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For example, I have formic acid and sodium formate. How do I know the NIE(net ionic equation) equation is $\ce{HCHO_2 + H_2O <=> H_3O+ + CHO_2-}$ and not $\ce{HCHO_2 + OH- <=> H_2O + CHO_2-}$ ? If I added $\ce{HCl}$ or $\ce{NaOH}$, would that help decide which equation? Is a buffer always added to water? Why or why not?

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  • $\begingroup$ Both reactions happen. The first with the equilibrium constant Ka, the second with the equilibrium constant 1/Kb=Ka/Kw. When I drink water, I do not add buffer, so buffer is not always added to water. ( Sorry, I cannot help myself, the question is formulated rather poorly and not clearly.) $\endgroup$
    – Poutnik
    Jan 28, 2021 at 5:32
  • $\begingroup$ For eventual writing and formatting of chemical formulas or equations, see how to use MathJax with mhchem extension $\endgroup$
    – Poutnik
    Jan 28, 2021 at 6:01
  • $\begingroup$ @Poutnik fixed it $\endgroup$
    – Sponge bob
    Jan 28, 2021 at 6:24
  • $\begingroup$ For next time, note that \ce{} need not underscores to display subscripts in chemical formulas. $\endgroup$
    – Poutnik
    Jan 28, 2021 at 6:52
  • $\begingroup$ @Poutnik I don’t know about your water but my tap water contains hydrogencarbonate which technically is a buffering agent ;) $\endgroup$
    – Jan
    Feb 1, 2021 at 12:06

1 Answer 1

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$\ce{HCHO_2 + H_2O <=> H_3O+ + CHO_2-}$ and $\ce{HCHO_2 + OH- <=> H_2O + CHO_2-}$ are both reactions that can happen.

If you added $\ce{HCl}$ (of high concentration), you added a strong acid, and there will be extra hydrogen ions. Therefore, it will drive your first reaction backwards due to the extra hydronium: $\ce{H_3O+ + CHO_2- <=> HCHO_2 + H_2O}$.

If you added $\ce{NaOH}$ (of high concentration), you added a strong base, and there will be extra hydroxide ions. Therefore, it will drive your second reaction forwards: $\ce{HCHO_2 + OH- <=> H_2O + CHO_2-}$.

Distilled water has no buffer added to it, and it is not a buffer itself because of extremely low concentrations of $\ce{H_3O+}$ and $\ce{OH-}$. Tap water might have buffers in it. Lakewater have buffers to prevent sudden changes in pH. For example, it might have limestone at the bottom of the lake, which serves as a buffer against strong acids. As an example, when acid rain falls, it brings with it sulfuric acid, and the buffers prevent the pH from dramatically decreasing, by the equation $\ce{H2SO4 + CaCO3 => CaSO4 + H2O + CO2}$. With the buffers, the fish living in the lake will not be affected by the sudden changes in pH, otherwise, they might stress or even die (but that's a question for Biology SE).

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