Need Help Understanding K_a & K_b for Bicarbonate Ion

[kirsten_2009]

Homework Statement

Hi everyone, I need help understanding something. My assignment is talking about the dissociation of CaCl2 and NaHC03- in water and heats of solution etc and says: "Write out the K_a and K_b for the bicarbonate ion with chemical equations". Problem is though that I don't even know what K_a and K_b is...I've never seen this in my life, my high school teacher must of missed this completely and now they're asking this of me in University! So, if anyone could explain to me what this is and how I go about finding a solution I would MOST SINCERELY appreciate it! Help please! :)

Homework Equations

The Attempt at a Solution

 

[danago]

K_a and K_b are called the acid and base dissociation constants. They are basically a way to measure how strong a particular acid or base is.

Im assuming that you have learned about chemical equilibrium? If you have the following reaction:

$$A+B \rightarrow C+D$$

Then the equilibrium constant is given by:
[tex]K=\frac{[D][C]}{[A]}[/tex]

This number will represent how far the reaction proceeds in the forward direction before reaching chemical equilibrium.

If you recall from your study of acids and bases, the strenght of an acid is how well it ionizes in a solution. For example, HCl in water will almost fully ionize into H⁺ and Cl^- ions and is therefore a strong acid. However, HF is a weak acid and will only partially ionize. To quantify the extent to which it does actually ionize (i.e. the strength of the acid), the equilibrium constant of HF ionizing is used, which is called K_a.

The same applies for a base. So to summarize, K_a and K_b are really just equilibrium constants that are given a special name when applied to acid/base reactions.

For more info, have a look at http://en.wikipedia.org/wiki/Acid_dissociation_constant, or else do a google search--there is heaps of information out there!

 

[Borek]

Check introduction to acid base equilibrium and Brønsted-Lowry's theory (you will need the second page to understand how to write K_b for bicarbonate).

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