Predicting the Position of Equilibrium

Everything we have mentioned so far has been the qualitative method for comparing acidity of different protons. In other words, we never said how much more acidic one proton is over another, and we never said exactly how acidic each proton is. We have talked only about relative acidities which proton is more acidic  

There is also a quantitative method of measuring acidities. All protons can be given a number that quantilies exactly how acidic they are. This value is called pKa. It is impossible to figure out the exact pA a by just looking at the structure of a compound. The pA a must be determined empirically through experimentation. Many professors require that you know some general pAa s for certain classes of compounds (for instance, all alcoholic protons, RO-H, will have the same ballpark pAa). Most textbooks will have a chart that you can memorize. Your instructor will tell you if you are expected to memorize this chart. Either way, you should know what the numbers mean. 

The smaller the pA a, the more acidic the proton is. This probably seems strange, but that s the way it is. A compound with a pA a of 4 is more acidic than a compound with a pKa of 7. Next, we need to know what the difference is between 4 and 7. These numbers measure orders of magnitude. So the compound with a pA a of 4 is 103 times more acidic (1000 times more acidic) than a compound with a pA a of 7. If we compare a compound with a pA a of 10 to a compound with a pA a of 25, we find that the first compound is 1015 times more acidic than the second compound (1,000,000,000.000,000 times more acidic). 

Now that we know how to compare stability of charge, w e can begin to predict which side of an equilibrium will be favored. Consider the following scenario  

Another w ay of looking at this is the following. In the equilibrium above, we see an A on one side and a B on the other side. The equilibrium will favor 

If B is more stable, then the equilibrium will lean so as to favor the formation of B  

The position of equilibrium can be easily predicted by comparing the relative stability of negative charges. 

EXERCISE 3.34 Predict the position of equilibrium for the following reaction  

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Increasing the pressure shifts the position of the above equilibrium to the right. The system has responded to counteract the effect of the change (an increase in pressure). From kinetic theory, two moles of gas molecules exert less pressure on the walls of a vessel than four moles. As predicted, the position of equilibrium has altered in such a way as to reduce the total pressure. 

If we know the relative strengths of two acids, we can predict the position of equilibrium between one of the acids and the conjugate base of the other, as illustrated in Example 15.7. 

How to Predict the Position of Equilibrium of an Acid-Base Reaction... 

Predict the position of equilibrium for each acid-base reaction that is, does each lie considerably to the left, does each lie considerably to the right, or are the concentrations evenly balanced (See Examples 8.4,8.12)... 

Use the approach we developed in Section 2.4 to predict the position of equilibrium in acid-base reactions. Equilibrium favors reaction of the stronger acid and stronger base to form the weaker acid and the weaker base. It is helpful to remember that even though ammonium ions are positively charged, they are much weaker acids than carboxylic acids. 

Complete the following acid-base reactions and predict the position of equilibrium for each. Justify your prediction by citing values of pKg for the stronger and weaker acid in each equilibrium. For values... 

Predict the position of equilibrium and calculate the equilibrium constant, for each acid-base reaction. 

Using pAT values given in Table 4.1, predict the position of equilibrium in this acid-base... 

In this section, we will learn to predict the position of equilibrium just by comparing conjugate bases without using pA values. To see how this works, let s examine a generic acid-base reaction ... 

In each case below, identify the acid and the base. Then draw the curved arrows showing a proton transfer reaction. Draw the products of that proton transfer, and then predict the position of equilibrium. 

L 10.2 PREDICTING THE POSITION OF EQUILIBRIUM FOR THE DEPROTONATION OF A TERMINAL ALKYNE... 

SkillBuilder 3.10 Predicting the Position of Equilibrium Without the Use of pJfa Values... 

Predicting the Position of Equilibrium for the Deprotonation of a Terminal Alkyne... 

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