Acids and Bases

Properties of Acids:

  • Sour taste
  • Changes the color of litmus paper
  • Reacts with certain metals to produce hydrogen gas
  • Conducts electricity in an aqueous solution
  • Reacts with carbonates and bicarbonates to produce carbon dioxide gas

Properties of Bases:

  • Bitter taste
  • Changes the color of litmus paper
  • Conducts electricity in an aqueous solution
  • Feels slippery

Strength of Acid/Base:

The strength of an acid depends on the polarity of the bond and the ease with which that bond is broken

Acid strength increases with increasing polarity and decreasing bond energy

Strong acids are strong electrolytes because they have more ions (more charged particles), aq solutions mean that charges can move more

The strength of base depends on the extent to which it dissociates or adds OH- to the solution

Arrhenius Acid/Base:

A Swedish chemist Savante Arrhenius was the first to classify acids and bases. When examining their properties in an aqueous solution he found that…

Acids are substances that dissociate (separate) in water to produce hydrogen ions.

Bases are substances that dissociate in water to produce hydroxide ions.

An Arrhenius acid produces H3O+ or H+ (hydronium) ions in solution

An Arrhenius base produces OH- (hydroxide ion) ions in solution

Acids are the reactant that gives up its H and contributed to the H3O+

Ex. HNO3 + H2O -> H3O+ + NO3-

HNO3 is the acid because it gives its H to H2O to form H3O+

Organic acids contain the carboxyl group COOH and they are generally weak acids

Arrhenius base is a cation + OH- ions. When a base completely dissociates in water to yield OH- ions, it’s called alkaline (like the metals on the periodic table)

Bronsted Lowry Acid/Base:

The Arrhenius definition of acids and bases only explained acids and bases that actually contain H+ and OH-. His definition was limited because it didn’t explain everything found in water.

Danish chemist Johannes Bronsted and English physicist Thomas Martin Lowry came up with a way to address the holes in the Arrhenius definition.

 They created the Bronsted Lowry theory which defined acids and bases by their ability to accept or donate a proton.

Bronsted-Lowry Definition:

  • Acid is a proton (H+) donor, it donates an H+ ion to a bronsted base
  • Base is a proton (H+) acceptor, it accepts an H+ ion from a bronsted acid

(water doesn’t always have to be present)

Monoprotic acids are acids that can donate only one proton (hydrogen ion) per molecule

Ex. HCL

The ionization of polyprotic occurs in stages, the acid loses its hydrogen atom one at a time

Stage 1 produces the greatest concentration of ions

Lewis Acid/Base:

Lewis acid: any species that can accept an electron pair to form a covalent bond with another species

H+ doesn’t always have to be the main event

Any compound in which the central atom has 3 valence electrons can react as a lewis acid because it gets its octet

Bronsted Lowry Conjugate Acid/Base:

When acids and bases react, they reach chemical equilibrium (shown by two arrows ⇄), a point where the reaction is reversible and the products and reactants are reacting at the same speed to establish a ratio of the concentration of products/reactants

Suppose a bronsted lowry acid gives up a proton, the remaining ion or molecule can accept that proton and act as a base which is called a conjugate base

Aka its what’s left after the H is donated

In every conjugate acid-base pair, the acid has one more proton than its conjugate base

The acid always produces a weaker conjugate base

OH in a molecule is called a hydroxyl group

Hydroxyl containing molecules can be acidic or amphoteric which are any species that can react with an acid or a base

For a compound to be acidic a water molecule must be able to attract a hydrogen atom from a hydroxyl group. This can occur more easily when the O-H bond is very polar

Neutralization:

When acids and bases react, they form salts. A salt is the combination of a cation (+) from the acid and an anion (-) from the base. When the two bond they have a neutral (0) charge.

Ex. KOH +HCL -> KCL +H2)

KCL is the salt that is formed

Neutralization reactions occur in…

  • Metal oxides and metal hydroxides:

  • Acid + metal oxide (base) -> salt + water

  • Acid + metal hydroxide (base) -> salt + water

  • Carbonates and hydrogen carbonates

  • Acid + metal carbonate -> salt + water + carbon dioxide

  • Acid + metal hydrogen carbonate -> salt + water + carbon dioxide

The Cl- and Na+ are spectator ions and will cancel out which is why only H2O is left in the net ionic equation.

Kw:

Acid/Base reaction is not necessary for H3O+ formation

[H3O+] = 1*10^-7 M

[OH-] = 1*10^-7 M

This concentration remains constant and is called the ionization constant of water aka kw

Kw = 1*10^-14 M^2

The ionization of H2O increases as temperature increases thus Kw goes up as T goes up

Because [H3O+] and [OH-] are the same in pure water, H2O is neutral

Anything where  [H3O+] = [OH-] is neutral

[H3O+] > [OH-] = acidic so therefore the [H3O+] > 1*10^-7 M

[H3O+] < [OH-] = basic so therefore [OH-] > 1*10^-7 M

Ex. [H3O+] = 1*10^-5

1*10^-5 > 1*10^-7

 [H3O+] > 1*10^-7 M

Acidic

pH and pOH:

In 1909, Soren Sorenson, a Danish biochemist came up with a way of simplifying and identifying the concentration of acidity or base in a solution: pH

Expressing acidity in terms of [H3O+] and [OH-] is cumbersome because values are so small

pH is french fro pouvoir hydrogene meaning hydrogen power

The pH of a solution is the negative of the common logarithm of the [H3O+]

It ranges from 0 (most acidic) to 14 (most basic), 7 is neutral

pH = -log[H3O+]

Ex. if [H3O+] = 1*10^-7, pH = 7

pOH of a solution is defined as the negative common logarithm of the [OH-]

pOH = -log[OH-]

Ex. [OH-] = 1*10^-6, pOH = 6

pH + pOH = 14

Ex. [H3O+] = 1 *10^-4

pH = 4

pOH = 10

When finding [H3O+] or [OH-] from pH:

[H3O+] = 10^-pH

[OH-] = 10^-pOH

Titration Curve:

A titration determines the concentration of an unknown solution. During titration, the titrant or reagent (a solution with a known concentration) is slowly added into a solution of unknown concentration until the endpoint of the reaction is reached. The endpoint is reached when a color change has occurred.

The equivalence point of the titration is the point at which exactly enough titrant has been added to react with all of the substance being titrated with no titrant left over. In other words, all the acid reacts with all the base and the solution is neutral. When the solution stays the color changes, which meant that the pH is greater than 7 and the solution has more base than acid. It just hit the equivalence point and if we went any further it would just be excess OH.