Free Energy and Equilibrium Constant Equilibrium Constants can be related to free energy via the

formula:

G = -RT In K

R universal gas constant = 8.314 JK-1mol-1

T Temperature in K

K equilibrium constant

Acid-Base Equilibria

Acids and bases can be defined in a number of ways:

Arrhenius-

Acids are proton generators: they dissociate to give H+

Bases are OH- generators: they dissociate to give OH-

Bronsted-Lowry

Acids are proton donors

Bases are proton acceptors

Acid-Base Equilibria

When a Bronsted-Lowry acid dissociates in water, it reacts reversibly with water in an acid-dissociation equilibrium, where a hydronium ion, H3O+, is formed (conjugate acid of water).

Acid-Base Equilibria

Bronsted-Lowry bases have at least one lone pair of electrons (where the proton attaches)

Strong and Weak Acids and Bases

Strong acids and bases dissociate almost completely in water.

Weak acids and bases only partially dissociate in water.

Weak acids have strong conjugate bases.

Dissociation of Water Water can act as both an acid or a base.

Water dissociates

22() = 3+ + ()

Ion-product constant for water: = [3

+][]

Dissociation of Water

= [3+]

For water at 25C:

[3+] = 1.0 107 M

= 1.0 107M

= [3+] = (1.0 107)(1.0 107)

= 1.0 1014

pH

= log[3+]

pH

Example 1:

If a sample of lemon juice contains [H3O+] = 2.5 x 10-3 M, what is

the pH of the lemon juice?

Example 2:

Calculate the pH of an aqueous ammonia solution that has an [OH-] concentration of 1.9 x 10-3M.

Example 3:

Calculate the concentration of OH- ions in a solution with pH = 12.1.

Electrochemistry

The area of chemistry concerned with the inter-conversion of chemical and electrical energy.

A device for interconverting chemical and electrical energy is called an electrochemical cell (battery).

Two types of electrochemical cells:

Galvanic a spontaneous chemical reaction generates an electric current.

Electrolytic - an electric current drives a nonspontaneous reaction.

Consider the redox reaction:

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

Recall:

Oxidation is loss of electrons.

Reduction is gain of electrons.

Zn is therefore oxidised to Zn2+ and Cu2+ is reduced to Cu.

Galvanic Cell

Galvanic Cell The metal strips in the half cells are referred to as electrodes.

The electrode at which oxidation occurs is called the anode.

The electrode at which reduction occurs is called the cathode.

A salt bridge connects the two half cells and is needed to complete the electrical circuit.

Cell Notation

For the reaction:

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

Abbreviated cell notation can be written as:

A single line | represents a phase boundary, e.g. between solid electrode and aqueous solution.

A double line || represents a salt bridge.

Cell Notation Anode is always written on the left of the salt bridge.

Cathode is written on the right.

Reactants written first, followed by the products.

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

Electrons flow from anode to cathode.

Current flows from cathode to anode.

Standard Cell Potentials The standard cell potential E is the cell potential when both

reactants and products are in their standard states:

solutes at 1 M concentrations

gases at a partial pressure of 1 atm,

solids and liquids in pure form,

with all at a specified temperature, usually 25 C.

Standard Cell Potentials

The standard potential of any galvanic cell is the sum of the standard half-cell potentials for oxidation at the anode and reduction at the cathode:

Ecell = Eox + Ered

Electrode potentials are measured against a reference half-cell called the standard hydrogen electrode (S.H.E.)

Consists of a platinum electrode in contact with Hydrogen gas and aqueous H+ ions under standard conditions ( 1atm H2(g), 1M H+(aq), 25C)

Standard Reduction Potentials

For:

Because the Cu2+/Cu half-reaction is a reduction, the corresponding half-cell potential, E= 0.34V, is called a standard reduction potential.

Standard Reduction Potentials

In a cell in which this half-reaction occurs in the opposite direction, the corresponding half-cell potential has the same magnitude but opposite sign.

The half-reactions are written as reductions rather than as oxidations.

The listed half-cell potentials are standard reduction potentials, also known as standard electrode potentials.

The half-reactions are listed in order of decreasing standard reduction potential, meaning a decreasing tendency to occur in the forward direction and an increasing tendency to occur in the reverse direction.

Standard Reduction Potentials

The standard potential for the following galvanic cell is 0.92 V:

Look up the standard reduction potential for the Al3+/Al half-cell, and calculate the standard reduction potential for the Cr3+/Cr half-cell.