Chem 1220 Unit 2

Arrhenius def of Acids and Bases

Acids: makes H+ ions

Bases: makes OH-

Only works in WATER

narrowest definition

Brønsted–Lowry def of Acids and Bases

Acids: donates proton H+

Bases: accepts proton H+

Doesn’t have to be water

medium scope

Lewis def of Acids and Bases

Acids: accepts an electron pair to form a new bond

Bases: donates an electron pair

H+ ions not required

broadest definition

Amphoteric substances

can act as an acid or a base

amphiprotic substances

acts as an acid or a base, doing so by donating or accepting a proton (H+)

All amphiprotic substances are amphoteric, but not all amphoteric substances are amphiprotic

The value of K_w changes with ___________.

temperature

The ‘normal’ pH scale only applies at ___K

298K

Six common strong acids

HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄

Strong Bases

LiOH, NaOH, KOH, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂

Weak acids do what?

partially dissociate

Weak bases so what?

They DO NOT dissociate, they react with water

Conjugate acid-base pairs have inversely proportional strength:

the stronger the acid, the weaker its conjugate base

As the H⁺ concentration goes down,

the pH goes up

Bond polarity

The difference in electronegativity between H and the other element. periodic trend for electronegativity across a period increases from left to right (excepting the noble gases. Within the same period, acid strength increases with increasing bond polarity or increasing electronegativity difference.

For binary acids, we have two considerations for those trends:

Bond polarity & bond strength

Bond strength

where the strength of the H-X bond decreases as we go down a group. Then, within the same group, acid strength increases with decreasing bond strength

Ternary acids and bases, or compounds with a bonding pattern E-O-H

The identity of E will determine whether such a compound, when combined with water, acts as a Brønsted–Lowry acid, a Brønsted–Lowry base, or both.

E-O-H When E is a Group 1 or 2 metal,

The compound is a strong base

E-O-H When E is a hydrocarbon like CH₃ or CH₃CH₂

we have a compound called an alcohol

most alcohols are about as acidic as water, which also means that they are about as basic as water (pH 7)

If E is something more electronegative than C -- especially if it is Cl, Br, I -- then we'll have a Brønsted–Lowry ____.

acid

If we attach some electronegative atoms to E, we'll get progressively stronger acids. 

inductive effect

The relationship between greater acidity and a greater number of highly electronegative atoms (or atoms with higher electronegativity values)

to figure out whether an aqueous solution of a salt is acidic, basic, or pH-neutral

Begin by splitting the compound into its cation and anion; then, evaluate how each ion behaves in aqueous solution with respect to Bronsted-Lowry acid-base chemistry

Anions:

• Conjugates of weak acids tend to be weak bases.  For example, CH₃CO₂⁻ is the conjugate of the weak acid CH₃CO₂H, and CH₃CO₂⁻ acts as a weak base.  

• Conjugates of strong acids are so weak that they effectively do not act as weak bases; instead, they are spectators.  For example, NO₃⁻ is the conjugate of the strong acid HNO₃, and NO₃⁻ acts as a spectator.  Other spectators that are conjugates of strong acids: Cl⁻, Br⁻, I⁻, ClO₃⁻, ClO₄⁻.

There are some noteworthy exceptions to the rules above, and all of them have to do with polyprotic acids. 

• H₂SO₄ is a strong acid, and only the first dissociation is strong.  The conjugate of H₂SO₄ is HSO₄⁻, and that acts as a weak acid.

• H₂CO₃ is an example of a weak polyprotic acid.  The conjugate of H₂CO₃ is HCO₃⁻, and HCO₃⁻ can act as both a weak acid and a weak base.

Cations:

• Counterions of strong bases are spectators with zero acid-base chemistry.  These include Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ca²⁺, Sr²⁺, Ba²⁺.

• Conjugate acids of weak bases are acidic.  These include cations like NH₄⁺, CH₃NH₃⁺, (CH₃)₃NH⁺.

• Metal cations aside from the Groups 1 and 2 cations are weak acids.  It may seem strange that Fe³⁺(aq) is acidic, but aqueous iron(III) is better written as Fe(H₂O)₆³⁺, and one of the waters loses an acidic hydrogen:

  • Fe(H₂O)₆³⁺(aq)  ⇌  Fe(H₂O)₅(OH)²⁺(aq) + H⁺(aq)

The higher the oxidation number,

the stronger the acid

The higher the Δelectronegativity

the lower the PK_a

A stronger acid, has a ______ conjugate base

weaker

A stronger base, has a ________ conjugate acid

weaker

Stronger acids have _____ pK_a values

lower

Weaker acids have ______ pK_a values

higher

pK_{a =}

-logK_a

coordinate covalent bond

Both electrons in a bond are from one element

As you move down a group on the P-table, the elements get ____ acidic

more

buffer components appear on ________ sides of the arrow.  Always.

opposite

Half equivalence point

pH = pK_a

w/ a weak acid titration, the first part is a ______

buffer; solve for pH with the Henderson-Hasselbalch equation

pK_a of the strongest weak acid?

has the lowest pK_a

Strong Acids (how to tell chemically)

#O - #H > or = to 2

(weak acids are 1 or less)

Acid strength on P table

when H is with another element, the strength increases as you go down to the right

Higher K_a, means a ________ ____.

stronger acid

High K_a = ___ pK_a

low