11The Chemistry The Chemistry of Acids and of Acids and BasesBases

The Chemistry The Chemistry of Acids and of Acids and BasesBases

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Acids and BasesAcids and Bases

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Acids and BasesAcids and Bases

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Acids and BasesAcids and Bases

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Some Properties of Acids

Produce H+ ions in water

Taste sour

Corrode metals

Electrolytes

React with bases to form a salt and water

pH is less than 7

Turns blue litmus paper to red “Blue to Red ACID”

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Some Common Acids

HNO3 Nitric Acid

HCL Hydrochloric Acid

H2SO4 Sulfuric Acid

H3PO4 Phosphoric Acid

H2CO3 Carbonic Acid

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Some Properties of Bases

Produce OHProduce OH-- ions in water ions in water

Taste bitter, chalkyTaste bitter, chalky

Are electrolytesAre electrolytes

Feel soapy, slipperyFeel soapy, slippery

React with acids to form salts and waterReact with acids to form salts and water

pH greater than 7pH greater than 7

Turns red litmus paper to blue “Basic Blue”Turns red litmus paper to blue “Basic Blue”

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Some Common Bases

NaOHNaOH sodium hydroxidesodium hydroxide

KOHKOH potassium hydroxidepotassium hydroxide

Ba(OH)Ba(OH)22 barium hydroxidebarium hydroxide

Mg(OH)Mg(OH)22 magnesium hydroxidemagnesium hydroxide

Al(OH)Al(OH)33 aluminum hydroxidealuminum hydroxide

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Acid/Base definitions

• Definition #1: Arrhenius (traditional)

Acids – produce H+ ions

HCl → H+(aq) + Cl-(aq)

Bases – produce OH- ions

NaOH → Na+(aq) + OH-(aq)

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Acid/Base Definitions

• Definition #2: Brønsted – Lowry

Acids – proton (hydrogen ion H+) donor

HNO3 + H2O→ H3O+(aq) + NO3-(aq)

Bases – proton acceptor

NH3 + H2O → NH4+(aq) + OH-(aq)

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Brønsted-Lowry Model

• According to the B-L model, the reaction can be represented as an acid (HA) donating a proton to a water molecule to form a new acid (the conjugate acid) and a new base (a conjugate base).

• Conjugate representing how the species reacts in the reverse reaction.

– HA (aq) + H2O → H3O+(aq) + A-(aq)

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Conjugate PairsConjugate Pairs

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Strong vs. Weak Acids/Bases

• The term strong vs. weak for acids and bases does not refer to concentration or danger.

• A strong acid or base completely ionizes in water.

• A weak acid or base partially ionizes in water.

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ACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIES

• The Brønsted definition means HThe Brønsted definition means H22O is O is both an acid and a base. both an acid and a base.

• It is called It is called AmphotericAmphoteric; a substance that ; a substance that can behave as an acid or base.can behave as an acid or base.

HH22O + HO + H22O O → H→ H33OO++(aq) + OH(aq) + OH--(aq)(aq)

1515The The pH scalepH scale is a way of is a way of expressing the strength expressing the strength of acids and bases. of acids and bases. Instead of using very Instead of using very small numbers, we just small numbers, we just use the NEGATIVE use the NEGATIVE power of 10 on the power of 10 on the Molarity of the HMolarity of the H++ (or (or OHOH--) ion.) ion.

Under 7 = acidUnder 7 = acid 7 = neutral 7 = neutral

Over 7 = baseOver 7 = base

1616Calculating the pH

pH = - log [H+](Remember that the [ ] means Molarity)

Example: If [H+] = 1 X 10-10

pH = - log 1 X 10-10

pH = - (- 10)

pH = 10

Example: If [H+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = - (- 4.74)

pH = 4.74

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Try These!Try These!

Find the pH of Find the pH of these:these:

1) A 0.15 M solution 1) A 0.15 M solution of Hydrochloric of Hydrochloric acidacid

2) A 3.00 X 102) A 3.00 X 10-7-7 M M solution of Nitric solution of Nitric acidacid

1818pH calculations – Solving for pH calculations – Solving for H+H+pH calculations – Solving for pH calculations – Solving for H+H+

If the pH of Coke is 3.12, [HIf the pH of Coke is 3.12, [H++] = ???] = ???

Because pH = - log [HBecause pH = - log [H++] then] then

- pH = log [H- pH = log [H++]]

Take antilog (10Take antilog (10xx) of both) of both sides and get sides and get

1010-pH -pH == [H[H++]][H[H++] = 10] = 10-3.12-3.12 = 7.6 x 10 = 7.6 x 10-4-4 M M *** to find antilog on your calculator, look for “Shift” or “2*** to find antilog on your calculator, look for “Shift” or “2nd nd

function” and then the log buttonfunction” and then the log button

1919pH calculations – Solving for pH calculations – Solving for H+H+

• A solution has a pH of 8.5. What is the A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the Molarity of hydrogen ions in the solution?solution?

pH = - log [HpH = - log [H++]]

8.5 = - log [H8.5 = - log [H++]]

-8.5 = log [H-8.5 = log [H++]]

Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

pH = - log [HpH = - log [H++]]

8.5 = - log [H8.5 = - log [H++]]

-8.5 = log [H-8.5 = log [H++]]

Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

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More About WaterMore About Water

HH22O can function as both an ACID and a BASE.O can function as both an ACID and a BASE.

In pure water there can beIn pure water there can be AUTOIONIZATIONAUTOIONIZATION

Equilibrium constant for water = KEquilibrium constant for water = Kww

KKww = [H = [H33OO++] [OH] [OH--] =] = 1.00 x 101.00 x 10-14-14 at 25 at 25 ooCC

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KKww = [H = [H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC

In a In a neutral neutral solution [Hsolution [H33OO++] = [OH] = [OH--]]

so Kso Kww = [H = [H33OO++]]22 = [OH = [OH--]]22

and so [Hand so [H33OO++] = [OH] = [OH--] = 1.00 x 10] = 1.00 x 10-7-7 M M

OH-

H3O+

OH-

H3O+

AutoionizationAutoionization

2222pOH

• Since acids and bases are Since acids and bases are opposites, pH and pOH are opposites, pH and pOH are opposites!opposites!

• pOH does not really exist, but it is pOH does not really exist, but it is useful for changing bases to pH.useful for changing bases to pH.

• pOH looks at the perspective of a pOH looks at the perspective of a basebase

pOH = - log [OHpOH = - log [OH--]]Since pH and pOH are on opposite Since pH and pOH are on opposite

ends,ends,pH + pOH = 14pH + pOH = 14

2323The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater?

The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?

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HNO3, HCl, H2SO4 and HClO4 are among the only known strong acids.

Strong and Weak Strong and Weak Acids/BasesAcids/Bases

The strength of an acid (or base) is determined by the amount of IONIZATION.

The strength of an acid (or base) is determined by the amount of IONIZATION.

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Strong and Weak Strong and Weak Acids/BasesAcids/Bases

• Generally divide acids and bases into STRONG or Generally divide acids and bases into STRONG or WEAK ones.WEAK ones.

STRONG ACID:STRONG ACID: HNOHNO3 3 (aq) + H(aq) + H22O (l) --->O (l) --->

HH33OO+ + (aq) + NO(aq) + NO33- - (aq)(aq)

HNOHNO33 is about 100% dissociated in water. is about 100% dissociated in water.

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• Weak acidsWeak acids are much less than 100% ionized in are much less than 100% ionized in

water.water.

One of the best known is acetic acid = CHOne of the best known is acetic acid = CH33COCO22HH

Strong and Weak Strong and Weak Acids/BasesAcids/Bases

Strong and Weak Strong and Weak Acids/BasesAcids/Bases

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• Strong Base:Strong Base: 100% dissociated in 100% dissociated in water.water.

NaOH (aq) ---> NaNaOH (aq) ---> Na+ + (aq) + OH(aq) + OH- - (aq)(aq)

Other common strong Other common strong bases include KOH andbases include KOH and Ca(OH)Ca(OH)22..

CaO (lime) + HCaO (lime) + H22O -->O -->

Ca(OH)Ca(OH)22 (slaked lime) (slaked lime)CaOCaO

HONORS ONLY!HONORS ONLY!

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• Weak base:Weak base: less than 100% ionized less than 100% ionized in waterin water

One of the best known weak bases is One of the best known weak bases is ammoniaammonia

NHNH3 3 (aq) + H(aq) + H22O (l) O (l) NH NH44+ + (aq) + OH(aq) + OH- - (aq)(aq)

Strong and Weak Strong and Weak Acids/BasesAcids/Bases

Strong and Weak Strong and Weak Acids/BasesAcids/Bases

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Weak BasesWeak Bases

3030pH testing

• There are several ways to test pHThere are several ways to test pH

–Blue litmus paper (red = acid)Blue litmus paper (red = acid)

–Red litmus paper (blue = basic)Red litmus paper (blue = basic)

–pH paper (multi-colored)pH paper (multi-colored)

–pH meter (7 is neutral, <7 acid, >7 pH meter (7 is neutral, <7 acid, >7 base)base)

–Universal indicator (multi-colored)Universal indicator (multi-colored)

– Indicators like phenolphthaleinIndicators like phenolphthalein

–Natural indicators like red cabbage, Natural indicators like red cabbage, radishesradishes

3131Paper testing

• Paper tests like litmus paper and pH Paper tests like litmus paper and pH paperpaper

– Put a stirring rod into the solution Put a stirring rod into the solution and stir.and stir.

– Take the stirring rod out, and Take the stirring rod out, and place a drop of the solution from place a drop of the solution from the end of the stirring rod onto a the end of the stirring rod onto a piece of the paperpiece of the paper

– Read and record the color change. Read and record the color change. Note what the color indicates. Note what the color indicates.

– You should only use a small You should only use a small portion of the paper. You can use portion of the paper. You can use one piece of paper for several one piece of paper for several tests.tests.

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pH meter

• Tests the voltage of the Tests the voltage of the electrolyteelectrolyte

• Converts the voltage to Converts the voltage to pHpH

• Very cheap, accurateVery cheap, accurate

• Must be calibrated with Must be calibrated with a buffer solutiona buffer solution