What endothermic reactions can be effectively reversed by electrolysis?

Not only newbie, but total layman here; please be patient with me.


Given are either two different elements, two different molecules, or one element and one molecule (chemical **A** and chemical **B**). **A** and **B** are preferably not solid in the temperature range from approx. −20 °C to approx. +40 °C.


They are placed in a container in the aforementioned temperature range (−20 °C to +40 °C). Once the heat increases above +40 °C, **A** and **B** engage in an *endothermic* reaction preferably resulting in a single chemical **C**, a liquid (potentially a gas, but I'm not sure if the physics or chemistry will add up further down) that will preferably remain in liquid (or potentially gas) state even if its temperature after the endothermic process goes down drops below +40 °C and down to, and not necessarily below −20 °C.


Assume **C** is placed in a different container where it is subjected to electrolysis or other procedure requiring no mechanical procedure, but electricity to reverse **C** to original **A** and **B**.


1. If any, what element(s) and/or molecule(s) may be **A** and **B**?
2. If any, what chemical is **C** resulting from **A** and **B**'s endothermic reaction above +40 °C?
3. Is it possible to revert **C** to **A** and **B** in any of the above described manners without the reaction producing thermal energy?




Any elemental, i.e. single step, reaction and many more complicated reactions that can reach equilibrium, can be reversed. The forward and reverse reactions in an equilibrium situation follow the same reaction path. The requirements are straight forward. At equilibrium there must be products, reactants, proper mixing, sufficient energy so enough molecules have the activation energy and an appropriate mechanism or catalyst for the reaction to proceed at a reasonable rate. If products are missing the reaction will proceed to the right to give products, if the reactants are missing the reaction will proceed to the left to give the reactants. When equilibrium is reached the activities of the products and reactants satisfy the equilibrium constant. It is important to include energy as a reactant and product in the reaction.


I will give one example that you can look up on the web to get more info, the lithium-ion battery in your cellphone. When it is fully charged there is an anode and cathode in a high energy state both of which involve Li+ ions; there is a potential difference between the anode and cathode, between + and -, and electrons cannot flow because of construction. When the battery is used electrons flow thru the device doing work and the circuit is completed by Li+ ion flow in the battery. after all the ions have flown and the high energy states of the cathode and anode have shrunk to zero the cell voltage is zero, no more cell phone. Now the charger is plugged in and applies a higher reverse voltage hopefully bringing very thing back to original [well almost].


The same thing happens in simple chemical reactions. If O2 and H2 are mixed nothing happens until a catalyst or initiating energy is introduced; then the reaction starts the temperature rises. If the heat is contained there will be a very hot mix of H2, O2, and H2O at equilibrium. If the energy is removed by radiation, or doing work, or generating electricity, more and more water will form until at room T only the slightest trace of hydrogen and oxygen remains. Conversely if we start with pure water at room T, add a catalyst and slowly add energy either by heat or electricity water will break down into H2 and O2. Done carefully there can be equilibrium every step of the way for the reaction in both directions. Any reaction that is removed from equilibrium, provided the conditions are there to reach equilibrium, is spontaneous. In one direction it is exothermic driven by the loss of energy; in the other direction it is entropy driven by the increase of entropy. At equilibrium Delta G = 0 and the change in energy is balanced by the change in entropy.


Careful observation of every day events shows many examples of steady state events that involve [almost] chemical equilibria: rain, snow, clouds, a steaming tea kettle your breath on a cool morning, putting on a sweater if cold, removing it if warm, global warming for a biggie.


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