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. N 2(g) + 2O 2(g)  2NO (g) H = 180 kj 2NO + O 2(g)  2NO 2(g) H = -112 kj N 2(g) + 2O 2(g)  2NO 2(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 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 = 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 ( o C)

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 Figure 10.10: Comparing the entropies of ice and steam.

Learning Goal 5

Electron Transfer Reactions Electron transfer reactions are oxidation- reduction or redox reactions. Electron transfer reactions are oxidation- reduction or redox reactions. Results in the generation of an electric current (electricity) or be caused by imposing an electric current. Results in the generation of an electric current (electricity) or be caused by imposing an electric current. Therefore, this field of chemistry is often called ELECTROCHEMISTRY. Therefore, this field of chemistry is often called ELECTROCHEMISTRY.

Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. OXIDIZING AGENT—electron acceptor; species is reduced. OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized. REDUCING AGENT—electron donor; species is oxidized. OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. OXIDATION—loss of electron(s) by a species; increase in oxidation number; increase in oxygen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen. OXIDIZING AGENT—electron acceptor; species is reduced. OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized. REDUCING AGENT—electron donor; species is oxidized.

OXIDATION-REDUCTION REACTIONS Indirect Redox Reaction A battery functions by transferring electrons through an external wire from the reducing agent to the oxidizing agent.

Electrochemical Cells An apparatus that allows a redox reaction to occur by transferring electrons through an external connector. An apparatus that allows a redox reaction to occur by transferring electrons through an external connector. Product favored reaction -- -> voltaic or galvanic cell --- -> electric current Product favored reaction -- -> voltaic or galvanic cell --- -> electric current Reactant favored reaction -- -> electrolytic cell ---> electric current used to cause chemical change. Reactant favored reaction -- -> electrolytic cell ---> electric current used to cause chemical change. Batteries are voltaic cells

AnodeCathode Basic Concepts of Electrochemical Cells

CHEMICAL CHANGE ---> ELECTRIC CURRENT With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.” Zn is oxidized and is the reducing agent Zn(s) ---> Zn 2+ (aq) + 2e- Zn is oxidized and is the reducing agent Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e- ---> Cu(s)

To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire. To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire. CHEMICAL CHANGE ---> ELECTRIC CURRENT This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery.

Electrons travel thru external wire. Salt bridge allows anions and cations to move between electrode compartments. Salt bridge allows anions and cations to move between electrode compartments. Electrons travel thru external wire. Salt bridge allows anions and cations to move between electrode compartments. Salt bridge allows anions and cations to move between electrode compartments. Zn --> Zn e- Cu e- --> Cu <--AnionsCations--> OxidationAnodeNegativeOxidationAnodeNegative Reduction Cathode Positive Reduction Cathode Positive RED CAT

Terms Used for Voltaic Cells

Calculating Cell Voltage Balanced half-reactions can be added together to get overall, balanced equation. Balanced half-reactions can be added together to get overall, balanced equation. Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) Zn(s) ---> Zn 2+ (aq) + 2e- Cu 2+ (aq) + 2e- ---> Cu(s) Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) If we know E o for each half-reaction, we could get E o for net reaction.

TABLE OF STANDARD REDUCTION POTENTIALS 2 E o (V) Cu e- Cu H + + 2e- H0.00 Zn e- Zn-0.76 oxidizing ability of ion reducing ability of element To determine an oxidation from a reduction table, just take the opposite sign of the reduction!

Zn/Cu Electrochemical Cell Zn(s) ---> Zn 2+ (aq) + 2e-E o = V Cu 2+ (aq) + 2e- ---> Cu(s)E o = V Cu 2+ (aq) + Zn(s) ---> Zn 2+ (aq) + Cu(s) E o = V E o = V Cathode, positive, sink for electrons Anode, negative, source of electrons +

Charging a Battery When you charge a battery, you are forcing the electrons backwards (from the + to the -). To do this, you will need a higher voltage backwards than forwards. This is why the ammeter in your car often goes slightly higher while your battery is charging, and then returns to normal. In your car, the battery charger is called an alternator. If you have a dead battery, it could be the battery needs to be replaced OR the alternator is not charging the battery properly.

Dry Cell Battery Anode (-) Zn ---> Zn e- Cathode (+) 2 NH e- ---> 2 NH 3 + H 2

Alkaline Battery Nearly same reactions as in common dry cell, but under basic conditions. Anode (-): Zn + 2 OH - ---> ZnO + H 2 O + 2e- Cathode (+): 2 MnO 2 + H 2 O + 2e- ---> Mn 2 O OH -

Mercury Battery Anode: Zn is reducing agent under basic conditions Cathode: HgO + H 2 O + 2e- ---> Hg + 2 OH -

Lead Storage Battery Anode (-) E o = V Pb + HSO > PbSO 4 + H + + 2e- Cathode (+) E o = V PbO 2 + HSO H + + 2e- ---> PbSO H 2 O