Acids & Bases

Chapter 15 & 16

Acids

• Have a sour taste

• Affect indicators

• React with bases to produce salt & water

• Conduct an electric current

• Examples include salad dressing, rainwater, car batteries, & soft drinks

Bases

• Taste bitter

• Feel slippery

• Affect indicators

• React with acids to produce salt & water

• Conduct an electric current

• Examples include soap, baking soda, & drain cleaners

Solutions of acids & bases

• The amounts of hydrogen ions (H+) & hydroxide ions (OH-) determine whether a solution is acidic, basic, or neutral.

– Acidic solutions contain more H+ than OH-

– Basic solutions contain more OH- than H+

– Neutral solutions contain equal amounts

Arrhenius model

• In 1883, Svante Arrhenius proposed the Arrhenius model of acids & bases to explain how pure water could become acidic or basic.

Arrhenius model

• According to the Arrhenius model

– An acid is a substance that contains hydrogen & ionizes to produce H+ in solution

HClO4 H+ + ClO4-

– A base is a substance that contains a hydroxide group & ionizes to produce OH- in solution

NaOH Na+ + OH-

Bronsted-Lowry Model

• Johannes Bronsted & Thomas Lowry proposed a better model for acids & bases that focuses on the role of the proton, or H+.

• According to the Bronsted-Lowry model:– An acid donates a proton, or H+

– A base accepts a proton, or H+

Explanation

When an acid, HX, dissolves in water, it donates a H+ to a water molecule. The water acts as a base & accepts the H+.

HX + H2O H3O+ + X-

On accepting the H+, the water becomes an acid, H3O+. The hydronium is an acid because it has an extra H+ it can donate. On donating its H+, the acid becomes a base, X-. The reaction can also occur in the reverse direction.

Conjugates

• The products of an acid-base reaction are known as the conjugate acid-base pair.

– The conjugate acid is the species produced when a base accepts a H+ from an acid.

– The conjugate base is the species that results when an acid donates an H+ to a base.

Problem

Label the acid, base, conjugate acid, & conjugate

base in each of the reactions.

HF + H2O H3O+ + F-

NH3 + H2O NH4+ + OH-

Amphoteric:substances, like water, that can act as acids or

bases

Strong Acids

• Strong acids are acids that ionize completely in solution.

HCl + H2O H3O+ + Cl-

• Strong acids:HCl HBr HI HClO4 HNO3 H2SO4

Weak Acids

• Weak acids are acids that ionize only partially in solution.

• All other acids are weak acids

HC2H3O2 + H2O H3O+ + C2H3O2-

• In the reaction, the acetic acid does not ionize completely & some of the acetic acid ions remain in molecular form in the solution.

Cont’d…

• The double arrow in the reaction means the reaction is an equilibrium reaction.

• The equilibrium constant expression is written by placing the concentration of the product in the numerator & the concentration of the reactants in the denominator.

• Water is a constant so it is not added to the expression.

• Ka is the acid ionization constant.

– The weaker the acid, the smaller the Ka.

– The stronger the acid, the larger the Ka.

Strong & Weak Bases

• The same application that apply for acids, apply for bases except using OH-.

• Strong bases ionize completely in solution• Weak bases ionize only partially in solution.• The equilibrium equation for bases use the

base ionization constant, Kb.

• The strong bases are:

NaOH KOH Ca(OH)2 RbOH CsOH Ba(OH)2

Water

• Water always ionizes itself a little.

H2O H+ + OH-

• This gives us a special constant that applies to the self-ionization of water.

Kw= [H+][OH-]

Kw is the ion product constant for water.

Cont’d…

• In pure water at 298K, [H+] & [OH-] always equal 1.0x10-7 M.

• Therefore, Kw= (1.0x10-7)(1.0x10-7)

Kw= 1.0x10-14

• You can use Kw to calculate the concentration of either ion if you know the concentration of the other.

Problem

Using the ion product constant for water, calculate [OH-] if

[H+]= 3.0x10-2 M

pH scale

• Chemists use a pH scale to express hydrogen ion concentrations.

pH= -log [H+]

• pH means the power of hydrogen• pH ranges from 0 to 14• 0 to 7 is acidic• 7 to 14 is basic• 7 is neutral• pH decreases as [H+] increases

pOH scale

• Chemists can also use the pOH scale to express the basicity, or alkalinity, of a solution.

pOH= -log [OH-]

• pOH ranges from 0 to 14• 0 to 7 is basic• 7 to 14 is acidic• 7 is neutral

A chemist can calculate pH or pOH if the other is

known using:

pH + pOH = 14.00

Problems

Calculate pH.

• [H+]= 1.0x10-2 M

• [H+]= 3.0x10-6 M

• [OH-]= 8.2x10-6 M

• [OH-]= 4.3x10-4 M

Neutralization

• Occurs when an acid and a base react to form a salt & water

HCl + NaOH NaCl + H2O

Titration

• The process of determining the concentration of an unknown solution by dripping, or titrating, a known strong acid or base into it

• If the unknown solution is acidic, we titrate the solution with a strong base of a known concentration.

Steps to Titration

• An indicator is a chemical dye added to the solution whose color is affected by acidic or basic solutions.

Cont’d…

• As you add the acid or base to the unknown solution, pH slowly rises.

• When the acid is neutralized, pH rises rapidly & the indicator in the solution changes color.

• The color change is called the end point.