Download Report

11/30/2010
Unit 7 Topics
Unit 7 Lecture Notes
Balancing and Classifying
Chemical Reactions
• Chemical reactions
– Definition, driving force, evidence for
• Chemical equations
– Writing
• Formulas, word equations
– Balancing
– Classifying
• 6 reaction types
Standards Correlation & Goals for Today
3a. Students know how to
describe chemical reactions
by writing balanced
equations
(1) What is a Chemical Rxn?
• A process in which one or more substances
are converted into new substances with
different physical and chemical properties
What are they?
Why and how do they happen?
How do you describe them with words and symbols?
(1) What is a Chemical Rxn?
• This happens when bonds between atoms in
compounds break, atoms rearrange, & new
bonds form new substances!
(2) Why Rxns Happen
• Atoms bond b/c they are more stable together
than alone
• Atoms are stable when they have 8 e- in their
valence shell so they gain, lose, or share e- with
other atoms to achieve this. (Octet Rule)
Covalent Bond (C & H sharing e-)
Ionic Bond (Na losing e-, Cl ganing e-)
1
11/30/2010
(4) Evidence That a Rxn Occurred
(3) How Reactions Start
• Some rxns require more energy than others
• Some rxns are spontaneous
– Iron + oxygen = rusting
• Some rxns need a “kick”
– Paper + oxygen + spark = burning
•
•
•
•
Color changes
Solid forms (“precipitate”)
Bubbles are produced
Heat is produced or absorbed
• Some rxns need constant energy
– Breaking down water molecules
by electrolysis
Electrolysis Demo
(5) Anatomy of a Chemical Equation
(5) Anatomy of a Chemical Equation
• A chemical equation describes a rxn
using symbols instead of words
• Reactants
– “Methane and
oxygen combine to
produce carbon
dioxide and water”
is symbolized as:
CH4 + O2 CO2 + H2O
(5) Anatomy of a Chemical Equation
• Coefficients
– Whole # before element or compound formula
– Tells how many moles there are
• Subscripts
– Whole # written below and to the right of each
atom in a molecule
or compound
– Tells how many of
Subscripts
each type of atom
there is
– Substances that enter into the chemical rxn
– Left side of arrow
• Products
– Substances that are produced by a rxn
– Right side of
arrow
(5) Anatomy of a Chemical Equation
• Symbols to know
– + means “reacts with”
– means “yields” or “produces”
–
means the rxn is reversible
– Catalysts (something that speeds up the rxn but is
not part of the rxn) are written above the arrow
•
or “heat” means heat was the catalyst
• Could also be a chemical substance (e.g, NaI)
2
11/30/2010
(5) Anatomy of a Chemical Equation
• States of matter are written to the right of the
substances
– (s) means solid (may also see ↓)
• If on the product side, called a precipitate; may be soluble,
insoluble, or slightly soluble
(6) Word Equations
• To move between symbols and words, you
must know the symbols and remember your
nomenclature rules!
• Write detailed word equations for these rxns
– (l) means liquid
– (g) means gas (may also see ↑)
– (aq) means aqueous (dissolved in water; “in solution”)
• HCl (g) = hydrogen chloride gas
• HCl (aq) = hydrochloric acid
(6) Word Equations
• Wrtite the formula in which magnesium reacts
with nitrogen to produce magnesium nitride
UNIT 7 WORKSHEET #1
Word Equations
• Write the formula in which silver (I) nitrate
reacts with copper to form copper (II) nitrate
and silver.
Standards Correlation & Goals for Today
3a. Students know how to
describe chemical reactions
by writing balanced
equations
What are they?
Why do equations have to be balanced?
What are the procedures for balancing them?
(7) Skeleton vs. Balanced Equations
• To show the true ratio of substances involved
and produced in a rxn, equations must be
“balanced”
– Balanced means that there
are the same # of each type
of atom on both sides
of the arrow
– Unbalanced
equations are
called skeleton
equations
3
11/30/2010
(8) Balanced Equations &
The Law of Conservation of Matter
• Balanced chemical equations reflect the law of
conservation of matter
(8) Balanced Equations &
The Law of Conservation of Matter
• Is this equation balanced?
– mass cannot be gained or lost in a chemical rxn
• Same # & type of every atom exist before & after rxn
– They’re just rearranged to make new substances
• Total mass of reactants = Total mass of products
(9) How to Balance Equations
• Change the coefficients!
– The whole #s written before the formulas that
indicate the # of atoms/molecules/moles present
• No decimal coefficients allowed
• Never change the subscripts!
– This would change the identity of the substance!
• H2O is not the same thing as H2O2
(9) How to Balance Equations
•
•
Na +
Cl2 NH4NO2 (9) How to Balance Equations
• Balancing is mostly trial and error but the
basic procedure is this:
– Count the # of atoms on each side
– Starting with the elements that occur in only 1
substance on each side,
– Balance each kind of atom in succession and then
adjust coefficients
as needed
(9) How to Balance Equations
• The balanced equation must have coefficients
in the lowest whole # ratio possible
NaCl
N2 +
• Is this equation balanced?
– Is this equation balanced correctly?
– 6 Ca + 4 AlCl3 -> 4 Al + 6 CaCl2
H2O
• Stuck?
•
FeCl2 +
Na3PO4 NaCl +
Fe3(PO4)2
– Check your formulas
– An incorrect subscript
can make an equation
impossible to balance
4
11/30/2010
Standards Correlation & Goals for Today
UNIT 7 WORKSHEET #2
Balancing Equations
3a. Students know how to
describe chemical reactions
by writing balanced
equations
What types of reactions are there?
How do you recognize each type?
How do you predict the products of a reaction?
(10) Classifying Chemical Reactions
• Most chemical reactions can be classified into
6 general categories
– Combination (synthesis)
– Decomposition
– Single replacement
– Double replacement
– Combustion
– Acid-Base
(11) Combination/Synthesis Rxn
• Two or more reactants combine to form a
single product
• A + B AB
– A, B stand for elements or compounds
– AB stands for a compound consisting of both A & B
• Na + Cl NaCl
• 4 Fe + 3 O2 2 Fe2O3
Demo Kit – Rxn Types
(12) Decomposition Rxn
• A single compound is broken down into 2
or more smaller compounds or elements
• AB A + B
• The reverse of a combination rxn
• 2H2O 2H2 + O2
• CaCO3 CaO + CO2
(13) Single Replacement Rxn
• An uncombined element replaces one that is
in a compound
• Reactants are always 1 element and 1
compound
• A + BX B + AX
• Mg + CuSO4 MgSO4 + Cu
• Cu + ZnNO3 CuNO3 + Zn
5
11/30/2010
(13) Single Replacement Rxn
• Predict the products in the following single
replacement reactions:
• Fe + CuSO4 • Zn + CuCl2 • Cl2 + NaBr • Mg + HCl (14) Double Replacement Rxn
• Atoms or ions from 2 different compounds
replace each other
• There are 2 compounds as reactants
+ 2 compounds as products
– Reactants usually 2 ionic comp.’s in aqueous soln
• AX + BY AY + BX
• CaCO3 + 2 HCl CaCl2 + H2CO3
• AgNO3 + NaCl AgCl + NaNO3
(15) Combustion Rxn
• Reaction in which something burns
• O2 must be one of the reactants
• CO2 and H2O must be among the products
– In the complete combustion of a hydrocarbon (CxHy), they
are the only products
• Results in so much heat being released, a
flame is produced
• 2 C8H18 + 25 O2 16 CO2 + 18 H2O
(13) Single Replacement Rxn
• Not all elements will replace all other
elements
• A more chemically reactive
element will replace a
less reactive element
– Fe + CuSO4 FeSO4 + Cu
– Iron is more reactive than
copper. Will the fwd rxn
occur/ The reverse?
(14) Double Replacement Rxn
• Predict the products in the following double
replacement reactions:
• BaCl2(aq) + Na2CO3(aq)
• Al2(SO4)3(aq) + 2Na3PO4(aq)
• 3BaCl2(aq) + 2H3PO4(aq)
(16) Acid-Base Rxn
• Acid + Base = Water + Salt
– Acid: H+ is cation
– Base: OH- is anion
– Water: H2O
– Salt: ionic compound that results from this
type of double-replacement rxn
• HCl(aq) + NaOH(aq) --> NaCl(aq) + H2O(l)
6
11/30/2010
(16) Acid-Base Rxn
• Strong Acids and Bases
• 100% of them break apart into ions (“ionize”)
when in solution.
• HCl H+ + Cl-
UNIT 7 WORKSHEET #3
Classifying Reactions, Predicting Products
• Examples of strong acids
• HCl, HBr, HI, HClO3, HClO4,HBrO3, HBrO4 HIO3,
HIO4, H2SO4, HNO3
• Examples of strong bases
• NaOH, KOH, CsOH, Ca(OH)2
Unit 7 Test Review Game
EXTRA SLIDES
(15) Double Replacement Rxn
Solubility Rules
• Not all pairs of compounds will react
• Most DR rxns will not occur unless both
compounds are dissolved in water so they can
separate into ions
• DR rxn is likely to proceed if at least 1 of the
products is
– a molecular compound
– a precipitate (a solid that separates from a solution), or
– a gas that can’t be dissolved in water
7
11/30/2010
8