Today is Monday,June 8th, 2015

Pre-Class:Ever drank acid

before?

P.S. You need a paper towel.

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In This Lesson:Acids and Bases(Lesson 3 of 4)

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:

• Hydrosulfuric acid

H2SExample

Anion Suffix Acid Name

-ide Hydro___ic acid

-ate ___ic acid

-ite ___ous acid

2-

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 old-fashioned 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

Svante Arrhenius

Meh.

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 toArrhenius Acids and Bases

• Acids have these formulas:– HX (aq)– HaXbOc (aq)

• Bases are ionic compounds and contain either:– OH- (hydroxide)– CO3

2- (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:

http://www.denstoredanske.dk/@api/deki/files/3673/=268426.501.jpg

http://lowres-picturecabinet.com.s3-eu-west-1.amazonaws.com/38/main/83/423546.jpg

Thomas Martin LowryJohannes Brønsted

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+ + OH-

(the means the reaction can go either way)

H

Acid

Base

Conjugate Acid

Conjugate Base

Brønsted-Lowry Acids and Bases

HCl + H2O H3O+ + Cl-

H

Acid

Base

Conjugate Acid

Conjugate Base

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

pH pOH

[H+] [OH-]

1410

-pH -log[H

+] 10-p

OH

-log[O

H-]

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 non-conductor, 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 base-causing

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. Balanced equation.2. Concentration of known solution (usually given).3. Moles of known solution solute.4. Moles of unknown solution solute.5. 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

• 0.0015 mol HCl

UNKNOWN – NaOH• 345 mL = 0.345 L• ? M

• 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.