Chapter 18: Acids and Bases

Strong Base

A base that dissociates entirely into metal ions and hydroxide ions. EX: NaOH, KOH, RbOH, CsOH, Ca(OH)2, Ba(OH)2

Hydronium Ion

A hydronium ion (H3O+) is a hydrogen ion which has a water molecule attached to it by a covalent bond.

Neutralization Reaction

A reaction in which an acid and a base in aqueous solution react to produce a salt and water. Double-replacement reaction.

Explanation for Acid Strength

Acid strength can be explained somewhat by the Brønsted-Lowry model because if the base on the left side has a most stronger attraction for H+ than does the conjugate base, so the acid is strong and wants to dissociate to join the base on the left side (normally H2O). Vice versa is true (acid will want to not dissociate if the conjugate base has stronger attraction to H+=weak acid).

Physical Properties of Acids

Acids taste sour. Acidic solutions can also conduct electricity. Pure water doesn't conduct electricity, but adding acids produces ions that allow it to become a conductor.

Strong Acids

Acids that ionize completely. Because strong acids produce the maximum number of ions, they are good conductors of electricity. EX: HCl, HI, HClO4, HNO3, H2SO4

Chemical Properties of Acids

Acids turn litmus paper red. Magnesium and zinc react with aqueous solutions of acids to produce hydrogen gas and metal carbonates and hydrogen carbonates also react with aqueous solutions of acids to produce CO2 (bubbling).

Le Châtelier's Principle

Adding extra H+ ions to the system causes stress to the system, then the added H+ ions react with the OH- ions to form more water molecules and thus the concentration of OH- ions decreases.

Arrhenius to Brønsted-Lowry to Lewis

All substances classified as acids and bases by the Arrhenius model are classified as acids and bases by the Brønsted-Lowry model. In addition, some substances not classified as base by the Arrhenius model are classified as bases by the Brønsted-Lowry model. Lewis proposed that an acid is an ion or molecule with a vacant atomic orbital that can accept (share) an electron pair. A base is an ion or molecule with a lone electron pair that it can donate (share).

What are acids and bases?

All water solutions contain hydrogen ions (H+) and hydroxide ions (OH-). The relative amounts of the two ions determine whether an aqueous solution is acidic, basic, or neutral. Neutral solutions are neither acidic nor basic.

Acid Anhydride

An acid anhydride is an oxide that can combine with water to form an acid. Other oxides combine with water to form bases. In general, oxides of metallic elements form bases; oxides of nonmetals form acids.

Lewis Model

An acid is an electron-pair acceptor and a base is an electron donor. This includes all substances classified as acids and bases under the Brønsted-Lowry model and many more.

Types of Acids

An acid that can donate only one hydrogen ion is called a monoprotic acid. Ionizable hydrogen atoms are often the hydrogen atoms bonded to other elements by polar bonds. Some acids donate more than one hydrogen ion. Acids that contain two ionizable hydrogen atoms are called diprotic acids. Acids with three hydrogen ions to donate are called triprotic acids. The term polyprotic acid can be used for any acid that has more than one ionizable hydrogen atom. All polyprotic acids ionize in steps.

Weak Acid

An acid that ionizes only partially in dilute aqueous solution. Weak acids produce fewer ions and thus cannot conduct electricity as well as strong acids. EX: HF, HC2H3O2, H2S, H2CO3, HCLO

Physical Properties of Bases

Bases taste bitter and feel slippery. Basic solutions can also conduct electricity. Pure water doesn't conduct electricity, but adding bases produces ions that allow it to become a conductor.

Acid-Base Indicator

Chemical dyes whose colors are affected by acidic and basic solutions.

Expressing H+ Ions

Concentrations of H+ ions are often small numbers expressed in scientific notation. Because these numbers are cumbersome, chemists have developed an easier way to express H+ ion concentrations. Chemists express concentrations of hydrogen ions using a pH scale based on common logarithms.

Neutral Solution

Contains equal concentrations of hydrogen ions and hydroxide ions.

Acidic Solution

Contains more hydrogen ions than hydroxide ions.

Basic Solution

Contains more hydroxide ions than hydrogen ions.

Molarity of Acids/Bases to pH

For all strong monoprotic acids, the concentration of the acid is the concentration of H+ ions. Thus, you can use the molarity of the acid to calculate pH. This also works for bases. For polyprotic acids and bases, the concentration of [H+] or [OH-] is however many H+ or OH- ions one molecule produces multiplied by the molarity of the solution. You have to use Ka and Kb values to determine the concentrations of H+ and OH- ions in solutions of weak acids and weak bases.

Calculating Ka from pH

From the pH, determine [H+]. If monoprotic, this is equal to [A-]. [HA] is equal to the molarity of the acid - [H+].

Measuring pH

Indicators, such as litmus paper, can be used to measure pH. Electronic devices/electrodes can also be used to measure pH.

Salt

Ionic compound made up of a cation from a base and an anion from an acid.

Weak Base

Ionizes only partially in dilute aqueous solution. EX: Ethylamine, Methylamine, Ammonia, Aniline

Titration

Method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration. You could perform this with the acid as the unknown and the base as the known and vice versa. Titration Procedure: Measured volume of acidic or basic solution of unknown concentration placed in beaker. The electrodes of a pH meter are immersed in this solution and the initial pH of the solution is read and recorded. A buret is filled with the titrating solution of known concentration. This is called the standard solution, or titrant. Measured volumes of the standard solution are added slowly and mixed into the solution in the beaker. The pH is read and recorded after each addition. This process continues until the reaction reaches the equivalence point, which is at the point where moles of H+ ions from the acid are equal to moles of OH- ions from the base. Not all equivalence points are at 7.

Electrical Conductivity

One property of acidic and basic solutions is that they conduct electricity. The electrical conductivities of different solutions differ, even if they are of the same molarity of different substances. This is because ions carry electric current through the solution and different substances ionize differently (either completely, partially, or not at all).

Why is pure water neutral?

Pure water produces equal numbers of H+ and OH- ions in a process called self-ionization, in which water molecules react to form a hydronium ion and a hydroxide ion. H2O (l) + H2O (l) → ← H3O+ (aq) + OH- (aq) H2O (l) → ← H+ (aq) + OH-(aq)

Kw

Recall that pure water contains equal concentrations of H+ and OH- ions produced by self-ionization. In self-ionization, equal numbers of hydronium and hydroxide ions are formed. The equation for the equilibrium can be simplified as: H2O→ ← H+ (aq) + OH- (aq) The double arrow indicates that this is an equilibrium. Recall that the equilibrium constant expression was the concentrations of the products / concentrations of the reactants. Since all terms have a coefficient of 1, all terms are to the first power. The concentration of pure water is constant, so it does not appear in the denominator. Kw=[H+][OH-]

Brackets

Signify concentration/molarity.

Ka

Since the concentration of H2O is considered to be constant in dilute aqueous solutions, it can be combined with Keq to give a new equilibrium constant, Ka. Keq [H2O] = Ka = [H3O+][X-] / [HX] Ka is called the acid ionization constant.

Buffer

Solutions that resist changes in pH when limited amounts of acid or base are added. Mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. The mixture of ions and molecules in a buffer solution resist changes in pH by reacting with any hydrogen or hydroxide ions added to the buffered solution. A buffer solution's capacity to resist pH change can be exceeded by the addition of too much acid or too much base.

Conjugate Acid

Species produced when a base accepts a hydrogen ion.

Conjugate Base

Species that results when an acid donates a hydrogen ion.

Brønstead-Lowry Model

States that an acid is a hydrogen ion donor and a base is a hydrogen ion acceptor. HX (aq) + H2O (l) → ← H3O+ (aq) + X- (aq) HX is an acid. H2O is a base. H3O+ is the conjugate acid. X- is the conjugate base. The forward reaction is the reaction of an acid and a base. The reverse reaction is also the reaction of an acid and a base.

Arrhenius Model

States that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in an aqueous solution. A base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in an aqueous solution. This model is somewhat faulty because it does not include all acids and bases. For example, ammonia (NH3) does not have a hydroxide group, but produces hydroxide ions in a solution and is a base.

Buffer Capacity

The amount of acid or base a buffer solution can absorb without a significant change in pH. The greater the concentration of the buffering molecules and ions in the solution, the greater this is. A buffer solution is most effective when the concentrations of the conjugate acid and base are equal or nearly equal.

Salt Hydrolysis

The anions of the dissociated salt accept hydrogen ions from water or the cations of dissociated salt donate hydrogen ions to water. Salts can produce acidic, basic, or neutral solutions.

Keq

The equilibrium constant, Keq, provides a quantitative measure of the degree of ionization of the acid. Keq=[H3O+][X-] / [HX][H2O]

pH

The negative logarithm of the hydrogen ion concentration. pH=-log[H+] At 298 K, acidic solutions have a pH of less than 7. 0.0 is strongly acidic. Basic solutions have a pH of more than 7. 14.0 is strongly basic.

pOH

The negative logarithm of the hydroxide ion concentration. pOH=-log[OH-] A solution with a pOH of less than 7.0 is basic, 7.0 is neutral, and higher than 7.0 is acidic. With both pH and pOH scales, a change of one unit represents a tenfold in ion concentration.

End Point

The point at which the indicator used in a titration changes color. Normally at the equivalence point.

Base Ionization Constant

The value of the equilibrium constant expression for the ionization of a base. The smaller the value of Kb, the weaker the base.

Acid Ionization Constant

The value of the equilibrium constant expression for the ionization of a weak acid. The weakest acids have the smallest Ka values because their solutions have the lowest concentrations of ions and the highest concentrations of un-ionized molecules (reactant) in the denominator.

Ion Product Constant

The value of the equilibrium constant expression for the self-ionization of water. Experiments show that in pure water, both [H+] and [OH-] are 1.0x10^-7 and therefore the product of [H+] and [OH-] in pure water at 298K is 1.0x10^-14. Because of this, if the concentration of H+ ions increases, the concentration of OH- ions must decrease and vice versa.

Conjugate Acid-Base Pair

Two substances related to each other by the donating and accepting of a single hydrogen ion.

Amphoteric

Water and other substances that can act as both bases and acids.

Electrical Conductivity for Bases

What is above about acids can be applied to bases, except with OH- ions rather than H+ ions. The conductivity of a base depends on the extent to which the base produces OH- ions in aqueous solution.

Kb

[HA] [OH-] / [A-]

pH and pOH relation

pH+pOH=14.00 Therefore, if you know one of the four of pH, pOH, [H+], or [OH-], you can figure out the other three.