In all chemical reactions there is possibility that the reverse reaction could occur. A chemical reaction that reaches equilibrium is one where the forward reaction occurs at the same rate as the reverse reaction. For example:
2NO2 (g) <=> N2O4 (g)
NO2 (g) + NO2 (g) <=> N2O4 (g)
Now this reaction reaches equilibrium and because the rate or speed of the forward reaction equals the rate of speed of the reverse reaction the amount of each chemical stays constant. If we were to put NO2 only in the reaction vessel and seal the container, a graph of the changes in concentration (Molarity or partial pressure) would like the following.
Notice that concentration of the NO2 is large initially and decreases as the reaction proceeds.
Notice the initial concentration of N2O4 is zero (it has not been made yet) and increases as the reaction proceeds.
Both NO2 and N2O4 eventually level out and at this point equilibrium has been reached!
If the rate of 2NO2 combining to make N2O4 (forward reaction) is as fast as rate of N2O4 decomposing into 2NO2 (reverse reaction) then the concentration of both chemicals become CONSTANT at equilibrium.
So what does this look like? Well it is pretty boring in that it appears that nothing is going on because the reactants and products stay constant. In truth there plenty going on because forward reaction and the reverse reaction atr continuously going on but there is no NET change.
Also notice part people that the reactants (NO2 ) and products ( N2O4 ) ARE NOT Equal at equilibrium. IN this example or reaction NO2 is favored at equilibrium. Why would nature favor the NO2? Which of the 2 chemicals has the greatest dispersed energy? The 2 pieces ( NO2 ) or the larger whole (N2O4)?
The 2 smaller pieces!! Nature favors energy that spreads out.
Nature favors a pathway that has the greatest amount of dispersed energy. We call this Entropy (S)! So we know that the universe works by always providing a pathway for energy to go from a high concentrated source to a lower energy source. In other words, Chemical or physical processes can only occur if the universe provides a pathway for it to happen. This pathway is dependent upon the Entropy (S) to increase in the universe or an increase in the change of Entropy (ΔS). This is the second Law of Thermodynamics!
ΔSuniverse = increase ( IF There is a pathway!!)
That is why Heat flows to Cold always!! There are limits to what can be done with matter and it all depends on Energy dispersing! If Energy is being diluted or dispersed into the Universe then the process is SPONTANEOUS under a set of defined conditions.
If the Energy is not dispersing into the universe then there is NO PATHWAY for the reaction or process to occur!! This process is NONSPONTANEOUS under a set of defined conditions.
If you think of it this way then you can understand why a change of how fast a reaction occurs (RATE) has nothing to do with spontaneity. In the case of the reaction above 2 particles (NO2) combine to make larger one (N2O4) or 1 particle breaks into 2 particles in the reverse. This ratio is upheld no matter how many more of these multiples I have. If I have 20 pieces (NO2) I will make 10 larger one (NO2 ) and if it occurs faster by adding a catalyst then whole number ratio is upheld!
Now in getting back to the equilibrium condition in above diagram or the reaction:
2NO2 (g) <=> N2O4 (g)
The forward reaction is moving as fast as the reverse. Since there is no net change there is no increase in Entropy in the universe or there is no real preferred pathway when equilibrium is reached. At this position the Energy has been dispersed already!!! This is the death of the chemical reaction in one direction!! All reactions move in a direction to exhaust there energy into the universe until it empties! That point is equilibrium!!!!
Thus all reactions move toward equilibrium, and thus get less spontaneous over time as they approach equilibrium or reach the death of the chemical reaction! The father away a reaction is from equilibrium the more spontaneous it is!!
This diagram illustrates what I am talking about:
At point C, there 100% NO2 and 0% N2O4 thus there is no competing reaction because there is no NO2 to combine to make the N2O4. The reverse reaction is at it maximum spontaneity and the forward reaction is at its minimum. There is a Shift in the reaction to move in the reverse. Over time its spontaneity will steadily decrease until it reaches the point of equilibrium and the “death of the reaction”.
Notice that the equilibrium position is at the bottom!! This is where the death of the reaction or where there is ZERO Spontaneity or No more release of energy into the Universe.
ΔSuniverse = O
At point A we have 100% NO2 and 0% N2O4. At this point there is tremendous pathway to go forward! There is No competing backwards reaction!! The reverse reaction is impossible here. It so spontaneous in the forward that the reverse is impossible!
At point B, there is 60% NO2 and 40% N2O4. Here we have a competing reaction reverse reaction thus the forward reactions is not as spontaneous and the reaction is getting closer to its death. The reverse reaction is still not as spontaneous as the forward but it is closing in, thus the overall Net direction is still forward but this pathway is not nearly as great as point A.
This shift is what we learn in Le Chateliers Principle –
Regents definition: When a stress is placed on a reaction at equilibrium, the reaction will shift forward of reverse to regain its balance.
REAL Definition: When a stress (a change in concentration) is placed on a reversible reaction the reaction will become more spontaneous in one direction in order to keep dispersing energy until it is run out (and no more work can be done). In a battery this means 0.00 volts!!
We have learned that voltage is the Energy to move current or charge around circuit either by the pushing of chemicals that spontaneously oxidize and in combination with chemicals that pull charge by spontaneously reducing. The voltage of a half reaction tells us about the spontaneity, ability to reduce or oxidize, or the pathway to do so based on the amount of dispersed energy that get released into the universe! Voltages give us a measured value of the available pathway for the redox reaction to occur.
So my question is, Why does a battery have a voltage drop? Why does it die?
Equilibrium and the death of chemical reaction is NOT unique to chemical changes only!
In physical changes (no bonds broken just attractions) equilibrium also will STOP phase changes!
That is why the melting point stays at one Temperature!
Example: Heat + H2O (s) <—> H2O (l)
If you start with a cooler of just ice AND ASSUME NO HEAT enters your cooler what will Happen?
At first when you have 100% ice and 0% liquid water and there WILL HAVE the greatest spontaneity 0r PATHWAY to melt or go forward.
The cooler will eventually get to the point where there is ice and water mixture and reaches equilibrium and thus the rate of melting equals the rate of freezing (or refreezing). At this point no more can be work is done there is no more spontaneity in either direction.
If no heat leaked into the cooler then then you would have an the same amount of ice and liquid water in the cooler forever.
freezing melting equilibrium animation:
Because heat leaks into the container (darn entropy!) the forward reaction becomes more spontaneous because of Le Chateliers Principle and eventually all ice will melt because at higher temperatures liquid water can disperse its energy (is more random) than the ice.
Ok then can we manipulate the physical process? We do this all the time. How about placing salt on ice?
We all know that salt will melt ice BUT WHY?
How can I manipulate the above physical process so that the Ice will melt even if it is currently at equilibrium?
We make the forward direction move spontaneous!!! We help give the forward process an increased pathway!!
Remember that ice and water at equilibrium and if we can BLOCK the water molecules from refreezing the RATE of melting will now exceed the RATE of refreezing and melting will occur!!
Salt will physical get in the way of liquid water reaching the crystal pattern of the ice MORE than the ice crystals melting!
H2O (s) <—> H2O (l)
So really we are limiting the reverse pathway!! Hey if we make the reverse pathway less favorable don’t we make the forward MORE FAVORABLE AND MORE SPONTANEOUS!!!