Hein's Chem Flashcards

Electrochemistry HL

electromotive force (emf)

electromotive force (emf)

the energy supplied by a source divided by the electric charge transported through the source

emf in a voltaic cell

emf is equal to the electric potential difference for zero current through the cell

Cell potential

the potential difference between the cathode and the anode when the cell is operating (always less than the maximum voltage that can be delivered by the cell.

What does the cells potential depend on?

• concentration of the species (reactant and product)

• operating temperature (Usually 298K or 25C)

Standard Cell Potential (Eocell)

cell potential taken under the standard conditions of 1 mol dm^-3 concentration for reactants in solution and 100 kPa for gaseous reactants

How is cell potential created

generated by the movement of electrons from the anode (- electrode) to the cathode (+ electrode) via the external circuit - EMF

Calculating Standard Cell Potential (Eocell)

• Eorhe - standard electrode potential at the cathode

• Eolhe - standard electrode potential at the anode

Standard Electrode Potential

is the potential (voltage) of the reduction half-equation under standard conditions measured relative to the SHE. Solute concentration is 1 mol dm-3 or 100 kPa for gases

Standard electrode potentials in the Data booklet

table 24

Calculating Standard Cell Potential (Eocell) for a spontaneous reaction

Eorhe (cathode) is taken as the more positive value and the Eothe (anode) is taken as the more negative value

• find the difference in their standard electrode potentials

Calculating Standard Cell Potential (Eocell) Daniell Voltaic cell

Standard Hydrogen Electrode (SHE)

a reference electrode consisting of an inert platinum (Pt) electrode in contact with 1 mol dm^-3 hydrogen ions (H+) and hydrogen gas (H2) at 100 kPa and 298K

Eo of SHE

0 at all temperatures

How to determine the standard electrode potential of a half-cell

connect the half-cell, under standard conditions to the SHE, using a connecting wire with a voltmeter attached and a salt bridge between the two solutions

Calculating Cu^2+(aq)|Cu(s) Standard Electrode Potential

Salt bridge of KNO3

when is a redox reaction spontaneous?

Eocell is positive

when is a redox reaction non-spontaneous?

Eocell is negative

Gibbs free energy equation

• n is amount, in mol, of electrons transferred in the balanced equation

• F is Faraday's constant (section 2 data booklet)

• Eocell is the standard cell potential (Eocell = erhe - elhe)

Gibbs free energy spontaneity

• ∆G is + not spontaneous

• ∆G is - spontaneous

More positive Eocell

More easily oxidized

More negative Eocell

Less easily oxidized (makes it a stronger oxidizing agent)

Eocell when balancing equations

Don't multiplee the values because Eocell is an intensive physical property

Sponteneity of reaction

Electrolysis of concentrated aqueous sodium chloride (NaCl)

• Electrolyte: NaCl(aq)

• Electrodes: inert Pt (Platinum)

Cathode in Electrolysis of concentrated aqueous sodium chloride (NaCl)

Na+ (aq) and H2O(l)

Anode in Electrolysis of concentrated aqueous sodium chloride (NaCl)

Cl- (aq) and H2O(l)

(Since the data booklet shows the Standard electrode potentials for reduction equations the sign must be flipped to get it for the oxidation reaction)

What is formed at each electrode in the Electrolysis of concentrated aqueous sodium chloride (NaCl)

• Cathode: Hydrogen Gas (H2)

• Anode: Chlorine Gas (Cl2)

Why is Cl2 gas formed in the Electrolysis of concentrated aqueous sodium chloride (NaCl)

The over-voltage required for the formation of oxygen is much larger than that required for the formation of chlorine, thus meaning chlorine gas is produce

• Oxygen must go from O to O^2-

• Chlorine only has to go from Cl^- to Cl

Over-voltage

voltage in a circuit or part oof it is raised above its upper design limit

Uses of the Electrolysis of concentrated aqueous sodium chloride (NaCl) also known as Brine

Basis of the chlor-alkali industry

Uses of Chlorine Gas

• PVS

• Bleaching agent

• Disinfectant

• water purification

Uses of Hydrogen Gas

• Fuel

• Haber process

Uses of Sodium Hydroxide (NaOH)

• Soap

• Paper

• Chemical industry

Observations in the Electrolysis of concentrated aqueous sodium chloride (NaCl)

• Cathode: bubbles of colorless hydrogen gas and Flammable gas (Small pop with pure hydrogen and louder with hydrogen and air mixture)

• Anode: Pale yellow gas and pungent odor

• Electrolyte: pH increases with increasing [OH]

Electrolysis of dilute aqueous sodium chloride (NaCl)

Over voltage does not occur so the half equations at each electrode can be worked out by looking at the Eo values

Products in the Electrolysis of dilute aqueous sodium chloride (NaCl)

• Cathode: H+ ions discharged

• Anode: OH- discharged

Electrolysis of aqueous copper(II) sulfate (CuSO4) with inert graphite electrodes

• Electrolyte: CuSO4(aq)

• Electrodes: inert graphite

Cathode in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with inert graphite electrodes

Cu^2+ (aq) and H2O(l)

Anode in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with inert graphite electrodes

SO4^2- (aq) and H2O(l)

(sulfates tend not to oxidize due to the +6 oxidation state of sulfur)

What is formed at each electrode in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with inert graphite electrodes

• Cathode: Copper (Cl(s))

• Anode: Oxygen gas (O2)

Observations in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with inert graphite electrodes

• Cathode: layer of pink-brown solid copper deposited on the cathode

• Anode: bubbles of colorless oxygen gas produced

• Electrolyte - PH will decrease as a result of increasing [H+]

Electrolysis of aqueous copper(II) sulfate (CuSO4) with active copper electrodes

• Electrolyte: CuSO4

• Electrodes: active copper

Cathode in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with active copper electrodes

Anode in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with active copper electrodes

What is happens at each electrode in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with active copper electrodes

• Cathode: Copper (Cu(s)) formed

• Anode: Copper (Cu(s)) lost

Observations in the Electrolysis of aqueous copper(II) sulfate (CuSO4) with active copper electrodes

• Cathode: layer of brown solid deposited on cathode (copper is pure), mass of the cathode increases

• Anode - sludge of impurities forms beneath the anode, mass of anode decreases

• Electrolyte - color does not change

Purpose of Electrolysis of aqueous copper(II) sulfate (CuSO4) with active copper electrodes

used as a method of electrorefining copper - the anode is impure copper and the cathode is pure copper

Electroplating

using electrolysis to coat a thin layer (typically 10`-3 to 10^-4 mm thick) of one metal onto the other (cathode)

Purpose of Electroplating

used to minimize corrosion or for decoration

Electroplating example

• Electrolyte is Na[Ag(CN)2] (sodium dicyanoargentate)

• Anode: silver bar

• Cathode: metal object that is being plated

What is needed to electrolysis water?

• dilute solution of sulfuric acid (H2SO4 (aq))

• dilute solution of sodium hydroxide (NaOH (aq))

Using inert (pt) platinum electrodes

Electrolysis of Water using dilute sulfuric acid (H2SO4 (aq))

• Cathode: H+ (aq)

• Anode: SO4^2-

Observations in the Electrolysis of Water using dilute sulfuric acid (H2SO4 (aq))

• Cathode: bubbles of colorless oxygen gas, pH increases as [H+] increase

• Anode: bubbles of colorless oxygen gas, pH increases as [H+] increase

• Electrolyte: 2 mol of hydrogen will be formed for every 1 mol of oxygen

Factors affecting yield

• Current (I)

• Time (t)

• Charge on the ion (z)

Current affect on yield

I ∝ Q

proportional to the amount of e- passing through the circuit and thus proportional to the amount of mol of product formed

• current increases then product increase

Time affect on yield

t ∝ Q

informs how long the e- pass trough the circuit

Charge on the ion affect on yield

amount, in mol, of e- needed to discharge 1 mol of an ion at electrode

Charge (Q) equation

Q=It

Anode (+) Rule

• If high concentration halogens present → formed

• No halogens → O2 (g) formed

Cathode (-) Rule

• If Cu, Ag, Au, or Pt present → formed

• If Cu, Ag, Au, or Pt not present → H2 (g) formed

Faraday's Law and Yield