Mr. Matthew TotaroLegacy High SchoolHonors Chemistry

ChemicalReactions

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Chemical Reactions• Reactions involve

chemical changes in matter resulting in new substances.

• Reactions involve rearrangement and exchange of atoms to produce new molecules.Elements are not

changed into new elements during a reaction.

Thermite Reaction

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Example of a Chemical Reaction

Reactants Products

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Evidence for a Chemical Reaction• Look for evidence of a new substance.

• Visual clues (permanent).Color change. Precipitate formation.

Solid that forms when liquid solutions are mixed.

Gas bubbles. Large energy changes.

Container becomes very hot or cold (transfer of heat).

Emission of light.• Other clues.

New odor. Whooshing sound from a tube. Permanent new state.

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Evidence of a Chemical Change

Color Change

Formation of Solid PrecipitateFormation of a Gas

Emission of LightRelease or Absorption of Heat

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Evidence of Chemical Change, Continued

• In order to be ABSOLUTELY SURE that a chemical reaction has taken place, you need to go down to the molecular level and analyze the structures of the molecules at the beginning and at the end.

Is boiling watera chemical change?

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Practice—Decide Whether Each of the Following Involve a Chemical

Reaction.• Photosynthesis• Heating sugar until it turns black• Heating ice until it turns liquid• Digestion of food• Dissolving sugar in water• Burning of alcohol in a flambé dessert

Yes, CO2 and H2O combine into carbohydrates

Yes, sugar decomposing

No, molecules still same Yes, food decomposing and combining

with stomach acid

No, molecules still same

Yes, alcohol combining with O2 to make CO2 and H2O

Chemical Equations

Concise representation of a chemical reaction

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The Combustion of Methane

• Methane gas burns to produce carbon dioxide gas and gaseous water.Whenever something burns it combines

with O2(g).

Anatomy of a Chemical Equation

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

Anatomy of a Chemical Equation

Reactants appear on the left side of

the equation.

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

Anatomy of a Chemical Equation

Products appear on the right side of the

equation.

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

Anatomy of a Chemical Equation

The states of the reactants and products are written in

parentheses to the right of each compound.

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

Anatomy of a Chemical Equation

Coefficients are inserted to balance

the equation.

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

Subscripts and Coefficients Give Different Information

• Subscripts tell the number of atoms of each element in a molecule

Subscripts and Coefficients Give Different Information

• Coefficients tell the number of molecules

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Symbols Used in Equations

• Symbols used to indicate state after chemical.(g) = gas; (l) = liquid; (s) = solid.(aq) = aqueous = dissolved in

water.• Energy symbols used above the

arrow for decomposition reactions. ∆ = heat. hv = light.shock = mechanical.elec = electrical.

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The Law of Conservation of Mass

Law of Conservation of Mass“We may lay it down as an

incontestable axiom that, in all the operations of art and

nature, nothing is created; an equal amount of matter

exists both before and after the experiment. Upon this principle, the whole art of

performing chemical experiments depends.”--Antoine Lavoisier, 1789

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Conservation of Mass in a Reaction

• In a chemical reaction, matter cannot be created or destroyed.Therefore, the total mass cannot

change.And the total mass of the reactants

will be the same as the total mass of the products.

• In a chemical reaction, all the atoms present at the beginning are still present at the end.If all the atoms are still there, then

the mass will not change.

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Balancing Example• When magnesium metal burns in air,

it produces a white, powdery compound magnesium oxide.

Mg(s) + O2(g) → MgO(s)

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Another Balancing Example

• Under appropriate conditions at 1000°C, methane gas reacts with oxygen gas to produce gaseous carbon dioxide and steam

CH4(g) + O2(g) → CO2(g) + H2O(g)

Reaction Types

Synthesis (Combination) Reactions

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

• only one product

A + B AB

Synthesis (Combination) Reactions

• Examples:N2 (g) + 3 H2 (g) 2 NH3 (g)

C3H6 (g) + Br2 (l) C3H6Br2 (l)

2 Mg (s) + O2 (g) 2 MgO (s)

• Two or more substances react to form one product

2H2 (g) + O2 (g) 2H2O (g)

Decomposition Reactions

• a compound breaks down into 2 or more simpler substances

• only one reactant

AB A + B

Decomposition Reactions

• Examples:CaCO3 (s) CaO (s) + CO2 (g)

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

2 NaN3 (s) 2 Na (s) + 3 N2 (g)

• One substance breaks down into two or

more substances

Combustion Reactions

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

• the burning of any substance in O2 to produce heat

A + O2 B

Combustion Reactions

• Examples:CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (g)

• Rapid reactions that produce a

flame• Most often

involve hydrocarbons reacting with oxygen in the

air

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Whoosh Bottle Demo is a Combustion

Reaction

C. Johannesson

Single Replacement• one element replaces another in a

compoundmetal replaces metal (+)

nonmetal replaces nonmetal (-)

A + BC B + AC

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Single Replacement Example

C. Johannesson

Single ReplacementCu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

C. Johannesson

AB + CD AD + CB

Double Replacement

• ions in two compounds “change partners”

• cation of one compound combines with anion of the other

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Double Replacement Example

C. Johannesson

Double Replacement

Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)

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Practice—Classify the Following Reactions as Synthesis, Decomposition, Single

Displacement, or Double Displacement.

3 Mg(s) + 2 FeCl3(aq) 3 MgCl2(aq) + 2 Fe(s)

CO2(g) + H2O(l) H2CO3(aq)

3 KOH(aq) + H3PO4(aq) K3PO4(aq) + 3 H2O(l)

)(CO )CaO( )(CaCO 23 gss

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Practice—Classify the Following Reactions as Synthesis, Decomposition, Single

Displacement, or Double Displacement, Continued.

3 Mg(s) + 2 FeCl3(aq) 3 MgCl2(aq) + 2 Fe(s)

CO2(g) + H2O(l) H2CO3(aq)

3 KOH(aq) + H3PO4(aq) K3PO4(aq) + 3 H2O(l)

)(CO )CaO( )(CaCO 23 gss

Single replacement.

Double replacement.

Decomposition.

Synthesis.

Reaction Prediction

Synthesis Reactions

A + B AB

Al (s) + O2 (g)

Two substances combine to form a single compound.

Aluminum reacts with oxygen to produce aluminum oxide

Decomposition Reactions

K2CO3 (s) ∆

One compound breaks down into multiple substances (usually because of the

presence of heat).

AB A + B

Metal carbonate + heat → metal oxide + carbon dioxide gas

Combustion Reactions

CH4 (s) + O2 (g)

One substance reacts with Oxygen to form Carbon Dioxide and Water

A + O2 CO2 + H2O

Single Replacement Reactions

Cu (s) + AgNO3 (aq)

A + BC B + AC

A more active metal replaces a metal in a compound to form a new compound and an element

Double Replacement Reactions

AgNO3 (aq) + KCl (aq)

AB + CD AD + CB

Two compounds exchange partners to form two new compounds