Chem 27 Lab 32 Galvanic Cells, the Nernst Equation

What is the purpose of the galvanic cells experiment?

-To measure the relative reduction potentials for a number of redox couples
-To develop an understanding of the movement of electrons, anions, and cations in a galvanic cell
-To study factors affecting cell potentials
-To estimate like concentration of ions in solution using the Nernst equation

What are the two types of electrochemical cells?

galvanic (voltaic) and electrolytic

What doe the two types of electrochemical cells have in common?

both employ the principle of oxidation-reduction (redox) reactions

In galvanic cells, redox reactions occur ________ as is common with all portable batteries

spontaneously

What drives electrolytic cells?

nonspontaneous redox reactions, which are reactions that require energy to occur

When copper wire is placed into a silver ion solution, copper atoms spontaneously ______ electrons; copper atoms are oxidized/.reduced.

donate; oxidized

When copper wire is placed into a silver ion solution, silver ions spontaneously ______ electrons; silver ions are oxidized/.reduced.

accept; reduced

______ ions migrate to _____ atoms to pick up electrons and form ______atoms at the _________-solution interface; the _______ ions that form then move into the solution away from interface.

Silver; copper; silver; copper; copper

What is an interface?

the boundary between two phases; in this case, the boundary that separates the solid metal from the aqueous solutions.

What is a redox couple?

an oxidized and reduced form of an ion/substance appearing in a reduction or oxidation half-reaction, geneally associated with galvanic cells

The electron is a product/reactant in oxidation half-reactions

product

The electron is a product/reactant in reduction half-reactions

reactant

What does the galvanic cell designed to take advantage of?

the spontaneous transfer of electrons

What does a galvanic cell do instead of having electrons being transferred at the interface of the copper metal and the silver ions in solution?

a galvanic cell separates the copper metal from the silver ions to force to force the electrons to pass externally through a wire

half-cell

a part of the galvanic cell that hosts a redox couple

anode

electrode at which oxidation occurs
-known as the negative electrode
-electrons are lost

cathode

electrode at which reduction occurs
-known as the positive electrode
-electrons are gained

electrode

usually the metal (reduced state) of the redox couple

solution in half-cell

a solution containing the corresponding cation (oxidized state)

external circuit

the movement of charge as electrons though a wire connecting the two held-cells, forming one-half of the electrical circuit in a galvanic cell

salt bridge

paper moistened with a salt solution, or an inverted tube containing a salt solution, that bridges two half-cells to complete the solution part of the electrical circuit

internal circuit

the movement of charge as ions through solution from one half-cell to the other, forming one-half of the electrical circuit in a galvanic cell

What does the internal circuit ensure

by completing the electrical circuit, it ensures that electrical neutrality is maintained in each half cell when the cell operates

How is electrical neutrality maintained when copper metal is oxidized to copper (II) ions in the Cu²⁺/Cu half-cell?

either NO₃⁻ anions must enter (from the salt bridge) or copper (II) ions must leave the hand-cell

How is electrical neutrality maintained when silver ions are reduced to form silver metal in its half-cell?

either NO₃⁻ anions must leave (into the salt bridge) or cations must enter its half-cell

What causes the cell potential of a galvanic cell?

the difference in tendencies of the two metals to oxidize (donate electrons) or of their ions to reduce (accept electrons)

reduction potential

the tendency for an ion (or molecule) to accept electrons

what is the value used to identify the relative ease of reduction of a half-reaction

a measure reduction potential

potentiometer/multimeter

when placed in the external circuit between two electrodes, measures the cell potential Ecell

Ecell

a value that represents the difference between tendencies of the metal ions in their respective half-cells to undergo reduction
-Ex. the difference between the reduction potentials of the two redox couples
-measured as a positive value

Ecell equation for reduction potential

Ecell=Ecathode-Eelectrode

In a galvanic cell with silver and copper, what has a greater reduction potential? Why? Where does it go in the Ecell equation?

Ag⁺, it has a greater tendency than the copper ion does to be in the reduced (metallic) state; goes before copper so Ecell remains positive

When does the measured cell potential correspond to the standard cell potential?

when the concentrations of all the ions are 1 mol/L and the temperature of the solution is 25°C

What may result from deviation from the theoretical values of the reduction potentials?

surface activity at the electrodes or activity of the ions in solution

When is the Nernst equation applicable?

Applicable to redox systems that are not at standard conditions, most often when the concentrations of the ions in solution are not 1 mol/L

Nernst equation (at 25°C)

Ecell=E°cell-(0.0592/n)logQ
n=moles of electrons exchanged according to the cell reaction
Q = reaction quotient

Q

reaction quotient

mass action expression

Q=[product]/[reactant]

mass action expression

the product of the molar concentrations of the products divided by the product of the molar concentrations of the reactants, each concentrations raised to the power of its coefficient in the balanced cell equation

When does Q = mass action expression

dilute ionic concentrations

Do solids appear in the mass action expression?

No; the concentrations (density) of solids are constant and therefore do not appear in the mass action expressions

When E°cell and the concentration of the cathode ion are contant

Ecell=constant - (0.0592/n)p[anode metal]
p[anode metal] = -log[anode ion]`

When first connecting the alligator clips to the electrodes, what do you do if the multimeter reads a negative number?

switch the clips so it equals a positive number.

If the measured Ecell= EA+,A-EB+/B is smaller than 0, what does this tell you?

EA+,A is the anode
EB+/B has a larger tendency to be reduced

The electrode at which reduction occurs is called _____ and its the (sign__) electrode

cathode, +

The electrode at which oxidation occurs is called _____ and its the (sign__) electrode

anode -

What is the oxidizing agent in the Zn-Mg call?

Zn²⁺