Password Lectures 1-9: Falcon10 Password Lectures 10-16: Wryneck Lecture 16 CM1001

Determination of Electrode Potential E o red = - E o ox = V The standard reduction potential of the Fe 2+ /Fe electrode is V Fe 2+ (aq) + 2e -  Fe (s) E o red = V

Corrosion Many metals, particularly Iron, undergo corrosion when exposed to air and water. Losses caused by corrosion of metals total billions of euro worldwide Corrosion is an electrochemical process. Iron in the presence of water tends to oxidise and give up electrons Anode Fe (s) → Fe 2+ (aq) + 2e - The electrons pass through the Iron to the edge of the drop, where they reduce Oxygen from the air to hydroxide ions

CathodeO 2 + 2H 2 O + 4e - → 4OH - The Iron (II) ions and hydroxide ions diffuse together and combine, forming insoluble Iron (II) hydroxide. Fe OH - → Fe (OH) 2(S) This precipitate is rapidly oxidized by Oxygen to rust, an Iron (III) compound with the approximate composition of Fe 2 O 3.H 2 O Corrosion

Corrosion Prevention Iron can be coated with paint, enamel or another metal. Also can treat with superheated steam to coat with an adherant oxide of Fe 3 O 4. Form an alloy with another metal, e.g,. Fe, Cr and Ni. Cathodic protection: e.g. an underground pipeline of iron in contact with the soil can be protected by connecting it with a wire to a more active metal, e.g. Zn, Mg or Al. The iron becomes a cathode at which at which Oxygen is reduced, rather than an anode where iron is oxidized. Need to replace more active metal periodically

Tutorial Questions 1. Define energy and list 4 different forms of energy. Ans. Energy is the capacity to do work and comes in the forms of potential, kinetic, electrical and thermal energy. 2. State the Law of Conservation of Energy. Energy of reactants = total energy of products + energy lost (or gained) 3. Define Enthalpy and Enthalpy change. The energy of a substance under constant pressure is called Enthalpy (H). Enthalpy of a substance cannot be measured, but the change in enthalpy can easily be measured as the heat lost or absorbed when a chemical or physical change occurs under constant pressure.

4. Name four different enthalpy states? Ans. Formation, fusion, combustion and vapourisation. 5. State Hess’ Law. Ans: When a reaction occurs at constant P or V, the resulting energy change is the same if the reaction occurs in 1 or more steps. 6. What is Entropy? Give and example of a reaction which increases entropy. Ans. Entropy is a measure of the order within a system. The melting of ice or the evaporation of water. Tutorial Questions

7. What does the Free Energy State ΔG predict? Give the free energy equation. Ans: Spontaneity of a reaction. ΔG = ΔH – T ΔS 8. The reaction of Hydrogen gas with Chlorine gas has a standard enthalpy (ΔH o 298 ) = -185kJ and a standard entropy change ΔS o 298 = 141JK -1. Calculate the standard free energy change of the reaction. Is the reaction likely to be spontaneous? H 2(s) + Cl 2(g) → 2HCl (g) Tutorial Questions

Ans: ΔG = ΔH T ΔS kJ – (298K)(141JK -1 ) -185,000 J – 42, 018 J -227kJ A negative free energy value indicates a spontaneous reaction 9. What does a negative value for an enthalpy change indicate? Ans: An exothermic reaction. A reaction which loses heat to the surroundings. Tutorial Questions

10. What is a redox reaction? Identify the reacants and products which are oxidised and reduced in the following equation. Zn (s) + 2 Ag + (aq) → Zn Ag (s) Ans: A redox reaction is one where electron transfer takes place between two chemical species. The species which losses electrons is oxidised and the species which gains electrons is reduced. Zinc is oxidised in the reaction and Silver is reduced. 11. What is a redox couple? Ans: Two members of the couple are the same element or compound, but of different oxidation states e.g., Ag + (aq) /Ag (s) and Mg 2+ (aq) /Mg (s) Tutorial Questions

12. How is the relative oxidizing or reducing power of a redox couple expressed? Against what reference is the redox couple measured? Describe the reference. Ans: The relative oxidizing or reducing strength of redox couples are expressed in terms of their Standard Reduction Potential E (in Volts). The E° of a redox couple is measured with reference to the Standard Hydrogen Electrode (S.H.E.). The S.H.E. is assigned potential of 0V. This consists of a H 2 gas electrode, a piece of platinum dipped in a solution of hydrogen ions (1M) with hydrogen gas (1atm) bubbling over the surface of the platinum. Tutorial Questions

13. A galvanic cell at 25 o C has an anode consisting of an Iorn strip immersed in 1M solution of Iorn (II) perchlorate (a Fe 2+ / Fe electrode) and a cathode consisting of a copper strip immersed in a 1M solution of Copper (II) perchlorate (a Cu 2+ / Cu electrode, E o red = V) A potentiometer shows E o cell = V The cell reaction is Fe (s) + Cu 2+ (aq)  Fe 2+ (aq) + Cu (s) What is the reduction potential of the Fe 2+ /Fe electrode? Tutorial Questions

Answer: The standard cell potential (0.777 V) is the sum of the standard electrode potentials of the anode and cathode. Iron metal is oxidised in the anode half reaction Copper (II) ion is reduced in the cathode half-reaction. Anode: Fe (s) → Fe 2+ (aq) + 2e - E = E o ox Cathode: Cu 2+ (aq) + 2e -  Cu (s) E = E o red = 0.337V _________________________________________________ SumFe (s) + Cu 2+ (aq) → Fe 2+ (aq) + Cu E o cell = E o ox + E o red 0.777V = E o ox V Thus E o ox = E o cell – V = V Tutorial Questions

The Iron electrode is the anode of the cell and is engaged in an oxidation half-reaction. In order to find the standard reduction potential for the electrode, we must reverse the sign of the oxidation potential E o red = - E o ox = V The standard reduction potential of the Fe 2+ /Fe electrode is V Fe 2+ (aq) + 2e -  Fe (s) E o red = V Tutorial Questions