1

Acids and Bases

(courtesy of L. Scheffler, Lincoln High School, 2010)

2

Acids• React with certain metals to produce

hydrogen gas.• React with carbonates and bicarbonates

to produce carbon dioxide gas

• Have a bitter taste• Feel slippery. • Many soaps contain bases.

Bases

3

Properties of AcidsProduce H+ (as H3O+) ions in water (the hydronium ion

is a hydrogen ion attached to a water molecule)

Taste sour

Corrode metals

Good Electrolytes

React with bases to form a salt and water

pH is less than 7

Turns blue litmus paper to red “Blue to Red A-CID”

Some Common Acids

HCHC22HH33OO22 acetic acidacetic acid in vinegarin vinegar

HClHCl hydrochloric acidhydrochloric acid stomach acidstomach acid

HH33CC66HH55OO77 citric acidcitric acid fruitsfruits

HH22COCO22 carbonic acidcarbonic acid soft drinkssoft drinks

H3H322POPO44phosphoric acidphosphoric acid soft drinkssoft drinks

5

Properties of Bases Generally produce OHGenerally produce 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 blueTurns red litmus paper to blue ““BBasicasic BBluelue””

Some Common Bases

NaOHNaOH sodium hydroxidesodium hydroxide lyelye

KOHKOH potassium hydroxidepotassium hydroxide liquid soapliquid soap

Ba(OH)Ba(OH)22 barium hydroxidebarium hydroxide stabilizer for plasticsstabilizer for plastics

Mg(OH)Mg(OH)22 magnesium hydroxidemagnesium hydroxide ““MOMMOM”” Milk of magnesia Milk of magnesia

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

7

Arrhenius DefinitionArrhenius

Acid - Substances in water that increase the concentration of hydrogen ions ((H+ or hydronium ions H3O+).

Base - Substances in water that increase concentration of hydroxide ions (OH-).

Categorical definition – easy to sort substances into acids and bases

Problem – many bases do not actually contain hydroxides

Practice Classify as an acid or a base 1. Taste bitter 2. Taste Sour 3. Feels slimy or slippery 4.Turns litmus paper blue 5. Turns litmus paper red 6. Gives off hydrogen gas when it reacts

with some metals

8

Practice

Classify the following as an Arrehnius Acid or Base and identify what is substance produces in water

1.HNO3

2.KOH

3.Ca(OH)2

4.H2SO4

9

When neutralization occurs, an acid and a base react together to form a salt and water. Write a balanced equation to represent the neutralization of sulfuric acid and calcium hydroxide, then calculate the mass in grams of calcium hydroxide needed to neutralize 250 mL of 0.01 M solution of sulfuric acid.

10

11

Bronsted-Lowry Definition

Acid - substance that donates a proton.

Base - substance that accepts a proton.

HA + B HB+ + A-

Ex HCl + H2O H3O+ + Cl-

Acid Base Conj Acid Conj Base

A “proton” is really just a hydrogen atom that has lost it’s electron!

The classification depends on how the substance behaves in a chemical reaction

ExampleH2SO4 + NH3 HSO4

- + NH4+

H2SO4 goes to HSO4-

Did it gain or lose a proton?

Is it a BL acid or base?

NH3 goes to NH4+

Did it gain or lose a proton?

Is it a BL acid or base?

12

Identify the BL acid and base

1. HC2H3O2 + H2O C2H3O2- + H3O+

1. HCO3- + HCl H2CO3 + Cl-

13

14

Conjugate Base - The species remaining after an acid has transferred its proton.

Conjugate Acid - The species produced after base has accepted a proton.

HA & A- - conjugate acid/base pair

A- - conjugate base of acid HA

B & HB+ - conjugate acid/base pair

HB+ - conjugate acid of base :B

Conjugate Acid Base Pairs

A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor

acidconjugate

basebase conjugate

acid

16

Note: Water can act as acid or base

Acid Base Conjugate Acid Conjugate Base

HCl + H2O H3O+ + Cl-

H2PO4- + H2O

H3O+ + HPO4

2-

NH4+ + H2O

H3O+ + NH3

Examples of Bronsted-Lowry Acid Base Systems

17

Lewis

Acid - an electron pair acceptor

Base - an electron pair donor

Lewis Definition

Brønsted-Lowry vs. Lewis

All B/L bases are Lewis bases BUT, by definition, a B/L base cannot donate its electrons to anything but a proton (H+)

While B/L is most useful for our purposes, Lewis allows us to treat a wider variety of reactions (even if no H+ transfer occurs) as A/B reactions

19

Acid Strength Strong Acid - Transfers all of its protons to water;

- Completely ionized; - Strong electrolyte; - The conjugate base is weaker and has a negligible tendency to be protonated.

Weak Acid - Transfers only a fraction of its protons to water;

- Partly ionized; - Weak electrolyte; - The conjugate base is stronger, readily accepting protons from water

As acid strength decreases, base strength increases. The stronger the acid, the weaker its conjugate base The weaker the acid, the stronger its conjugate base

20

Acid Dissociation ConstantsDissociation constants for some weak acids

21

Base Strength Strong Base - all molecules accept a proton; - completely ionizes; - strong electrolyte; - conjugate acid is very weak, negligible tendency to donate protons.

Weak Base - fraction of molecules accept proton; - partly ionized; - weak electrolyte; - the conjugate acid is stronger. It more readily donates protons.

As base strength decreases, acid strength increases. The stronger the base, the weaker its conjugate acid. The weaker the base the stronger its conjugate acid.

22

Common Strong Acids/Bases

Strong BasesStrong BasesSodium Hydroxide

Potassium Hydroxide

*Barium Hydroxide

*Calcium Hydroxide

*While strong bases they are not very soluble

Strong AcidsStrong AcidsHydrochloric Acid

Nitric Acid

Sulfuric Acid

Perchloric Acid

A/B Behavior & Chemical Structure

1. Binary Acids• Hydrogen and another element

2. Polyprotic Acids• Have more than 1 Hydrogen to give away

3. Oxyacids • have O in compound

4. Carboxylic Acids • have –COOH in compound

Wait, water can go both ways? amphoteric substances can behave

as either an acid or base depending on what they react with.

water and anions with protons (H+) attached are the most common amphoteric substances

Autoionization of Water

H2O + H2O OH- + H3O+

@ 25 @ 25 ooC the concentrations for both C the concentrations for both

[H[H33OO++] and [OH] and [OH--] = 1.00 x 10] = 1.00 x 10-7-7 and and

[H[H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 = K = Kww

OH-

H3O+

OH-

H3O+

Since [H[H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 = K = Kww

when [H[H33OO++]=[OH]=[OH--] ] the solution is neutral

when [H[H33OO++]>[OH]>[OH--] ] the solution is acidic

when [H[H33OO++]<[OH]<[OH--] ] the solution is basic

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

Under 7 = acid 7 = neutral

Over 7 = base

pH of Common pH of Common SubstancesSubstances

pH 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, [H+] = ???

Because pH = - log [H+] then

- pH = log [H+]

Take antilog (10x) of both sides and get

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

and then the log button

Calculating the pH

pH = - log [H+](Remember that the [ ] mean 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

pH calculations – Solving for H+ A solution has a pH of 8.5. What is the

Molarity of hydrogen ions in the solution?

pH = - log [H+]

8.5 = - log [H+]

-8.5 = log [H+]

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

10-8.5 = [H+]

3 X 10-9 = [H+]

pH = - log [H+]

8.5 = - log [H+]

-8.5 = log [H+]

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

10-8.5 = [H+]

3 X 10-9 = [H+]

pOH Since acids and bases are opposites,

pH and pOH are opposites!

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

pOH looks at the perspective of a basepOH = - log [OH-]

Since pH and pOH are on opposite ends

pH + pOH = 14

33

The pH Scale

pH [H3O+ ] [OH- ] pOH

pH testing There are several ways to test pH

Blue litmus paper (red = acid)Red litmus paper (blue = basic)pH paper (multi-colored)pH meter (7 is neutral, <7 acid, >7

base)Universal indicator (multi-colored)Indicators like phenolphthaleinNatural indicators like red cabbage,

radishes

pH indicators Indicators are dyes that can be

added that will change color in the presence of an acid or base.

Some indicators only work in a specific range of pH

Once the drops are added, the sample is ruined

Some dyes are natural, like radish skin or red cabbage

36

Indicators

37

pH and acidity

The pH values of several common substances are shown at the right.

Many common foods are weak acids

Some medicines and many household cleaners are bases.

39

Neutralization An acid will neutralize a base, giving a

salt and water as products Examples Acid Base Salt water

HCl + NaOH NaCl + H2O

H2SO4 + 2 NaOH Na2SO4 + 2 H2O

H3PO4 + 3 KOH K3PO4 + 3 H2O

2 HCl + Ca(OH) 2 CaCl2 + 2 H2O

A salt is an ionic compound that is formed from the positive ion (cation) of the base and the negative ion (anion) of the acid

40

Titration & Titration Curves Titration: the adding of one solution of an

known concentration into another solution standard solution: a solution with a known

concentration Titration curve: a graph showing pH vs

volume of acid or base added The pH shows a sudden change near the

equivalence point The Equivalence point (a.k.a. stoichiometric

point) is the point at which the moles of OH- are equal to the moles of H3O+

42

Buffer Solutions - Characteristics A solution that resists a change in pH.

It is pH stable. A weak acid and its conjugate base

form an acid buffer. A weak base and its conjugate acid

form a base buffer. We can make a buffer of any pH by

varying the concentrations of the acid/base and its conjugate.