Pearson Prentice Hall Pearson Prentice Hall

Physical Science: Physical Science: Concepts in ActionConcepts in Action

Chapter 7 Chapter 7

Chemical Chemical ReactionsReactions

7.1 Describing Reactions 7.1 Describing Reactions

Objectives:Objectives: 1. Interpret chemical equations 1. Interpret chemical equations

in terms of reactants, products in terms of reactants, products and conservation of massand conservation of mass

2. Balance chemical equations 2. Balance chemical equations by manipulating coefficientsby manipulating coefficients

3. Explain what a mole is and 3. Explain what a mole is and calculate molar masscalculate molar mass

Reactants & ProductsReactants & Products During a chemical equation, reactants During a chemical equation, reactants

(the stuff you start with) are changed into (the stuff you start with) are changed into products (the stuff you finish with)products (the stuff you finish with)

The products actually are composed of The products actually are composed of the same stuff you started with, but the the same stuff you started with, but the molecules have been rearranged by molecules have been rearranged by chemical bonding to produce completely chemical bonding to produce completely new compounds with unique physical new compounds with unique physical propertiesproperties

A basic chemical equation looks like this: A basic chemical equation looks like this:

REACTANTS PRODUCTSREACTANTS PRODUCTS

A chemical reaction is when A chemical reaction is when atoms interact with each other to atoms interact with each other to form new substancesform new substances

What you put in = reactantWhat you put in = reactant What comes out = productWhat comes out = product Mass of reactants = mass of Mass of reactants = mass of

productsproducts MASS IS CONSERVEDMASS IS CONSERVED

Conservation of MassConservation of Mass Definition: a chemical equation is Definition: a chemical equation is

a chemical reaction in which a chemical reaction in which reactants and products reactants and products (separated by an arrow) are (separated by an arrow) are expressed as formulasexpressed as formulas

The Law of Conservation of Mass The Law of Conservation of Mass states that mass is neither states that mass is neither created nor destroyed during a created nor destroyed during a chemical reactionchemical reaction

Balancing Balancing Equations/Manipulating Equations/Manipulating

CoefficientsCoefficients In order to show that mass is conserved In order to show that mass is conserved

during a reaction, a chemical equation during a reaction, a chemical equation must be balancedmust be balanced

Definition: a coefficient is the number Definition: a coefficient is the number that appears BEFORE a formulathat appears BEFORE a formula

Recall that subscripts are used to Recall that subscripts are used to balance a formulabalance a formula

Coefficients are used to balance the Coefficients are used to balance the equationequation

Steps to balancing a chemical Steps to balancing a chemical equation: equation:

1. Write out the chemical equation1. Write out the chemical equation 2. Balance the most unusual element 2. Balance the most unusual element

or compound first, use subscripts or compound first, use subscripts (charges) as a guide (charges) as a guide

3. Continue with other elements, 3. Continue with other elements, ending with the most commonending with the most common

Example: Balance the chemical Example: Balance the chemical equationequation

ZnClZnCl2 2 + HF → ZnF + HCl + HF → ZnF + HCl

Moles & Molar MassMoles & Molar Mass Definition: a mole is the counting unit Definition: a mole is the counting unit

that measures the amount of substancethat measures the amount of substance A mole contains 6.02 x 10A mole contains 6.02 x 102323 particles particles

(molecules, atoms, ions) of substance(molecules, atoms, ions) of substance Definition: molar mass is the mass of Definition: molar mass is the mass of

one moleone mole Molar mass is calculated by adding up Molar mass is calculated by adding up

the atomic masses of each element the atomic masses of each element (multiplied by the subscript)(multiplied by the subscript)

Example: COExample: CO22 carbon=12g oxygen x 2 carbon=12g oxygen x 2 = 32g & 12 + 32= 44g = 32g & 12 + 32= 44g

The molar mass of COThe molar mass of CO22 is 44 g is 44 g

7.2 Types of Reactions7.2 Types of Reactions Objectives:Objectives: 1. Classify chemical reactions as 1. Classify chemical reactions as

synthesis (also called synthesis (also called combination), decomposition combination), decomposition (decomp), single replacement (decomp), single replacement (single displacement) double (single displacement) double replacement (double replacement (double displacement) or combustion displacement) or combustion reactionsreactions

Synthesis or Combination Synthesis or Combination ReactionsReactions

There are millions of substances There are millions of substances but only five general types of but only five general types of reactionsreactions

The first is a The first is a synthesis reactionsynthesis reaction where two substances combine where two substances combine to form a new more complex to form a new more complex substancesubstance

general form general form A + B → ABA + B → AB Example: 2HExample: 2H22 + O + O22 → 2H → 2H22OO

DecompositionDecomposition

The second reaction type is The second reaction type is decompositiondecomposition, where one large , where one large substance breaks down to two or substance breaks down to two or more smaller substancesmore smaller substances

general form AB → A + Bgeneral form AB → A + B Example: 2NaClOExample: 2NaClO33 → 2NaCl + 3O → 2NaCl + 3O22

Single Replacement Single Replacement (Displacement)(Displacement)

The next reaction type is a The next reaction type is a single replacementsingle replacement reaction reaction where atoms of one element where atoms of one element take the place of an element in a take the place of an element in a compoundcompound

general form XA + B → A + XBgeneral form XA + B → A + XB example 3CuClexample 3CuCl22 + 2Al → 2AlCl + 2Al → 2AlCl33

+ 3Cu+ 3Cu

Double Replacement Double Replacement (Displacement)(Displacement)

A reaction type with a similar A reaction type with a similar name is the name is the double double displacement displacement where two where two compounds exchange their compounds exchange their elements elements

general form AX + BY → AY + BXgeneral form AX + BY → AY + BX example : AgNOexample : AgNO33 + HCl → AgCl + HCl → AgCl

+ HNO+ HNO33

CombustionCombustion The final type of reaction is a The final type of reaction is a

combustion combustion reaction where a reaction where a compound (usually a hydrocarbon) compound (usually a hydrocarbon) and oxygen burnand oxygen burn

With enough oxygen the products With enough oxygen the products are carbon dioxide and waterare carbon dioxide and water

without enough the products are without enough the products are carbon monoxide and watercarbon monoxide and water

general form A + Ogeneral form A + O22 → H → H22O + COO + CO22

example 2CHexample 2CH44 + 4O + 4O22 → 4H → 4H22O + 2COO + 2CO22

7.3 Energy Changes in 7.3 Energy Changes in ReactionsReactions

Objectives:Objectives: 1. Describe the energy changes 1. Describe the energy changes

that take place during chemical that take place during chemical reactionsreactions

2. Classify chemical reactions as 2. Classify chemical reactions as exothermic or endothermicexothermic or endothermic

3. Explain how energy is 3. Explain how energy is conserved during chemical conserved during chemical reactionsreactions

Energy Changes & Chemical Energy Changes & Chemical ReactionsReactions

Indications of a chemical reaction Indications of a chemical reaction include color change, gas formation, include color change, gas formation, and temperature changeand temperature change

Atoms are just rearranged as bonds Atoms are just rearranged as bonds are broken and new ones formedare broken and new ones formed

Breaking bonds requires energyBreaking bonds requires energy Forming bonds releases energyForming bonds releases energy

Definition: chemical energy is Definition: chemical energy is the energy stored in the the energy stored in the chemical bonds of a substancechemical bonds of a substance

Chemical reactions involve the Chemical reactions involve the breaking of chemical bonds in breaking of chemical bonds in the reactants & the formation of the reactants & the formation of chemical bonds in the productschemical bonds in the products

Q: Does breaking chemical bonds Q: Does breaking chemical bonds require energy or release require energy or release energy?energy?

Exothermic & EndothermicExothermic & Endothermic Definition: Reactions that release Definition: Reactions that release

energy are exothermicenergy are exothermic Definition: Reactions that absorb Definition: Reactions that absorb

energy are endothermicenergy are endothermic ENERGY IS CONSERVED. ENERGY IS CONSERVED. Example: for an exothermic Example: for an exothermic

reaction energy stored in bonds reaction energy stored in bonds (chemical energy) = energy (chemical energy) = energy released during reactionreleased during reaction

Conservation of EnergyConservation of Energy

Law of Conservation of Energy: Law of Conservation of Energy: during a chemical reaction, during a chemical reaction, energy is neither created nor energy is neither created nor destroyeddestroyed

In exothermic reactions, the In exothermic reactions, the chemical energy of the reactants chemical energy of the reactants is converted to heat plus the is converted to heat plus the chemical energy of the productschemical energy of the products

7.4 Reaction Rates7.4 Reaction Rates

Objectives:Objectives: 1. Explain what a reaction rate is1. Explain what a reaction rate is 2. Describe factors that affect 2. Describe factors that affect

chemical reaction rateschemical reaction rates

What a Reaction Rate isWhat a Reaction Rate is

Definition: the reaction rate is Definition: the reaction rate is the rate at which reactants the rate at which reactants change into products over timechange into products over time

Reaction rates tell you how fast a Reaction rates tell you how fast a reaction is goingreaction is going

Factors that Affect Reaction Factors that Affect Reaction RatesRates

Reactions can happen at different Reactions can happen at different speedsspeeds

Factors influencing reaction rates:Factors influencing reaction rates: Temperature: rxn’s go faster at Temperature: rxn’s go faster at

higher temphigher temp Surface area: large surface area Surface area: large surface area

speeds up reactionspeeds up reaction Large particles have less surface area than Large particles have less surface area than

the same amount of small particles the same amount of small particles (blocks)(blocks)

Stirring or agitation (shaking) Stirring or agitation (shaking) increases exposure of reactants & increases exposure of reactants & increases reaction rateincreases reaction rate

Concentrated solutions react faster Concentrated solutions react faster (better chance of atoms hitting each (better chance of atoms hitting each other)other)

Rxn’s go quicker at higher pressure Rxn’s go quicker at higher pressure Catalysts speed up the reaction by Catalysts speed up the reaction by

lowering the energy needed (do not lowering the energy needed (do not influence the products)influence the products)

Enzymes are biological catalysts Enzymes are biological catalysts

7.5 Equilibrium7.5 Equilibrium

Objectives:Objectives: 1. Identify and describe physical 1. Identify and describe physical

and chemical equilibriaand chemical equilibria 2. Describe the factors affecting 2. Describe the factors affecting

chemical equilibriumchemical equilibrium

Physical & Chemical Physical & Chemical EquilibriumEquilibrium

Definition: equilibrium is a state in Definition: equilibrium is a state in which the forwards and reverse which the forwards and reverse paths take place at the same ratepaths take place at the same rate

Some reactions are reversible, a Some reactions are reversible, a double arrow indicates the reaction double arrow indicates the reaction can go either waycan go either way

Equilibrium is when the amount of Equilibrium is when the amount of reactants being turned into products reactants being turned into products is the same as the amount of is the same as the amount of products being turned into reactantsproducts being turned into reactants

reactants ↔ products reactants ↔ products

When a physical change does When a physical change does not go to completion, a physical not go to completion, a physical equilibrium is established equilibrium is established between the forward and reverse between the forward and reverse changeschanges

HH22O(l) HO(l) H22O(g) O(g) Definition: a reversible reaction Definition: a reversible reaction

is a reaction in which the is a reaction in which the conversion of the reactants into conversion of the reactants into products can happen products can happen simultaneously (at the same simultaneously (at the same time)time)

When a chemical reaction does When a chemical reaction does not go to completion, a chemical not go to completion, a chemical equilibrium is established equilibrium is established between the forward and reverse between the forward and reverse reactionsreactions

2SO2SO22(g) + O(g) + O22(g) 2SO(g) 2SO33(g)(g) Q: What happens during Q: What happens during

chemical equilibrium?chemical equilibrium?

Factors Affecting Chemical Factors Affecting Chemical EquilibriumEquilibrium

LeChatelier’s principle predicts LeChatelier’s principle predicts what will happen to equilibrium what will happen to equilibrium as a change is introduced to the as a change is introduced to the systemsystem

LeChatelier’s principle: if a LeChatelier’s principle: if a change is made to a system at change is made to a system at equilibrium the system will shift equilibrium the system will shift to oppose or relieve the changeto oppose or relieve the change

CONDITIONCONDITION• Tempera-Tempera-

tureture

• PressurePressure

• Concentra-Concentra-tion tion

REACTIONREACTION• Increasing temp Increasing temp

favors the reaction favors the reaction that absorbs energythat absorbs energy

• Increasing pressure Increasing pressure favors the reaction favors the reaction that produces less that produces less gasgas

• Increasing Increasing concentration favors concentration favors the reaction that the reaction that produces less of that produces less of that substancesubstance

Example: NExample: N22 (g)(g)+ H+ H2 (g) 2 (g) ↔ NH↔ NH3 (g)3 (g) + + heat heat

Which way will the equilibrium shift Which way will the equilibrium shift when the reaction is cooled? when the reaction is cooled? When the pressure is raised? When the pressure is raised? When more hydrogen is added?When more hydrogen is added?