pearson prentice hall physical science: concepts in action chapter 7 chemical reactions
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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?
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