Acid and Bases

Acid and Bases

þ Produce H+ (as H3O+) ions in water (the hydronium

ion is a hydrogen ion attached to a water molecule)

þ Taste sour

þ Corrode metals

þ Electrolytes: release ions in solution

þ Neutralize with bases to form a salt and water

þ pH is less than 7

þ Turns blue litmus paper to red

Some Properties of Acids

Produce OH-(Hydroxide Ions) ions in water

Taste bitter, chalky

Are electrolytes

Feel slippery(forms soap w/ skin oils)

Neutralize with acids to form salts and water

pH is greater than 7

Turns red litmus paper to blue

Some Properties of Bases

HCl Hydrochloric Acid Stomach acidHNO3 Nitric Acid Jewelry makingH2SO4 Sulfuric Acid Paper makingH3PO4 Phosphoric Acid Preservative in Coca-

ColaCH3COOH Acetic Acid Vinegar

NaOH Sodium hydroxide lyeKOH Potassium hydroxide liquid soapBa(OH)2 Barium hydroxide stabilizer for plasticsMg(OH)2 Magnesium hydroxide “MOM” Milk of

magnesia Al(OH)3 Aluminum hydroxide Maalox (antacid)

Some Common Acids & Bases

Definition #1: Arrhenius (traditional)

Acids – produce H+ ions (or Hydronium ions H3O+)

Bases – produce OH- (Hydroxide) ions

(problem: some bases don’t have hydroxide ions!)

Acid/Base definitions

Arrhenius acid is a substance that produces H+ (H3O+) in water

Arrhenius base is a substance that produces OH- in water

Acid/Base DefinitionsDefinition #2: Brønsted –

Lowry

Acids – proton donor

Bases – proton acceptor

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

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

acidconjugate

basebase conjugate acid

ACID-BASE THEORIESThe Brønsted definition means

NH3 is a BASE in water — and water is itself an ACID

BaseAcidAcidBaseNH4

+ + OH-NH3 + H2O

Conjugate Pairs

Acids & Base Definitions

Lewis acid - a substance that accepts an electron pair

Lewis base - a substance that donates an electron pair

Definition #3 – Lewis

Formation of hydronium ion is also an excellent example.

Lewis Acids & Bases

• Electron pair of the new O-H bond originates on the Lewis base.

H H

H

BASE

••••••

O—HO—HH+

ACID

The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use pH

pH = - log [H+]Example: If [H+] =1E-10

pH = - log( 1E-10)pH = - (- 10)pH = 10

Example: If [H+] = 1.8E-5pH = - log 1.8E-5pH = - (- 4.74)pH = 4.74

Calculating the pH

Find the pH of these:1) A 0.15 M solution of Hydrochloric

acid (answer: .8239)2) A 3.00 X 10-7 M solution of Nitric acid (answer: 6.5229)

Try These!

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

10-pH = [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

pH calculations – Solving for H+

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

pH calculations – Solving for H+

pH = - log [H+]8.5 = - log [H+]-8.5 = log [H+]Antilog -8.5 = antilog (log [H+])10-8.5 = [H+]3.16E-9 = [H+]

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

pOH

More About WaterH2O can function as both an ACID and a

BASE.In pure water there can be

AUTOIONIZATIONH2O H30+ + OH-

Equilibrium constant for water = Kw

Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC

Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oCIn a neutral solution [H3O+] = [OH-]

so Kw = [H3O+]2 = [OH-]2

and so [H3O+] = [OH-] = 1.00 x 10-7 M

More About WaterOH-

H3O+

Autoionization

Kw = [H+1][OH-1] = 1E-14Used to find unknown [H+1] or [OH-1] if the

other is known or givenIf [H+1] = .0003M, find the [OH-1] 1E-14 = [.0003M] [OH-1] [OH-1]=3.33E-11M

Kw: the Constant of Water or How to find the missing #

The 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? (answer: 1.51E-5M)

The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?(answer: 7.3979)

[OH-]

[H+] pOH

pH

10 -pOH

10 -pH-Log[H+]

-Log[OH-]

14 - p

OH

14 - p

H

1.0 x

10-14

[OH

- ]

1.0 x

10-14

[H

+ ]

Calculating [H3O+], pH, [OH-], and pOHProblem 1: A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M. Calculate the [H3O+], pH, [OH-], and pOH of the two solutions at 25°C.

Problem 2: What is the [H3O+], [OH-], and pOH of a solution with pH = 3.67? Is this an acid, base, or neutral?

Problem 3: Problem #2 with pH = 8.05?

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

Strong and Weak Acids/Bases

The strength of an acid (or base) is determined by the amount of IONIZATION. STRONG=100% IONIZED

Weak acids are much less than 100% ionized in water.

One of the best known is acetic acid = CH3COOH

Weak Acids/Bases

Strong Base: 100% dissociated in water.

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

Strong Bases

Other common strong bases include KOH and Ca(OH)2.

CaO (lime) + H2O -->

Ca(OH)2 (slaked lime)CaO

Weak base: less than 100% ionized in water

One of the best known weak bases is ammonia

NH3 (aq) + H2O (l) NH4+ (aq) + OH-

(aq)

Weak Bases

Ionization Constants for Acids/Bases

Acids ConjugateBases

Increase strength

Increase strength

HONORS ONLY!

Equilibrium Constants for Weak Acids

Weak acid has Ka < 1 Leads to small [H3O+] and a pH of 2 - 7

HONORS ONLY!

Consider Acetic Acid(weak), HC2H3O2 (HOAc)Diss. Eq: HC2H3O2 + H2O H3O+ + C2H3O2

-

Equilibrium Expression:

Equilibria Involving Weak Acids and Bases

Ka [H3O+][OAc- ][HOAc]

1.8 x 10-5

(K is designated Ka for ACID)

K gives the ratio of ions (split up) to molecules (don’t split up)

HONORS ONLY!

Step 1: Diss. Eq: HC2H3O2 + H2O H3O+ + C2H3O2-

Step 2. Write Ka expression

Equilibria Involving A Weak AcidYou have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, H3O+, OAc-, and the pH.

Ka 1.8 x 10-5 = [H3O+][OAc- ][HOAc]

x2

1.00 - xThis is a quadratic. Solve using quadratic

formula.

or you can make an approximation if x is very small! (Rule of thumb: 10-5 or smaller is ok)

HONORS ONLY!

Step 3. Solve Ka expression

Equilibria Involving A Weak AcidYou have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, H3O+, OAc-, and the pH.

Ka 1.8 x 10-5 = [H3O+][OAc- ][HOAc]

x2

1.00 - x

First assume: X<<<1.00 because Ka is so small.

Ka 1.8 x 10-5 = x2

1.00

Now we can more easily solve this approximate expression without Quadratic Equation.

HONORS ONLY!

Step 4. Solve Ka approximate expression

Equilibria Involving A Weak AcidYou have 1.00 M HOAc. Calc. the equilibrium concs. of HOAc, H3O+, OAc-, and the pH.

Ka 1.8 x 10-5 = x2

1.00

x = [H3O+] = [OAc-] = 4.2 x 10-3 M

pH = - log [H3O+] = -log (4.2 x 10-3) = 2.37[HOAc] = 1.00 – X = 1.00 - 4.2E-3M = .9958M

HONORS ONLY!

Calculate the pH of a 0.0010 M solution of formic acid, HCO2H.

HCO2H + H2O H3O+ + HCO2-

Ka = 1.8 x 10-4 Approximate solution [H3O+] = 4.2 x 10-4 M, pH = 3.37Exact Solution [H3O+] = [HCO2

-] = 3.4 x 10-4 M [HCO2H] = 0.0010 - 3.4 x 10-4 = 0.0007 M pH = 3.47

Equilibria Involving A Weak AcidHONORS ONLY!

pH meterTests the voltage of

the electrolyteConverts the voltage

to pHVery cheap, accurateMust be calibrated

with a buffer solution

pH indicatorsIndicators are dyes that will change

color in the presence of an acid or base.

Indicators only work in a specific range of pH

Some dyes are natural, like radish skin or red cabbage

Phenolphthalein turns at pH 7.00

Setup for Titrating an acid with a base

Titration 1. Add solution of [known] from the buret with [unknown] in the flask.

2. Reagent (base) reacts with compound (acid) in solution in the flask.

3. Indicator shows when exact stoichiometric reaction has occurred. (moles Acid = moles Base)

4. This is called END POINT where NEUTRALIZATION has occurred (Acid=Base). The color turns pink if Phenolphthalein is the indicator used.

Titration Math(short version)

M1 • V1 = M2 • V2

Remember to add 4 drops Phenolphthalein to each sampleStep 1: Approximate Run. Use a stock base and a 25 ml

sample of the acid to get an idea of the [ACID]. Calculate approximate concentration of the acid. If needed, make a new base with a concentration the same as the acid.

Step 2: Using the new base, titrate a second 25ml sample of the acid trying to get within 1ml of accuracy. 25ml of acid should require about 25ml of base if they are close in concentration.

Step 3: Pin Point Run. Titrate a third 25ml sample of acid with the base that you made and try to get within 1 drop accuracy.

Report results to the Instructor.

Typical Titration is done in 3 Steps

 New Base Needed? How to make Base.

Titration using a pH MeterSet up the buret and the chemicals the same except no

Phenolphthalein is addedSet up the Nova with a pH sensor attached and insert the

probe into the Unknown solution. Proceed with the addition of the base solution until the graph looks like the one attached

The volume where the largejump takes place is the EndPoint, moles Base=moles AcidBegin calculations