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Nailing Corrosions.ppt
- 1. Nailing corrosion The electrochemistry of rusting Education in Chemistry March 2016 www.rsc.org/eic
- 2. H2O H2O H2O H2O H2O H2O Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ H2O(l) + e- ⇌ ½H2(g) + OH-(aq) -0.83 V Fe2+(aq) + 2e- ⇌ Fe(s) -0.45 V Small circle = holds on to electrons more strongly Larger circle = holds onto electrons more weakly Excess electrons – negative charge Positively charged ion in solution ATTRACTION
- 3. H2O H2O H2O H2O H2O H2O Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ H2O H2O H2O H2O H2O H2O Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ H2O H2O + + + + Larger circle = holds onto electrons more weakly Smaller circle = holds onto electrons more strongly Ions dissolve more readily Fewer ions dissolve More electrons on metal Fewer electrons on metal Potential for electrons to flow
- 4. Fe2+ Fe2+ Fe2+ OH- OH- H2O H2O H2O OH- OH- ½ O2 H2O H2O H2O Fe2+ Fe2+ Fe2+ H2O H2O H2O OH- OH- ½ O2 H2O OH- OH- ½ O2 H2O Fe2+(aq) + 2e- ⇌ Fe(s) -0.45 V ½O2(g) + H2O(l) +2e- ⇌ 2OH-(aq) +0.40 V ½ O2 What needs to happen next … ? Oxygenated water near surface – able to take electrons from iron • Oxygenated water accepts electrons • Hydroxide ions formed • Indicator turns pink • Iron ions dissolve • Indicator turns blue
- 5. Fe2+ Fe2+ Fe2+ OH- OH- ½ O2 H2O H2O H2O OH- OH- ½ O2 H2O H2O H2O Fe2+ Fe2+ Fe2+ H2O H2O H2O OH- OH- ½ O2 H2O OH- OH- ½ O2 H2O Cl- Na+ Na+ Cl-
- 7. Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Mg2+ Mg2+ Mg2+(aq) + 2e- ⇌ Mg(s) -2.37 V Fe2+(aq) + 2e- ⇌ Fe(s) -0.45 V
- 8. Fe2+ Fe2+ Fe2+ OH- OH- ½ O2 H2O OH- OH- ½ O2 H2O Fe2+ Fe2+ Fe2+ OH- OH- ½ O2 H2O OH- OH- ½ O2 H2O Mg2+ Mg2+ ½ H2 OH- H2O ½ H2 OH- H2O ½ H2 OH- H2O ½ H2 OH- H2O ½ H2 OH- H2O ½ H2 OH- H2O Mg2+ Mg2+ ½ H2 OH- ½ H2 OH- H2O OH- OH- ½ O2 H2O H2O Mg2+(aq) + 2e- ⇌ Mg(s) -2.37 V H2O(l) + e- ⇌ ½H2(g) + OH-(aq) -0.83 V Fe2+(aq) + 2e- ⇌ Fe(s) -0.45 V ½O2(g) + H2O(l) +2e- ⇌ 2OH-(aq) +0.40 V
- 9. Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Fe2+ Cu2+ Cu2+ Fe2+(aq) + 2e- ⇌ Fe(s) -0.45 V Cu2+(aq) + 2e- ⇌ Cu(s) +0.34 V
- 10. Fe2 + Fe2 + Fe2 + OH- OH- ½ O2 H2O H2O H2O OH- OH- ½ O2 H2O H2O H2O Fe2 + Fe2 + Fe2 + H2O OH- OH- ½ O2 H2O OH- OH- ½ O2 H2O Cu2 + Cu2 + Cu2 + H2O H2O H2O OH- OH- ½ O2 H2O OH- OH- ½ O2 H2O Fe2+(aq) + 2e- ⇌ Fe(s) -0.45 V Fe2+(aq) + 2e- ⇌ Fe(s) -0.45 V ½O2(g) + H2O(l) +2e- ⇌ 2OH-(aq) +0.40 V
Editor's Notes
- Here’s the structure of a metal. The blue circles represent iron ions within a solid metallic structure. The red circles represent water molecules. The orange dots represent electrons. These are currently free to move throughout the metallic structure. The electrostatic force of attraction between the negatively charged delocalised electrons and the positive metal ions holds the metallic structure together. The size of the circle is NOT relevant to size of the particle – the size of the circle is meant to represent how strongly the particle can attract an electron. The smaller the circle, the more strongly it holds onto electrons (this is a massive simplification because standard electrode potentials are not just affected by ionisation energy or electron affinity. It’s just a workaround to try to introduce the idea that some species are better reducing agents than others). Why did I choose small circles to hold onto electrons more strongly? Because students are expected to know that ionisation energies are higher for smaller ions of the same size. It was a sensible compromise. Metal ions can dissolve into water and move through solution once there. However, electrons can only move through metal or be transferred TO another substance (smaller circle) in a chemical reaction. Questions: Can the iron give electrons to the water? Remember, the better the species is at giving away electrons, the larger it is. Can iron ions dissolve into the water? The metal started as neutral – what charge will it be now? Are there any forces between the Fe2+ and the metal? What does that mean about where the Fe2+ can go next? Do you think this process can continue forever? The blue line around the outside of the circle indicates that if a lot of these ions dissolve into solution, we’ll see a blue colour – what do you expect to see and where? *Show the first petri dish
- While some iron ions may dissolve into solution in the immediate vicinity of the metal, unless there is a way to move electrons away from the metal, this process will quickly reach an equilibrium and not continue any further.
- So water on its own doesn’t react. But if we put O2 in – notice that O2 and H2O are much better at accepting electrons / worse at giving them away. Where have I placed the O2? We dissolve an ion – what do the electrons want to do? Before this was as far as we went, but now there is somewhere these electrons can go. Pink starts to form and, through further reactions, rust. * Show 2nd petri dish What is going to happen to the ions (attract)? We are completing a circuit. Electrons flow up the metal and ions flow through the solution. To begin with there aren’t a lot of ions in the solution to help redistribute the charge. What will happen if we put in a lot of ions like salt?
- Now we can start to think about how to play around with this system. Can you think of any ways in which we could stop the rusting from taking place? Paint etc.
- Now we’re going to look at some other options. Cu and Mg. (The image here is of a zinc sacrificial anode although in the video I keep talking about magnesium to avoid throwing more complexity into the explanation – CC BY SA Joe Mabel: https://commons.wikimedia.org/wiki/File:Chittenden_Locks_during_large_lock_maintenance_009_-_new_zinc_anode.jpg) Remember: to start the process we need to give electrons to water. Up until now, these electrons have been coming from the iron metal as it dissolved into solution.
- Show Mg/Fe. Without any water what’s going to happen?