Chemical Reactions

1

• Chemical Reactions Equations

2
Chemical Equations

• Shows what takes place during a
• chemical reaction
• Reactant (substances reacting) on left
• Arrow - (?) at center yields or produces
• Products (new substances formed) on
• right

• A B ? C D

PRODUCTS
REACTANTS
3
Coefficients how many? appears in FRONT of
the formula

• molecules for covalent substances
• moles of atoms
• units for ionic substances

3CO2 ? 2Mg ? 4MgO ?
3 molecules of carbon dioxide 2 moles of
magnesium 4 units of magnesium oxide

• Phases
• solid (s), liquid (l), gas (g), aqueous (aq)

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Example
CH4 2O2 ? CO2 2H2O
reactants
products

• Names of reactants methane oxygen
• Names of products carbon dioxide water

• How many of each (coefficient)
• __CH4 ____O2 ____CO2 ___H2O

1
2
2
1
1
1
2
2

• Ratio of coefficients ___ ____ ___ ___

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Expressing Chemical Equations

• Word carbon oxygen yields carbon dioxide
• Remember your diatomic gases (BrINClHOF)
• C O2 ? CO2

Formula
Diagram

?
6
Exothermic Endothermic Processes see RB pg
34 table 2.5

• Exothermic release energy surrounding temp
  increases NRG on the product side
• A B ? AB energy
• CH4 3 O2 ? CO2 2 H2O energy
• Endothermic absorb energy surrounding temp
  decreases NRG on the reactant side
• AB energy? A B
• H2O(s) energy ? H2O (l)

Endothermic video http//www.youtube.com/watch?v5
RJLvQXce4A
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Law of Conservation of Mass/Matter/NRG

• Matter is neither created nor destroyed in
  chemical reactions. In any chemical total numbers
  kinds of atoms must remain unchanged in the
  reaction.
• For chemical equations, this is called a balanced
  equation.
• Charge, energy, mass, and atoms are conserved
  (unchanged) in balanced reactions

4 H 2 O
4 H 2 O
36 g
4 g
32 g
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How well did you focus? Partner Chat

• Obtain a plastic baggie containing a substance.
  List 2 physical observations of the substance.
  Feel the outside of the baggie. Note the
  temperature.
• Add some water to the baggie and seal mixing the
  two substances. List 2 physical observations of
  the substance. Feel the outside of the baggie.
  Note the temperature. When finished dispose of
  the baggie into the garbage.
• summary
• All reactions involve heat (energy). The
  substance in the baggie is NH4Cl(s) . When mixed
  with water, the solid dissolves forming NH4Cl(aq)
  . The reaction for this is as such
• NH4Cl(s) NH4 (aq)
  Cl - (aq)
• reflection answer on a separate sheet of paper
• Was the reaction endothermic or exothermic?
  Explain/give evidence to support your answer.
• Would the term heat go on the reactant side or
  the product side of the equation?
• According to Reference Table I, was your answer
  correct? What is the amount of heat involved per
  1 mole of substance produced? (Hint look for
  this reaction)
• In terms of entropy ( s), does it increase,
  decrease or RTS as the reaction proceeds from
  reactant to product? (Hint look at phases)
• Would this substance when placed in water conduct
  electricity? Support your answer.
• When finished check your homework answers on the
  side overhead screen.

H2O
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Chemical Change chemical composition changes

• Signs of a Chemical Reaction
• Evolution of heat and light
• Formation of a gas
• Formation of a precipitate (solid)
• Color change

Physical Change chemical composition remains
the same

• Examples
• Ripping, tearing, breaking
• Boiling, melting, freezing, vaporing a substance
  (a phase change)
• dissolving

• Examples
• Burning, rusting, oxidizing
• Flammable, explosive, reacting

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II. Balancing Equations

• Chemical Reactions

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A. Balancing Steps

• 1. Write the unbalanced equation.
• 2. Count atoms on each side.
• 3. Add coefficients to make s equal.
• Coefficient ? subscript of atoms
• 4. Reduce coefficients to lowest possible
  ratio, if necessary.
• 5. Double check atom balance!!!

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B. Helpful Tips

• Balance one element at a time.
• Update ALL atom counts after adding a
  coefficient.
• If an element appears more than once per side,
  balance it last.
• Balance polyatomic ions as single units.
• 1 SO4 instead of 1 S and 4 O

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C. Balancing Example

• Aluminum and copper(II) chloride react to form
  copper and aluminum chloride.

3
3
2
2
1 1 1 1 2
3
2 ?
? 2 ? 6
3 ? 6 ?
? 3
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III. Types of Chemical Reactions

• Chemical Reactions

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Target I can recognize the types of chemical
reactions

• TASK 1 Individual
• Read silently pages 37-40 in your review book.
  Complete the section on page 7 in your guide
  (PREFERABLY IN PENCIL) that is based on the
  reaction tag you have.
• Task 2 Square OFF (teams of 4)
• Taking turns, teach your type of reaction to
  the group so they can complete the reactions 1-4
  on page 7.
• Guide pg 8 - complete 1-20. Each person in the
  group should be prepared to explain and/or answer
  questions from the teacher upon completion.
• TASK 3 Teacher
• Check for understanding Reaction 5
  Combustion
• TASK 4 Independent
• Homework Review book pg 40 59

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A. Combustion

• the burning of any substance in O2 to produce heat

A O2 ? B H2O
CH4(g) 2O2(g) ? CO2(g) 2H2O(g)
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A. Combustion

• Products
• contain oxygen
• hydrocarbons form CO2 H2O

Na(s) O2(g) ?
Na2O(s)
4 2
C3H8(g) O2(g) ?
CO2(g) H2O(g)
5 3
4
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B. Synthesis or Direct Combination

• the combination of 2 or more substances to form a
  compound
• only one product

A B ? AB
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B. Synthesis

• H2(g) Cl2(g) ? 2 HCl(g)

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B. Synthesis

• Products
• ionic - cancel charges

Al(s) Cl2(g) ?
AlCl3(s)
2 3 2
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C. Decomposition

• a compound breaks down into 2 or more simpler
  substances
• only one reactant

AB ? A B
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C. Decomposition

• 2 H2O(l) ? 2 H2(g) O2(g)

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C. Decomposition

• Products
• binary - break into elements

KBr(l) ?
K(s) Br2(l)
2 2
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D. Single Replacement

• one element replaces another in a compound
• metal replaces metal ()
• nonmetal replaces nonmetal (-)

A BC ? B AC
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D. Single Replacement

• Cu(s) 2AgNO3(aq) ? Cu(NO3)2(aq) 2Ag(s)

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D. Single Replacement

• Products
• metal ? metal ()
• nonmetal ? nonmetal (-)
• free element must be more active (check activity
  series-Table J)

Fe(s) CuSO4(aq) ?
Cu(s) FeSO4(aq)
Br2(l) NaCl(aq) ?
N.R.
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E. Double Replacement

• ions in two compounds change partners
• cation of one compound combines with anion of the
  other

AB CD ? AD CB
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E. Double Replacement

• Pb(NO3)2(aq) K2CrO4(aq) ? PbCrO4(s) 2KNO3(aq)

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E. Double Replacement

• Products
• switch negative ions
• one product must be insoluble (check solubility
  table)

Pb(NO3)2(aq) KI(aq) ?
PbI2(s) KNO3(aq)
2
2
NaNO3(aq) KI(aq) ?
N.R.
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Study of QUANTITATIVE relationships between
substances to determine the PROPORTIONS in a
chemical relationship

• IV. Stoichiometry

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• The COEFFICIENTS of the FORMULAS in the balanced
  equation indicate the relative number of moles,
  molecules and volume (at STP) 0 OC 1 atm
• When working with chemical equations it is
  assumed that
• 1. The reaction is a forward reaction (no side
  reactions).
• 2. The reaction goes to completion.
• 3. The reactants are completely used.
• LIMITING REACTANT- LIMITS THE AMOUNT OF PRODUCT
  FORMED

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• Remember Law of Conservation of Mass-
• MASS and ENERGY are conserved in EVERY chemical
  reaction (so is the charge)
• 1) Determining Mole Ratio (doesnt give mass
  ratio)
• Use the coefficient to get a mole ratio
•  
• 2 C2H6 7 O2 ? 4 CO2 6 H2O
•  
• Mole ratio
• This means that 2 moles of C2H6 combine with 7
  moles of O2 to produce 4 moles of CO2 and 6 moles
  of water.
•  

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2). Problem Solving using Stoichiometric
calculations - start with the given (use factor
label method or set up as a proportion)
3/4 c. brown sugar 1 tsp vanilla extract 2 eggs 2
c. chocolate chips Makes 5 dozen cookies.
2 1/4 c. flour 1 tsp. baking soda 1 tsp. salt 1
c. butter 3/4 c. sugar

• I have 5 eggs. How many cookies can I make?

5 eggs
5 doz. 2 eggs
12.5 dozen cookies
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A) Mole-Mole and volume-volume Problems (gases
only at STP) REMEMBER moles and volume are
directly related 1 mol 22.4 L

• Example 1 mol-mol
• How many moles of water will be produced from the
  complete combustion of 3 moles of ethane (C2H6 )?

2 C2H6 7 O2 ? 4 CO2 6 H2O
? mol
3 mol
2 mol C2H6 3 mol C2H6
6 mol H2O x mol H2O
9 mol H2O
Equation Problem
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Ex. 2 vol-vol

• How many liters of ethane can be completely
  oxidized by a reaction with 63 liters of oxygen
  gas?

2 C2H6 7 O2 ? 4 CO2 6 H2O
? L
63 L
7 L O2 63 L O2
2 L C2H6 X L C2H6
Equation Problem
18 L O2
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• Ex 3 mol-mol
• Assuming at STP, how many moles of ammonia are
  produced when 0.60 mol of nitrogen reacts with
  hydrogen. (this is called the Haber Process)

N2 3 H2 ? 2 NH3
? mol
.60 mol
.3 mol N2
2 mol NH3 .60 mol NH3
1 mol N2 x mol N2
Ex 4 vol mol Using the above equation, how
many liters of nitrogen are needed to produce 5.5
moles of NH3?
1 mol N2 X mol N2
2 mol NH3 5.5 mol NH3
2.75 mol N2
x 22.4 L 61.6 L N2 1 mol N2
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Stoichiometry Steps

• 1. Write a balanced equation.
• 2. Identify known unknown.
• 3. Line up conversion factors.
• Mole ratio - moles ? moles
• Molar mass - moles ? grams
• Molar volume - moles ? liters gas

• Mole ratio - moles ? moles

Core step in all stoichiometry problems!!
4. Check answer.
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Conversion factors in stoich
LITERS OF GAS AT STP
Molar Volume (22.4 L/mol)
MASS IN GRAMS
NUMBER OF PARTICLES
MOLES
Molar Mass (g/mol)
6.02 ? 1023 particles/mol
Molarity (mol/L)
LITERS OF SOLUTION
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B). Mass-Mass Problems (convert g? mol? mol? g)

• Ex 1. How many grams of silver will be formed
  from 12.0 g of copper?
• Cu 2AgNO3 ? 2Ag Cu(NO3)2

? g ? mol
12.0 g Cu x 1 mol Cu .19 moles Cu
64 g/mol Cu
a. g-mol
1 mol Cu 2 mol Ag 10.5 mol Ag .19 mol
Cu x mol Ag
b. mol-mol ratio
10.5 mol Ag x 108 g Ag 1134 g Ag
1 mol Ag
c. mol-g
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B). Mass-Mass Problems (convert g? mol? mol? g)

• Ex 2. What mass of silver nitrate is required to
  liberate 54 g of silver?
• Cu 2 AgNO3 ? Cu(NO3) 2 2 Ag

x g ? mol
54 g ? mol
54 g Ag x 1 mol Ag .5 moles Ag
108 g/mol Ag
a. g-mol
2 mol AgNO3 2 mol Ag .5 mol AgNO3
X mol AgNO3 .5 mol Ag
b. mol-mol ratio
.5 mol AgNO3 x 170 g AgNO3 85 g AgNO3
1 mol AgNO3
c. mol-g
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3Wa4 12Ch 16Ma ? 4Wa3Ch3Ma4

• Suppose you only have 6 tbsp of chocolate and 2
  marshmallows, How many servings of hot chocolate
  can you make?
• Why??
• Which ingredient limited the amount of hot
  chocolate you can make?

½ serving
if it takes 4 ma to make 1 serving, then 2
yields ½ serving
marshmallows
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3Wa4 12Ch 16Ma ? 4Wa3Ch3Ma4
Group Activity Limiting Reactants

• 3 cups of water
• 12 tablespoons of chocolate
• 16 marshmallows
• Produce 4 servings of Hot Chocolate
• Using the balanced equation and the factor label
  method determine the amount of water, chocolate
  and marshmallows needed to produce 1 serving.

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3Wa4 12Ch 16Ma ? 4Wa3Ch3Ma4

• 1 serving x 3 c. water ¾ c water
• 4 servings
• 1 serving x 12 T chocolate 3 T
  chocolate
• 4 servings
• 1 serving x 16 marshmallows 4
  marshmallows
• 4 servings

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FLM Stoichiometry mol-mol

• How many moles of KClO3 must decompose in order
  to produce 9 moles of oxygen gas?

2KClO3 ? 2KCl 3O2
? mol
9 mol
2 mol KClO3 x mol KClO3
3 mol O2 9 mol O2
6 mol KClO3
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FLMStoichiometry L-mol-mol-g

• How many grams of KClO3 are reqd to produce 9.00
  L of O2 at STP?

2KClO3 ? 2KCl 3O2
? g
9.00 L
9.00 L O2
1 mol O2 22.4 L O2
2 mol KClO3 3 mol O2
122.55 g KClO3 1 mol KClO3
32.8 g KClO3
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FLM Stoichiometry g-mol-mol-g

• How many grams of silver will be formed from 12.0
  g copper?

Cu 2AgNO3 ? 2Ag Cu(NO3)2
12.0 g
? g
12.0 g Cu
1 mol Cu 63.55 g Cu
2 mol Ag 1 mol Cu
107.87 g Ag 1 mol Ag
40.7 g Ag
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FLM Stoichiometry L-mol-mol-g

• How many grams of Cu are required to react with
  1.5 L of 0.10M AgNO3?

Cu 2AgNO3 ? 2Ag Cu(NO3)2
1.5L 0.10M
? g
63.55 g Cu 1 mol Cu
1.5 L
.10 mol AgNO3 1 L
1 mol Cu 2 mol AgNO3
4.8 g Cu