[tieKh]

Hi guys,

It's been a while since I did any real chemistry and I'm just wondering about something that came up at work recently. When we open our reagents we give them a secondary shelf life as the manufacturer understandably won't stand by the product once opened. The dates we use are predecided but there's no real reason why these we're chosen other than common knowledge in many cases.
So, what I'm wondering is, what exactly is happening that makes chemicals (in general) degrade or deteriorate? I know for solutions or hygroscopes, the environment is probably a big factor. And assuming there's been no contamination or human error, and air exposure is minimal what other factors are at play?
I suppose, the sort of thing I'm thinking about is that I've always been told inorganic salts basically last forever if they're kept well and I never questioned that. But when I try to look for a reason, I can't find a study demonstrating that they do last. I tried looking at thermodynamic and kinetic stability, but I'm out of practice and Pchem was never my strongest area.
I know it's very broad, and it's going to depend a lot on the species but if you have any good reading, your recommendations are very wlecome! Thanks

[Arkcon]

This is a difficult question to answer because you're asked it in the most general way possible.  Maybe if you started with a few specific examples, you can combine them all to figure out what you want to know.

[Furanone]

So many different compounds could react and thus degrade in quality in different ways and to different degrees.

Just some considerations:

light sensitivity - many compounds with conjugated dienes will be sensitive to light and could degrade with increased eposure to light and UV radiation. Best to store in amber glass and in the dark to increase storage life.
moisture sensitivity - as you mention many compounds depending on their hydrophilicity will be more or less hygroscopic and absorb moisture from atmosphere. Working in a low ERH (Equilibrium Relative Humidity) Lab (<45% ERH) will help increase storage and closing caps on bottle quickly after taking your sample.
pH sensitivity - Related to above moisture sensitivity, as samples absorb moisture the pH can change usually decrease affecting many things such as color, and even chemical structure (oxidizing nitrites to nitrates).
oxygen sensitivity - exposure to oxygen can catalyze many reactions where compounds can be oxidized such as lipid oxidation, peroxidation, etc.
temperature sensitivity - as temperature increases by 10 C reaction rates generally double so this means changes will happen faster when stored at warmer temperatures. If enzymes, enzymatic activity can be impaired. Likewise at very cold temperatures where ice crystallization can occur in water solvent, damage can take place from crystals puncturing living cells, or destroying enzyme activity.
chemical reactivity - some chemical functional groups are more reactive than others such as acids and alcohols can form esters over time, aldehydes are reactive, so i a mix, the equilibrium can shift over time when conditions have changed.
volatility - for smaller MW molecules there can be volatility losses over time when openly exposed to atmosphere for significant periods changing functionality especially if a mix where some compounds volatilize while others do not, so best to cap bottle immediately following pouring a sample.

Therefore, store in cold, dark areas with tightly-capped bottles!

[monkeymagic]

Assuming no external influence, will a chemical composition last eternally? E.g. Will H2O or NaCL ever split back up again of their own accord if the composition is protected from external influences?

[Corribus]

Eternally is a very long time. It's not even certain if a proton is eternally stable.

That said, once chemical equilibrium is reached in a closed system, it will not change without some external force or stimulus is applied, in which case the system is no longer really closed, is it? 

It's kind of a nonsense question, though (no offense). Supposing you sealed water in some container. Will it last eternally? No. Every material is porous to some small degree -the water will eventually evaporate. Just so with salt, I imagine. Which is to say, there's no such thing as a truly closed system (unless you maybe count the universe itself), which violates the premise of the question.

[Enthalpy]

The question is too general, as different compounds have different stability. Concentrated H₂O₂ is about impossible to keep eternally despite "some people" would like to. If the container is open, external influences decompose the peroxide slowly (or even quickly, for instance a rat falling into). If you close the container, after a variable time the peroxide detonates.