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Can we force a reversible reaction in equilibrium to become a "complete" one? If so, will the equilibrium constant change?

It's said about reversible reactions that they never go to completion & I know that's because in reversible reactions, the mixture obtained in equilibrium is more stable than the extremes (i.e. either reactant or product) But if we force the reaction to proceed in a certain direction, say in the direction of products, will it ever go to 100% completion or near 100%?

My second question is regarding the equilibrium constant. I have read about equilibrium constant that its value doesn't change if we alter the initial concentration of reactants, which I understand why, but consider this situation: if we force equilibrium to proceed in the direction of products by altering $p,T,V$ (only and not adding or removing any substance), the equilibrium will get disturbed and obtain a new equilibrium. Wouldn't the equilibrium constant for this new equilibrium be different from the earlier one?

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  • $\begingroup$ My bad , i should have been clearer, the first one is also a Q, my Q is that can rev che rexns be made to go to completion somehow by altering the parameters , or will the eqb keep on acquiring new eqb states & not go to completion. $\endgroup$ – user47024 Feb 12 '15 at 8:55
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The two questions are sort of related, so i'll answer to the best of my ability.

A reaction in dynamic equilibrium can get close to completion depending on its equilibrium constant, which is a ratio between products and reactants at a certain temperature. No, the equilibrium constant won't change if you add more reactants in an attempt to push the reaction to completion. We refer to Le Chatelier's principle which states that if one were to add another parameter, say a reactant, the reaction will act to stabilize the stress that has been placed on it.

Take this reaction $\ce{PCl3 + Cl2 <=> PCl5}$. If I were to add more $\ce{PCl3}$, the change we would witness is a shift in equilibrium, that is to say, the system will react to consume more reactants and produce more products. What we see essentially is the system reacting to stabilize the changes we made.

This principle isn't just limited to adding reactants or products, however. We see the system tries to stabilize changes made to pressure, temperature, and volume. If we increase pressure, how will the system act to stabilize it? Pressure is a result of the gas particles hitting the walls of the container. Thus we would see the system reduced the number of gas particles in order to reach equilibrium again. Essentially, it will shift to the side with the least amount of gas particles. Volume and pressure are obviously related in this principle. An increase in volume is a decrease in pressure and a decrease in volume is an increase in pressure.

Take this reaction for example $\ce{CaCO3(s)<=>CaO(s) + CO2(g)}$. If we increase the volume (decrease the pressure), we would see the system shift to the right because it has the most moles of gas and we need more gas particles to fill the volume and stabilize the pressure.

Lastly, when changing heat we view it as a product or a reactant for exothermic and endothermic reactions respectively. An exothermic reaction has heat written as a product $\ce{A <=> B} + \text{heat}$ and an endothermic reaction has heat written as reactant. Thus, when we change the heat of an endothermic or exothermic reaction, we are changing its reactants or products and we treat it in the same way we treat a change in concentration.

The point is that the equilibrium constant remains constant, otherwise it would be called the equilibrium variable. We do, however, see a change in equilibrium position.

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  • $\begingroup$ I downvoted this answer because it does not answer the question "But if we force the reaction to proceed in a certain direction, say in the direction of products, will it ever go to 100% completion or near 100%?". This answer currently just repeats what is commonly given in introductory texts about the Le Chatelier's Principle. $\endgroup$ – Gaurang Tandon Feb 25 '18 at 2:22
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To make it short and simple no you cannot because that is how the reaction works, however you can change the equilibrium constant by increasing or decreasing the temperature. One thing to point out is that adding or decreasing pressure, concentration can effect the equilibrium and make it shift to a certain direction. K will change if you make a change that the reaction cannot fix like temperature.

But one thing to note is that if we do get the reaction to go one direction then there is no concept of equilibrium and reversible reactions. There won't be a K because it will always go a certain way. You can't change the K when there isn't one.

  1. You can force it.
  2. No equilibrium constant if it is a "complete" reaction.

This is just my insight into this with what I've learned in college chemistry.

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