How Can We Describe Chemical Reactions?
Chemistry Unit 8: Guided Notes
New Skills
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Identify different types of reactions
Balance chemical reactions
Predict solubility and replacement reactions
Write net-ionic equations
Academic Language:
Aqueous solution
Chemical equation
Chemical reaction
Coefficient
Combustion reaction
Complete ionic equation
Decomposition reaction
Double-replacement reaction
Insoluble
Net ionic equation
Precipitate
Product
Reactant
Single-replacement reaction
Soluble
Solute
Solvent
Spectator ion
Synthesis reaction
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Unit 8 Homework:
CALM: http://calm.indiana.edu/
8.1 Reactions and Equations
 10 questions
8.2 Classifying Chemical Reactions
 10 questions
8.3 Solubility
 10 questions
8.4 Reactions in aqueous solutions
 10 questions
8.5 Accumulating Content and Skills
 10 questions
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Chemistry Unit 8: Learning Goals and Objectives
8.1 Reactions and Equations – Chemical reactions are represented by balanced
chemical equations
o Recognize evidence of chemical change.
o Represent chemical reactions with equations.
o Balance chemical equations
8.2 Classifying Chemical Reactions – There are four types of chemical reactions:
synthesis, combustion, decomposition, and replacement reactions.
o Classify chemical reactions
o Identify the characteristics of different classes of chemical reactions
8.3 Solubility - When solutions containing ionic substances are mixed, ions interchange
and can form solids.
o Identify new possible ionic compounds in a reaction
o Define the terms soluble and insoluble
o Predict solids based on solubility rules
8.4 Reactions in aqueous solutions - Double –replacement reactions occur between
substances in aqueous solutions and produce precipitates, water, or gases.
o Describe aqueous solutions
o Write complete ionic and net ionic equations for chemical reactions in aqueous
solutions.
o Predict whether reactions in aqueous solutions will produce a precipitate,
water, or a gas.
8.5 Accumulating Content and Skills:– Chemistry content is continuous and builds on
prior knowledge and skills. This section will combine this unit with previous units.
o
Apply knowledge and skills from previous units to content learned in this unit.
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8.1 Reactions and Equations – Chemical reactions are represented by balanced
chemical equations
Chemical Reactions
Objective: Recognize evidence of chemical change.
Chemical reaction- a process by which the atoms of one or more substances are
rearranged to form different substances.
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Evidence of a chemical reaction is a …
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Evidence:
Representing Chemical Reactions
Objective: Represent chemical reactions with equations.
Chemical equations – statements that show chemical reactions by the use of chemical
formulas and conserved matter with the relative amounts of substances in the
reaction.
Parts of an equation reaction:
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Reactants –
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Products –
Common symbols
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Representing Reactions
Word equation – use of words for reactants and products
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Skeleton equation – chemical formulas used for reactants and products but not
balanced.
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Chemical Equation - Is a statement that uses chemical formulas to show the identities
and relative amounts of the substances involved in a chemical reaction.
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Balancing Chemical Reactions
Objective: Balance chemical equations
Coefficient - in a chemical equation it is the number written in front of a reactant or
product, describing the lowest whole-number ratio of the amounts of all the reactants
and products.
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Steps for balancing equations:
1. Write the skeleton equation:
a. Make sure chemical formulas are correct
b. Put in symbols and physical states
liquid sodium carbonate + aqueous calcium chloride yields solid calcium
carbonate + aqueous sodium chloride
2. Count the atoms of the elements in the reactants
a. Group intact polyatomic ions as a single substance
Na2CO3(l) + CaCl2(aq)  CaCO3(s) + NaCl(aq)
Sodium:
Carbonate:
Calcium:
Chloride:
3. Count the atoms of the elements in the products
a. Group intact polyatomic ions as a single substance
Na2CO3(l) + CaCl2(aq)  CaCO3(s) + NaCl(aq)
Sodium: 2
Sodium:
Carbonate: 1
Carbonate:
Calcium: 1
Calcium:
Chloride: 2
Chloride:
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4. Change the coefficients to make the number of atoms of each element equal on
both sides of the equation
a. Never change subscripts
Na2CO3(l) +
CaCl2(aq) 
CaCO3(s) +
NaCl(aq)
5. Write the coefficients in their lowest possible ratios
Na2CO3(l) +
CaCl2(aq) 
CaCO3(s) +
2NaCl(aq)
6. Go back and check math
Example: aqueous sodium hydroxide + aqueous calcium bromide yields solid
calcium hydroxide and aqueous sodium bromide
***practice problems p287 #4-6***
***p288 #7-13**
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8.2 Classifying Chemical Reactions – There are four types of chemical reactions:
synthesis, combustion, decomposition, and replacement reactions. Some reactions can fit
more than one reaction type.
Types of Chemical Reactions
Objective: Classify chemical reactions
Objective: Identify the characteristics of different classes of chemical reactions
Chemists classify reactions in order to organize the many types.
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Synthesis – a chemical reaction in which two or more substances react to produce a
single product.
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When two compounds react
o A + B  AB; when two elements react.
o AB + CD
 ABCD; AB + BC  ABC
Combustion – oxygen combines with a substance and releases energy in the form of
heat and light.
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Example:
o Element and oxygen react: A + O2  AO
o Compound and oxygen react: AB + O2  AO + B
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Decomposition – a single compound breaks down into two or more elements or new
compounds
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o Compound breaks down into two elements: AB  A + B
o Compound breaks down to form new compounds: ABCD  AC + BD
Replacement / Displacement
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Single Displacement – the atoms of one element replace the atoms of another
element in a compound.
o A + BX  B + AX
Activity Series
o A metal will not always replace another metal in a compound dissolved
in water because of differing reactivities. - some metals are more
reactive than others
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o Halogens frequently replace other halogens in replacement reactions.
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Metals/Halogens are listed in order of reactivity. A less reactive metal/halogen
will not replace a more reactive metal/halogen.
***p 291 #14-17; page 292 #18-20, p295 #21-24***
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Double replacement – also called metathesis - the exchange of ions between two
compounds.
o AX + BY  AY + BX; A and B are cations, XY are anions
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Metathesis reactions often forms one of three products:
o Precipitate
o Water
o Gas
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***p297 #25-28***
***p298 #29-34***
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8.3 Solubility - When solutions containing ionic substances are mixed, ions
interchange and can form solids.
Objective: Identify new possible ionic compounds in a reaction
Objective: Define the terms soluble and insoluble
Ionic Compounds in Solutions
Ionic compounds in aqueous solutions mix and exchange partners (double
replacement).
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example: Na2SO4 + CaCl2 
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Solubility
Objective: Predict solids based on solubility rules
Solubility rules are used to determine the state of matter of products in an aqueous
solution.
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Soluble
Insoluble
Solubility Rules
1. Most nitrate (NO3) salts are soluble
2. Most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the
ammonium ion (NH4+) are soluble.
3. Most chloride, bromide, and iodide salts are soluble
a. Exceptions: Ag+, Pb2+, Hg22+
4. Most sulfate salts are soluble .
a. Exceptions: Bas+, Pb2+, Hg22+, and Ca2+
5. Most hydroxides are only slightly soluble (treat as insoluble).
a. Exceptions: Na+, K+
6. Most sulfide (S2-), carbonate (CO32-), chromate (CrO42-), and phosphate (PO43-)
salts are only slightly soluble (treat as insoluble).
a. Exceptions: any containing Alkali metals and ammonium.
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Solubility Summary:
Rule
Soluble
Insoluble
#1
#2
#3
#4
#5
#6
Nitrates (NO3-)
Group 1, NH4+
Halogens
Sulfates (SO42-)
No exceptions
No exceptions
Ag+, Pb2+, Hg22+
Bas+, Pb2+, Hg22+, Ca2+
Hydroxides (OH-)
S2-, CO32-, CrO42- PO43-
Example Problem: Aqueous silver nitrate mixes with aqueous sodium chloride;
what solid will be produced from this solution.
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8.4 Reactions in aqueous solutions - Double –replacement reactions occur
between substances in aqueous solutions and produce precipitates, water, or gases.
Objective: Describe aqueous solutions
Aqueous solution – contains one or more dissolved substances (called solutes) in
water.
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Solution –
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Solutes –
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Solvent –
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There are many possible solutes—sugar and alcohol are molecular compounds
that exist as molecules in aqueous solutions.
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Objective: Write complete ionic and net ionic equations for chemical reactions in aqueous
solutions.
Types of Aqueous Equations
Ionic Equation: equations that show ionic detail and dissociation within reactions.
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Formula Equationo 2NaOH(aq) + CuCl2(aq)  2NaCl(aq) + Cu(OH)2(s)
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Complete ionic equationo 2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq)  2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s)
o Spectator Ions-
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Net ionic equation –
o 2OH-(aq) + Cu2+(aq)  Cu(OH)2(s)
Some reactions produce more water molecules.
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HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq)
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Without spectator ions :
Gases that are commonly produced are carbon dioxide, hydrogen cyanide, and
hydrogen sulfide.
2HI(aq) + Li2S(aq) → H2S(g) + 2LiI(aq)
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
H2CO3(aq) decomposes immediately.
H2CO3(aq) → H2O(l) + CO2(g)
Two reactions can be combined and represented by a single chemical reaction
(wait, don’t write down anything from the picture)
Reaction 1: HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
Reaction 2:
H2CO3(aq) → H2O(l) + CO2(g)
Combined Equation:
HCl(aq) + NaHCO3(aq) + H2CO3(aq) → H2CO3(aq) + NaCl(aq) + H2O(l) + CO2(g)
Overall Equation:
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Objective: Predict whether reactions in aqueous solutions will produce a precipitate,
water, or a gas.
Types of Reactions in Aqueous Solutions: Double replacement
1. Precipitate is formed- when a compound forms from ions, an exothermic
reaction takes place. The ions by themselves are less stable and therefore of
higher energy than when combined in a compound. This change in energy
(increase in stability) ‘drives’ the reaction.
Example: 2NaOH(aq) + CuCl2(aq)  2NaCl(aq) + Cu(OH)2(s)
Net:
2. Water is formed- even though a double reaction takes place, solutions may look
the same since water will still be the dominant substance.
Example: HBr(aq) + NaOH(aq)  H2O(l) + NaBr(aq)
Net:
3. Gas is formed- for the same reasons that a precipitate is formed, a gas will also
form. Common gases in double replacement reactions are: carbon dioxide,
hydrogen cyanide, and hydrogen sulfide.
Example: 2HI(aq) + Li2S(aq)  H2S(g) + 2LiI(aq)
Net:
***practice problems p302 #35-39***
***p304 #40-44, p306 #45-49, p308 #50-56***
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8.5 Accumulating Content and Skills:– Chemistry content is continuous and
builds on prior knowledge and skills. This section will combine this unit with previous
units.
Objective: Apply knowledge and skills from previous units to content learned in this unit.
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Why do polyatomic ions stay intact in an aqueous solution?
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How do naming rules change when working with gases vs. aqueous acids?
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How does lattice energy relate to solubility rules?
Key Concepts
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Some physical changes are evidence that indicate a chemical reaction has
occurred.
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Word equations and skeleton equations provide important information about a
chemical reaction.
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A chemical equation gives the identities and relative amounts of the reactants
and products that are involved in a chemical reaction.
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Balancing an equation involves adjusting the coefficients until the number of
atoms of each element is equal on both sides of the equation.
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Classifying chemical reactions makes them easier to understand, remember,
and recognize.
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Activity series of metals and halogens can be used to predict if singlereplacement reactions will occur.
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In aqueous solutions, the solvent is always water. There are many possible
solutes.
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Many molecular compounds form ions when they dissolve in water. When some
ionic compounds dissolve in water, their ions separate.
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When two aqueous solutions that contain ions as solutes are combined, the ions
might react with one another. The solvent molecules do not usually react.
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Reactions that occur in aqueous solutions are double-replacement reactions.
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