AP Chemistry- Electrochemistry

Define electrochemistry:

Branch of chemistry that deals with the relationship between chemical changes and electric currents

Are oxidation states an actual charge?

no

What are oxidation states used for?

a way of keeping track of whether electrons are lost or gained

What is oxidation?

when an element loses electrons and the oxidation number increases

What is reduction?

when an element gains electrons and the oxidation number decreases

Define oxidizing agent:

something that removes electrons from something else and itself is reduced

Define reducing agent:

something that gives up an electron and itself is oxidized

Acronym for balancing REDOX reactions:

WHE

Define voltaic cells:

voltaic cells utilize a spontaneous redox reaction which converts chemical energy to electrical energy to produce electricity and do work

In order for a spontaneous redox reaction to occur and produce electricity what must be true?

two half reactions must take place in separate compartments

When will a REDOX reaction be spontaneous?

when the free element is more active than the ion

What happens at the anode compartment in a voltaic cell?

oxidation

What happens at the cathode compartment in a voltaic cell?

reduction

Because the two half reactions are in separate compartments in a voltaic cell, how do electrons travel?

through a wire that produces an electron current

How is the electron current measured in a voltaic cell?

by a voltmeter

How are the charges balanced in a voltaic cell?

by a salt bridge

What does the salt bridge permit?

ions to migrate

Where do anions and cations go in a salt bridge?

anions go to the anode and cations go to the cathode

What happens to the mass of the anode in a voltaic cell? Why?

it decreases because it undergoes oxidation and the atoms at the anode are becoming ions in solution

What happens to the mass of the cathode in a voltaic cell? Why?

it increases because the ions in the solution are becoming atoms that join onto the electrode

What can a voltaic cell have instead of a salt bridge?

a porous barrier

Why do electrons move from the anode to the cathode?

the potential energy is higher in the anode than in the cathode and substances are always more favorable at low energy

How is the difference in PE measured between two electrodes?

in volts

Define volt:

potential energy difference required to impart one joule of energy to a charge of one coulomb

Define cell EMF:

the cell potential/ the cell voltage and its measured in volts

What does EMF stand for?

electromotive force

For a reaction that is spontaneous, what is the cell potential?

+

is EMF an intensive or extensive property? Why?

it is intensive because it is acquired from dividing two extensive properties (joules and coulombs)

Can you multiply half cell potentials?

NOOOOO

How to use the cell potential with Hess's Law:

E0 cell= E cathode + E anode

What is true about more positive reduction potentials?

the greater the driving force for the reduction and more likely it is to be at the anode

What is true about more negative reduction potentials?

less likely to be reduced and more likely to be oxidized and be at the cathode

What are the signs of delta G and EMF and K in a spontaneous voltaic cell?

delta G= -

EMF= +

K>1

Formula for calculating delta G for a cell:

delta G0= -nFE

Nerst Equation for when cell is at nonstandard conditions:

E= E0-(RT/nF) ln Q

What is a simpler Nerst Equation:

E= E0 (-.0592/n) (log q)

If two compartments have different concentrations then when is the reaction spontaneous?

when the cathode compartment is more concentrated than the anode compartment

Why will a reaction only be spontaneous if the cathode is more concentrated under nonstandard conditions?

because the solution is the product of the anode compartment and the reactant in the cathode compartment thus q can be written as the concentration of the anode divided by the cathode

What does Q =

[anode]/[cathode]

Define corrosion:

a spontaneous redox reaction in which a metal is attacked by some substance in the enviroment such as the oxygen in the air and is converted to an unwanted compound

3 ways to prevent corrosion:

1) if element itself forms a protective layer which does not flake off

2) if cover surface of the metal with covering, it prevents oxygen and/or water from reaching the internal metal

3) can couple the metal with another metal that is more easily oxidized which acts as a sacrificial anode

Issue with covering the surface of the metal with paint:

if coating breaks off, the flaking will begin

Define sacrificial anode:

Cathodic protection (acts as the cathode)

Define electrolytic cell:

opposite of voltaic cell and requires electrical energy for non-spontaneous redox reaction to occur

What does an electrolytic cell need?

an external source of electrical energy= direct source or battery

Where are the anode and the cathode in an electrolytic cell?

in one compartment because don't need to worry about the redox reaction not taking place without a current

What does electroplating have that makes it different?

active electrodes

Define active electrodes:

electrodes that are part of the reaction not just the site of the reaction