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ReactionsReactions

CH.3Balancing Reactions

Reaction Types

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A Chemical ReactionA Chemical Reaction

Reactants Products

Types of Chemical reactions:

There are many types of reactions butmost can be classified into a few simple

reactions types.

How do you know a chemical reaction has occurred?

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Chemical equationsChemical equations

Chemist’s shorthand to describe a reaction.

It shows:•All reactants and products•The state of all substances•Any conditions used in the reaction

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

ReactantReactant Products Products

A balanced equation shows the relationshipbetween the quantities of all reactants and products.

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Balancing chemical equationsBalancing chemical equations

Each side of a chemical equation must have the same number of each type of atom.

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

Reactants Products1 Ca 1 Ca

1 C 1 C

3 O 3 O

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Balancing chemical equationsBalancing chemical equations

Step 1Step 1Count the number of atoms of each element on each side of the equation.

Step 2Step 2 Determine which atom numbers are not balanced.

Step 3Step 3 Balance one atom at a time by using coefficients in front of

one or more substances.

Step 4Step 4Repeat steps 1-3 until everything is balanced.

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Chemical ChangesChemical Changes

Temperature Changes

Light

Gas formation

Precipitation

Color changes

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Types of Chemical ReactionsTypes of Chemical ReactionsReactions involving electron transfer

–synthesis or combination–decomposition–Electrochemical or single replacement–Combustion of hydrocarbons.

Reactions that involve rearrangement but not necessarily involve electron transfer–Metathesis or Double Substitution–Precipitation reactions and –Acid Base Reactions

Polymerization reaction

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Properties ofProperties ofaqueous solutionsaqueous solutions

There are two general classes of solutes.

ElectrolyticElectrolytic•ionic compounds in polar solvents•dissociate in solution to make ions•conduct electricity•may be strong (100% dissociation) or

weak (less than 100%)

NonelectrolyticNonelectrolytic•do not conduct electricity•solute is dispersed but does not

dissociate

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Dissolving ionic compoundsDissolving ionic compounds

When an ionic solid dissolves in water, the solvent removes ions from the crystal.

NaCl + H2O Na+(aq) + Cl- (aq)

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Dissolving covalent Dissolving covalent compoundscompounds

Covalent compounds do not dissociate.C6H12O6 C6H12O6 (aq)

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Ionic equationsIonic equations

When ionic substances dissolve in water, they dissociate into ions.

AgNO3 Ag+ + NO3

-

KCl K+

+ Cl-

When a reaction occurs, only some of the ions are actually involved in the reaction.

AgAg++ + NO3

- + K

+ + ClCl- AgClAgCl(s(s) + K+ +

NO3-

H2O

H2O

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Ionic equationsIonic equations

To help make the reaction easier to see, we commonly list only the species actually involved in the reaction.

Molecular equationMolecular equationKCl + AgNO3 AgCl(s) + KNO3

Full ionic equationFull ionic equation

AgAg++ + NO3

- + K

+ + ClCl- AgClAgCl(s)(s) + K+ + NO3

-

Net ionic equationNet ionic equation

AgAg++ + ClCl-- AgClAgCl(s)(s)

NO3- and K

+ are referred to as spectatorspectator ions.

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Some simple solubility rulesSome simple solubility rules

• All acids are soluble.

• All Na+, K+ and NH4+ salts are soluble.

• All nitrate and acetate salts are soluble.

• All chlorides except AgCl and Hg2Cl2 are soluble. PbCl2 is slightly soluble.

• All sulfates are soluble except PbSO4, Hg2SO4, SrSO4 and BaSO4. Ag2SO4 and CaSO4 are slightly soluble.

• All sulfides are insoluble except those of the Group IA (1), IIA (2) and ammonium sulfide.

• All hydroxides are insoluble except those of the group IA(1) and Ba(OH)2. Sr(OH)2 and Ca(OH)2 are slightly soluble.

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Metathesis ReactionsMetathesis Reactions

Precipitation reactionsthe formation of a solid upon mixing two

solutions.Gas formation

The formation of a gas when two mixtures are reacted

Acid Base titrationThe neutralization of acids and bases to

form a salt in water.

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PrecipitationPrecipitation

NaCl + AgNO3 = AgCl (ppt) + NaNO3

Pb(NO3)2 + K2CrO4 = PbCrO4 (ppt) + KNO3

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Gas FormationGas Formation

Vinegar + Baking soda = Carbon dioxide

CH3COOH + NaHCO3 = NaCH3COO + H2O + CO2

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Acid Base NeutralizationAcid Base Neutralization

Vinegar + Ammonia = Salt + water

CH3COOH + NH4OH = NH4CH3COO + H2O

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Common REDOX ReactionsCommon REDOX Reactions

Combustion

Corrosion

Photosynthesis

Kreb’s Cycle

Synthesis and Decomposition

Single Replacement

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Oxidation numbers and Oxidation numbers and the periodic tablethe periodic table

Some observed trends in compounds.Some observed trends in compounds.

• Metals have positive oxidation numbers.

• Transition metals typically have more than one oxidation number.

• Nonmetals and semimetals have both positive and negative oxidation numbers.

• No element exists in a compound with an oxidation number greater than +8.

• The most negative oxidation numbers equals 8 - the group number

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Oxidation NumbersOxidation Numbers

1. An atom in its elemental state has an oxidation number of 0.

2. An atom in a monatomic ion has an oxidation number identical to its charge.

3. An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have it were a monatomic ion.

4. The sum of the oxidation numbers is 0 for a neutral compound and equal to the net charge for a polyatomic ion.

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Identifying oxidation-Identifying oxidation-reduction reactions.reduction reactions.

Oxidation-Reduction - REDOXOxidation-Reduction - REDOX

A chemical reaction where there is a net change in the oxidation number of one or more species.

Both an oxidation and a reduction must occur during the reaction.

Mg (s) + Cl2 (g) MgCl2 (s)

Here the oxidation number of Mg has changed fromzero to +2. Cl has changed from zero to -1.

Here the oxidation number of Mg has changed fromzero to +2. Cl has changed from zero to -1.

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REDOX reactionsREDOX reactions

OxidationOxidationAn increase in oxidation number.

ReductionReductionA decrease in oxidation number.

If the oxidation number of any element changes in the course of a reaction, the reaction is oxidation-reduction.

Example.Example.

2 Fe(NO3)3 (aq) + Zn(s) 2 Fe(NO3)2 (aq) + Zn(NO3)2 (aq)

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Half ReactionsHalf Reactions

The reactions that show the loss or gain of electrons.

2 Mg + O2 2 MgO

What are the oxidation states of each of the atoms in the above reaction?

Mg Mg+2

O2 2 O2-

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OxidationOxidation

2 Mg + O2 2 MgO

Mg Mg+2

12 p, 12 e 12 p, 10 e

Mg Mg+2 + 2 e

The loss of electrons

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Single replacement reactionSingle replacement reaction

Where one element displaces another in a chemical compound.

H2 + CuO Cu + H2O

• In this example, hydrogen replaces copper.

• This type of reaction always involves oxidation and reduction (REDOX).

• Since one species is replacing another, there are no spectator ions.

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Single ReplacementSingle Replacement

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Synthesis ReactionSynthesis Reaction

The formation of a new compound from several substances

2 H2 + O2 = 2 H2O2 Na + Cl2 = 2 NaCl

CaO (lime water) + CO2 = CaCO3 (s)

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SynthesisSynthesis

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Decomposition ReactionsDecomposition Reactions

HgO = Hg + O2

H2O2= H2O + O2

with MnO2 catalyst

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1774: Antoine Lavoisier solves the 1774: Antoine Lavoisier solves the question of the burning process and question of the burning process and

chemical processeschemical processes

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CombustionCombustion

The burning of a substance in the presence of oxygen.Gasoline burns when oxygen is taken into your engine, the products of the reaction are carbon dioxide and water

C8H18 + O2 = CO2 + H2OFe + O2 = Fe2O3

(rust)

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