Chemical equilibrium

 

Many chemical reactions are reversible. What does that mean?

A + B react to C + D, but C + D again react back to A + B.

Reactions going outward and reverse are called reversible reactions.

after a certain time in such reactions there will be an equilibrium.

Have a look at this funny example first:

Mario and Bruno are in a school yard and want to try a special game with 100 small balls. Bruno is standing 100 meters away from Mario. 100 balls are with Mario now. Mario, who is somewhat larger and 2 years older than Bruno, claimed he could send across all 100 balls to Bruno, even if Bruno would shoot them back repeatedly to Mario.

 

The game has just begun. With great force Mario raises the balls over to pass to Bruno. Bruno sends these balls as fast as possible back to Mario. After about 2 hours Mario and Bruno are quite exhausted. Still balls are going and coming back. However it seems to have slowed down. And the velocity of going to Bruno and rolling or flying back by him seems to be almost the same. Recently it starts to rain. The mother of Bruno calls from the balcony of the neighboring house, he should finally come home.

Mario now is with 40 and Bruno with 60 balls on the ground.

An equilibrium is reached. We divide the number of balls at Bruno by the number of bales at Mario: 60/40 = 1,5

Mario wants to repeat the same game. This time with a school friend, which is 2 years older than Mario. His name is Paul.  Again after 2 hours the same numbers of balls go from Mario to Paul and back to Mario. Again Mario has missed its target , sending all 100 balls to the partner. Furthermore: In Mario now are located even 60 balls and Paul only 40.  Again like above playing with Bruno, we divide the existing balls of Paul by those of Mario. 40/60 = 2/3 or 0.67 

The two examples above provide that all chemical reactions which are not spontaneous or highly exothermic result in equilibrium reactions. The equilibrium is reached as soon as the speed of the forward and reverse reaction is equal.

The law of mass action

What you have seen above and results of those divisions are not but the

equilibrium constant K. Although it’s not a chemical example, but it’s like the law of mass action works chemically, according to convention.

General formulation:

For a chemical reaction

A + B ===> C + D

Products on the right side in the counter / Reactants on the left side in denominator.

If a plurality of products or reactants are involved in the reaction they are multiplied.

K = C * D / A * B

Stoichiometric numbers of the substances involved are potentiated. Their unit is the number of moles, n. n = 1 = 1 mole

aA + cB  ===>  xC + z D

K = Aexpa * Bexpc / Cexpx * Dexpz

Generally

The constant K depends on temperature!

Mostly, reactions take place in a liquid medium. Number of moles per liter, mol/l are used as unit.

That’s why the constant is also called Kc.

Returning to the example above.

The ballgame Mario against Bruno ends with the equilibrium constant K = 1.5. K is larger than 1, K > 1. This means nothing more than the balance is on the right side. (side of the products)

In the game Mario against Paul, for the constant we get K = 2/3 or 0.67. K is lower than 1, K < 1, the equilibrium lies to the left side. (side of the reactants)

This can also be applied to the law of mass action chemically. Have a look at the following table:

 

 

 

 

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