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It is generally said that reactants react so that they can achieve a lower energy state. Then why does a reversible reaction occur in the first place? Since turning reactants into products increases their energy - an undesirable effect from the point of view of reacting molecules.

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First, let's tackle what an irreversible reaction is. Baking a cake, for example, is commonly considered "irreversible." The eggs, flour, milk, etc. which you started with are no longer present in their original forms after the cake has been baked.

Now, can you reconstitute the eggs, flour, milk, etc.? Of course you could take the molecules of the cake and piece them together to make eggs, flour, and milk, but doing so would be a very formidable task. A lot of energy will have to be expended to reconstitute the egg, for example, because eggs are made of protein and with heat (baking) proteins are denatured (lose their original 3D conformation and possibly their linear makeup or amino acid sequence as well).

Another example of an irreversible reaction is combustion of hydrocarbons - i.e. burning gasoline in your car. This reaction is definitely reversible in the long run - it just takes a few million years, a lot of heat, and a lot of pressure for the hydrocarbons to be formed again. But for semantic's sake we say that combustion is irreversible because it's not reversible on a really relevant time scale.

So in general, the difference between irreversible and reversible isn't exactly well defined - i.e. it might be hard to reverse a reaction (turn a cake back into eggs, etc.) or it might take a long time to reverse a reaction, but what's hard? What's long? Tellinging, the IUPAC (International Union of Pure and Applied Chemistry) does not try to define irreversible vs. reversible reaction.

A deeper treatment would involve discussion of equilibrium constants (symbolized by capital K) but I'll let another poster get into this if necessary.

It is generally said that reactants react so that they can achieve a lower energy state. Then why does a reversible reaction occur in the first place?

Good question. Remember that we can always add energy to make an unfavorable reaction proceed. For example, the sodium ion, which is isoelectronic with neon, is stable with a full octet of electrons. However, we can still take away more electrons. It just takes a rather sizable application of energy.

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Thanks for the compliment. So what it really means that in reversible reactions( those reactions which are easily reversed ),the energy released and other factors like temperature, pressure etc. themselves favour the backward reaction. Also am I wrong in thinking that backward reaction takes place easily since the difference between two energy states is small enough that in same conditions both energy states can be achieved? –  viham Aug 23 at 3:04
    
It is not the case that in all reversible reactions the backwards reaction is favored. Reversible reactions are fundamentally equilibria. –  Dissenter Aug 23 at 4:39

There are several reasons for the reverse reaction to be favored; one is temperature. In the following reaction: $$\ce{2NO_2<=> N_2O_4 + energy}$$ The right is the favored product, unless the temperature is increased. With enough heat, then the reactant is favored.

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