burning
burning of Mg metal
• Predict the products. Write and balance 2Mg(s) + O2(g) 2MgO(s)
the following synthesis reaction rusting of iron
equations. 4Fe(s) + 3O2(g) 2Fe2O3(s)
• Sodium metal reacts with chlorine gas burning non-metals
C(s) + O2(g) CO2(g)
Na(s) + Cl2(g) 
• A compound may combine with free
• Solid Magnesium reacts with fluorine gas element to from another compound
Mg(s) + F2(g)  2CrCl2(s) + Cl2(g) 2CrCl3(s)
• Aluminum metal reacts with fluorine gas • Two compound may combine together to
form a single compound.
Al(s) + F2(g) 
MgO(s) + CO2(g) MgCO3(s)
CaO(s) + H2O(l) Ca(OH)2 (s)
2. Decomposition Reaction
Decomposition Exceptions
a compound is broken down into simpler
compounds or constituent elements
1 Reactant  Product + Product • Carbonates and chlorates are special case
• In general: decomposition reactions that do not go
XY  X + Y requires energy to the elements.
Example: • Carbonates (CO32-) decompose to carbon
dioxide and a metal oxide
HYDROLYSIS - decompose water • Example: CaCO3  CO2 + CaO
with electricity - hydrogen
fuel cells • Chlorates (ClO3-) decompose to oxygen gas
2H2O(l) 2H2(g) + O2(g) and a metal chloride
electricity • Example: 2 Al(ClO3)3  2 AlCl3 + 9 O2
MnO2
2KClO3(s) Heat
2KCl(s) + 3O2(g)
decomposition of mercury(II) oxide
2HgO(s) 2Hg(s) + O2(g)
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6. 12/2/2008
Practice
Identify the type of reaction for each of the
3. Combustion Reactions
following synthesis or decomposition
reactions, and write the balanced equation: • Combustion reactions
occur when a
Solid Lead (IV) oxide decomposes hydrocarbon reacts with
PbO2(s)  oxygen gas.
Aluminum nitride decomposes • This is also called
burning!!! In order to
AlN(s) 
burn something you
N2(g) + O2(g)  Nitrogen monoxide need the 3 things in the
BaCO3(s)  “fire triangle”:
1) A Fuel (hydrocarbon)
Co(s)+ S(s)  (make Co be +3) 2) Oxygen to burn it with
NH3(g) + H2CO3(aq)  3) Something to ignite
NI3(s)  the reaction (spark)
Combustion Reactions 4. Single Replacement Reactions
• In general:
CxHy + O2  CO2 + H2O • Single Replacement Reactions occur when
• Products in combustion are ALWAYS carbon one element replaces another in a
dioxide and water. (although incomplete burning compound.
does cause some by-products like carbon • A metal can replace a metal (+) OR
monoxide) a nonmetal can replace a nonmetal (-).
• Combustion is used to heat homes and run • element + compound product + product
automobiles (octane, as in gasoline, is C8H18)
A + BC  AC + B (if A is a metal) OR
• Example A + BC  BA + C (if A is a nonmetal)
C5H12 + 8O2  5CO2 + 6H2O Ex:
• Write the products and balance the Zn(s) + CuSO4(aq) ZnSO4 (aq) + Cu(s)
following combustion reaction: Zinc (metal) displaces copper (metal)
C10H22 + O2  F2(g) + CuCl2(aq) CuF2(aq) + Cl2(g)
Fluorine (non metal) displaces chlorine
(nonmetal)
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7. 12/2/2008
Single Replacement Reactions Metals Nonmetals
Relative reactivity of uncombined
Most Active Li F2 Most Active
• Write and balance the following single K O2
replacement reaction equation: Ba Cl2
• Zinc metal reacts with aqueous hydrochloric acid Ca Br2
Na I2 Least Active
Zn(s) + HCl(aq)  ZnCl2 + H2(g) Mg
Al
Note: Zinc replaces the hydrogen ion in the
elements
Mn
reaction Zn
Cr
• Sodium chloride solid reacts with fluorine gas Fe
NaCl(s) + F2(g)  NaF(s) + Cl2(g) Co
Ni
Note that fluorine replaces chlorine in the compound
Sn
• Aluminum metal reacts with aqueous copper Pb
(II) nitrate H2
Cu
Al(s)+ Cu(NO3)2(aq) Hg
Ag
Pt
Practice Example 8.8: (from Goldberg- Page 218) Least Active Au
5. Double Replacement Reactions Practice
• Reaction that has the interchanging of two ions from • Predict the products. Balance the equation
two different compounds. 1. HCl(aq) + AgNO3(aq) 
• Compound + compound  product + product 2. CaCl2(aq) + Na3PO4(aq) 
• general form: 3. Pb(NO3)2(aq) + BaCl2(aq) 
AB + CD----> AD + CB
CD----> 4. FeCl3(aq) + NaOH(aq) 
• Example:
Example: 5. H2SO4(aq) + NaOH(aq) 
Pb(NO3)2
Pb(NO3)2 + 2 KI ----> PbI2
----> + 2 KNO3 6. KOH(aq) + CuSO4(aq) 
• During a reaction the cations (or anions) switch • State the type, predict the products, and
places. balance the following reactions:
• The products usually consist of a precipitate. 1. BaCl2 + H2SO4 
• Example: 2. C6H12 + O2 
AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq) 3. Zn + CuSO4 
• Another example:
4. Cs + Br2 
K2SO4(aq) + Ba(NO3)2(aq)  KNO3(aq) + BaSO4(s)
5. FeCO3 
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