Chapter 6Chapter 6

Chemical ReactionsChemical Reactions

Section 1: The Nature Section 1: The Nature of of Chemical Reactions

Chemical reactions occur when occur when substances undergo chemical changes substances undergo chemical changes to form NEW substances.to form NEW substances.– Examples: Grow, ripen, decay , burn, rust, Examples: Grow, ripen, decay , burn, rust,

change color, produce a gas, digestchange color, produce a gas, digest– Atoms are rearranged as bonds are broken Atoms are rearranged as bonds are broken

and new ones are formed.and new ones are formed.

Reaction Format:Reaction Format:

HH22O --- O --- >> H H22 and O and O22

ReactantsReactants ProductsProducts

Energy and ReactionsEnergy and Reactions

Endothermic reactions:Endothermic reactions:– When more energy is required to When more energy is required to

break bonds in the reactants than is break bonds in the reactants than is given off by forming bonds in the given off by forming bonds in the productsproducts

– Example: Example: photosynthesis– Products might feel colderProducts might feel colder

Exothermic reactions:Exothermic reactions:– More energy is released as the More energy is released as the

products form than is absorbed to products form than is absorbed to break bonds in the reactantsbreak bonds in the reactants

– Examples: zinc reacting with HCl, Examples: zinc reacting with HCl, reaction of isooctane and oxygenreaction of isooctane and oxygen

– Temperature in the surroundings Temperature in the surroundings risesrises

– Quiz

Section 2: Reaction Types Synthesis reaction: substances Synthesis reaction: substances

join to form a more complex join to form a more complex compound than the reactantscompound than the reactants

A + B ---A + B --->> AB AB

2Na + Cl2Na + Cl22 --- --->> 2NaCl 2NaCl

2H2 + O2 ---> 2H2O

Decomposition Decomposition ReactionReaction More complex substances are More complex substances are

broken apart to form simpler broken apart to form simpler productsproducts

AB ---AB --->> A + B A + B

HH22O ---O --->> H H22 + O + O22

Combustion Combustion

Oxygen is always a reactant, Oxygen is always a reactant, water is also a common productwater is also a common product

The oxidation reaction of an The oxidation reaction of an organic compound releasing heatorganic compound releasing heat

CHCH44 + O + O22 --- --->> CO CO22 + H + H22OO

Single-Replacement Single-Replacement ReactionsReactions

One element takes the place of One element takes the place of another element in a compoundanother element in a compound

AX + B ---AX + B --->> A + BX A + BX

3CuCl3CuCl22 + 2Al --- + 2Al --->> 2AlCl 2AlCl33 + 3Cu + 3Cu

Double-Replacement Double-Replacement ReactionReaction

A compound forms from the A compound forms from the apparent exchange of atoms apparent exchange of atoms between 2 compoundsbetween 2 compounds

AX + BY AX + BY BX + AY BX + AY

Pb(NOPb(NO33))22 + K + K22CrOCrO44 PbCrO PbCrO44 + + KNOKNO33

REDOX reactionREDOX reaction

Oxidation-reduction reaction: a chemical Oxidation-reduction reaction: a chemical change in which a substance is oxidized change in which a substance is oxidized (loses electrons) and another is reduced (loses electrons) and another is reduced (gains electrons):(gains electrons):

Fe + OFe + O22 Fe Fe22OO33

Other examples: respiration and combustionOther examples: respiration and combustionReviewDrawings

Diatomic ElementsDiatomic Elements

These elements, when alone, These elements, when alone, exist diatomically (2 atoms – exist diatomically (2 atoms – share electrons):share electrons):

HH22, N, N22, O, O22, Cl, Cl22, Br, Br22, I, I22, F, F22

Section 3: Section 3: Balancing Chemical Balancing Chemical EquationsEquations

Law of Conservation of Mass: the number of Law of Conservation of Mass: the number of atoms on the left side must equal the atoms on the left side must equal the number of atoms on the right.number of atoms on the right.

Rules for balancing:Rules for balancing:– ONLY coefficients (in front of the formula) can be ONLY coefficients (in front of the formula) can be

added to either or both sides of the equation.added to either or both sides of the equation.– NO chemical formulas can be changed (subscripts NO chemical formulas can be changed (subscripts

cannot be changed)cannot be changed) Hints:Hints:

– Save solo elements for last.Save solo elements for last.– If there is a certain polyatomic ion on both sides, If there is a certain polyatomic ion on both sides,

don’t count the atoms within the ion separately.don’t count the atoms within the ion separately.– Know diatomic elements: HKnow diatomic elements: H22, N, N22, O, O22, F, F22, Cl, Cl22, Br, Br22, I, I22 . .

Try some balancing:Try some balancing:

QuizQuiz More . . . More . . .

Section 4: Rate of Section 4: Rate of reactionsreactionsChemical reactions can be increased by:Chemical reactions can be increased by: Increasing temperatureIncreasing temperature Increasing the concentration of one Increasing the concentration of one

substance in an equilibrium reaction substance in an equilibrium reaction favors the reaction that produces less of favors the reaction that produces less of that substance: Le Chatelier’s principlethat substance: Le Chatelier’s principle

Adding energyAdding energy Increasing pressureIncreasing pressure CatalystsCatalysts Increasing surface areaIncreasing surface area

EquilibriumEquilibrium

Chemical equilibrium is a state of Chemical equilibrium is a state of balance in which the rate of reaction is balance in which the rate of reaction is equal in both directions and the equal in both directions and the concentration of products and reactants concentration of products and reactants remains the sameremains the same

Different factors can affect equilibrium: Different factors can affect equilibrium: – Increasing temperature favors a reaction Increasing temperature favors a reaction

that absorbs energythat absorbs energy– Increasing pressure favors a reaction that Increasing pressure favors a reaction that

reduces the production of gasreduces the production of gas– Increasing concentration favors a reaction Increasing concentration favors a reaction

that produces less of that substancethat produces less of that substance