1 acids and bases (courtesy of l. scheffler, lincoln high school, 2010)
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Acids and Bases
(courtesy of L. Scheffler, Lincoln High School, 2010)
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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
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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
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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)
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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
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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
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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.
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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?
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Identify the BL acid and base
1. HC2H3O2 + H2O C2H3O2- + H3O+
1. HCO3- + HCl H2CO3 + Cl-
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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
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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
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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
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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
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Acid Dissociation ConstantsDissociation constants for some weak acids
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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.
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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
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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
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Indicators
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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.
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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
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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+
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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.
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