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Chapter 18

Q. 18.5

Q. 18.5

Predicting the Net Direction of an Acid-Base Reaction

Problem Predict the net direction and indicate whether K_c is greater or less than 1 for each of the following reactions (assume equal initial concentrations of all species):

(a) H_2PO_4^−(aq)  +  NH_3(aq)  \xrightleftharpoons[]{}  NH_4^+(aq)  +  HPO_4^{2−}(aq)

(b) H_2O(l)  +  HS^−(aq) \xrightleftharpoons[]{}  OH^−(aq)  +  H_2S(aq)


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Plan We identify the conjugate acid-base pairs and consult Figure 18.8 to see which acid and base are stronger. The reaction proceeds in the direction in which the stronger acid and base form the weaker acid and base. If the reaction aswritten proceeds to the right, then [products] is higher than [reactants], so K_c  >  1.

Solution (a) The conjugate pairs are H_2PO_4^−/HPO_4^{2−} and NH_4^+/NH_3. Since H_2PO_4^− is higher on the list of acids, it is stronger than NH_4^+; since NH_3 is lower on the list of bases, it is stronger than HPO_4^{2−}. Therefore,

The net direction is to the right, so K_c  >  1.
(b) The conjugate pairs are H_2O/OH^− and H_2S/HS^−. Since H_2S is higher on the list of acids, and OH^− is lower on the list of bases, we have

The net direction is to the left, so K_c  <  1.


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