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My book says that in acidic water, the negative fatty acid ions become fatty acids. However, they did not include a chemical equation to demonstrate this, so I made my own: $$\ce{C17H35COO- (aq) + HCl (aq) -> C17H35COOH (aq) + Cl- (aq)}$$. But according to this equation, the fatty acid ion is a base, since it gets a proton. So this must be wrong. Can anyone point out my mistakes, and explain what happens when a negative fatty acid ion comes in contact with acidic water, and becomes a fatty acid?

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  • $\begingroup$ Here's a thought: Try adding vinegar to a solution of soap and water. Will this be enough to break down all soap, and cancel out the effect of the soap molecules totally? Why / Why not? $\endgroup$ – Lars Tuff Feb 24 '16 at 2:57
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Your equation is correct. Hydrochloric acid ($\ce{HCl}$) is a stronger acid than stearic acid ($\ce{C17H35COOH}$),i.e. it is dissociated into $\ce{H+}$ and $\ce{X-}$, the anionic rest, to a greater degree. Therefore, it protonates the stearate ion. One can also say that the conjugated base of stearic acid (the stearate ion) is a stronger base than chloride, the conjugated base of hydrochloric acid.

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Actually, soap in acidic water just means reactions with the hydrogen ions in acid and whatever other element in the acids is not a matter to be concerned about. For example, sodium stearate $\ce{C17H35COONa}$ decomposes in in acidic water soap:

\begin{align} \ce{C17H35COONa &-> \underset{\text{(stearate ion)}}{C17H35COO-} + \underset{\text{(sodium ion)}}{Na+}} \\ \ce{C17H35COO- + \underset{\text{(from any acid)}}{H+} &-> \underset{\text{(stearic acid)}}{C17H35COOH}} \end{align}

Due to the formation of stearic acid, soap cannot act efficiently in acidic water.

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