Chemistry Chapters 16 and 17

nickel-cadmium battery

(NiCd battery) secondary battery that uses cadmium, which is a toxic heavy metal; heavier than lithium ion batteries, but with similar performance characteristics

Both ΔH and ΔS are positive

. This condition describes an endothermic process that involves an increase in system entropy. In this case, ΔG will be negative if the magnitude of the TΔS term is greater than ΔH. If the TΔS term is less than ΔH, the free energy change will be positive. Such a process is spontaneous at high temperatures and nonspontaneous at low temperatures.

dry cell

A common primary battery The dry cell is a zinc-carbon battery. The zinc can serves as both a container and the negative electrode.

Think about the example of diamond and graphite

C (s, diamond) -> C (s, graphite) The phase diagram for carbon indicates that graphite is the stable form of this element under ambient atmospheric pressure, while diamond is the stable allotrope at very high pressures, such as those present during its geologic formation. In other words, carbon is the element at ambient atm while a diamond can be created at higher atm Thermodynamic calculations of the sort described in the last section of this chapter indicate that the conversion of diamond to graphite at ambient pressure occurs spontaneously, yet diamonds are observed to exist, and persist, under these conditions. Though the process is spontaneous under typical ambient conditions, its rate is extremely slow, and so for all practical purposes diamonds are indeed "forever." Situations such as these emphasize the important distinction between the thermodynamic and the kinetic aspects of a process. In this particular case, diamonds are said to be thermodynamically unstable but kinetically stable under ambient conditions.

Rudolf Clausius

In a later review of Carnot's findings, Rudolf Clausius introduced a new thermodynamic property that relates the spontaneous heat flow accompanying a process to the temperature at which the process takes place. This new property was expressed as the ratio of the reversible heat (qrev) and the kelvin temperature (T).

system consisting of two flasks connected with a closed valve

Initially there is an ideal gas on the left and a vacuum on the right. When the valve is opened, the gas spontaneously expands to fill both flasks. Recalling the definition of pressure-volume work from the chapter on thermochemistry, note that no work has been done because the pressure in a vacuum is zero. w=−PΔV=0 (P=0 in a vaccum) Note as well that since the system is isolated, no heat has been exchanged with the surroundings (q = 0). The first law of thermodynamics confirms that there has been no change in the system's internal energy as a result of this process. ΔU=q+w=0+0=0

distributions

Microstates with equivalent particle arrangements (not considering individual particle identities) are grouped together and are called Since entropy increases logarithmically with the number of microstates, the most probable distribution is therefore the one of greatest entropy

Describe how matter is redistributed when the following spontaneous processes take place: (a) A solid sublimes. (b) A gas condenses. (c) A drop of food coloring added to a glass of water forms a solution with uniform color.

Sublimination is the conversion of a solid (relatively high density) to a gas (much lesser density). This process yields a much greater dispersal of matter, since the molecules will occupy a much greater volume after the solid-to-gas transition. Condensation is the conversion of a gas (relatively low density) to a liquid (much greater density). This process yields a much lesser dispersal of matter, since the molecules will occupy a much lesser volume after the gas-to-liquid transition. The process in question is dilution. The food dye molecules initially occupy a much smaller volume (the drop of dye solution) than they occupy once the process is complete (in the full glass of water). The process in question is dilution. The process, therefore, entails a greater dispersal of matter. The process may also yield a more uniform dispersal of matter since the initial state of the system involves two regions of different dye concentrations (high in the drop, zero in the water), and the final state of the system contains a single dye concentration throughout.

An electric current

The flow or movement of charge is

The spontaneity of a process is ______ correlated to the speed of the process.

The spontaneity of a process is not correlated to the speed of the process. A spontaneous change may be so rapid that it is essentially instantaneous or so slow that it cannot be observed over any practical period of time.

ΔH is positive and ΔS is negative.

This condition describes an endothermic process that involves a decrease in system entropy. In this case, ΔG will be positive regardless of the temperature. Such a process is nonspontaneous at all temperatures

Both ΔH and ΔS are negative.

This condition describes an exothermic process that involves a decrease in system entropy. In this case, ΔG will be negative if the magnitude of the TΔS term is less than ΔH. If the TΔS term's magnitude is greater than ΔH, the free energy change will be positive. Such a process is spontaneous at low temperatures and nonspontaneous at high temperatures.

ΔH is negative and ΔS is positive.

This condition describes an exothermic process that involves an increase in system entropy. In this case, ΔG will be negative regardless of the temperature. Such a process is spontaneous at all temperatures

processes that reduce the number of microstates

Wf < Wi, yield a decrease in system entropy, ΔS < 0

For processes involving an increase in the number of microstates

Wf > Wi, the entropy of the system increases, ΔS > 0

secondary battery

`battery that can be recharged

sacrificial anode

a highly anodic metal in a system to prevent corrosion in other parts of that system more active, inexpensive metal used as the anode in cathodic protection; frequently made from magnesium or zinc

a positive ΔG indicates

a non spontaneous response

A negative value for ΔG indicates

a spontaneous response

battery

an electrochemical cell or series of cells that produces an electric current An ideal battery would never run down, produce an unchanging voltage, and be capable of withstanding environmental extremes of heat and humidity. There are two basic types of batteries: primary and secondary

voltaic cell

another name for a galvanic cell

standard entropies

are given the label S 298° for values determined for one mole of substance at a pressure of 1 bar and a temperature of 298 K.

standard entropy change (ΔS°)

change in entropy for a reaction calculated using the standard entropies, usually at room temperature and denoted ΔS∘298

standard free energy of formation (ΔG∘f)

change in free energy accompanying the formation of one mole of substance from its elements in their standard states ΔG∘298=ΔG°=∑νΔG∘298(products)−∑νΔG∘298(reactants)=[xΔG∘f(C)+yΔG∘f(D)]−[mΔG∘f(A)+nΔG∘f(B)].

standard free energy change (ΔG°) and formula

change in free energy for a process occurring under standard conditions (1 bar pressure for gases, 1 M concentration for solutions ΔG=ΔH−TΔS

Faraday's constant (F)

charge on 1 mol of electrons; F = 96,485 C/mol e−

corrosion

degradation of metal through an electrochemical process

electroplating

depositing a thin layer of one metal on top of a conducting surface

Ludwig Boltzmann

developed a molecular-scale statistical model that related the entropy of a system to the number of microstates possible for the system

fuel cell

devices that produce an electrical current as long as fuel and oxidizer are continuously added; more efficient than internal combustion engines

overpotential

difference between the theoretical potential and actual potential in an electrolytic cell; the "extra" voltage required to make some nonspontaneous electrochemical reaction to occur

cell potential

difference in electrical potential that arises when dissimilar metals are connected; the driving force for the flow of charge (current) in oxidation-reduction reactions

electrolytic cell

electrochemical cell in which electrolysis is used; electrochemical cell with negative cell potentials

galvanic cell

electrochemical cell that involves a spontaneous oxidation-reduction reaction; electrochemical cells with positive cell potentials; also called a voltaic cell

anode

electrode in an electrochemical cell at which oxidation occurs; information about the anode is recorded on the left side of the salt bridge in cell notation

cathode

electrode in an electrochemical cell at which reduction occurs; information about the cathode is recorded on the right side of the salt bridge in cell notation

inert electrode

electrode that allows current to flow, but that does not otherwise participate in the oxidation-reduction reaction in an electrochemical cell; the mass of an inert electrode does not change during the oxidation-reduction reaction; inert electrodes are often made of platinum or gold because these metals are chemically unreactive.

active electrode

electrode that participates in the oxidation-reduction reaction of an electrochemical cell; the mass of an active electrode changes during the oxidation-reduction reaction

electrical potential

energy per charge; in electrochemical systems, it depends on the way the charges are distributed within the system; the SI unit of electrical potential is the volt (1 V=1JC)

Nernst equation

equation that relates the logarithm of the reaction quotient (Q) to nonstandard cell potentials; can be used to relate equilibrium constants to standard cell potentials

current

flow of electrical charge; the SI unit of charge is the coulomb (C) and current is measured in amperes (1 A=1Cs)

concentration cell

galvanic cell in which the two half-cells are the same except for the concentration of the solutes; spontaneous when the overall reaction is the dilution of the solute

galvanized iron

method for protecting iron by covering it with zinc, which will oxidize before the iron; zinc-plated iron

cathodic protection

method of protecting metal by using a sacrificial anode and effectively making the metal that needs protecting the cathode, thus preventing its oxidation

half-reaction method

method that produces a balanced overall oxidation-reduction reaction by splitting the reaction into an oxidation "half" and reduction "half," balancing the two half-reactions, and then combining the oxidation half-reaction and reduction half-reaction in such a way that the number of electrons generated by the oxidation is exactly canceled by the number of electrons required by the reduction

electrical work (wele)

negative of total charge times the cell potential; equal to wmax for the system, and so equals the free energy change (ΔG)

What is the driving force in a two flask system with a closed valve that opens and disperses the matter inside?

not a consequence of any energy accompanying the process Instead, the driving force appears to be related to the greater, more uniform dispersal of matter that results when the gas is allowed to expand. Initially, the system was comprised of one flask containing matter and another flask containing nothing. After the spontaneous process took place, the matter was distributed both more widely (occupying twice its original volume) and more uniformly (present in equal amounts in each flask).

spontaneous process

one that occurs naturally under certain conditions

circuit

path taken by a current as it flows because of an electrical potential difference

microstate (W)

possible configuration or arrangement of matter and energy within a system

Alkaline batteries

primary battery that uses an alkaline (often potassium hydroxide) electrolyte; designed to be an exact replacement for the dry cell, but with more energy storage and less electrolyte leakage than typical dry cell

electrolysis

process using electrical energy to cause a nonspontaneous process to occur

reversible process

refers to a process that takes place at such a slow rate that it is always at equilibrium and its direction can be changed (it can be "reversed") by an infinitesimally small change is some condition NOTE: no real processes are truly reversible, rather they are classified as irreversible.

Nicholas Léonard Sadi Carnot

researched the efficiency of steam-powered machinery

lead acid battery

secondary battery that consists of multiple cells; the lead acid battery found in automobiles has six cells and a voltage of 12 V

cell notation

shorthand way to represent the reactions in an electrochemical cell

Primary batteries

single-use batteries because they cannot be recharged Dry cells and (most) alkaline batteries are examples of primary batteries.

entropy (S)

state function that is a measure of the matter and/or energy dispersal within a system, determined by the number of system microstates often described as a measure of the disorder of the system

oxidation half-reaction

the "half" of an oxidation-reduction reaction involving oxidation; the half-reaction in which electrons appear as products; balanced when each atom type, as well as the charge, is balanced

reduction half-reaction

the "half" of an oxidation-reduction reaction involving reduction; the half-reaction in which electrons appear as reactants; balanced when each atom type, as well as the charge, is balanced

standard cell potential

the cell potential when all reactants and products are in their standard states (1 bar or 1 atm or gases; 1 M for solutes), usually at 298.15 K; can be calculated by subtracting the standard reduction potential for the half-reaction at the anode from the standard reduction potential for the half-reaction occurring at the cathode

standard hydrogen electrode (SHE)

the electrode consists of hydrogen gas bubbling through hydrochloric acid over an inert platinum electrode whose reduction at standard conditions is assigned a value of 0 V; the reference point for standard reduction potentials

The third law of thermodynamics

the entropy of a pure, perfect crystalline substance at 0 K is zero. With only one possible microstate, the entropy is zero

second law of thermodynamics

the entropy of the universe increases for a spontaneous process

Thermodynamics vs kinetics

the most stable states of a kinetic reaction are those of the reactants, in which an input of energy is required to move the reaction from a state of stability, to that of reacting and converting itself to products. Kinetics is related to reactivity. In contrast, the most stable state of a thermodynamically favorable reaction is the products, because the reaction occurs spontaneously, without the need for energy to be added. Thermodynamics is related to stability. Therefore, something that is unreactive will desire to stay in the form of reactants, which will require an input of energy to cause the reaction to go forward, converting reactants into products

Thermodynamics

the study of relationships between the energy and work associated with chemical and physical processes provides the predictive ability of if a process will occur under predictive conditions

standard reduction potential (E°)

the value of the reduction under standard conditions (1 bar or 1 atm for gases; 1 M for solutes) usually at 298.15 K; tabulated values used to calculate standard cell potentials

Gibbs free energy change (G) and formula

thermodynamic property defined in terms of system enthalpy and entropy; all spontaneous processes involve a decrease in G G=H−TS

two types of changes in a system

those that occur spontaneously and those than occur by force Processes have a natural tendency to occur in one direction under a given set of conditions. Water will naturally flow downhill, but uphill flow requires outside intervention such as the use of a pump.

lithium ion battery

very popular secondary battery; uses lithium ions to conduct current and is light, rechargeable, and produces a nearly constant potential as it discharges

Unspontaneous process

will not take place unless it is "driven" by the continual input of energy from an external source. A process that is spontaneous in one direction under a particular set of conditions is nonspontaneous in the reverse direction. At room temperature and typical atmospheric pressure, for example, ice will spontaneously melt, but water will not spontaneously freeze.