Time for Chemical Equilibrium

Name: Kevin
Status: student
Grade: 9-12
Location: CA

Question: How long does it take for chemical equilibrium to be reached?
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Kevin-   Sorry, the main answer is "It depends."
On which reaction, on temperature, on concentrations, on catalysts,...

"Equilibrium" is the end result of a chemical reaction.
Knowing the equilibrium tells you what state the mix is going towards,
   but tells you almost nothing about how fast it'll get there.
How ironic.  Talk about dealing with the devil...
Unfortunately "how fast" tends to be a different branch of thought called
  "chemical kinetics" or "reaction rates".

Scientists who make phase diagrams have that problem all the time,
  when trying to decide how much tin can stay dissolved in solid lead,
   or other solid-solid solutions.
It is easy when the metals are hot and equilibrium is reached kind of fast.
Then the boundary lines fade away towards the bottom of the page,
cause that metal-mix is just not going to get to equilibrium any time soon.
While the mixture is cold, it might be trying to change gradually
 for a few days or for millions of years.

If a chemical reaction has been started,
  and we know what its equilibrium should be,
  and by measuring we see that most of the expected changes are done,
then we know we are have pretty much arrived at equilibrium...
And we know that the reaction time is less than the time since this reaction started.

What is interesting/worthwhile  about equilibrium problems is
 knowing in advance what mix of reactants and products will result
 from a particular attempt to make a reaction.

Frankly, it is usually most useful for reactions where
  the reaction time is kind of quick,
  so we can do multiple experiments with different starting mixes, or some such.
Sometimes designing the end result with numbers
  can be done quicker, cheaper, and smarter
  than really doing endless experiments with small variations.
Sometimes it is useful in very slow reactions (like rocks aging) because
  it helps us decide whether the reaction is done yet,
without having to watch the whole process.

Jim Swenson
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Kevin,

Think of chemical equilibrium as the state in which the forward rate of reaction 
(how fast the reactants are becoming products) and the reverse rate of the reaction 
(how fast the products are becoming reactants) are equal.

This would mean that as fast as the reactants are disappearing (to become products) 
would be just as fast as the reactants are being formed (from the products). As such,
while there are chemical reactions going on, there is no change in the concentrations 
of the reactants and/or products.

Now, remember that concentrations have an effect on the rate of the reaction. If a 
reaction solution is very concentrated, the reactants will collide with each other 
more, and react more often and faster. So, in the beginning a reaction may have a 
very fast rate of forward reactions. But as time progresses, the forward rate will 
slow down because there is a lower and lower concentration of reactants.

At the same time the reverse rate is getting faster and faster as more and more 
products are being formed.

Eventually the two rates will be equal and this is the point that we have chemical 
equilibrium.

So, this means that the time it takes to achieve chemical equilibrium will depend on 
the rate constant of the reaction (how generally fast the concentration changes over 
time) and the initial concentration of the reactants and products.

Suppose that the starting solution is made up of some reactants and some products, 
then the equilibrium is achieved quite quickly. On the other hand, if the solution 
is pure reactant (or product), then it may take a while. Also, if the reactions (both 
forward and reverse) generally happen very fast, then the equilibrium might be reached 
really fast even if the solution starts as pure reactant (or product).

Greg (Roberto Gregorius)
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Kevin,

There is no quick answer to your question.  The best answer is it depends.

It depends on 
(1) the reaction itself, particularly the reactants and the 
thermodynamics of the reaction, 
(2) the concentration of the reactants, 
(3) the temperature of the reaction, 
(4) the pressure of the reaction if a gas is involved, 
(5) are reactants added to the reaction is it proceeds, and/or are products being 
removed?  All these factors usually fall under the topic of LeChatelier's 
principles. 

Warren Young
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