This discussion focuses on the mechanisms of recharging secondary batteries and the fundamental differences between secondary and primary cells. Secondary cells, such as lithium-ion batteries, can be recharged because their chemical reactions are reversible, allowing the original reactants to be regenerated. In contrast, primary cells, like Leclanché batteries, undergo irreversible reactions where products are consumed in side reactions, preventing recharging. The concept of local energy minima is also explored, emphasizing that secondary cells can be pushed back to a charged state without encountering competing local minima.
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ok i have figured it out.could you please look at this video from 1:28 to 3:58 question is the cell which is described in this video from 1:28 to 3:58 is secondary cell which is rechargeable,why can't the same occur in primary cell which is not rechargeable,how the chemistry of these two cells (secondary cell &primary cell )differ so that these cells get opposite characteristics :i.e in one cell recharging is possible and it is not possible in the other cell.Borek said:Can you write reactions that take place during discharging of a battery? (pick whatever type you like, just select one that can be recharged).
if you don't mind can you please explain concept of local energy minimum.just tell me basics later i will try to figured it out by myself.Borek said:In general it has to do with the irreversibility of the reaction. In primary cells if you put the energy back into the system, it will not go back to the original state. Say, in Leclanche battery, instead of reversing the reaction you will just decompose water present (and produced in the battery reaction) into hydrogen and oxygen - leaving ZnCl2 and Mn2O3 intact, so the battery doesn't recharge.
Thermodynamically - when you try to recharge primary cell it won't get to the original state, but to some local energy minimum. Secondary cells are those that don't have such competing local minimum, so they can be pushed only back to the charged state. Note, that asking "why secondary cells behave this way" is putting the cart before the horse - we call cells that behave this way "secondary", so it is behavior that classifies the cells, not the the classification that tells us how the batteries behave.
it is very complicated concept .but good explanation thanks.Borek said:Actually I think my previous post was a little bit misleading. When recharging the battery we are not necessarily traveling to a minimum. We are moving in a direction that is chemically "the most easy way up".
You do know what a minimum, global minimum and local minimum are in general?
Every chemical system has its energy (actually it would be better to talk about Gibbs free energy, but let's not get into too many details at once). Any change to the system (like discharging a battery) means moving the system on the energy plot (be it curve, or surface, or multidimensional surface). Every system has a tendency of spontaneously moving in the direction of some energy minimum (which one - depends on the shape of the energy function around, think about water flowing down in the mountains - direction of the flow depends on the slope). Discharging a battery mean moving a system (that was initially somewhere on the slope) to its local minimum (in which the system is much more stable than the original battery was; battery was stable because the reagents were artificially separated so that they didn't react spontaneously). When you attempt to recharge the battery (which is not a spontaneous process, so the system can travel up on the energy landscape) you are trying to push it up - but it may prefer to not travel the the starting point, but in some other direction (say, where the slope is less stiff).
Pharrahnox said:primary cells differ from secondary cells, in that in primary cells, the products of the discharging reaction are used up in side reactions