How easily reversible are chemical reactions?

In chemistry books (if I recall correctly - nearly all) the described equations are reversible; so reversing an exothermic reaction you need energy. I get gas(es) in pyrolysis of plastics and I can break water to hydfrogen and oxygen in electrolysis. Are there any truly irreversible reactions? Are there any more of these -lysis processes I could try?

If I remember correctly theoretically all reactions are reversible, but some are very slow. Some polyesters hydrolyze fairly easily. Ester interchange is common too. Vinyl addition polymers tend to be hard to get started, but once started may unzip as fast as they formed.

If you burn a dollar bill, I guess you can get back the hydrogen and carbon, but the great seal and Washington will be tough.

in a state of chemical equilibrium, there is no net change but many constant chemical reactions happening/reversing.

vinegar and water in a glass would be an example.

If you get any compound hot enough you will separate it into separate elements again. If you get hotter still, you’ll start transmuting elements, but there’s a pretty wide range of temperatures at which there are no chemical compounds but all elements are as stable as they ever are.

An example of what? You’ve listed a water-based solution and water, how is that supposed to give a reaction? Do you mean a solution of acetic acid in water, where you have an equilibrium between the acid, the water, and the ions of both?

The speed of a reaction is a function of its energetic pathway. NaCl dissolves easily in water, but it dissolves more easily in hot water than in cold water: the lower temperature means that the ions have less energy available to separate and move away from the solid. You can speed up dissolution by stirring: this increases the spread-away speed and the amount of contact between solid salt and water. A reaction may need to be started (a spark, a hot point), it may need to have its “activation energy” kept low throughout so it can continue (use of catalysts)… the activation energy will usually be higher from one side of the equation (the side favored by the reaction, the side of which you’ll get more if you let the reaction continue until equilibrium) than by the other.

The most irreversible yet apparently-simple reactions are those which go from very complex molecules to very small ones, such as combustion or hydrolisis of proteins. Burn a relatively-simple molecule such as glucose (C[sub]6[/sub]H[sub]12[/sub]O[sub]6[/sub]) get CO[sub]2[/sub] and H[sub]2[/sub]O - can you get the glucose back? Not through direct reversal of the reaction, and not by the system of “putting together enough CO[sub]2[/sub] and H[sub]2[/sub]O and a reducing agent”.

But that apparently simple reaction is actually multi-step: you can oxidize glucose “just a little”, to produce different compounds (we don’t do it because these compounds are obtained much more easily and cheaply in other ways). When you have a chain of reactions the further along you get the harder it is to go all the way back. In chemistry, one of the goals of reaction optimization is “one pot sequences” where by adding different reagents in sequence and changing reaction conditions you get

…A+B -> C (+D -> E (+F -> G…)))

By the time you’re at G, there will be some A and B left in the pot but getting back to “mostly A and B, with the rest at the impurity level” is just not going to happen.

yes that is one example of the many types of chemical equilibrium.

johnpost, do you realize acetic acid and vinegar are not the same thing?

yes i do. i know the difference between glacial acetic acid and wine vinegar and use each appropriately. vinegar, as people commonly use the term, contains acetic acid. it is a example a nonscientist might easily relate to.

Only if you explain it properly, just dropping the terms with no additional explanation doesn’t explain anything. “Vinegar and water” is not a reaction, it’s half of a salad dressing.

The acetic acid whatever its source and water or salt is not an example of a reaction, just ionization.

Instead look at something like esterification. Put a compound with a hydroxyl and one with a carboxyl together. Add a little xylene as an azeotrope, and a trap for the water and heat to reflux. The carboxyl and hydroxyl will react yielding an ester linkage and water. As long as you keep removing the water, the reaction will proceed. You will also have the formed esters breaking down and the reactants interchanging. You can add water but the reverse reaction will go slowly. Add too much and things will boil over.

You must select compounds that won’t break down or boil away before getting hot enough to react quickly enough.