Question

Why is NaHCO3 a base?

(I need someone to explain it using the ka values)

H2CO3 <--> HcO3- + H+ ka = 4.3x10^-7
HCO3- <--> CO3^2- + H+ ka = 5.61x10^-11

if I underestand correctly, the kb for the second equation is greater than the ka. Thus the reaction will not proceed forward, but rather produce more HCO3-. This is a basic tendency. Why is it more powerful than the first reaction?

thanks everyone!

Answer 1

There is a mistake in your values, the first acidity of H2CO3 is about 4x10^-7

Answer 2

Question still open--

you're correct though Vincent, thanks

Answer 3

What's to explain? The K's for both reactions are very small, which tells you equilibrium lies to the left. The question that determines whether you have an acidic or basic solution is what you start with.

If you start with lots of H2CO3 (e.g. by bubbling CO2 through water), then although K is small, it isn't zero, so at least SOME of the H2CO3 will ionize, releasing extra H+ into the water and lowering the pH. You have an acidic solution.

On the other hand, if you start with lots of HCO3- (e.g. by dissolving a few tsp of NaHCO3 in water), then because the K is small quite a lot of the HCO3- will rip off an H+ from nearby waters, becoming H2CO3 and releasing OH- from the wounded water. You have a basic solution.

You can go a little further: knowing that K is quite small in both cases, you can conclude that the conjugate base (the anion, e.g. HCO3-) is a stronger base than the conjugate acid (e.g. the H2CO3) is acid. So, for example, if you compare a 1M solution of H2CO3 and NaHCO3, you can expect the pH of the first to be closer to 7 than the pH of the latter.

I suspect part of your confusion comes from the fact that you are somehow mentally equating both sides of the chemical equation, in terms of their initial amounts, and thinking that the size of the K is all that determines the final composition of the solution. That is not the case. The initial concentrations matter quite a bit. You have to consider BOTH facts -- the value of K, and the initial composition of the solution, to figure out the final composition of the solution, and hence the pH.

Answer 4

Another problem may be that you mentally associate "acidity" with the presence of a dissolved acid, like H2CO3, and "basicity" with the presence of a dissolved base, like HCO3-.

But that's not the case. A solution is NOT acidic by virtue of the mere presence of an acid, strange as that may sound. Being an acid is a dynamic characteristic -- it says how the compound reacts with water (among other things). It is the presence of the result of this reaction, the H+, that makes the solution acidic. This is why the acidity of a solution has nothing directly to do with the concentration of the acid. What matters is the concentration of H+ -- which is determined by the initial concentration of acid, and K.

Here's an analogy. Michael Jordan was an awesome basketball player. But just because he's located inside a gym doesn't mean an awesome basketball game is going on. It's what he does that determines that question. If he just sits around, or he doesn't have any teammates -- or an opposing team -- then no awesome game will be going on. Similarly, the presence of an acid is not all by itself proof that a solution is acidic. You have to ask yourself what the acid is DOING.

Answer 5

NaHCO3 is an amphoteric substance, i.e. a base and an acid at the same time.

Depending on the strength of the conjugate acid (\(H_2CO_3 )\) and base (\(CO_3^{2-}\)), i.e. depending on the values of the Ka's, the solutions of amphoteric substances can be acidic, neutral or alkaline.

For such substances, in standard conditions, pH is given by:

\(pH=\frac12 (pK_1+pK_2)\)

(Correct) numerical values for \(H_2CO_3/HCO_3^-/CO_3^{2-}\) lead to pH = 8.3 which is an alkaline solution.

So rather than saying that NaHCO3 "is a base" (which is technically not true, because it is ALSO an acid), you should say that "its solution is alkaline".