Chemical Reactions

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Chemical Reactions

• Chapter 10

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Part I Counting Atoms

• How Many Atoms in a Molecule?

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Counting Atoms

• Most substances that we encounter are compounds,
  not elements.
• A chemical compound is a pure substance formed
  from the combination of two or more different
  elements.
• The properties of the compound may be completely
  unlike those of the elements that form it.
• The formula for a compound lists the symbols of
  the individual elements followed by subscripts
  which indicate the number of atoms of that
  element.
• (If no subscript is given, it is understood to be
  1.) E.g., NaCl, H2O, C12H22O11.

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Counting Atoms

• A molecular formula gives the actual number of
  atoms of each element in a molecule of a
  compound.
• Hydrogen peroxide H2O2
• Water H2O
• Glucose C6H12O6
• A structural formula uses lines to represent
  covalent bonds, and shows how the atoms in a
  molecule are joined together
• HOOH
• HOH
• OCO

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Counting Atoms

• Example How Many Atoms?

6 Cs 12 Hs 6 Os 24 atoms
C6H12O6
3 Ks1 Ps 4 Os 8 atoms
K3PO4
2 Cs 5 Hs 1 Os 1 H 9 atoms
C2H5OH
2 Hs and 2 Os 4 atoms
H2O2
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Counting Atoms with Polyatomic Ions - Al2(SO4)3
17 ATOMS

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Counting Atoms with Polyatomic Ions

• When counting atoms with polyatomic ions
• Count number of atoms in one polyatomic ion
• Ions inside the parentheses
• Multiply by number of polyatomic groups in the
  molecule (number outside the parenthesis)
• Examples
• Al2(SO4)3 - 2 Als 3(1 S 4 Os) 2 3(5)
  215 17 atoms
• Mg(NO3)2 1 Mg 2(1 N 3 Os) 1 2(4) 18
  9 atoms

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Hydrates

• Hydrates are ionic compounds which also contain a
  specific number of water molecules associated
  with each formula unit. The water molecules are
  called waters of hydration.
• The formula for the ionic compound is followed by
  a raised dot and H2O
• Example MgSO47H2O.
• They are named as ionic compounds, followed by a
  counting prefix and the word hydrate
• CuSO45H2O copper(II) sulfate pentahydrate
• BaCl26H2O barium chloride hexahydrate
• MgSO47H2O magnesium sulfate heptahydrate (Epsom
  salts)

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How Many Atoms in a Hydrate?

• When counting atoms in the hydrate, count the
  water atoms also.
• Example
• CuSO45H2O
• 1 Cu 1 S 4 Os 5(2 Hs 1 Os)
• 1145(3)
• 615 21 atoms
• BaCl26H2O
• 1 Ba 2 Cl 6(2 Hs 1 Os)
• 126(3)
• 3 18 21 Atoms

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Part II Conservation of Mass
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Conservation of Mass

• In a normal chemical reaction, the mass of
  substances in a closed system will remain
  constant, no matter what processes are acting
  inside the system.
• How ever many atoms a reaction starts with, ends
  with the same number.
• Atoms dont change their identity in a chemical
  reaction
• Number of atoms for EACH ELEMENT STAYS THE SAME
  in a chemical reaction
• The elements just rearrange their organization
• The beginning MASS of the reaction EQUALS the
  ending MASS of the reaction

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Conservation of Mass

• Total Mass stays the same in a chemical reaction

2g H2 16g O2 yields 18g H2O

• Number and Identity of Atoms stays the same in a
  chemical reaction
• 2 H2 1 O2 yields 2 H2O

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Part III Writing Reactions

• How Do You Write a Chemical Reaction?

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III. Chemical Reactions

• Definition process by which the atoms of one or
  more substances are rearranged
• KEY new substances are formed
• KEY No Atoms are Gained or Lost
• A chemical reaction is the process by which atoms
  of one or more substances are rearranged into new
  substances
• Chemical change occurs
• How do you know?

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III. Evidences of a Chemical Reaction

• 1) gas production
• 2) light production
• 3) temperature change (endo/exothermic)
• 4) precipitate formed (solid from 2 liquids)
• 5) permanent color change

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III. Energy Changes

• Energy is stored in compounds as chemical
  potential energy
• due to specific arrangements of atoms.
• A chemical reaction changes the potential energy
  present.

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Energy Changes

• When energy is lost as heat, it is called an
  __________________.

exothermic reaction
These reactions get hotter.

• When energy is gained heat is added for a
  reaction to occur. These are called
  ______________________,

endothermic reactions
These reactions get colder.

• Energy in a reaction is shown with
• ?H (heat)
• kJ
• Joules

• Heat
• energy

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III. Chemical Reactions

• Representing Chemical Reactions
• Reactants the stuff you start with
• An arrow which means yields, or becomes
• Products the stuff you end up with
• Principle of Conservation of Mass applies to
  chemical reactions.
• Why?

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III. Chemical Reactions

• Word Equations
• Reactant-A Reactant-B yields Product-AB
• Example
• Sodium(s) Chlorine(g) ? Sodium Chloride(s)
• The small letters in paretheses () indicate the
  state of the reactant or product (solid, liquid,
  gas, or aqueous solution)
• (s) solid
• (l) liquid
• (g) gas
• (aq) aqueous dissolved in water

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Part IV Balancing Equations

• Applying Conservation of Mass to Equations

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VI. Chemical Equations

• Step 1 Write a Skeleton Equation
• Skeleton Equation uses chemical formulas and
  symbols instead of words
• Words Sodium Chlorine gas yields Sodium
  Chloride
• Symbols Na(s) Cl2(g) ? NaCl
• Skeleton Equations are not complete equations,
  but are the first step in writing a complete
  equation

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IV. Chemical Equations

• Chemical Equation is BALANCED
• Balanced means that conservation of mass is
  upheld
• All atoms in reactants are also in products
• No more, no less
• Just rearranged

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IV. Chemical Equations

• Balancing Equations
• Use a number before the compound/element symbol
  to indicate how many of them are needed
• Called a COEFFICIENT
• Written in front of the atom/compound
• KEY Coefficient is a MULTIPLIER
• Number of atoms per molecule is SUBSCRIPT
• Change ONLY the COEFFICIENTS to balance the
  equation

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IV. Chemical Reactions

• Steps to Balance Equations
• Write the skeleton equation
• Count the atoms of EACH element in the reactants
• Count the atoms of EACH element in the products
• Change the coefficients to make the number of
  atoms of each element equal on both sides of the
  equation
• Write the coefficients in the lowest possible
  ratio
• Check your work
• NEVER CHANGE A SUBSCRIPT

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IV. Chemical Equations

• Write the skeleton equation
• Al O2 ? Al2O3

This is not balanced because the numbers dont
match
3. Multiply coefficients until they match
multiply the entire units
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Al O2 ? Al2O3
Go to 6 Oxygens
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IV. Balancing Equations
Al O2
2
Al2O3
Multiply each atom by 2
4
3
Balanced
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IV. Balancing Equations 2
The work of balancing a chemical equation is in
many ways a series of trials and errors.
Consider the equation given below. Does this
represent a balanced chemical equation?
N2 H2 ?
NH3
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IV. Balancing Equations 3
To balance this reaction, it is best to choose
one kind of atom to balance initially. Let's
choose nitrogen in this case.
2 Nitrogen Atoms in Reactants requires 2 Ammonia
molecules in Product to balance the nitrogen
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IV. Balancing Equations 2

• Once we know what the molecules are (N2, H2, and
  NH3 in this case) we cannot change them (only how
  many of them there are).
• The nitrogen atoms are now balanced, but there
  are 6 atoms of hydrogen on the product side
• only 2 of them on the reactant side.
• The next step requires multiplying the number of
  reactant hydrogen molecules by three to give

Balanced
                        
N2
3H2
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IV. Dont Forget Diatomic Elements

• Definition 7 elements that NEVER occur as
  singular atoms (always paired with an the same
  or different element)

H2 O2 F2 Br2 I2 N2 Cl2
Ex 2 HCl 2K ? 2 KCl H2
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IV. Balancing Equations 3
1. Start with an unbalanced equation
2. Draw boxes around the compounds so you dont
mess with the groups
Dont be threatened by how complex it looks!
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IV. Balancing Equations 2
3. Make an element inventory count number of
atoms for each element on each side of the
equation
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IV. Balancing Equations 3

• 4. Write coefficients in front of each of the
  boxes until the inventory for each element is the
  same both before and after the reaction
• Save Oxygen and Hydrogen for last, Treat
  Polyatomic like an atom.
• Lets start with Sodium

• We have 2 in products, so I need 2 in reactants

Multiply reactant with sodium by 2 and recount
atoms
N
Y
Y
N
N
Y
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IV. Balancing Equations 3

• Inventory Shows
• Reactant side has FOUR hydrogen atoms
• Product side has TWO hydrogen atoms
• Using your amazing powers of mathematics
• two hydrogen multiplied two becomes four hydrogen

Balanced
Y
Y
N
N
Y
Y
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Helpful Hints

• Balance hydrogen and oxygen last
• Balance polyatomic ions as a group if present on
  both reactants and products
• You can consider a polyatomic ion as a single
  element
• If the balancing starts to get very complex
• Stop
• Start over
• Select a different atom to balance first.

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Example Using PolyAtomics

• Before

MgCl2 NaOH ? Mg(OH)2 NaCl
1 Mg
1 Na
1 Mg
1 Na
2 Cl
1 Cl
1 OH
2 OH

• After

MgCl2 2 NaOH ? Mg(OH)2 2 NaCl
1 Mg
2 Na
1 Mg
2 Na
2 Cl
2 Cl
2 (OH)
2 (OH)
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Types of Chemical Reactions

• Part V

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Classifying Chemical Reactions

• Synthesis
• Decomposition
• Single replacement
• Double Replacement
• Combustion

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Synthesis

• Definition two or more substances react to form
  ONE product
• Product is usually bigger or more complex than
  either reactant
• A B ? AB

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Hey baby lets get jiggy.
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Synthesis

• reaction of two elements

2
2
3
___Al ___Cl2 ?
___AlCl3
Al3
Cl1-
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Synthesis

• reaction of two compounds
• reaction of element and compound

CaO H2O ?
Ca(OH)2
SO2 O2 ?
SO3
2
2
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Decomposition

• definition one substance breaks down into two
  or more simpler products

AB ? A B
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Break yoself fool!
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Decomposition

• Example reaction

2
2
3
__ NaN3 (s) ?
___ Na (s)
___ N2 (g)
2
2
1
__ CaO (s) ?
___ Ca (s)
___ O2 (g)
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Single Replacement Reactions

• Definition one element replaces another element
  in a compound to form new compound

A BX ?
AX B
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Im gon dance with yo lady
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Double Replacement

• Defn exchange of cations between two ionic
  compounds

A B C D ?
AD CB
switch
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(No Transcript)
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3 possible products of double replacement
reactions

• Precipitate
• Gas
• Water

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Reactivity Series (or Activity Series)

• More active will replace less active
• Less active will NOT replace more active
• metals

Li K Ca Na Mg Al Mn Zn Fe Ni Sn Pb Cu Ag Au
most active
least active
F Cl Br I

• halogens

most active
least active
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examples

• aluminum iron (III) oxide

Fe3
O2-
Stronger?
2
2
1
1

Al

Fe2O3
Al2O3
Fe
Al3
O2-
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examples

• silver copper (I) nitrate

Cu1
NO31-
Stronger?
Ag

CuNO3
NO RXN
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examples

• fluorine gas sodium bromide

Stronger?
2
2
1
1
F2

NaBr
Br2
NaF

Na1
F1-
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examples

• chlorine gas hydrogen fluoride

Stronger?
Cl2
HF

NO RXN
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Example Problem

• lithium iodide and aqueous silver nitrate react

Li1
I1-
Ag1
NO31-
Li I
Ag I (s)
AgNO3
LiNO3


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Combustion

• definition compound reacts with O2
• Hydrocarbon compound w/ only carbon and hydrogen

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Combustion

• Combustion of hydrocarbons
• ALWAYS produces CO2 and H2O

CxHy O2
CO2 H2O
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Ex problem

• show combustion of propane (C3H8) gas

C3H8
O2
CO2
H2O


1
5
3
4
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