Acid-Base Reactions

Brønsted-Lowry Acids and Bases

• The species remaining after a Brønsted-Lowry acid donates a proton is its conjugate base.

• The species formed when a Brønsted-Lowry base accepts a proton is its conjugate acid.

• Amphoteric: A species that can act as either an acid or a base.

• Hydroxyl group: A covalently bonded -OH group present in an acid.

• Brønsted-Lowry reactions involve conjugate acid-base pairs.

• The Brønsted-Lowry theory is useful for describing acid-base reactions in aqueous solutions.

• When a Brønsted-Lowry acid donates a proton, the remaining ion or molecule that can re-accept the proton is the conjugate base.

• Brønsted-Lowry acid-base reactions are equilibrium systems (work both ways) involving two conjugate acid-base pairs.

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

Strength of Conjugate Acids and Bases

• The extent of a Brønsted-Lowry acid-base reaction depends on the relative strengths of the acids and bases involved.

• Strong acids readily donate protons, resulting in very weak conjugate bases.

• Strong bases have a low tendency to attract and retain protons, resulting in very weak conjugate acids.

• General rule: The stronger an acid is, the weaker its conjugate base, and vice versa (strong-weak, weak-strong).

Using Strength to Predict Reactions

• The relative strengths of acids and bases can be used to predict the outcome of a reaction.

• The stronger acid reacts with the stronger base to produce the weaker acid and the weaker base.

• Proton-transfer reactions favor the production of the weaker acid and the weaker base.

• For an acid-base reaction to proceed to completion, the reactants must be much stronger acids and bases than the products.

• Some substances, like water, can act as either acids or bases (amphoteric).

• Whether a substance acts as an acid or a base depends on the strength of the acid or base it is reacting with.- Strong acid - act as a base

  • Strong base - act as an acid

  ---

-OH in a Molecule

• Molecular compounds containing OH groups can be acidic or amphoteric.

• For a compound to be acidic, a water molecule must be able to attract a hydrogen atom from a hydroxyl group.- More likely when O-H is very polar.

  ---

• Any feature of a molecule that increases the polarity of the O-H bond increases the acidity of the compound.

• Small, more electronegative nonmetal atoms at the upper right of the periodic table form compounds with acidic hydroxyl groups.

• Oxyacids are molecular electrolytes that contain one or more O-H bonds, such as chloric and perchloric acids.

• The number of oxygen atoms bonded to the atom connected to the -OH group affects the compound’s behavior.- More oxygen increases acidity.

  ---

• Electronegative oxygen atoms draw electron density away from the O-H bond, making it more polar.

• In acetic acid, a second oxygen atom is bonded to the carbon atom connected to the -OH group, making it acidic, unlike ethanol.

Neutralization Reactions

• Neutralization reactions produce water and a salt.

• Neutralization reactions involve the reaction of an acidic compound with a basic compound (they neutralize each other).- Example: NaHCO3 and H2C4H4O_6 produce baking powder

  • When water is added, carbon dioxide is produced.

  ---

Strong Acid-Strong Base Neutralization

• Acid-base reactions occur in aqueous solutions between strong acids and strong bases.

• In aqueous solutions, neutralization is the reaction of hydronium ions and hydroxide ions to form water molecules.

• Salt: An ionic compound composed of a cation from a base and an anion from an acid.

• In this reaction, there was an equal number of H_3O^+ and OH^- ions, and they are fully converted to water.

• Both a salt and water were produced.

Acid Rain

• Industrial processes produce gases like NO, NO2, CO2, SO2, and SO3.

• These gases dissolve in atmospheric water to produce acidic solutions, resulting in acid rain or snow.

• Rainwater is normally slightly acidic (very acidic rain is acid rain).

• Acid rain erodes statues and affects ecosystems, such as water environments and forests.

• Acid rain can decrease the biodiversity of ecosystems by eliminating fish populations in lakes and streams.

• Amendments to the Clean Air Act in 1990 set limits on SO_2 emissions from power plants, which have decreased but not eliminated acid rain in the US.