Unit 8

Question 1

Electrochemistry

Answer 1

The interconversion of chemical and electrical energy
* Spontaneous reactions can produce electricity and
electricity can cause non-spontaneous reactions to occur.

Question 2

Redox reactions

Answer 2

involve the movement of electrons from
one reagent to another

Question 3

Oxidation

Answer 3

loss of electrons

Question 4

reduction

Answer 4

gain of electrons

Question 5

Spontaneous Redox Reactions Around Us

Answer 5

Oxidation of copper (Cu)
by O2 forms a protective
surface layer
Oxidation of iron (Fe) by O2 (forming rust,
Fe2O3·nH2O(s)) is a problem, e.g., for the
Gardiner expressway
4
(CuO)
-The oxygen in these compounds is formally O2-, thus the metals (Cu and Fe)
have been oxidized to form Cu2+ and Fe3+.
O2 is the oxidizing agent.

Question 6

Balancing Redox Reactions: Half Rxn Method

Answer 6

Write down the two half-reactions, each of which contains the
oxidized and reduced forms of one of the species.
* Balance the atoms and charges in each half-reaction.
– First balance atoms other than O and H, then O, then H.
– Charge is balanced by adding electrons (e-) to the reactant side in the
reduction half-reaction and to the product side in the oxidation half-reaction.
* Electrons should cancel out in net rxn. If necessary, multiply one or
both half-reactions by an integer so that
– number of e- gained in reduction = number of e- lost in oxidation
* Add the balanced half-reactions, and include states of matter.
* CHECK that both atoms and charges balance in the final reaction

Question 7

Voltaic cells

Answer 7

spontaneous chemical reaction (DG < 0)
generates an electric current
– Batteries contain one or more voltaic cells
– A voltaic cell (the system) does work on the surroundings
(the load – e.g., your cell phone or a flashlight), converting
higher energy reactants in the cell into lower energy products
– These are also called “galvanic” cells

Question 8

Electrolytic cells

Answer 8

non-spontaneous reactions (
DG > 0) are
driven by electric current
– An external power source supplies free energy to run electrolytic
cells. The surroundings thus do work on the cell (system). Lower
energy reactants are converted to higher energy products in the cell.
– used for electroplating, purification of metals, and more

Question 9

Electrodes

Answer 9

are usually metal strips/wires connected by an
electrically conducting wire

Question 10

Anode

Answer 10

is the electrode where oxidation takes place.

Question 11

Cathode

Answer 11

is the electrode where reduction takes place

Question 12

Salt Bridge:

Answer 12

is often a U-shaped tube that contains a gel permeated
with a solution of an inert electrolyte (contains positive and negative
spectator ions –e.g., NaCl, Na2SO4, KCl, KNO3). The salt bridge keeps
half cells electrically neutral because ions flow in and out of the salt
bridge, counteracting charge build-up due to electron flow

Question 13

Why Does a Voltaic Cell Work?

Answer 13

t’s a competition for electrons. We know this reaction is spontaneous,
which tells us that Cu2+ wins, taking electrons away from Zn(s).
Let’s re-write the two half-reactions to compare the possible reduction
reactions
- This reduction reaction “wins.”
This is a more favorable reduction reaction.
Cu2+ is a stronger oxidizing agent (it wants the
electrons more) than Zn2+
- The differing abilities of the metals to gain electrons gives rise to a voltage
drop. This is also known as electromotive force (EMF) or cell potential

Question 14

Ecell

Answer 14

The cell potential of any electrochemical cell is the sum of the half-cell
potentials for the oxidation and reduction half-cells.
-Note that, the half-cell potentials are NOT MULTIPLIED,as E is an intensive property.
(Bigger cells with more moles of redox
components will last longer, but will have the same standard output voltage.

Question 15

The Standard Hydrogen Electrode

Answer 15

Although what we tabulate is a standard half cell potential
(usually Ered ̊), potentials are actually determined experimentally
from the difference in potential between two electrodes.
* The reference point is called the standard hydrogen electrode
(S.H.E.). The S.H.E. consists of a platinum electrode in contact
with H2 gas (1 atm) and aqueous H+ ions (1 M).
* The standard hydrogen electrode is assigned arbitrarily a value of
exactly 0.00 V.

Question 16

the standard cell potential (E°)
depends on:

Answer 16

the position of the equilibrium (K or Keq,
which we know in turn depends on DG°)
2) the amount of charge transferred in the
redox rxn (nF)

Question 17

Three Methods to Determine Equilibrium Constants

Answer 17

• K from equilibrium concentration data
  and/or partial pressures:
  thermodynamic equilibrium constants are mixed,
  including both partial pressures and concentrations
  if the standard states of reactants and products
  include both solutions and gasses
  -K from standard state thermochemical data
  -K from standard state electrochemical data
  if the equilibrium involves a redox reaction
  (remembering to account for the amount of
  charge transferred)

Question 18

Cell Potential Under Any Conditions

Answer 18

VSo far, we have only talked about E°, which are standard cell
potentials (i.e. the specialized condition of pure liquids & solids,
1 M solutions, 1 atm gases)
However, cell potentials change with the conditions.
For example, we’ve all had “dead” batteries whose
voltage has dropped to too low a value (because they have so
few reactants) to be used and therefore must be replaced or
recharged.
The cell potential under any conditions can be calculated using
our knowledge of the relationship between electrochemical cell
potentials and thermodynamics

Question 19

Concentration cells

Answer 19

capture the electrical energy from a concentration
difference

Question 20

Applications of Concentration Cells

Answer 20

Measuring ions selectively
– For example, if you want to determine K+ levels is blood serum, or F- in drinking water
– One compartment has a known concentration of the desired ion (e.g. K+ or F-). The
other compartment has an electrode inside a specialized membrane that only allows
the desired ion (e.g., K+) to pass
* Biological cells! Signal conduction in nerve cells, muscle contraction,
ATP production and more
– The insides of many biological cells are “negatively polarized” by (10s of mV) relative
to the outside due in part to chemical gradients of Na+ and K+ ions. Creating the
chemical gradients require energy (often from hydrolysis of ATP), and then that
energy can be released extremely quickly to do specific tasks, like send a signal from
our brain to move our hand. Mitochondria use large proton gradients to create ~200
mV potentials which provide energy that is ultimately used to make ATP.
* You are not responsible for knowing any of these specifics of these
applications, but should understand the principals of concentration cells

Question 21

Electrolysis: Forcing Electrons the Other Way ‘Round

Answer 21

is the process in which electrical energy is used to
drive a nonspontaneous chemical reaction.
* Processes in an electrolytic cell are the reverse of those in a
voltaic cell.
* We will discuss several applications of electrolysis:
– Production of gases H2, O2, Cl2
– Production of pure metals (for example, Na)
– Converting ions to pure metals - electroplating

Question 22

Electrolytic Cells Drive Non-spontaneous Rxns

Answer 22

An electrolytic cell uses electrical energy from an external source to drive
a nonspontaneous redox reaction.
- As with a voltaic cell, oxidation occurs at the anode and reduction takes
place at the cathode. Electrons still flow from anode to cathode.
An external power source (such as a battery) drives this reaction by
supplying the cathode (site of reduction: connected to the negative terminal
of the power source) with electrons and removing electrons from the anode
(site of oxidation: connected to the positive terminal).
The power source must supply sufficient voltage (> 0.48 V) to push the
electrons in the desired direction around the circuit.

Question 23

Electrolysis of Water:

Answer 23

Not spontaneous: To electrolyze pH 7 water, it is
necessary to apply a voltage greater than 1.24 V.
* The polarity of the power supply determines which electrode acts
as the site of reduction (the cathode: supplies electrons) and
which becomes the site of oxidation (anode: pulls in electrons
The set-up also requires the presence of an electrolyte (an inert salt such as
Na2SO4) in the water to carry the current. The electrolyte must be less easily
oxidized and reduced than water, so that it is not involved in any redox rxn

Question 24

Application: Electrolysis of Molten Salts (E.g. NaCl)

Answer 24

The power supply (here – a battery) drives this non-spontaneous reaction
by pulling electrons from the anode and pushing them into the cathode
2 Cl-(l) + 2 Na+(l) → 2 Na(l) + Cl2(g)
Application: Electrolysis of Molten Salts (E.g. NaCl)
Useful for
1) production of active metals
(here – sodium) and somes
gases (here – chlorine)
2) energy storage technology

Question 25

Electrolysis Application: Electroplating

Answer 25

Electroplating is the process of depositing pure metal onto a surface.