Chem - Ch. 10 Electrochemistry

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What is an oxidation number (or oxidation state)?

It describes how many electrons an atom is donating or accepting in the overall bonding of a molecule.

What is an oxidation-reduction (redox) reaction?

It is a reaction in which the oxidation numbers of any of the reactants CHANGE.

Atoms gain or lose e- as new bonds are formed.

Define oxidation.

When an atom LOSES e-; its oxidation number increases.
"OIL" or "LEO"

Define reduction.

When an atom GAINS e-; its oxidation number decreases.
"RIG" or "GER"

What is a reducing agent (reductant)?

It is the atom that is oxidized in a reaction and loses e- to another atom.
Reducing agents GIVE UP e- and reduces another atom which will gain those lost e-.

What is an oxidizing agent?

It is the atom that oxidizes another atom that loses the e-.
Oxidizing agent = reduced atom.

A redox reaction can be broken down into a pair of...

HALF-REACTIONS which will show the oxidation and reduction separately.

Half-reactions are composed of...

ion-electron equations.

Basically, redox reactions involve the transfer of...

e-!
The flow of electrons constitutes an electric CURRENT that can do work!
∴We can use a spontaneous redox reaction to generate an electric current!

What is a galvanic (or voltaic) cell?

It is a device that utilizes the work produced by the flow of electrons in redox reactions.

Describe the set up for a galvanic cell.

1. Electrode: comprised of a metal, labeled the ANODE, which gets OXIDIZED. The e- its atom loses travel along a wire to a second metal electrode...
2. CATHODE (2nd metal electrode) which gets REDUCED.
3. The flow of e- in the conducting wire is the electric current that's produced by the spontaneous redox reaction between electrodes.

What connects the two half-cells in a galvanic cell?

A salt bridge!

Anions from the salt bridge go to the...

ANODE.

Cations from the salt bridge go to the...

CATHODE.

The anode is ALWAYS the site of ___________ and the cathode is ALWAYS the site of ___________.

Anode = oxidation (a & o are vowels)
Cathode = reduction (c & r are consonants)

or "an ox, red cat"

What is a cell diagram?

It is a shorthand notation used to identify the species present in a galvanic cell.

They look like this:
Anode I Anodic solution (conc.) I I Cathodic solution (conc.) I Cathode

If conc. aren't given, assume 1M.

How can we determine if a redox reaction of a cell is spontaneous and can produce an electric current?

We would need to figure out the cell voltage.

How do we calculate the standard free energy change ΔG°() for a redox voltage whose cell voltage is E°?

ΔG° = -nFE°

n: number of moles of e- transferred
F: faraday (magnitude of the charge of one mole of e-)

The redox reaction in a cell will be spontaneous (-ΔG°) if...

the cell voltage (E°) is POSITIVE!

If the cell voltage is positive...

then the reaction is spontaneous.

If the cell voltage is negative...

then the reaction is non-spontaneous.

If we have to balance our reactions, do we multiple the potential (E°) by the coefficients?

NO! NEVER multiply the potential by a coefficient, even if you multiply a half-reaction by a coefficient to get the balanced equation for the reaction.

This is because potentials are INTRINSIC to the identities of the species involved, and do NOT depend on the number of moles of the species.

The more NEGATIVE the reduction potential...

the stronger the PRODUCT is as a reducing agent.

The more POSITIVE the reduction potential...

the stronger the REACTANT is as an oxidizing agent.

How does an electrolytic cell differ from a galvanic cell?

Electrolytic cell uses an external voltage source (i.e. battery) to create an electric current that forces a non-spontaneous redox rxn to occur = ELECTROLYSIS!

What is electroplating?

The process of using electrolytic cells to plate a thin layer of metal on top of another material.

What are the similarities between galvanic and eletcrolytic cells?

In both cells...
1. the anode is the site of oxidation and the cathode is the site of reduction
2. the e- in the external circuit always move from the anode to the cathode.

What are the differences between galvanic and eletcrolytic cells?

1. a galvanic cell uses a spontaneous redox reaction to create an electric current
2. an electrolytic cell uses an electric current to force a non-spontaneous redox rxn to occur.
3. galvanic cell: anode is - and cathode is +
4. electrolytic cell: anode is + and cathode is - (since e- are being forced to move where they DON'T want to go)

What is Faraday's Law of Electrolysis?

The amount of chemical change is proportional to the amount of electricity that flows through the cell.

If the reduction potential (E°) is negative, what does this mean about the spontaneity of the process?

If the reduction potential is negative, the process is NOT spontaneous.

Electrolytic cells involve ______________ chemical reactions. Where does oxidation and reduction occur in an electrolytic cell?

Electrolytic cells = NONSPONTANEOUS chemical reaction.

Define oxidation.

Loss of electrons and an increase in oxidation number

Oxidizing and reducing agents are ALWAYS _______________ in a redox reaction.

the REACTANTS, never the products!

How can we determine the oxidizing ability of a process if given only the reduction potentials?

Flip the reaction and flip the sign on the reduction potential to get the oxidizing potential!

An oxidizing agent is the species that gets...

REDUCED!

A reducing agent is the species that gets...

Oxidizing!
∴The species that is the best reducing agent is the one that is most easily OXIDIZED!

Larger atoms may form stronger covalent bonds because...

of a larger area of electron overlap!

What is the highest possible oxidation state of carbon?

+4

What is a byproduct of protein metabolism?

urea

When E° is positive, then ΔG is...

negative!

ΔG =-nFE

Electrolytic cells employ electricity generated somewhere else to drive...

a NONSPONTANEOUS chemical reaction!

Does the number of electrons flowing in the cell have anything to do with the cell potential?

NO!

Any equilibrium with gases present will be affected by changes in ____________ resulting in change of concentration if the numbers of gas molecules on the reactant and product sides are different.

pressure.

As a reaction proceeds from left to right in a galvanic cell, the voltage of the cell will...

DECREASE!

Think of the Nernst equation where E = E° - (0.0592/n)(log Q)

If a reaction proceeds to the right, Q will get larger as reactants are converted into products. As Q gets larger, log Q also gets larger thus, E will decrease.

Does the concentration terms for solids appear in the reaction quotient (Q)?

No!

According to the Nernst equation, E can be...

less than, greater than, or equal to E°.

In a voltaic cell, the anode is labeled as the ___________ electrode since...

anode = NEGATIVE electrode since free electrons are being formed there due to oxidation.

Thus, the cathode is the POSITIVE electrode since free electrons are consumed there by the reduction reaction.

In an electrolytic cells, these designations are reversed.

In an electrolytic cell, which is the positive electrode and the negative electrode?

+ electrode = anode
- electrode = cathode

If the equilibrium of a spontaneous reaction favors the products, then Keq will be...

greater than 1! Keq > 1

How can we determine which is the strongest oxidizing agent given the reduction potentials?

The strongest oxidizing agent is the one with the greatest tendency to be reduced!

A cell with a negative voltage (thus a nonspontaneous reaction) is considered to be...

"dead" and will produce NO current (current in the cell = 0)

Metals can never act as....

oxidizing agents becoming anions.

But if metals are in contact, the metal with the higher reduction potential (more corrosive resistant), will replace electrons lost to oxygen by taking them from the metal with the lower reduction potential.

All metal oxides are _________.

alkaline.

In an electrolytic cell, the anode is the ___________ electrode and the cathode is the ____________ electrode.

anode = positive electrode

cathode = negative electrode

*exact reverse of a galvanic cell

What is an electrolyte?

A compound which, when dissolved in water, will produce lots of ions.

Define ligation. Ligation is an example of...

Ligation = describes the process of forming a coordinate covalent bond between an electron pair donator (ligand = Lewis base) and an electorn pair acceptor (metal ion = Lewis acid).

Ligation = Lewis acid-base reaction

What are standard state conditions?

Temperature = 25°C = 298K
Pressure = 1am = 760 torr
Concentration of all solutes = 1 M

According to the "pseudo-law" of chemistry, pressure only has strong effects on chemical reactions that involve...

gaseous compounds. This is because the compressibility of gases is large, while those of solids and liquids are much smaller.

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