Chap 18 AcidsBases

1
Chap 18Acids/Bases

• Acid/base Theories
• Strong vs Weak
• Salts, Anhydrides,Buffers

2
Essential Questions

• 1. What are acids and bases(use theories)
• 2. What is the difference between strong and
  weak acids and bases
• 3. How do acids react with bases

3
How do you define an acid

• Use one of the acid base theories

4
What are the 3 theories to describe Acids and
bases

• Arrhenius,
• Bronsted-Lowry,
• Lewis

5
How do you describe an acid with The Arrhenius
theory

• Produce H
• Turn Litmus red
• Taste Sour
• React with metals to produce H2
• React w/ bases to produce a salt and
  water(neutralization)

6
How about a base

• Produce OH-
• Turn Litmus Blue
• Taste Bitter
• Slippery feel
• Neutralize Acids

7
Why is this theory called an operational
Definition

• It is a definition that can be tested in lab
• What is the problem with this definition
• It is too limited

8
What is the Bronsted-Lowry def of an acid

• Donate H ions(proton donor)
• What is the Bronsted-Lowry def of a base
• Accept H(proton acceptor)

9
This is called a conceptual def, why

• It is a statement that cant be proven in lab

10
What is a special component of this theory

• The theory is composed of conjugate acid-base
  pairs
• When an acid donates its H, the specie left is
  called its conjugate base. When a base accepts a
  proton, what is formed is its conjugate acid

11
Show me

• HCl H2O ? Cl- H3Oacid
  base conjugate conjugate base
  acid

12
What is the conjugate base of each of the
following

• To be a base it must be able to accept a proton
  (each of these must lose a proton to be able to
  accept one)
• (conj base is what is left after the acid donates
  its proton)
• (conj base acid minus a H

H3PO4
H2PO4-1
HCO3-1
CO3-2
NH4
NH3
13
What is the conjugate acid for

• Conjugate acid is what is formed after the base
  accepts its proton(conj acid base plus H)

H2O
H3O
HCO3-1
H2CO3
OH-1
H2O
14
Why do some compounds accept protons in one case
and then donate one in another

• These substances are amphoteric (amphiprotic)
• You will recognize them as substances that have
  multiple Hs in their standard form and will be
  at least one short. (They must be able to gain
  and lose

15
Show me

• H2CO3 std form
• HCO3-1 is missing an H so it could gain one (act
  like a base.)
• It also has one left so it could lose it(act
  like an acid).
• It has the capacity to both gain and lose a
  proton.
• Any other substance with similar properties
  will be amphoteric
• H3O ?H2O ? OH-1 so water is amphoteric

16
Why is this theory the predominate one

• It is broad enough to be used in our basic chem

17
What is the Lewis theory

• An acid is an electron pair acceptorA base is an
  electron pair donator

18
What is special about this theory

• It does not require water

19
Do acids and bases vary in their strength

• Yes
• There are strong acids(bases) and weak
  acids(bases)

20
How are they differentiated

• Strong are 100 dissociated in water
• Weak are less than 100
• (Acids ionize and bases dissociate(acids are
  molecular while bases are ionic)

21
How can you tell by looking

• MEMORIZE !
• strong acidsHCl, HNO3, H2SO4, HClO3, HClO4
  HBr, HBrO3, HI,
• The strong bases are Group I and Ca Ba
  hydroxides (LiOH, NaOH, KOH, RbOH,CsOH Ca(OH)2,
  Ba(OH)2)
• All others are weak

22
How can you tell cont.(Using a different theory)

• Bronsted-Lowry says a stronger acid will donate
  a proton to a weaker acid. So all is relative.
• A strong acid has a weak conjugate base A weak
  acid has a strong conjugate base, etc
• Another way to look at it is Strong acids have
  VERY large Ks

23
What is Kw

• The ionization constant for water.
• Kw 1x 10-14

24
What is its significance

• In any soln that has water the concentration of
  the Hydrogen (hydronium) ions and the hydroxide
  ions can never be greater than 1 x 10-14
• In fact they will always equal 1 x 10-14

25
How does it look

• Kw H OH-
• (Kw H3O OH-)
• H3O hydronium ion

26
Whats this hydronium ion

• Scientists have discovered that H ions
  cant(dont) exist by themselves in water.
  Because of the nature of water and the single
  proton in H, a bond forms between H and H2O
  making H3O (I will use H and H3O
  interchangeably)

27
What is its use

• In any soln, we can find the amount of each of
  these ions
• If the H (H3O) is larger than OH- the soln
  is acidic
• If the H (H3O) is smaller than OH- the soln
  is basic
• If the H (H3O) OH- the soln is neutral

28
How do we use this info

• pH is used as a way to describe the H (H3O)
  in a soln

29
What is pH

• the negative logarithm of the hydrogen ion in
  mole/liter
• pH -logH pH -logH3O

30
What is the range of the scale

• 0-14
• 7 being neutral
• lt7 acidic
• gt7 basic

31
What is pOH

• -logOH-
• So p
• negative log

32
How do you use this and Kw

• pH pOH 14

33
How many different categories of substances are
there in acid base theory

• acids,
• bases,
• salts,
• and anhydrides

34
How do you recognize each

• acids contain H
• bases OH
• salts anion from acid cation from base
• the salt NaCl is 1/2 acid and 1/2 baseHCl Cl-1
  NaOH Na1

35
Recognize cont.

• anhydride substance that is not an acid or base
  but becomes one when dissolved in water.
• acid anhydrides are nonmetal oxides SO2
• basic anhydrides are metal oxides Na2O

36
Is there anything else special about salts

• Since there are composed of parts of strong and
  weak acids and bases, these have strong and weak
  characteristics

37
What?

• Some salts undergo hydrolysis.
• A process in which a substance reacts with the
  water it is dissolving in.

38
How can you recognize this

• Remember that weak acids would rather be
  molecules ions. If a salt has a weak acid(base)
  portion, it will reactwith the water and turn
  into that weak acid(base)

39
Show me

• a salt is the offspring of two parents, one acid
  and one base.(neutralization reaction)
• NaCl comes from sodium hydroxide and
  hydrochloric acid HCl NaOH ? NaCl H2O
• Since these are both strong they exist 100 as
  ions and do not reach equilibrium. (not affected
  by the water.) However,

40
Show me cont.

• Na2CO3 comes from sodium hydroxide (strong) and
  carbonic acid (weak).
• Because the CO3-2 ions are weak they will reach
  equilibrium. They will find water(HOH) and grab
  the H from it to make HCO3-1. This leaves OH-1
  behind. And now the soln is basic.
• Thus a salt not an acid(base) can become
  acidic(basic) when dissolved in water

41
Whats the bottom line

• Salts are
• strong acid - strong base pH 7
• strong acid - weak base pH lt7
• weak acid - strong base pH gt7
• weak acid - weak base pH ?

42
How do I use this info to solve a problem with
salts

• 1. Determine that the substance is a salt (see
  previous slide)
• 2. Take the weak part of the salt and write the
  equation with it and water
• 3. Write the K expression
• 4. Build a Chart

43
How do I use this info to solve a problem with
salts contd

• 5. Since I do not know the concentration of the
  products let x the moles/liter of the
  acid/base that form
• 6. Use the coefficients in the balanced equation
  to determine the values for the other substances
• 7. Substitute and solve

44
Show me

• Determine the pH of a 0.02 M soln of NaF Ka
  3.4 x 10-7.1. NaF is a salt. The parents are
  NaOH and HF. HF is the weak one so
• 2. F- H2O ? HF OH-
• 3. KHKw HF OH- Ka F-

45
Show me

• Let x Moles/liter HF that form

46
Show me

• Substitute and solve
• KH 1x10-14 xx 3.4x10-7
  0.02-x
• 2.9x10-8 x2 .02-x
• 5.9x10-10 2.9x10-8x x201x2 2.9x10-8x
  5.9x10-10
• x2.4x10-5

47
Show me

• Substitute and solve
• x2.4x10-5 OH-
• H 1x10-14/2.4x10-5
• H 4.2x10-10
• pH 9.4

48
Show me

• Determine the pH of a 0.1 M soln of NH4Cl Kb
  1.8 x 10-5.1. NH4Cl is a salt. The parents are
  NH4OH and HCl. NH4OH is the weak one so
• 2. NH4 H2O ? NH3 H3O
• 3. KHKw NH3 H Kb NH4

49
Show me

• Let x Moles/liter HF that form

50
Show me

• Substitute and solve
• KH 1x10-14 xx 1.8x10-5
  0.1-x
• 5.6x10-10 x2 .1-x
• 5.6x10-11 5.6x10-10x x201x2 5.6x10-10x
  5.6x10-11
• x2.4x10-5

51
Show me

• Substitute and solve
• x2.4x10-5 H
• pH -log(2.4x10-5 )
• 4.6 pH

52
What are buffers

• solns that are resistant to change in pH they
  have a capacity to accept acid or base and not
  change the concentration of the ions very much.

53
How do I recognize a buffer

• It is a mixture of a weak acid and its conjugate
  base ( an acid and a salt that contains that weak
  acid)
• or a weak base and its conjugate acid(a base and
  a salt that contains that weak base)
• NaC2H3O2 and HC2H3O2
• CuOH and CuCl

54
How does it work?

• A mixture NaC2H3O2 and HC2H3O2
• sodium acetate is soluble and in solution will
  exist 100 as Na and C2H3O2-1 so there will be a
  lot of these ions present in the solution.

55
How does it work contd?

• HC2H3O2 being weak will only dissociate a small
  amount so there will be a lot of HC2H3O2
  molecules and not many ions
• since these are mixed there will be a large
  amount of HC2H3O2 and Na and C2H3O2-1

56
How does it work contd?

• Now lets add some acid (H) there are a lot of
  C2H3O2-1 react with so the H added are
  removed.(No change in pH).
• Lets add some base (OH-) there is a lot of
  HC2H3O2 that can ionize into H to react with the
  OH- (no change in pH)