Chemistry Lab/Acids and Bases

Acids and Bases was a topic for the event Chemistry Lab in 2009.

Acids and Bases (2009)

Acids and Bases is basically an acid/base titration lab. Be sure you know what a titration is, because it is not a good thing if you do not. This is a fairly quick and simple lab to complete, and it is more than worthwhile to double check your lab if you have enough materials. More repetitions of the lab can result in a more accurate answer. In a free-response style lab report, this might also get you some extra points for style and accuracy. Acid/base questions can range in difficulty from identifying if a solution was an acid based on its pH to balancing advanced reactions trying to find the acidic constant. In order to excel in this event you must be prepared for all levels.

pH and pOH

pH + pOH = 14

pH

pH is equal to the $-\log [H^+]$ or $-\log [H_3O^+]$.

pOH

pOH is equal to the $-\log[OH^-]$.

Acids

All acids have a pH less than 7

Arrhenius Acids

Arrhenius Acids are defined to be chemicals that, when put in water, produce hydronium ($H_3O^+$) ions.

Bronsted-Lowry Acids

Bronsted-Lowry Acids are defined to be chemicals that donate protons ($H^+$). This is a broader definition than the Arrhenius definition because it does not have to involve water.

Lewis Acids

Lewis Acids are defined to be chemicals that accept electron pairs.

Strong Acids

Strong Acids are acids that pretty much completely disassociate in water. Some examples of Strong Acids are: $HI, HBr, HClO_4, HCl, HClO_3, H_2SO_4$, and $HNO_3$

Weak Acids

Weak Acids are acids that only partially disassociate in water. They have a Ka to define how much. Weak Acids consist of pretty much everything that is not a strong acid. For example: $HCOOH, CH_3COOH, HOOCCHOHCHOHCOOH,$ and ${HSO_4}^-$

Bases

All bases have a pH greater than 7.

Arrhenius Bases

Arrhenius Bases are defined to be chemicals that, when put in water, produce hydroxide ($OH^-$) ions.

Bronsted-Lowry Bases

Bronsted Lowry Acids are defined to be chemical that accept protons ($H^+$). This is a broader definition than the Arrhenius definition because it does not have to involve water.

Lewis Bases

Lewis Bases are defined to be chemicals that donate electron pairs.

Strong Bases

Strong Bases are bases that pretty much completely disassociate in water. Examples include $LiOH, NaOH, KOH, RbOH,$ and $CsOH$.

Weak Bases

Weak Bases are bases that only partially disassociate in water. They have a Kb to define how much. A common example of a weak base is $NH_3$.

Equilibrium Constants

Take this reaction:

$aA + bB \to cC + dD$

The equilibrium constant is equal to:

$k=\frac{[C]^c * [D]^d}{[A]^a * [B]^b}$

Where all of the concentrations are the concentrations at equilibrium and where solids are excluded.

Acid Dissociation Constant

The acid equilibrium constant (Ka) is equal to

$\frac{[H^+][A^-]}{[HA]}$

for the following reaction:

$HA \to H^+ + A^-$

Base Dissociation Constant

The base dissociation constant (Kb) is equal to

$\frac{[BH^+][OH^-]}{[B]}$

for the following reaction:

$B + H_2O \to BH^+ + OH^-$

Dissociation Constant of Water

The dissociation constant of water (Kw) is equal to

$[H^+][OH^-] = 1*10^{-14}$

for the following reaction:

$H_2O \to H^+ + OH^-$

This is why pH + pOH = 14

Relationship between Ka and Kb

$\frac{[H^+][NH_3]}{[NH_4^+]} * \frac{[NH_4^+][OH^-]}{[NH_3]} = [H^+][OH^-]$

This method works for all acids and bases. Thus,

Ka $*$ Kb = Kw