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Chemical Reactions. Chapter 10. Part I: Counting Atoms. How Many Atoms in a Molecule?. 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. - PowerPoint PPT Presentation
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Chemical ReactionsChemical Reactions
Chapter 10Chapter 10
Part I: Counting AtomsPart I: Counting Atoms
How Many Atoms in a Molecule?How Many Atoms in a Molecule?
Counting AtomsCounting 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.
Counting AtomsCounting 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: H—O—O—H H—O—H O=C=O
Counting AtomsCounting Atoms
Example: How Many Atoms?Example: How Many Atoms?
2 H’s and 2 O’s = 4 atoms2 H’s and 2 O’s = 4 atomsHH22OO22
2 C’s+ 5 H’s + 1 O’s + 1 H = 9 atoms2 C’s+ 5 H’s + 1 O’s + 1 H = 9 atomsCC22HH55OHOH
3 K’s+1 P’s + 4 O’s = 8 atoms3 K’s+1 P’s + 4 O’s = 8 atomsKK33POPO44
6 C’s + 12 H’s + 6 O’s = 24 atoms6 C’s + 12 H’s + 6 O’s = 24 atomsCC66HH1212OO66
Counting Atoms with Polyatomic Counting Atoms with Polyatomic Ions - AlIons - Al22(SO(SO44))3 3
Al
Al
SO
O
OO
SO
O
OO
SO
O
OO
=17 ATOMS
Counting Atoms with Polyatomic Counting Atoms with Polyatomic IonsIons
When counting atoms with polyatomic ions;When counting atoms with polyatomic ions; Count number of atoms in one polyatomic ionCount number of atoms in one polyatomic ion
o Ions inside the parenthesesIons inside the parentheses
Multiply by number of polyatomic groups in the Multiply by number of polyatomic groups in the molecule (number outside the parenthesis)molecule (number outside the parenthesis)
Examples:Examples: AlAl22(SO(SO44))3 3 - 2 Al’s + 3(1 S + 4 O’s) = 2 + 3(5) = 2+15 = - 2 Al’s + 3(1 S + 4 O’s) = 2 + 3(5) = 2+15 =
17 atoms17 atoms Mg(NOMg(NO33))22 – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 = – 1 Mg + 2(1 N + 3 O’s) = 1 + 2(4) = 1+8 =
9 atoms9 atoms
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: MgSO4•7H2O.
They are named as ionic compounds, followed by a counting prefix and the word “hydrate” CuSO4•5H2O copper(II) sulfate pentahydrate BaCl2•6H2O barium chloride hexahydrate MgSO4•7H2O magnesium sulfate heptahydrate
(Epsom salts)
How Many Atoms in a Hydrate?How Many Atoms in a Hydrate?When counting atoms in the hydrate, When counting atoms in the hydrate,
count the water atoms also.count the water atoms also.Example:Example:
CuSO4•5H2Oo 1 Cu + 1 S + 4 O’s + 5(2 H’s + 1 O’s)o =1+1+4+5(3)o = 6+15 = 21 atoms
BaCl2•6H2Oo 1 Ba + 2 Cl + 6(2 H’s + 1 O’s)1 Ba + 2 Cl + 6(2 H’s + 1 O’s)o = 1+2+6(3)= 1+2+6(3)o =3 + 18 = 21 Atoms=3 + 18 = 21 Atoms
Part II: Conservation of MassPart II: Conservation of Mass
Conservation of MassConservation of Mass In a normal chemical reaction, the mass of In a normal chemical reaction, the mass of
substances in a closed system will remain substances in a closed system will remain constant, no matter what processes are acting constant, no matter what processes are acting inside the system. inside the system. How ever many atoms a reaction starts with, ends How ever many atoms a reaction starts with, ends
with the same number.with the same number. Atoms don’t change their identity in a chemical Atoms don’t change their identity in a chemical
reactionreaction Number of atoms for EACH ELEMENT Number of atoms for EACH ELEMENT STAYS THE STAYS THE
SAMESAME in a chemical reaction in a chemical reaction The elements just rearrange their organizationThe elements just rearrange their organization The beginning MASS of the reaction EQUALS the The beginning MASS of the reaction EQUALS the
ending MASS of the reactionending MASS of the reaction
Conservation of MassConservation of Mass
Total MassTotal Mass stays the same in a chemical stays the same in a chemical reactionreaction
2g H2 + 16g O2 yields 18g H2O Number and Identity of AtomsNumber and Identity of Atoms stays the same in a chemical stays the same in a chemical
reactionreaction 2 2 H2 + 1 O2 yields 2 H2O
H
H
H
H
O
O HH
O
HH
O
Part III: Writing ReactionsPart III: Writing Reactions
How Do You Write a Chemical How Do You Write a Chemical Reaction?Reaction?
III. Chemical ReactionsIII. Chemical Reactions
Definition – process by which the atoms of Definition – process by which the atoms of one or more substances are rearrangedone or more substances are rearranged KEY: new substances are formedKEY: new substances are formed KEY: No Atoms are Gained or LostKEY: No Atoms are Gained or Lost
A chemical reaction is the process by A chemical reaction is the process by which atoms of one or more substances which atoms of one or more substances are rearranged into new substancesare rearranged into new substances Chemical change occursChemical change occurs How do you know?How do you know?
III. Evidences of a Chemical III. Evidences of a Chemical ReactionReaction
1) gas production1) gas production
2) light production2) light production
3) temperature change (endo/exothermic)3) temperature change (endo/exothermic)
4) precipitate formed (solid from 2 liquids)4) precipitate formed (solid from 2 liquids)
5) permanent color change5) permanent color change
Energy is stored in compounds as Energy is stored in compounds as chemical potential energy chemical potential energy due to specific arrangements of atoms. due to specific arrangements of atoms.
A chemical reaction changes the potential A chemical reaction changes the potential energy present. energy present.
III. Energy ChangesIII. Energy Changes
•When energy is gained; heat is added for When energy is gained; heat is added for a reaction to occur. These are called a reaction to occur. These are called ______________________, ______________________,
Energy ChangesEnergy Changes
exothermic reaction
endothermic reactions
These reactions get hotter.These reactions get hotter.
•When energy is lost as heat, it is called an When energy is lost as heat, it is called an __________________. __________________.
These reactions get colder.These reactions get colder.
•Energy in a reaction is shown with:•ΔH (heat)•kJ•Joules
•Heat•energy
III. Chemical ReactionsIII. Chemical Reactions
Representing Chemical Reactions:Representing Chemical Reactions: ReactantsReactants – the ‘stuff’ you start with – the ‘stuff’ you start with An An ‘arrow’‘arrow’ which means ‘yields’, or ‘becomes’ which means ‘yields’, or ‘becomes’ ProductsProducts – the ‘stuff’ you end up with – the ‘stuff’ you end up with
Principle of “Conservation of Mass” Principle of “Conservation of Mass” applies to chemical reactions. applies to chemical reactions. Why?Why?
III. Chemical ReactionsIII. Chemical Reactions
Word Equations:Word Equations: Reactant-A + Reactant-B Reactant-A + Reactant-B yieldsyields Product-AB Product-AB Example:Example:
o Sodium(s) + Chlorine(g) → Sodium Chloride(s)Sodium(s) + Chlorine(g) → Sodium Chloride(s)o The small letters in paretheses () indicate the state The small letters in paretheses () indicate the state
of the reactant or product (solid, liquid, gas, or of the reactant or product (solid, liquid, gas, or aqueous solution)aqueous solution)
(s) = solid(s) = solid (l) = liquid(l) = liquid (g) = gas(g) = gas (aq) = aqueous = dissolved in water(aq) = aqueous = dissolved in water
Part IV: Balancing EquationsPart IV: Balancing Equations
Applying Conservation of Mass to Applying Conservation of Mass to EquationsEquations
VI. Chemical EquationsVI. Chemical Equations
Step 1: Write a Skeleton EquationStep 1: Write a Skeleton EquationSkeleton EquationSkeleton Equation uses chemical formulas uses chemical formulas
and symbols instead of words:and symbols instead of words: Words: Sodium + Chlorine gas yields Sodium Words: Sodium + Chlorine gas yields Sodium
Chloride Chloride Symbols: Na(s) + ClSymbols: Na(s) + Cl22(g) → NaCl(g) → NaCl Skeleton Equations are not complete Skeleton Equations are not complete
equations, but are the first step in writing a equations, but are the first step in writing a complete equationcomplete equation
IV. Chemical EquationsIV. Chemical Equations
Chemical Equation is BALANCEDChemical Equation is BALANCED Balanced means that “conservation of mass” Balanced means that “conservation of mass”
is upheldis upheld All atoms in reactants are also in productsAll atoms in reactants are also in products
o No more, no lessNo more, no lesso Just rearrangedJust rearranged
IV. Chemical EquationsIV. Chemical Equations
Balancing EquationsBalancing Equations Use a number before the compound/element symbol Use a number before the compound/element symbol
to indicate how many of them are neededto indicate how many of them are neededo Called a COEFFICIENTCalled a COEFFICIENTo Written in front of the atom/compoundWritten in front of the atom/compoundo KEY: Coefficient is a MULTIPLIERKEY: Coefficient is a MULTIPLIER
Number of atoms per molecule is SUBSCRIPTNumber of atoms per molecule is SUBSCRIPT Change Change ONLYONLY the COEFFICIENTS to balance the the COEFFICIENTS to balance the
equationequation
IV. Chemical ReactionsIV. Chemical Reactions Steps to Balance EquationsSteps to Balance Equations
1.1. Write the skeleton equationWrite the skeleton equation
2.2. Count the atoms of EACH element in the reactantsCount the atoms of EACH element in the reactants
3.3. Count the atoms of EACH element in the productsCount the atoms of EACH element in the products
4.4. Change the coefficients to make the number of atoms Change the coefficients to make the number of atoms of each element equal on both sides of the equationof each element equal on both sides of the equation
5.5. Write the coefficients in the lowest possible ratioWrite the coefficients in the lowest possible ratio
6.6. Check your workCheck your work NEVER CHANGE A SUBSCRIPTNEVER CHANGE A SUBSCRIPT
IV. Chemical EquationsIV. Chemical Equations1.Write the skeleton equation:
Al + O2 → Al2O3 2. Count Number of atoms for each element on both sides
This is not balanced because the numbers don’t match
3. Multiply coefficients until they match – multiply the entire units
2 Al + O2 → Al2O3
Go to 6 Oxygens
IV. Balancing EquationsIV. Balancing Equations
Al + O22 Al2O3
4 3Al + 2 Al2O3 O2
Multiply each atom by 2
IV. Balancing Equations 2IV. Balancing Equations 2The 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
IV. Balancing Equations 3IV. Balancing Equations 3To 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
2NH3H2+N2
IV. Balancing Equations 2IV. 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:
N2
2NH3H2+ 3H2
IV. Don’t Forget: Diatomic ElementsIV. Don’t Forget: Diatomic Elements
Definition – 7 elements that NEVER occur Definition – 7 elements that NEVER occur as as singular atoms (singular atoms (always paired with an always paired with an
the same or different elementthe same or different element))
HH22 O O22 F F22 Br Br22 I I22 N N22 Cl Cl22
Ex: 2 HCl + 2K 2 KCl + H2
IV. Balancing Equations 3IV. Balancing Equations 3
1. Start with an unbalanced equation
2. Draw boxes around the compounds so you don’t mess with the groups
Don’t be threatened by how complex it looks!
IV. Balancing Equations 2IV. Balancing Equations 2
3. Make an element inventory – count number of atoms for each element on each side of the equation
IV. Balancing Equations 3IV. Balancing Equations 34. 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.•Let’s start with Sodium
NN
YY
YY
NN
•We have 2 in products, so I need 2 in reactants
Multiply reactant with sodium by 2 and recount atoms
223 3
1111
1111
2211
HH
SOSO44
OO
NaNa
ProductProductReactantReactantElementElement Balanced?Balanced?
1 21 2 YY
1 21 2
3 4 3 4
NN
IV. Balancing Equations 3IV. 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
NN
YY
NN
YY
2244
1111
1122
2222
HH
SOSO44
OO
NaNa
ProductProductReactantReactantElementElement Balanced?Balanced?
1 21 2 YY
2 4 2 4 YY
Helpful HintsHelpful Hints Balance hydrogen and oxygen lastBalance hydrogen and oxygen last
Balance polyatomic ions as a group Balance polyatomic ions as a group if present on if present on both reactants and productsboth reactants and products You can consider a polyatomic ion as a single You can consider a polyatomic ion as a single
elementelement
If the balancing starts to get very complex:If the balancing starts to get very complex: StopStop Start overStart over Select a different atom to balance first.Select a different atom to balance first.
Example Using PolyAtomicsExample Using PolyAtomicsBeforeBefore
MgCl2 + NaOH Mg(OH)2 + NaCl
1 Mg 1 Mg
2 Cl 1 Cl1 Na 1 Na
1 OH 2 OH
MgCl2 + 2 NaOH Mg(OH)2 + 2 NaCl
1 Mg 1 Mg
2 Cl 2 Cl2 Na 2 Na
2 (OH) 2 (OH)
AfterAfter
Types of Chemical Types of Chemical ReactionsReactions
Part VPart V
Classifying Chemical ReactionsClassifying Chemical Reactions
SynthesisSynthesisDecompositionDecompositionSingle replacementSingle replacementDouble ReplacementDouble ReplacementCombustionCombustion
SynthesisSynthesis
Definition – two or more substances Definition – two or more substances react to form ONE productreact to form ONE product Product is usually bigger or more Product is usually bigger or more
complex than either reactantcomplex than either reactant
A + B A + B AB AB
Hey baby let’s get jiggy.
SynthesisSynthesis
reaction of two elementsreaction of two elements
___Al + ___Cl2 ___AlCl3
Al3+ Cl1-
22 2233
DecompositionDecomposition
definition – one substance breaks definition – one substance breaks down down into two or more simpler into two or more simpler productsproducts
AB AB A + B A + B
Break yoself fool!
DecompositionDecomposition
Example reaction:Example reaction:
__ NaN3 (s) ___ Na (s) + ___ N2 (g)2 2 3
__ CaO (s) ___ Ca (s) + ___ O2 (g)2 2 1
Single Replacement ReactionsSingle Replacement Reactions
Definition – one element replaces Definition – one element replaces another element in a compound to another element in a compound to form new compoundform new compound
A + BX A + BX AX + BAX + B
I’m gon’ dance with yo’ lady
Double ReplacementDouble Replacement
Defn – exchange of Defn – exchange of cationscations between two between two ionic compoundsionic compounds
A B + C D AD + CBswitch
3 possible products of double 3 possible products of double replacement reactionsreplacement reactions
PrecipitatePrecipitateGasGasWaterWater
Reactivity Series (or Activity Series)Reactivity Series (or Activity Series)
More active will replace less activeMore active will replace less active Less active will NOT replace more activeLess active will NOT replace more active
metalsmetals
Li K Ca Na Mg Al Mn Zn Fe Ni Sn Pb Cu Ag Au
most active least active
F Cl Br IF Cl Br I
most active least active
halogenshalogens
examplesexamples
aluminum + iron (III) oxidealuminum + iron (III) oxide
Al Fe2O3+
Stronger?
Al3+ O2-
Al2O3Fe +
Fe3+ O2-
2 21 1
examplesexamples
silver + copper (I) nitratesilver + copper (I) nitrate
Ag CuNO3+
Stronger?
Cu1+ NO31-
NO RXN
examplesexamples
fluorine gas + sodium bromidefluorine gas + sodium bromide
F2 NaBr+
Stronger?
Br2NaF +
Na1+ F1-
221 1
examplesexamples
chlorine gas + hydrogen fluoridechlorine gas + hydrogen fluoride
Cl2 HF+
Stronger?
NO RXN
Example ProblemExample Problem
lithium iodide and aqueous silver nitrate lithium iodide and aqueous silver nitrate reactreact
Li1+ I1- Ag1+ NO31-
Li I AgNO3 LiNO3Ag I (s)+ +
CombustionCombustion
definition – compound reacts with Odefinition – compound reacts with O22
HydrocarbonHydrocarbon – compound w/ only – compound w/ only carbon and hydrogencarbon and hydrogen
CombustionCombustion
Combustion of hydrocarbonsCombustion of hydrocarbons
ALWAYS produces ALWAYS produces COCO22 and and HH22OO
CxHy + O2 CO2 + H2O
Ex problemEx problem
show combustion of propane (Cshow combustion of propane (C33HH88) gas) gas
C3H8 O2 CO2 H2O+ +1 5 3 4
I sell propane and propane accessories!