Chemistry Lab/Acids and Bases

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Acids and Bases is a topic for Chemistry Lab for the 2019 season.

Acids and Bases

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.

Solutions

For more info on solutions see Chem Lab/Aqueous Solutions

Definition of acids and bases

There are three major definitions of acids: the Arrhenius acid, the Bronsted-Lowry acid, and the Lewis acid. Each is defined differently, although the definitions are related.

Arrhenius acids/bases

Arrhenius acids are defined to be chemicals that, when put in water, produce hydronium ([math]H_3O^+[/math]) ions. Similarly, Arrhenius bases produce hydroxide ([math]OH^-[/math]) ions in aqueous solution.

Bronsted-Lowry acids/bases

Bronsted-Lowry acids are defined to be chemicals that donate protons ([math]H^+[/math]), while Bronsted bases accept protons. This is a broader definition than the Arrhenius definition because the acid-base reaction does not have to occur in water, although we know through observation that in solution, Bronsted acids donate protons to water molecules and form hydronium ions, as defined by the Arrhenius definition. Similarly, Bronsted bases in aqueous solution deprotonate water into hydroxide ions.

Lewis acids/bases

Lewis acids are defined to be chemicals that accept electron pairs, while Lewis bases donate electron pairs. Many acids and bases under this definition do not even have an acidic proton to be exchanged, although Arrhenius and Bronsted acids all fit the definition of a Lewis acid.

Acid/base equilibria

Chemical reactions only very rarely react to completion. Reactants react forward to form products, but products can also react backwards to form reactants. The forward and reverse reactions will set up a state known as equilibrium, where the forward reaction and the reverse reaction balance each other out. The equilibrium constant Keq describes the ratios of the concentrations of all chemical species at equilibrium. Take this reaction:

[math]aA + bB \rightleftharpoons cC + dD [/math]

The equilibrium constant Keq is equal to:

[math]K_{eq}=\frac{[C]^c[D]^d}{[A]^a[B]^b}[/math]

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

For more info on equilibrium, see Chem Lab/Equilibrium.

Acid and Base Dissociation Constants

The reaction of an acid with water to form hydronium and a conjugate base also occurs in equilibrium. A special equilibrium constant, the acid dissociation constant Ka of any given acid, is used to record the equilibrium concentrations of the reactants and products of the dissociation of a proton from an acid in aqueous solution.

For the following reaction:

[math]HA + H_2O \rightleftharpoons H_3O^+ + A^-[/math]

The acid dissociation constant (Ka) is equal to

[math]\frac{[H_3O^+][A^-]}{[HA]}[/math]

Note that this is similar to the expression for the general equilibrium expression, although water, which is a reactant, is not included in this expression. That is because the concentration of water does not change in this reaction, since it is usually the solvent for the dissociation.

We can also define a base dissociation constant Kb. For the reaction

[math]B + H_2O \rightleftharpoons BH^+ + OH^-[/math]

The base dissociation constant (Kb) is equal to

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

pH and pOH

pH + pOH = 14

pH

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

pOH

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


Strong Acids

Strong acids are acids that almost completely disassociate in water. Some examples of strong acids are: [math]HI, HBr, HClO_4, HCl, HClO_3, H_2SO_4[/math], and [math]HNO_3 [/math]

Weak Acids

Weak acids are acids that only dissocite to a small extent in water. Their Ka value determines how much of the acid will dissociate in aqueous solution. Weak acids consist of all acidic species that are not strong acids. For example: [math]HCOOH, CH_3COOH, HOOCCHOHCHOHCOOH,[/math] and [math]{HSO_4}^-[/math]

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 ([math]OH^-[/math]) ions.

Bronsted-Lowry Bases

Bronsted Lowry Acids are defined to be chemical that accept protons ([math]H^+[/math]). 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 [math]LiOH, NaOH, KOH, RbOH,[/math] and [math]CsOH[/math].

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 [math]NH_3[/math].

Equilibrium Constants

Base Dissociation Constant

The base dissociation constant (Kb) is equal to

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

for the following reaction:

[math]B + H_2O \to BH^+ + OH^-[/math]

Dissociation Constant of Water

The dissociation constant of water (Kw) is equal to

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

for the following reaction:

[math]H_2O \to H^+ + OH^-[/math]

This is why pH + pOH = 14

Relationship between Ka and Kb

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

This method works for all acids and bases. Thus,

Ka [math]*[/math] Kb = Kw

Titrations

For more info on Titrations, see Chem Lab/Titration Race.

Links

  • Acid and base links [1]