Chemical Reactions Unit Learning Goal 4: Examine the Law of Conservation of Energy Learning Goal 5: Describe how electrochemical energy can be produced.

Chemical Reactions Unit

Learning Goal 4: Examine the Law of Conservation of Energy

Learning Goal 5: Describe how electrochemical energy can be produced in a chemical reaction

The Nature of Energy

Energy The ability to do work or produce heat. Potential Energy

Stored energy Kinetic Energy

Energy due to the motion of an object.

Law of Conservation of Energy

States that energy can be converted from one form to another but cannot be created nor destroyed. That is the energy in the universe is constant.

Temperature vs. Heat

Temperature A measurement of the random motions of the

components of a substance. Heat

The flow of energy due to a temperature difference.

H is negative in an

exothermic reaction.

Enthalpy (Heat Change)

Exothermic Energy is released from the reaction.

Feels hot.

Copyright © Houghton Mifflin Company

10-6

Figure 10.5: The energy changes accompanying the burning of a match.

Enthalpy (Heat Change)

Endothermic Energy is absorbed from the

environment. Feels cold.

H is positive in an endothermic

reaction.

Hess’s Law

Going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or two.

N2(g) + 2O2(g) 2NO (g) H = 180 kj

2NO + O2(g) 2NO2(g) H = -112 kj

N2(g) + 2O2(g) 2NO2(g) H = 68 kj

Characteristics of Enthalpy Changes

1. If the reaction is reversed, the sign of H is also reversed.

2. The magnitude of H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of H is multiplied by the same integer.

Thermodynamics

First Law of Thermodynamics The energy of the universe is constant.

E = q + w(delta) means a change in the function that

follows.

E = energy

q = heat

w = work

Units for Measuring HeatUnits for Measuring Heat

The Joule is the SI system unit for measuring heat.

The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree

1 cal = 4.184 j

Specific Heat Capacity

The amount of energy required to change the temperature of one gram of a substance by one Celsius degree.

Q = s x m x T Q = energy (heat required s = specific heat capacity m = mass of the sample (g) T = change of temperature (oC)

ENTROPY A measure of disorder or randomness. As randomness increases entropy (S)

increases. The entropy of the universe is always

increasing.

Copyright © Houghton Mifflin Company

10-14

Figure 10.10: Comparing the entropies of ice and steam.

Learning Goal 5

Electron Transfer ReactionsElectron Transfer ReactionsElectron Transfer ReactionsElectron Transfer Reactions

Electron transfer reactions are Electron transfer reactions are oxidation-oxidation-

reductionreduction or or redoxredox reactions. reactions.

Results in the generation of an electric Results in the generation of an electric

current (electricity) or be caused by current (electricity) or be caused by

imposing an electric current. imposing an electric current.

Therefore, this field of chemistry is often Therefore, this field of chemistry is often

called called ELECTROCHEMISTRY.ELECTROCHEMISTRY.

Terminology for Redox Terminology for Redox ReactionsReactionsTerminology for Redox Terminology for Redox ReactionsReactions

OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in oxygen.increase in oxidation number; increase in oxygen.

REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; increase oxidation number; decrease in oxygen; increase in hydrogen.in hydrogen.

OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; species —electron acceptor; species is reduced.is reduced.

REDUCING AGENTREDUCING AGENT—electron donor; species is —electron donor; species is oxidized.oxidized.

OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in oxygen.increase in oxidation number; increase in oxygen.

REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; increase oxidation number; decrease in oxygen; increase in hydrogen.in hydrogen.

OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; species —electron acceptor; species is reduced.is reduced.

REDUCING AGENTREDUCING AGENT—electron donor; species is —electron donor; species is oxidized.oxidized.

OXIDATION-REDUCTION REACTIONSOXIDATION-REDUCTION REACTIONS

Indirect Redox ReactionIndirect Redox Reaction

A battery functions by transferring A battery functions by transferring electrons through an external wire electrons through an external wire

from the reducing agent to the from the reducing agent to the oxidizing agent.oxidizing agent.

Electrochemical CellsElectrochemical CellsElectrochemical CellsElectrochemical Cells An apparatus that allows a An apparatus that allows a

redox reaction to occur by redox reaction to occur by transferring electrons transferring electrons through an external through an external connector.connector.

Product favored reaction Product favored reaction ---> ---> voltaic or galvanic cellvoltaic or galvanic cell ----> electric current----> electric current

Reactant favored reaction Reactant favored reaction ---> ---> electrolytic cellelectrolytic cell ---> ---> electric current used to electric current used to cause chemical change.cause chemical change.

Batteries are voltaic cellsBatteries are voltaic cells

AnodeAnode CathodeCathode

Basic Concepts Basic Concepts of Electrochemical Cellsof Electrochemical Cells

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENTCHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

Zn metal

Cu2+ ions

Zn metal

Cu2+ ions

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

Zn is oxidized Zn is oxidized and is the reducing agent and is the reducing agent Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+ is reduced is reduced and is the oxidizing agentand is the oxidizing agentCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)

To obtain a useful current, To obtain a useful current, we separate the oxidizing we separate the oxidizing and reducing agents so that and reducing agents so that electron transfer occurs thru electron transfer occurs thru an external wire. an external wire.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENTCHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

This is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell. cell.

A group of such cells is called a A group of such cells is called a batterybattery..

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

••Electrons travel thru external wire.Electrons travel thru external wire.Salt bridge Salt bridge allows anions and cations to allows anions and cations to move between electrode compartments.move between electrode compartments.

••Electrons travel thru external wire.Electrons travel thru external wire.Salt bridge Salt bridge allows anions and cations to allows anions and cations to move between electrode compartments.move between electrode compartments.

Zn --> ZnZn --> Zn2+2+ + 2e- + 2e- CuCu2+2+ + 2e- --> Cu + 2e- --> Cu

<--Anions<--AnionsCations-->Cations-->

OxidationOxidationAnodeAnodeNegativeNegative

OxidationOxidationAnodeAnodeNegativeNegative

RedReductionuctionCatCathodehodePositivePositive

RedReductionuctionCatCathodehodePositivePositive

RED CATRED CAT

Terms Used for Voltaic Terms Used for Voltaic CellsCells

Calculating Cell VoltageCalculating Cell Voltage

Balanced half-reactions can be added Balanced half-reactions can be added together to get overall, balanced together to get overall, balanced equation. equation.

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

If we know EIf we know Eoo for each half-reaction, we for each half-reaction, we could get Ecould get Eoo for net reaction. for net reaction.

TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALSTABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS

2

Eo (V)

Cu2+ + 2e- Cu +0.34

2 H+ + 2e- H 0.00

Zn 2+ + 2e- Zn -0.76

oxidizingability of ion

reducing abilityof element

To determine an oxidation from a To determine an oxidation from a reduction table, just take the opposite reduction table, just take the opposite sign of the reduction!sign of the reduction!

Zn/Cu Electrochemical Zn/Cu Electrochemical CellCell

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e- EEoo = +0.76 V = +0.76 VCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) EEoo = +0.34 V = +0.34 V------------------------------------------------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s) (aq) + Cu(s)

EEoo = +1.10 V = +1.10 V

Cathode, Cathode, positive, positive, sink for sink for electronselectrons

Anode, Anode, negative, negative, source of source of electronselectrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons ++

Charging a BatteryCharging a Battery

When you charge a battery, you are forcing the When you charge a battery, you are forcing the electrons backwards (from the + to the -). To do electrons backwards (from the + to the -). To do this, you will need a higher voltage backwards than this, you will need a higher voltage backwards than forwards. This is why the ammeter in your car often forwards. This is why the ammeter in your car often goes slightly higher while your battery is charging, goes slightly higher while your battery is charging, and then returns to normal.and then returns to normal.

In your car, the battery charger is called an In your car, the battery charger is called an alternator. If you have a dead battery, it alternator. If you have a dead battery, it could be the battery needs to be replaced OR could be the battery needs to be replaced OR the alternator is not charging the battery the alternator is not charging the battery properly.properly.

Dry Cell BatteryDry Cell Battery

Anode (-)Anode (-)

Zn ---> ZnZn ---> Zn2+2+ + 2e- + 2e-

Cathode (+)Cathode (+)

2 NH2 NH44++ + 2e- ---> + 2e- --->

2 2 NHNH33 + H + H22

Alkaline BatteryAlkaline Battery

Nearly same reactions Nearly same reactions as in common dry as in common dry cell, but under basic cell, but under basic conditions.conditions.

Anode (-): Anode (-): Zn + 2 OHZn + 2 OH-- ---> ZnO + H ---> ZnO + H22O + 2e-O + 2e-

Cathode (+): Cathode (+): 2 MnO2 MnO22 + H + H22O + 2e- ---> O + 2e- --->

MnMn22OO33 + 2 OH + 2 OH--

Mercury BatteryMercury Battery

Anode:Anode:

Zn is reducing agent under basic Zn is reducing agent under basic conditionsconditions

Cathode:Cathode:

HgO + HHgO + H22O + 2e- ---> Hg + 2 OHO + 2e- ---> Hg + 2 OH--

Lead Storage BatteryLead Storage Battery

Anode (-) Anode (-) EEoo = +0.36 V = +0.36 V

Pb + HSOPb + HSO44-- ---> PbSO ---> PbSO44 + H + H++ + 2e- + 2e-

Cathode (+) Cathode (+) EEoo = +1.68 V = +1.68 V

PbOPbO22 + HSO + HSO44-- + 3 H + 3 H++ + 2e- + 2e-

---> PbSO---> PbSO44 + +

2 H2 H22OO