Chemistry again! Book gives different Net Ionic Equations for same reaction?

Jimmy King

Many months and chapters ago, back in my first semester of chemistry, we learned all about net ionic equations. It was straightforward. I am leaving out the aqueous and liquid phase notations throughout.

HCl + NaOH -> NaCl + H2O

So ionic equation stuff is
H⁺ + Cl^- + Na⁺ + OH^- -> Na⁺ + Cl^- + HOH
H⁺ + OH^- -> H2O

I just went back and reviewed as well since I haven't done this in 6 months.

But now, in the same book, 8 chapters later, we are dealing with acid base equilibrium stuff and for the same HCl + NaOH reaction it tells me that the net ionic equation is:
OH^- + H₃O⁺ -> 2 H2O

There is no explanation for why there are two different ways of writing this, how I would know which one is being looked for if all I am told is to write the net ionic equation, etc. Please enlighten me as to what my book is leaving out and apparently just assuming I have inferred in this process.

bowen

https://www.stolaf.edu/depts/chemistry/courses/toolkits/125/js/naming/netion.htm

That explains it at the beginning.

Basically, they're both right, but the later one is more exact on what's going on.

Fuzzy Cumulonimbus Cloud

Free protons don't actually exist in a water equation so we write it as the hydronium ion: H3O+.

Kruite

In my chemistry studies I cannot recall a component that was truly a "free proton".

In acid base reactions you generally have an H+ that is piggy backing on whatever anion or slightly negative molecule that is in solution.

My organic chemistry prof had strange analogies that sometimes alienated himself amung staff, but he always described hydrogen as a bit of a hussy. She doesn't care who she's dancing with so long as she's dancing with somebody.

Jimmy King

Meh, sorry I took a few days to follow up. Work + school = crazy times. So, yeah, I guess it all more or less makes sense aside from I'm not comfortable with "we just changed how we're writing it because we did". I mean, is there some reason you would ever write it the first way, with just the H+? That is mostly what I don't get. Either one is technically correct and should be used and one is not and the other shouldn't be used or there's some reasoning for when you'd use one vs the other which went unexplained.

Fuzzy Cumulonimbus Cloud

Jimmy King wrote: »

Meh, sorry I took a few days to follow up. Work + school = crazy times. So, yeah, I guess it all more or less makes sense aside from I'm not comfortable with "we just changed how we're writing it because we did". I mean, is there some reason you would ever write it the first way, with just the H+? That is mostly what I don't get. Either one is technically correct and should be used and one is not and the other shouldn't be used or there's some reasoning for when you'd use one vs the other which went unexplained.

Physical chemistry explains why this is so. There are certain lies you must tell in general chemistry in order to explain the true approximations later on.
Sometimes it is convenient to pretend that a solution with low pH is full of H+ but when we actually want to look at equilibrium we would need the H3O+ equation. Also in organic chemistry, there are many ways to look at electron movement: sometimes it is convenient to say that the electrons move via H+ movement, other times we don't do that. It will be explained better as you go on.

Kruite

Overcome the assumption that H+ freely exists in solution. It doesn't in reality. Hydrogen constantly seeks to share it's one electron with anything in order to have a completed orbital.

Beyond my scope, but I believe you have free hydrogen ions in plasma. But that's also beyond the scope of your work so no worries.

The first equation as written is just a simplified version of what's happening to understand a net change. For one reaction it's not that helpful, but if you work with a system that has simultaneous reactions. Reverse reactions, and Co products such methods will become more useful.