ACID BASE UNIT

CHAPTER 19

The characteristic properties of acids result from the presence of the H+ ion generated when an acid dissolves in water. It also explains why acids neutralize bases and vice versa. Acids provide the H+ ion; bases provide the OH- ion; and these ions combine to form water.

H+(aq) + OH-(aq) -> H2O(l)

II. Acid/Base Strengths

1. Strong Acids: Ionize completely, are good conductors of electricity because they produce the maximum numbers of ions.

Examples: Hydrochloric acid (HCl), Perchloric acid (HCLO4), Nitric acid (HNO3), Hyrdoiodic acid (HI), and sulfuric acid (H2SO4)

The general reaction that occurs when an acid is dissolved in water can best be represented as:

 HX (aq) + H2O (l) -> H3O+ (aq) + X- (aq)Acid + Base -> conjugate acid + conjugate base

Every acid has associated with it a conjugate base, formed from the acid by the loss of a proton.

The conjugate base is everything that remains of the acid molecule after a proton is lost.

Every base has associated with it a conjugate acid, formed from the base by the addition of a proton.

HX (aq) + H2O (l) -> H3O+ (aq) + X- (aq)

The conjugate acid is formed when the proton is transferred to the base. A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton. In the example above, there are two conjugate acid-base pairs:

HX and X-, and H2O and H3O+.

HX (aq) + H2O (l) -> H3O+ (aq) + X- (aq)

A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton. In the example above, there are two conjugate acid-base pairs: HX and X-, and H2O and H3O+.

Practice Problems

Identify the conjugate acid-base pairs in the following reactions.

1. NH4+(aq) + OH-(aq) NH3(aq) + H20(l)

1. HBr(aq) + H20(l)H3O+(aq) + Br-(aq)

II. Strengths of Acids & Bases

1. Strength of acids

a. Strong Acids: Ionize completely and are good conductors of electricity

b. Examples: Hydrochoric (HCl), Hydrobromic (HBr), Hydroiodic (HI), Perchloic (HClO4), Nitric (HNO3), Sulfuric (H2SO4)

c. Weak Acids: Partially ionize and are weak conductors

d. Examples: Hydrofluoric (HF), Hydrocyanic (HCN), Acetic (HC2H3O2), Hydrosulfuric (H2S), Carbonic (H2CO3), Hypochlorous (HClO)

1. Strength of Bases

a. Strong Bases: dissociate entirely into metal ions and hydroxide ions.

b. Examples: Calcium Hydroxide (Ca(OH)2), Sodium Hydroxide (NaOH), Potassium Hydroxide (KOH), Barium Hydroxide (Ba(OH)2)

c. Weak Bases: only partially ionize into metal ions and hydroxide ions.

d. Examples: Ethylamine (C2H5NH2), Methlyamine (CH3NH2), Ammoina (NH3), Aniline (C6H5NH2)

pH Scale

Acidic and basic are two extremes that describe a chemical property of substances.

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value.

For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6.

The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value.

For example, pH 10 is ten times more alkaline than pH 9 and 100 times (10 times 10) more alkaline than pH 8.

Definition of pH and pOH

The concentrations of hydrogen ions and indirectly hydroxide ions are given by a pH number. pH is defined as the negative logarithm of the hydrogen ion concentration. The equation is:

pH = - log [H+] similarly, pOH = - log [OH-]

The following diagram illustrates the relationship between pH, H+, OH-

Example: If an acid has an H+ concentration of 0.0001 M, find the pH.

Solution:First convert the number to exponential notation, find the log, then solve the pH equation.

H+ = 0.0001M = 10-4; log of 10-4 = -4;pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH The purpose of the negative sign in the log

definition is to give a positive pH value.

Example: If the base has an OH- concentration of 0.001M, find the pH.

 Solution:

First find the pOH, (similar to finding the pH,) then subtract the pOH from 14.OH- = 0.001M = 10-3;

pOH = -log [OH-] = -log (10-3) = +3 = pOH

pH = 14 - pOH; pH = 14 - 3 = 11 = pHpH Principle: pH and pOH must always equal

pKw (14).

Acid and Base Indicators

The most common indicator is found on "litmus" paper. It is red below pH 4.5 and blue above pH 8.2.

        

turns bluestays sameBase

stays sameturns redAcid

Red LitmusBlue Litmus

Other commercial pH papers/solutions are able to give different colors for every main pH value.

Universal Indicator is a mixture of indicators is able to also provide a full range of colors for the pH scale.