![]() ![]() The hydrogen ions come from the atmospheric reaction between carbon dioxide and water, forming carbonic acid. The electrons migrate towards the cathodic region, where oxygen, with an electrode potential of +1.23 volts, is reduced to water in an acidic environment. Defects on the surface serve as the anodic regions where iron, with an electrode potential of −0.44 volts, readily oxidizes to iron(II). Rusting is a spontaneous electrochemical reaction. For example, iron oxidizes in the presence of oxygen, water, and hydrogen ions, to structurally unstable metal-oxides called rust, which continuously flakes off, exposing the underlying metal to further oxidation. For example, copper oxidizes in the presence of oxygen, carbon dioxide, and water, to form basic copper carbonate, a blue-green and structurally stable protective layer. It is the same phenomenon behind the blue-green color of the Statue of Liberty or a rusty anchor. ![]() This is in accordance with the Second Law of Thermodynamics which tells us that on the transformation of energy from one form to another form entropy always increases and free energy always decreases.Corrosion is the spontaneous oxidation of metal in the presence of an oxidizing agent, usually oxygen. The total entropy increases by the rusting which favours spontaneity. ![]() ∆G = - 2840 kj/mole, negative ∆G makes the reaction spontaneous. ∆G = ∆H – T ∆S= , ∆G = Gibbs free energy Huge gain in entropy by surrounding makes the rusting reaction spontaneous. ∆S total = ∆S system + ∆S surrounding = ) Jmol-1K The air molecules in the surroundings move faster and make more hard hitting collisions to speed up rusting.Įntropy and free energy change in rusting reaction: Rusting reaction really takes off and becomes a self – supporting raction when the surplus energy 1648 kJ mol-1, localized in Fe and O2 becomes dispersed to the surroundings as ‘heat’ and raises the entropy of surrounding. The reason is, even though there is significant affinity of oxygen for iron, O2 at normal room temperature around 298 K, has only few atoms moving exceptionally fast which can hit the Fe just right so that Fe-Fe and an O-O bonds breake and Fe-O bond can form. Why rusting initiation is a slow process? This surplus stored energy in Fe and O2, makes the corrosion reaction exothermic and it has huge implication in making rusting a spontaneous reaction. In other words, the bond energy of Fe-Fe-Fe-Fe atoms + O = O atoms > bond energy of Fe2O3. Iron atoms (as -Fe-Fe-Fe-) plus oxygen molecules of the air (O-O) have more energy localized within their bonds than does the product of their reaction, iron rust (iron oxide).The rust formation reaction is exothermic. The answer is iron plus oxygen to form iron oxide or rust, the reactants, iron and oxygen don't have to be at a high temperature to have energy localized within them. The first question that comes to our mind who provides energy for this chemical reaction? How iron gets activation energy to cross the hill at ambient temperature? Image below, represents the activation energy barrier for rusting. : ΔrH is enthalpy change of reaction and ∆S is entropy change for the reaction. What makes rusting a slow but self-supporting spontaneous chemical reaction?ĤFe (s) + 3O2 (g) ⟶ 2Fe2O3 (s) : ΔrH = -1648 kJ mol-1, ∆S system = -549.4 JK-1mol-1 at 298K. Current flows through the metal from the anode to the cathode.įe++ and OH- ions combine to produce hydroxides which dehydrate to Fe203.nH2O which is rust. Corrosion occurs at the anode, where metal oxidizes and dissolves. Differential O2 concentration between O2 starved point on metal and other normal part of metal sets up electrolytic corrosion cells consisting of anodes/cathodes because of potential difference. The drop on metal becomes barrier for O2 on metal surface. A drop of stationary water on iron surface is sufficient for the initiation process of rusting.
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