In This Lesson:
Acids and Bases
(Lesson 3 of 4)
Today is Thursday,
January 14th, 2016
Pre-Class:
Ever drank acid
before?
P.S. You need a
paper towel.
Stuff You Need:
Calculator
Periodic Table
Polyatomic Ion
List
Paper Towel
Today’s Agenda
• Salts
• Acids and Bases
– The many ways to think of them.
• pH and pOH
• Titrations
• Where is this in my book?
– P. 587 and following…
By the end of this lesson…
• You should be able to define acids and bases
using the Arrhenius and Brønsted-Lowry
definitions.
• You should be able to predict the products of
a neutralization reaction.
• You should be able to calculate pH, pOH, [H+],
and [OH-].
• You should be able to perform a titration.
Demo – Neutralization Reaction
• Vinegar (acetic acid) and Baking Soda (a base)
– relates to your homework!
Okay, first of all…
• It’s about time we discuss salt.
• Salt in chemistry terms is a very specific thing.
• A salt is an ionic compound formed from a
neutralization reaction between an acid and a
base.
– So it’s not just NaCl.
– More on neutralization reactions later.
Salts
http://cdn.meme.am/instances/45063314.jpg
Credit to Sam L.
May 2015
Dissociation
• Remember that all acids are dissolved in water
(aq).
• When the acid is dissolved in water, it will break
down.
• The process of the molecule breaking down is
called dissociation.
– Strong acids will dissociate completely (or nearly so).
– Weak acids will not dissociate much.
Naming Acids [REMINDER]
• First, cover the (H) and name the
anion normally.
– Sulfide.
• Next, use this key:
Example
H2
Anion Suffix
Acid Name
-ide
Hydro___ic acid
-ate
___ic acid
-ite
___ous acid
• Hydrosulfuric acid
2S
Remembering Acid Names
• “Ick, I ate it.”
– __ic is the acid suffix for stuff otherwise ending in
__ate.
• “Ite, I oust it.” OR “Riteous”
– __ous is the acid suffix for stuff otherwise ending in
___ite.
• You’ll have to come up with something for
hydro___ic acid. I’m not creative enough.
Practice
• HCl
– Cl- would be chloride, so it’s hydrochloric acid.
• H2SO4
– SO42- would be sulfate, so it’s sulfuric acid.
• HClO2
– ClO2- would be chlorite, so it’s chlorous acid.
• Phosphoric acid
– H3PO4, because “phosphoric” came from
“phosphate,” which is PO43-.
Properties of Acids
•
•
•
•
•
•
pH is lower than 7
Turn methyl orange and blue litmus paper red
Taste sour
React with active metals to produce H2
React with carbonates
Acids neutralize bases
Acids’ pH
• Acids’ pH values
are less than 7:
Acids Affect Indicators
• Blue litmus paper turns red.
• Methyl orange also turns red.
Acids Taste Sour
•
•
•
•
Citric acid in citrus fruits.
Malic acid in sour apples.
Lactic acid in sour milk and sore muscles.
Butyric acid in rancid butter.
Acids React with Metals
• Acids react with metals to form salts and
hydrogen gas:
– Mg + 2HCl  MgCl2 + H2 (g)
– Zn + 2HCl  ZnCl2 + H2 (g)
– Mg + H2SO4  MgSO4 + H2 (g)
Acids React with Carbonates
• Acids react with carbonates, often forming
water and carbon dioxide:
– 2HC2H3O2 + Na2CO3  2NaC2H3O2 + H2O + CO2
Acids React with Carbonates
• Acid rain’s effect on marble (CaCO3) in
Washington Square Park, Manhattan:
George Washington before…
George Washington after…
Acids Neutralize Bases
• Neutralization reactions are double replacement
reactions between an acid and a base.
– Specifically Arrhenius acids/bases (more later).
• They always produce a salt and water.
– HCl + NaOH  NaCl + H2O
– H2SO4 + 2NaOH  Na2SO4 + 2H2O
– 2HNO3 + Mg(OH)2  Mg(NO3)2 + 2H2O
• Try underlining the OH and H for water.
– Combine the rest to make the salt.
Neutralization Reaction Practice
• HCl + KOH  ?
– HCl + KOH  KCl + H2O
• H2SO4 + Ca(OH)2  ?
– H2SO4 + Ca(OH)2  CaSO4 + 2H2O
• HNO3 + NaOH  ?
– HNO3 + NaOH  NaNO3 + H2O
• H2CO3 + 2NaOH  ?
– H2CO3 + 2NaOH  Na2CO3 + 2H2O
Properties of Bases
• pH is greater than 7
• Turn phenolphthalein pink and red litmus
paper blue
• Taste bitter, feel slippery
• Bases neutralize acids
Bases’ pH
• Bases’ pH values
are greater than 7:
Bases Affect Indicators
• Red litmus paper turns blue.
• Phenolphthalein turns pink.
Bases Feel Slippery
• They dissolve oils and fatty acids on your
fingers to make soap, basically.
– Get it?
http://earnthis.net/wp-content/uploads/2013/11/4603414+_2348a6229619302f103f2d06551b60f6.jpg
Bases Neutralize Acids
• Milk of Magnesia is an oldfashioned stomachache cure.
– Contains Mg(OH)2 – magnesium
hydroxide.
• Magnesium hydroxide
neutralizes stomach acid,
producing water and
magnesium chloride (a salt).
– 2HCl + Mg(OH)2  MgCl2 + 2H2O
Acid/Base Definitions
• There are three different definitions of acids/bases:
• We will talk mainly about two of them:
– Arrhenius Acids/Bases
• Acids are H+ producers in solution.
• Bases are OH- producers in solution.
– Brønsted-Lowry Acids/Bases
• Acids are proton (H+) donors.
• Bases are proton (H+) acceptors.
– Lewis Acids/Bases
• Acids are electron pair donors.
• Bases are electron pair acceptors.
Acid/Base Definition Disclaimer
• Svante Arrhenius defined acids and
bases in the early 1900s.
• It was a major step forward for
chemistry, but today it does not
explain all the known acids and
bases.
• However, it remains a good starting
point.
• Arrhenius also first described
activation energy.
– Side note: Arrhenius also had a keen
interest in eugenics. Whoops.
http://en.wikipedia.org/wiki/File:Arrhenius2.jpg
Meh.
Svante Arrhenius
Arrhenius Acids
• Under the Arrhenius definition of acids, you’ll
also see the term H3O+.
• When an Arrhenius acid dissolves, it gives off
H+ ions (protons).
• Many of those protons then join with existing
water molecules, creating the hydronium ion
(H3O+).
– Thus, the presence of hydronium ions is indicative
of an Arrhenius acid.
H+ + H2O  H3O+
A Field Guide to
Arrhenius Acids and Bases
• Acids have these formulas:
– HX (aq)
– HaXbOc (aq)
• Bases are ionic compounds and contain either:
– OH- (hydroxide)
– CO32- (carbonate)
– HCO3- (bicarbonate/hydrogen carbonate)
• NH3 (ammonia) and others can be bases.
– Why? Because NH3 + H2O  NH4+ + OH-
Brønsted-Lowry Acids and Bases
• Two scientists
independently
developed the
definition that acids
donate protons and
bases accept them:
Johannes Brønsted
http://lowres-picturecabinet.com.s3-eu-west-1.amazonaws.com/38/main/83/423546.jpg
http://www.denstoredanske.dk/@api/deki/files/3673/=268426.501.jpg
Thomas Martin Lowry
Brønsted-Lowry Acids and Bases
• There’s something else, though, about acids
and bases under this definition. Let’s
introduce it with a story:
• Suppose, on your birthday, I give you a gift.
– A Horse, perhaps?
• Don’t look it in the mouth.
• I, therefore, am the “gifter” and you are the
“giftee.”
Brønsted-Lowry Acids and Bases
• However, once I’ve given you the Horse, I am
all out of gifts.
• Also, you could, if you wanted, give the Horse
away.
– But you probably won’t.
• This is just how acids and bases work under
the Brønsted-Lowry definition.
Brønsted-Lowry Acids and Bases
•
•
•
•
An acid gives away H+ (protons).
A base accepts H+ (protons).
A conjugate base has already given away H.
A conjugate acid has already received H.
• The key is to find the H transfer.
– Example next slide…
Brønsted-Lowry Acids and Bases
NH3 + H2O  NH4+ + OHH
Conjugate Base
Conjugate Acid
Acid
Base
(the  means the reaction can go either way)
Brønsted-Lowry Acids and Bases
HCl + H2O  H3O+ + ClH
Conjugate Base
Conjugate Acid
Base
Acid
Quick Vocabulary Words
• In the examples we just did, water behaved as an
acid in one and a base in the other.
– When something can act as an acid or a base, we call
it amphoteric.
• When we described the different terms in the
equation, one might say we were describing the
species in the reaction.
– Yep, there’s some biology lingo there…
• Acidic protons are written in front.
– So the hydrogen in C2H3O2- will not be transferred.
Quick Summary
• Arrhenius Acid:
– Anything that makes H+ in water.
• Arrhenius Base:
– Anything that makes OH- in water.
• Brønsted-Lowry Acid:
– Anything that donates H+.
• Brønsted-Lowry Base:
– Anything that accepts H+.
Practice
• Conjugate Pairs Worksheet
Practice
• Close your notebooks but hold the page with a
worksheet or something.
• Acids and Bases Review Sheet
– #1-7 (TO BE DONE INDEPENDENTLY)
• Circle any number you’re unsure of. When you try all 7, go
back to your notebook for the circled numbers.
• We’ll then “grade” it in groups – put an X next to each
incorrect answer and give yourself a score.
– #8-15 (TO BE DONE INDEPENDENTLY)
• Repeat above procedure.
– #16-23 (TO BE DONE INDEPENDENTLY)
• Repeat above procedure.
pH
• pH is the measure of the
concentration of an acid or
base.
• You can also measure
concentration with molarity,
but it’s tough.
– pH is easier.
– Thanks to Søren Sørensen for
proposing this one in 1909.
Søren Sørensen
pH
• pH stands for “potential Hydrogen” or “power
of Hydrogen.”
• Basically, it’s a measure of the presence of
hydrogen ions (H+), which make solutions
acidic.
• Don’t forget… [H+] means “concentration of
hydrogen ions.”
Calculating pH
• To calculate pH from the concentration of
hydrogen ions [H+], calculate its negative
logarithm:
• pH = -log [H+]
• To calculate [H+] from pH, use this formula:
• [H+] = 10-pH
• Concentration is usually in the form of
molarity (M).
Calculating pH Examples
• What is [H+] if pH = 9.9?
• [H+] = 10-9.9 = 1.259 x 10-10 M
• [H+] in an acid solution is 1.5 x 10-3 M. What is the
pH of the solution?
• pH = -log [1.5 x 10-3] = 2.82
• What is the pH of a solution with hydrogen ion
concentration of 4.2 x 10-10 M? Is it acidic or basic?
• pH = -log [4.2 x 10-10] = 9.38
• It’s basic.
Logarithm FAQ
• Just an FYI for the math nerds out there:
• A logarithm of a number X, unless another
base is specified, is the exponent by which 10
has to be raised to produce X.
– They’re like backwards-powers.
• In other words, log [10] = 1 because 101 = 10.
• Or, log [15] = 1.176 because 101.176 = 15.
Acids, Bases, and pH
• Anything above 7 is basic.
• Anything below 7 is acidic.
• Anything at 7 is neutral.
– Water (neutral) has an [H+] concentration of 1 x
10-7 M, or 0.0000001 M.
Practice
• pH Practice worksheet
– #1, 5, 6, 7, 10
pOH – Bizarro World
• Less frequently used is
pOH, a similar but opposite
scale.
• <7 = Basic
• >7 = Acidic
• For the same substance,
pH + pOH = 14.
http://www.nationofblue.com/content/attachments/5137d1297288243-bizarro-seinfeld.jpg
pH + pOH = 14
Calculating pOH
• To calculate pOH from the concentration of
hydroxide ions [OH-], calculate its negative
logarithm:
• pOH = -log [OH-]
• To calculate [OH-] from pOH, use this formula:
• [OH-] = 10-pOH
– Units are M again.
Calculating pOH Example
• What is [OH-] if pOH = 2.3? Is it acidic or
basic?
• [OH-] = 10-2.3 = 5.01 x 10-3 M
• pOH is less than 7, so it’s basic.
Practice
• pH Practice worksheet
– #2-4
pH and pOH Summary
• Calculating pH and pOH:
• pH = -log10[H3O+] or -log10[H+]
• pOH = -log10[OH-]
• Relationship between pH and pOH:
• pH + pOH = 14
• Finding [H+] or [H3O+] and [OH-]:
• [H3O+] or [H+] = 10-pH
• [OH-] = 10-pOH
pH and pOH Summary
• Acidic solutions have higher [H+] than [OH-].
• Basic solutions have higher [OH-] than [H+].
• Neutral solutions have equal [H+] and [OH-].
pH and pOH Summary
[H+]
-pOH
10
pOH
14
-log[OH-]
-log[H+]
10-pH
pH
[OH-]
1 x 10-14
pH and pOH Summary
pH Practice (last one)
• Multistep pH and pOH Problems worksheet
– #1, 2, 5
Self-Ionization of Water
• Though pure water is considered a nonconductor, there is a slight but measurable
conductivity due to self-ionization.
– Only about one in 2 billion water molecules does
this.
+
H2O + H2O



+
H3O+ + OH-
Ionization of Water
• In pure water at 25 °C:
• [H3O+] = 1 x 10-7 mol/L
• [OH-] = 1 x 10-7 mol/L
• Which is why water’s neutral.
– The concentration of acid-causing H3O+ and basecausing OH- are equal.
• Fun fact: Interestingly, the neutral pH value of 7
changes with different temperatures.
– Neutral pH at 100 °C, for example, is 6.14.
– At 0 °C, it’s 7.47.
Practice
• Acids and Bases Review
– #24-32
Acid/Base Strength
• Students frequently confuse pH (or pOH) with
acid/base strength.
– In reality, pH and pOH concern concentration,
which is more about how much than about how
strong.
• The strength of acids and bases is a product of
ionization.
• TED: George Zaidan and Charles Morton – The
Strengths and Weaknesses of Acids and Bases
More About Neutralization Reactions
• Chemists frequently use neutralization
reactions during the process of titration.
• Titration is a way for chemists to determine
the concentration of an acid or base solution
using the concentration of a known solution.
– During titration, the solution whose concentration
is known is called the standard solution.
Titration Procedure
• Let’s imagine that we’ve got an acid with an
unknown concentration (molarity).
• We’ll add a base indicator to the solution.
– It shouldn’t change color because we have an acid
in there.
Titration Procedure
• We’ll then slowly add a base with a known
concentration until the indicator changes color.
– When the indicator changes, that tells us that the acid
can no longer neutralize the base, meaning the
neutralization reaction is done.
• When the indicator changes color permanently,
we’ve reached our endpoint (when we stop
titrating).
• The endpoint is close to, but not exactly, the
equivalence point, which is when the acid and
base have neutralized each other.
Titration Practice
• When solving a titration problem, you need to write
the balanced reaction.
– Remember, acids + bases form water and a salt.
• Step 1: Find the moles (using the molarity) of the
known solution.
• Step 2: Use a mole ratio to find the number of
moles of the unknown solution.
• Step 3: Calculate the molarity of the unknown
solution using its volume and calculated moles.
Titration Problems
• Typically, you’ll need to find these things in this
order:
1.
2.
3.
4.
5.
Balanced equation.
Concentration of known solution (usually given).
Moles of known solution solute.
Moles of unknown solution solute.
Concentration of unknown solution.
• Generally, the problem takes the shape of:
1. Molarity formula for known.
2. Mole ratio.
3. Molarity formula for unknown.
Titration Practice Problem
• A 25 mL solution of H2SO4 (sulfuric acid) is
completely neutralized by 18 mL of 1.0 M
NaOH (sodium hydroxide). What is the
concentration of the sulfuric acid solution?
• Step 1: Find the balanced equation:
– H2SO4 + 2NaOH  Na2SO4 + 2H2O
H2SO4 + 2NaOH  Na2SO4 + 2H2O
• Step 2: Find the moles of the known solution.
– Remember, 25 mL of H2SO4 was neutralized by 18 mL of 1.0
M NaOH.
– That means there are 0.018 moles of NaOH present.
• Step 3: Use a mole ratio to find moles of unknown
solution.
– By mole ratio, we would need 0.009 moles of H2SO4 with
which to react.
• Step 4: Calculate the molarity of the unknown solution.
– If there are 0.009 moles of H2SO4 in 0.025 L, that means the
molarity of H2SO4 is 0.36 M.
Titration Practice Problem 2
• If it takes 30 mL of 0.05 M HCl to neutralize
345 mL of NaOH solution, what is the
concentration of the sodium hydroxide
solution?
• Step 1: Find the balanced equation.
– HCl + NaOH  NaCl + H2O
HCl + NaOH  NaCl + H2O
• Step 2: Find the moles of the known solution.
• 0.030 L of 0.05 M HCl = 0.0015 mol HCl
• Step 3: Use a mole ratio to find moles of
unknown solution.
• 1 mol HCl reacts with 1 mol NaOH.
• Therefore we are neutralizing 0.0015 mol NaOH.
• Step 4: Calculate the molarity of the unknown
solution.
• 0.0015 mol / .345 L = 0.0043 M NaOH
Organization
• One thing that really helps these problems is
to be organized.
• You’ll have to find the method that works for
you, but I’ll show you the one that works for
me.
HCl + NaOH  NaCl + H2O
KNOWN – HCl
• 30 mL = 0.03 L
• 0.05 M
UNKNOWN – NaOH
• 345 mL = 0.345 L
• ?M
• 0.0015 mol HCl
• 0.0015 mol NaOH
• 0.0043 M NaOH
Practice
• Titration Practice Problems
– #1
• Titration Practice Problems
– #2-5
• Acids and Bases Review Sheet
– #33
Titration Lab
• “My lab is…extra vinegary.”
Closure: Titration Joke
• Get it?
Closure: Titration Boss
• You use two burets for a titration.
• One measures out the unknown – a sample of HCl.
The initial buret reading is 5 mL and the final
reading is 19.2 mL.
• The other measures out the standard solution –
1.0 M NaOH. The initial buret reading is 2 mL. At
endpoint, the final buret reading is 14.8 mL.
• What is the concentration of the HCl?
Closure: Titration Boss
•
•
•
•
•
Equation: HCl + NaOH  NaCl + H2O
HCl volume = 19.2 mL – 5 mL = 14.2 mL
HCl concentration = ? M
NaOH volume = 14.8 mL – 2 mL = 12.8 mL
NaOH concentration = 1.0 M
• 0.0128 mol NaOH = 0.0128 mol HCl
• 0.0128 mol HCl in 0.0142 L = 0.90 M HCl
Closure
• Why is water neutral on the pH scale?
– When water dissociates, it forms an H+ ion,
making it acidic, and an OH- ion, making it basic.
– The protons and hydroxide ions cancel one
another out.