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• Chemists use chemical equations to describe reactions they observe in the laboratory or in nature.

• Chemical equations provide us with the means to

1. summarize the reaction

2. display the substances that are reacting

3. show the products

4. indicate the amounts of all component substances in a reaction.

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The Chemical EquationThe Chemical Equation

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• Chemical reactions always involve change.

• Atoms, molecules or ions rearrange to form new substances.

• The substances entering the reaction are called reactants.

• The substances formed in the reaction are called products.

• During reactions chemical bonds are broken and new bonds are formed.

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• A chemical equation uses the chemical symbols and formulas of the reactants and products and other symbolic terms to represent a chemical reaction.

• A chemical equation is a shorthand expression for a chemical change or reaction.

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Balancing Equations

___W8 + ___B12 ___WBWB___W8 + ___B12 ___WBWB

•ReactantsReactants •ProductsProducts

Making Hot dogs:How many packages wieners & buns to buy so none is left over.

33 22 2424

http://www.youtube.com/watch?v=oYIHLUxzRr8

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Al + Fe2O3 Fe + Al2O3

reactants productsAl + Fe2O3 Fe + Al2O3

Chemical Equation

iron oxygen bonds break

aluminum oxygen bonds form

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Coefficients (whole numbers) are placed in front of substances to balance the equation and to indicate the number of units (atoms, molecules, moles, or ions) of each substance that is reacting.

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Al + Fe2O3 Fe + Al2O3

coefficient2 2

coefficient

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Conditions required to carry out the reaction may be placed above or below the arrow.

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Al + Fe2O3 Fe + Al2O3

coefficient2 2

coefficient

heat

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The physical state of a substance is indicated by symbols such as (l) for liquid.

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2Al(s) + Fe2O3(s) 2Fe(l) + Al2O3 (s)

All atoms present in the reactant must also be present in the products.

In a chemical reaction atoms are neither created nor destroyed.

(s) (l) (s)(s)

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Symbols UsedSymbols Usedin Chemical Reactionsin Chemical Reactions

Symbols UsedSymbols Usedin Chemical Reactionsin Chemical Reactions

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placed between substances

+symbol

plusmeaning

location

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placed between substances

symbol

resonancemeaning

location

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symbol

yieldsmeaning

between reactants and products

location

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symbol

equilibriummeaning

between reactants and products

location

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(s)symbol

solidmeaning

after formulalocation

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(l)symbol

liquidmeaning

location after formula

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(g)symbol

gasmeaning

location after formula

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(aq)symbol

aqueousmeaning

after formulalocation

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symbol

heatmeaning

written above location

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hsymbol

light energymeaning

written above location

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symbol

gas formationmeaning

after formulalocation

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symbol

precipitate formation

meaning

after formulalocation

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Writing andWriting andBalancing EquationsBalancing Equations

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• To balance an equation, adjust the number of atoms of each element so that they are the same on each side of the equation.

• Golden Rule of Balancing:

Balance with coefficients only!

Do not mess with the subscripts!!!

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Steps for Steps for Balancing EquationsBalancing Equations

Steps for Steps for Balancing EquationsBalancing Equations

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Step 1 Identify the reaction. Write a description or word equation for the reaction.

Mercury(II) oxide decomposes to form mercury and oxygen.

mercury(II) oxide → mercury + oxygen

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HgO Hg + O2

– The formulas of the reactants and products must be correct.

– The reactants are written to the left of the arrow and the products to the right of the arrow.

Step 2 Write the unbalanced (skeleton) equation.

The formulas of the reactants and products can never be changed.

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Step 3a Balance the equation.– Count and compare the number of atoms of each

element on both sides of the equation.

– Determine the elements that require balancing.

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2HgO 2Hg + O2

THE EQUATION IS BALANCED

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sulfuric acid + sodium hydroxide → sodium sulfate + water

Balance the Equation

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H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)2

Balance the Equation

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H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)2 2

THE EQUATION IS BALANCED

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butane + oxygen → carbon dioxide + water

Balance the Equation

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C4H10 (g) + O2 (g) → CO2(g) + H2O(l)4

Balance the Equation

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C4H10 (g) + O2 (g) → CO2(g) + H2O(l)4 5

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C4H10 (g) + O2 (g) → CO2(g) + H2O(l) 4 513 2

Remove fractions by multiplying by the lowest common multiple

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C4H10 (g) + O2 (g) → CO2(g) + H2O(l)1052 8 10

THE EQUATION IS BALANCED

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What Information Does What Information Does an Equation Tell Us?an Equation Tell Us?

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The meaning of a formulais context dependent.

The formula H2O can mean:

1. 2 H and 1 O atom

2. 1 molecule of water

3. 1 mol of water

4. 6.02 x 1023 molecules of water

5. 18.02 g of water

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In an equation formulas can represent units of individual chemical entities or moles.

H2 + Cl2 2HCl→

1 molecule H2 1 molecule Cl2 2 molecules HCl

1 mol H2 1 mol Cl2 2 mol HCl

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FormulasNumber of molecules

Number of atomsNumber of molesMole weights

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Types of Chemical Types of Chemical EquationsEquations

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Combination (synthesis) Decomposition (analysis) Single-Displacement (substitution) Double-Displacement (metathesis) Combustion (oxidation)

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Combination ReactionsCombination ReactionsCombination ReactionsCombination Reactions

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A + B AB

Two reactants combine to form one product.

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ExamplesExamples

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2Ca(s) + O2(g) 2CaO(s)

Metal + Oxygen → Metal Oxide

4Al(s) + 3O2(g) 2Al2O3(s)

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S(s) + O2(g) SO2(g)

Nonmetal + Oxygen → Nonmetal Oxide

N2(g) + O2(g) 2NO(g)

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2K(s) + F2(g) 2KF(s)

Metal + Nonmetal → Salt

2Al(s) + 3Cl2(g) 2AlCl3(s)

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Na2O(s) + H2O(l) 2NaOH(aq)

Metal Oxide + Water → Metal Hydroxide

CaO(s) + 2H2O(l) 2Ca(OH)2(aq)

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SO3(g) + H2O(l) H2SO4(aq)

Nonmetal Oxide + H2O(l) → Oxy-acid

N2O5(g) + H2O(l) 2HNO3(aq)

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Decomposition ReactionsDecomposition ReactionsDecomposition ReactionsDecomposition Reactions

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AB A + B

A single substance breaks down togive two or more different substances.

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ExamplesExamplesExamplesExamples

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2Ag2O(s) 4Ag(s) + O2(g)

Metal Oxide → Metal + Oxygen

Metal Oxide → Metal Oxide + Oxygen

2PbO2(s) 2PbO(s) + O2(g)

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Carbonate → CO2(g)

CaCO3(s) CaO(s) + CO2(g)

2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g)

Hydrogen carbonate → CO2(g)

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Miscellaneous Reactions

2KClO3(s) 2KCl(s) + O2(g)

2NaNO3(s) 2NaNO2(s) + O2(g)

2H2O2(l) 2H2O(l) + O2(g)

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Single Displacement Single Displacement ReactionsReactions

Single Displacement Single Displacement ReactionsReactions

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A + BC AC + B

One element reacts with a compound toreplace one of the elements of that compound.

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Mg(s) + HCl(aq) H2(g) + MgCl2(aq)

2Al(s) + 3H2SO4(aq) 3H2(g) + Al2(SO4)3(aq)

salt

Metal + Acid → Hydrogen + Salt

salt

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Na(s) + 2H2O(l) H2(g) + NaOH(aq)

Ca(s) + 2H2O(l) H2(g) + Ca(OH)2(aq)

Metal + Water → Hydrogen + Metal Hydroxide

metal hydroxide

metal hydroxide

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Metal + Water → Hydrogen + Metal Oxide

metal oxide

Fe(s) + 4H2O(g) 4H2(g) + Fe3O4(s)

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Double Displacement Double Displacement ReactionsReactions

Double Displacement Double Displacement ReactionsReactions

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AB + CD AD + CB

Two compounds exchange partners with each other to produce two different compounds.

The reaction can be thought of as an exchange of positive and negative groups.

A displaces C and combines with DB displaces D and combines with C

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The Following Accompany Double Displacement Reactions

• formation of a precipitate

• release of gas bubbles

• release of heat

• formation of water

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ExamplesExamples

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Acid Base Neutralization

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l)

acid + base → salt + water

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Formation of an Insoluble Precipitate

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)

↓

↓

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Metal Oxide + Acid

CuO(s) + 2HNO3(aq) Cu(NO3)2(aq) + H2O(l)

CaO(s) + 2HCl(aq) CaCl2(s) + H2O(l)

metal oxide + acid → salt + water

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Formation of a Gas

H2SO4(aq) + 2NaCN(aq) Na2SO4(aq) + 2HCN(g)

NH4Cl(aq) + NaOH(aq) NaCl(aq) + NH4OH(aq)

NH4OH(aq) NH3(g) + H2O(l)

indirect gas formation

↑

↑

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Combustion ReactionsCombustion ReactionsCombustion ReactionsCombustion Reactions

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Hydrocarbon reacts with oxygen to produce carbon dioxide and water

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ExamplesExamples

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Combustion of Methane

CH4 (g) + O2 (g) CO2 (g) + H2O (l)

Balance by first balancing C, then H, and finally O

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1 CH4 (g) + O2 (g) 1 CO2 (g) + H2O (l)

1 CH4 (g) + O2 (g) 1 CO2 (g) + 2 H2O (l)

1 CH4 (g) + 2 O2 (g) 1 CO2 (g) + 2 H2O (l)

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Combustion of Ethane

C2H6 (g) + O2 (g) CO2 (g) + H2O (l)

Balance by first balancing C, then H, and finally O.Use fractions if needed to balance, then change to whole numbers.

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1 C2H6 (g) + O2 (g) 2 CO2 (g) + H2O (l)

1 C2H6 (g) + O2 (g) 2 CO2 (g) + 3 H2O (l)

1 C2H6 (g) + O2 (g) 2 CO2 (g) + 3 H2O (l)

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2

2 C2H6 (g) +7 O2 (g) 4 CO2 (g) + 6 H2O (l)

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