3.1 REDOX AND STANDARD ELECTRODE POTENTIAL

Role of platinum electrode

Inert electrode carry the charge

How to measure volume of gas produced

Gas syringe measure volume of gas produced at specified times

Which way do electrons flow in a cell

The more positive of system is the oxidising agent

Accepting electrons via external circuit

Vanadium ions and colours

VO2 + (+5) yellow

VO 2+ (+4) blue

V 3+ (+3) green

V 2+ (+2) purple

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Anode and cathode identifying

Anode - Oxidised (more negative electrode potential)

Cathode - Reduction (more positive)

Standard cell potential formula

Ecell∘ ​=Ecathode∘​ −Eanode∘​

Postitve - more negative

Oxidising agent

Accepts electrons from substance ( reduced )

Reducing agent

Donates electrons (oxidised)

Half-equation definition

Shows oxidation or reduction tracks electron transfer

Half cells

Metal or electrolyte combination(oxidation or reduction)

Standard electrode potential definition

“The standard electrode potential (E°) is the e.m.f. of a half-cell compared with the Standard Hydrogen Electrode under standard conditions.”

• 298 K (25 °C)

• Solutions at 1 mol dm⁻³

• Gases at 100 kPa pressure

Electrode potentials?

Oxidising agent - more positive

Reducing more negative

Standard hydrogen electrode

E= 0 provide a baseline potential

Set up - Platinum electrode (coated platinum black increase surface area)

When is platinum electrode used in cells

No solid metal present to act as the electrode

Inert, does not react, conducts electrons and provide surface for redox reaction

Pt electrode purpose

Provide electrical connection when no solid metal present

Why is platinum used as electrode

Chemical inert

Good conductor of electricity

Provide surface for electron transferr

When do you not need a platinum electrode

When solid metal present that can act as electrode

Only use when ions or gases in soloution

Standard conditions

Concentration 1moldm-3

Temperature 298k

Pressure (gas) 1atm

Manganate reaction

MnO4 - + 8H+ + 5e- → Mn2+ + 4H2O

Cell EMF (electromotive force) definition

Potential difference (voltage ) between 2 half cells under Standard conditions

Represent driving force for electron flow in cell

Calculating cell EMF

Ecell = E reduction(cathode) - E oxidation (anode)

Significance of cell EMF (feasibility)

The sign of Ecell determines whether a reaction is feasible

Positive Ecell

• Reaction is feasible

• Electrons will flow naturally

• Cell produce electrical energy

Negative Ecell

• Not feasible

• Only occur if external energy is supplied

Zero

• System at equilibrium

Why does EMF indicate feasibility

Electrons flow from

• More negative electrode potential →positive electrode potential

• Reducing agent → Oxidising agent

Reaction is feasible if

The oxidising agent has a higher E than the reducing agent

Hydrogen fuel cells

A hydrogen fuel cell converts chemical energy from hydrogen and oxygen into electrical energy using redox reactions

• Reactants continuously supplied

• Products continuously removed

2H2 + O2 → 2H2O

Half reactions in hydrogen fuel cells

Anode (oxidation)

Hydrogen is oxidised

H2→ 2H+ + 2e-

Hydrogen lose electrons produce protons

Cathode (reduction)

Hydrogen is reduced

O2 +4H+ +4e- → 2H20

Standard electrode potential definition

EMF measured on high resistance voltmeter when half cell containing-

1moldm-3 conc and all gases 1atm 298K

Standard hydrogen electrode definition

Inert platinum electrode

1moldm-3 Soloution of H+

Hydrogen gass pressure 1atm bubbled over temp 298k

Structure of hydrogen fuel cell

Anode (hydrogen electrode)

Cathode (oxygen electrode)

Electrolyte membrane

Catalyst Platinum

Functions

• Electrodes allow electron transfer

• Electrolyte allow ion movement

• External circuit

Benefits of hydrogen fuell cells

• Convenient store and release energy

• High energy efficiency convert chemical → electrical energy avoid energy loss via combustion

• Continuous operations - operate as long as fuel is supplied

• Water product - no Co2

Disadvantages of fuel cells

1. Hydrogen production (not a primary energy source)

Electrolysis of water os steam reform methane (produces CO2)

2. Hydrogen gas hard to store (low density high pressure low temp)

3. Catalyst cost platinum rare and expensive

Ammeter

Measure current (flow of electrons) A

Half cells structure

1moldm-3 of soloutions

Electrode

Slat bridge

Wire

High resistance voltmeter

Wire and high resistance voltmeter function in half cells

Allow electrons flow

High resistance voltmeter measure potential difference

Salt bridge function

Complete circuit ion flow without mixing

Gel soaked KNO3

Corrosion

Oxidation occur

So electrode potential of metal must be lower than acid for metal to be oxidised

Half cells method of Cu2+ and Zn2+

Measure 50cm of Cu(SO4) and ZnSO4 in separate beakers

Place copper foil if CuSO4 soloution and zinc foil in ZnSO4 soloution

Connect zinc foil strip and Cu foil strip in ammeter

Connect voltmeter across foils

Place 2 beakers next to each other

Soak filter paper in saturated KNO3

Measure potential difference