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 HClO 4  H + + ClO 4 - –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 + H 2 O H 3 O + + X - On accepting the H +, the water becomes an acid, H 3 O +. 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 + H 2 O H 3 O + + F - NH 3 + H 2 O NH OH -

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

Strong Acids Strong acids are acids that ionize completely in solution. HCl + H 2 O  H 3 O + + Cl - Strong acids: HCl HBr HI HClO 4 HNO 3 H 2 SO 4

Weak Acids Weak acids are acids that ionize only partially in solution. All other acids are weak acids HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2 - 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. K a is the acid ionization constant. –The weaker the acid, the smaller the K a. –The stronger the acid, the larger the K a.

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, K b. The strong bases are: NaOH KOH Ca(OH) 2 RbOH CsOH Ba(OH) 2

Water Water always ionizes itself a little. H 2 O 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= 1.0x 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 + H 2 O

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.