The Basics of Acids, Bases, and Equilibrium
A Brief Intro
In this hub I will outline the absolute basics of acids and bases. We will cover the various definitions, discuss their equilibrium constants, touch a bit on acid rain, and even discuss body chemistry!
Just Where Do We Encounter Acids and Bases In Everyday Life?
Acids and bases are all around us. We have hydrochloric acid (HCl) in our stomach, carbonic acid (H2CO3) in our blood, phosphoric acid (H3PO4) in our soft drinks, and hydrosulfuric acid (H2SO4) in our rain water! Bases are no slouch either when it comes to prevalence! We have sodium hydroxide (NaOH) in our soaps and calcium carbonate (CaCO3) in our bones, for example!
What Are Acids and Bases?
The definition of an acid or a base has been debated for some time. First we'll start with the Arrhenius definition of acid and bases to describe them. According to the Swedish scientist Svante Arrhenius, an acid is a substance that donates a hydrogen ion (H+) and a base is a substance that donates hydroxide ions (OH-). So to put it simply:
Arrhenius Definition of Acids and Bases:
Acids: HCl (aq) <--> H+ (aq) + Cl- (aq)
Bases: NaOH (aq) <--> Na+ (aq) + OH- (aq)
Bronsted-Lowry Definition of Acids and Bases:
The Bronsted-Lowry definition of acids and bases is even more general. It just says that acids are anything that donates protons and that bases are anything that accepts protons! The image above and to the right illustrates this.
Strong Acids and Bases
Not all acids and bases are considered equal. Some are definitely much stronger than others and we can elucidate this from their disassociation constants. The greater this constant (Ka for acids, Kb for bases), the stronger the acid or base!
Here's a list of them:
These acids completely disassociate in water.
How To Calculate pH
If you are given the concentration of hydrogen ions (H+) or hydronium ions (H3O+) then calculating pH is very simple. Just get out your calculator and take the -log of this concentration!
-log( [H+] )
-log (0.00697) = pH of 2.157
How to Calculate pOH
If you are given a concentration of hydroxide ion (OH-) just do the same thing and subtract the resulting pOH from 14 to get your pH!
How Does Kb Relate to Ka?
If you find yourself needing to convert from Ka to Kb or vice versa, all you need to do is remember that Ka * Kb = Kw with Kw equaling 1.0 x 10-14. So if you are given the Kb of a base and need to find the Ka of its conjugate acid, just divide the Kb from Kw!
How Does Molecular Structure Affect Acid Strength
Acid strength is influenced primarily by bond strength and the difference in electronegativities across that bond. For example, larger atoms bonded to hydrogen, such as the chlorine in HCl, form relatively weak bonds and the large difference in electronegativies makes the HCl bond particularly weak. Ｔhis results in a strong acid.
On the other hand, HF, or hydrofluoric acid, is not a strong acid. Why is this? The fluorine ion is highly electronegative?! Well, this has everything to do with atom size. Since the fluorine atom is small, it is more tightly bound to the hydrogen and does not dissociate enough to be considered a strong acid (it is still pretty potent though!).
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