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Chapter 17: Problem 14

Consider the equilibrium $$\mathrm{B}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HB}^{+}(a q)+\mathrm{OH}^{-}(a q)$$ Suppose that a salt of \(\mathrm{HB}^{+}(a q)\) is added to a solution of \(\mathrm{B}(a q)\) at equilibrium. (a) Will the equilibrium constant for the reaction increase, decrease, or stay the same? (b) Will the concentration of \(\mathrm{B}(a q)\) increase, decrease, or stay the same? (c) Will the \(\mathrm{pH}\) of the solution increase, decrease, or stay the same?

Short Answer

Expert verified
In summary, when a salt of HB+ is added to the equilibrium system: (a) The equilibrium constant will stay the same, as it is only dependent on temperature; (b) The concentration of B will increase, as the reaction shifts in the reverse direction to consume HB+ and OH-; and (c) The pH of the solution will decrease, due to a decrease in OH- concentration and an increase in H3O+ concentration.

Step by step solution

01

Equilibrium constant

Le Chatelier's principle helps us predict the effect of a change in conditions on a chemical equilibrium. According to this principle, when a change is applied to a system at equilibrium, the system shifts in a direction that opposes the change to re-establish the equilibrium. However, the equilibrium constant does not change in this case since the equilibrium constant is only dependent on temperature. Therefore, adding a salt of HB+ will not change the equilibrium constant. Answer (a): The equilibrium constant will stay the same.
02

Reactant concentration

When a salt of HB+ is added to the equilibrium system, the concentration of HB+ increases. According to Le Chatelier's principle, the system will shift in a direction that reduces the impact of this change. As a result, the reaction will shift in the reverse direction, consuming HB+ and OH- and producing more B and H2O. Thus, the concentration of B will increase. Answer (b): The concentration of B will increase.
03

pH changes

From the previous analysis, we know that the reaction will shift in the reverse direction, producing more B and consuming OH-. The decrease in OH- concentration will cause a rise in the concentration of H3O+ (hydronium ions) since the product of H3O+ and OH- concentrations is always equal to Kw (the ion product of water), \(K_w= [H_3O^+][OH^-]\). Since the pH is defined as -log[H3O+], if the concentration of H3O+ increases, then the pH will decrease. Answer (c): The pH of the solution will decrease.

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Most popular questions from this chapter

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