Chemistry SM-1232Week 8 Lesson 2

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademySpring 2008

Class Today• Today Chapter 14: Acids and Bases• Chapter 14 Quiz Monday!• Quiz Answers Posted Monday Night! • NO MAKEUPS!!!• Chapter 13 and 14 Test Wednesday.• We will have class next FRIDAY MAY 1st• Wiki Project next month! Details to follow soon!

Concentration

• Another term for molarity is concentration.• You’ve used this before with oranje juice from

concentrate. It’s comes in a concentrated form and then you have to dilute it down.

• Concentration is written by putting a molecule in brackets like this [HCl], which would mean the concentration of HCl.

• A solution that is 1.0M in HCl can be written like [HCl] = 1.0M

Strong Acids

• Strong acids fully dissociate in water. That means water tears every molecule of the acid into H+ ions and base – ions.

• HCl H+ + Cl-

• Good electrolyte, meaning strong acid solutions conduct electricity since so many charges are in solution.

Mono or multiprotic

• Some acids only release 1 proton (H+ ion). Others release more than one.

• HCl is monoprotic• Sulfuric acid H2SO4 is diprotic

• Phosphoric acid H3PO4 is triprotic

Weak Acids• Weak acids do not completely ionize• HF is a weak acid• In solution it become H+ and F- when it ionizes, but

there are strong electrostatic attractions F- and H+ so they come back together.

• HF + H2O H3O+ + F-

• As a consequence weak acids are poor electrolytes and electricity is not conducted through their solutions well.

Tug of War

• The solvent pulls the charges apart and dissolves them, but the oppositely charged particles are attracted to each other.

• Generically speaking • HA + H2O H3O+ + A-

• If the acid is strong the products are favored and the reactants are barely present. If the acid is weak the reactants are favored and the products are barely present.

Rule of thumb

• HA + H2O H3O + A-• HA is the acid• A- is the conjugate base

• The stronger the acid the weaker the conjugate base.

Strong Bases

• A strong base completely dissociates in solution into the ions that make it up.

• NaOH Na+ + OH-

• A 1M solution of NaOH will have [Na+]= 1M and [OH-]=1M.

• Strong bases make good electrolytes.• For the purpose of this class any hydroxide is a

strong base.

Weak Bases

• Weak bases do not fully dissociate in water. Most weak bases do not have a hydroxide ion as part of them.

• In order to act in a basic manner the weak base reacts with water to steal a proton, and that interaction creates the OH- ion.

• NH3 + H2O NH4+ + OH-

Weak Bases

• Generically speaking• B + H2O BH+ + OH-

• B= base• BH+= conjugate acid

• The stronger the base the weaker the conjugate acid.

• The book has a list of common weak bases on page 505

Time to hurt you!

• Water is amophoteric. It can act like a base or an acid.

• HCl + H2O H3O+ + Cl-

• NH3 + H2O NH4+ + OH-

• Water can act like it’s own acid and base• H2O + H2O H3O+ + OH-

Water’s Ion Product Constant

• H2O + H2O H3O+ + OH-

• Scientists have measured how much this happens at 25C in pure water.

• The concentration (aka molarity) of each ion is 1.0e-7M. Aka [H3O+]=[OH-]=1.0e-7

• If you multiply the concentration of [H3O+]X[OH-] you get 1.0e-14.

• This is considered the ion product constant for water (Kw).

Kw

• Kw= [H3O+]X[OH-]• Kw= 1.0e-7 X 1.0e-7• Kw= 1.0e-14

Quick Math Break

• Answer the following • 1.0e6 X 1.0e7=• 1.0e 6 / 1.0e12=• 1.0e6 X 4.5=• 1.0e6 / 3.2= 3.1e5• 1.0 e-13/ 1.0 e-4 =

How does this help us?

• It becomes a 3 variable problem!• Kw=[H3O+] X [OH-]• We know Kw and if we know H3O+ or OH- we

can solve for the other one!

• Work this one out. If [H3O+]=1e-3 then what must the [OH-] be equal to?

Solution

• Kw=[H3O+] X [OH-]

• Kw/[H3O+] = [OH-]

• 1e-14/1e-3 = [OH-]• 1e-11 = [OH-]

Acidic or basic?

• In the previous example you found that the acid concentration was 1e-3 and the base concentration was 1e-11.

• Is there more acid or base in solution? So, is the solution acidic or basic?

• It’s acidic! If you don’t see why write out the number 1e-3 and write out the number 1e-11.

Handy Reference

• In acidic solutions [H3O+] > [OH-]

• In neutral solutions [H3O+] = [OH-]

• In basic solutions [OH-] > [H3O+]

Work through this

• You drop acid into water such that the [H+]=6.44e-2. What is [OH-]?

pH: More brain pain!

• There is another scale chemists use to determine how acidic or basic something is.

• It’s called the pH scale.• It’s based on how many H+ ions are in solution.• Things that are acid have pH ranges from 1-7.• Neutral solutions have pH 7.• Basic solutions are 7-14.

It’s a relative scale

• A pH of 1 is very acidic. This is like pure sulfuric acid or pure phosphoric acid.

• pH of 3-5 is like fruit that is slightly acidic.• pH of 7-8 is approxiate to eggs• pH of 9-10 are many dish soaps• pH of 14 is like Liquid Plumber or highly

concentrated basic solutions

pH and Logarithms

• The pH scale is logarithmic. This means every time we step up from 1 to 2 or 3 to 4 it actually means the concentration went up by a factor of ten.

• When you and I count 10, 100, 1,000, 10,000 the logarithmic scale would count 1, 2, 3, 4.

pH scale

• P mean the –log• H is talking about [H+] which is the same as

[H3O+]

• So…• pH= -log[H3O+]

Another way to look at logs

• Log 101 =1 log102=2 log103=3 log104=4

• Log10-1 = -1 log10-2=-2 log10-3=-3

• -Log10-1=1 –log10-2=2 –log10-3=3

Example

• You’re [H+]=1.39e-5. What’s the pH? Is it acidic or basic?

• You’re [H+]=3.82e-12. What’s the pH? Is it acidic or basic?

Calculating [H3O+] from pH

• If the pH is 4.8 what is the [H3O+]• pH= -log [H3O+]• 4.8=-log [H3O+]• -4.8= log [H3O+]• 10-4.8= 10 log [H3O+]

• 10-4.8=[H3O+]• 1.6e-5=[H3O+]• Or 10 raised to the negative pH= [H3O+]

pOH scale

• It’s basically (get it) the same as the pH scale, but now we’re measuring the amount of [OH-] in solution.

• pOH=-log[OH-]

• If the concentration of [OH-] = 1.88e-13 what is the is the pOH. Is it acidic or basic?

[OH-] from p[OH]

• 10 raised to the negative p[OH] = [OH-].

• What is the [OH-] if the p[OH] is 12.3?

pH and pOH

• pH+pOH=14• So, if you know pH you can figure out pOH

quickly.

Work

• Chapter 14 HW due Monday.• Quiz on Chapter 14 Monday.• Quiz answers on wiki Monday night.• Test on Chapter 13 and 14 Wednesday April

29th. We will have class Friday May 1st to start chapter 15.