CHEM112 - Exam 3 eh

Does the reducing agent act at the anode or the cathode in a galvanic cell? Explain briefly.

A reducing agent gets oxidized, since oxidation occurs at the anode, the reducing agent acts at the anode.

Define all the terms in the equation: DeltaG= -nFE

DeltaG: Gibbs free energy change n: Number of moles of electrons transferred into the reaction F: farday's constant E is the cell potential

In the Nernst equation, E= E^o - (RT/nF)lnQ, under what circumstances does E = 0? Your answer must make reference to some experimental condition, not just assign a specific numerical value to a variable listed in the equation.

E = 0, when the battery is fully discharged, in other words, the system has reached equilibrium. At equilibrium, Q=K

What is a sacrifical anode? Give an example?

A sacrificial anode is readily oxidized material that is connected through a conducting wire to an oxidizable material that you want to protect from oxidation. An example would be a bar of magnesium connect to an iron bridge. What the iron is subjected to oxidizing conditions, electrons from the Mg bar into the Fe bridge and keep the bridge reduced. When the Mg bar is completely oxidized you replace it with a new one.

A) What is the second law of thermodynamics? B) How do physical and/or chemical changes that occur in a s system alter the entropy of a system's surroundings?

A) All spontaneous processes results in an increase in entropy of the universe B) If the system's deltaH is negative (heat is dumped into the surroundings) the entropy of the surroundings increases. If the system deltaH is positive (heat is removed from the surroundings) then the entropy of the surroundings decreases.

A) What is the function of the salt bridge in an electrochemical cell? B) How would constructing an electrochemical cell that did not contain a salt bridge alter its function?

A) To prevent charge build up and thereby maximize energy yield from the cell. B) The build up of electrostatic charge on the cell would cause the system to come to equilibrium (more more work could be done) prior to reaching true electrochemical equilibrium.

Why is the cathode of galvanic cell considered to be the positive electrode?

Because electron flow through the external circuit to the cathode. Electrostatic-ally negative things move towards positive things.

Define all terms in the equation E=E^o-(RT/nF)lnQ

E: Cell potential under conditions given E^o: Cell potential under standard conditions R: Gas constant T: Temperature in Kelvin n: Number of electrons transferred F: Faraday constant Q: Reaction quotient

What conditions must be met for potential E to qualify as a standard cell potential E^o?

E=E^o when all reactants and products are in their standard states - solutes at 1M concentrations, gasses at Px= states, and solids and liquids in pure form at 25C.

What happens to a reducing agent?

It gets oxidized.

What happens to an oxidizing agent?

It gets reduced.

What reaction occurs at the anode?

Oxidation reaction

How do you determine the increasing strength of oxidizing agents?

Oxidizing agents get reduced, so the strongest has the most positive E^o values. (Look at the left side of the arrow).

How do you determine the strength of reducing agents?

Reducing agents get oxidized, so look at the reducing agents on the right side of the arrow.

What type of reaction occurs at the cathode?

Reduction reaction

State the 3rd Law of Thermodynamics

The entropy of a perfectly ordered crystalline substance at 0 Kelvin is zero

Explain how corrosion occurs on iron surfaces exposed to the "elements".

The metal and a surface water droplet constitute a tiny galvanic cell. Iron is oxidized in an annode region on the surface, and O2 is reduced near the edge of the droplet at a cathode region. In the anode region, electrons flow from anode to cathode through the metal, while ions flow through the water droplet. Dissolved O2 oxidizes Fe2+ further to Fe3+ before it is deposited as rust. (Fe2O3*H2O)

Explain briefly, but with mathematical justification why batteries tend to be less efficient at lower temperatures.

The problem has both kinetic and equilibrium components. From a kinetic point of view, reactions produced more slowly in the cold: (lowercase) k=pZe^-E/RT. As T gets smaller, K gets smaller. From an equilibrium stand point, Nernst equation is our guide. Generally, a charged battery will have Q << 1. Therefore lnQ will be negative. This negative times the - in front of -RT/nF*LnQ makes that term positive. As T acts as a multiplier, the higher the T, the bigger the multiplier, and the bigger E is.

What is the function of the salt bridge in a galvanic cell?

The salt bridge functions to mitigate the charge build up that would occur as the half-cell reactions proceed.

Briefly explain how and where element heavier than He were/are produced.

This comes from the interlude on p.930. With our math/physics background we can go back roughly 13.7*10^9 years to the big bang. Elementary particles such as protons, neutrons and electrons started to form O shortly (seconds) after the bang. Shortly (minutes) later 4^He^2+ nuclei started to form. Most matter remained in this state until the universe cooled sufficiently for nuclei to bind e- and form atoms. Gravitational attractive forces lead to formation of stars. In the core of more massive stars elements heavier than than He were formed (C,O,Si,Mg,Fe) as the stars were burning low. Elements heavier than Fe form in gravitational collapses associated with supernovas.