Lect w7 152_abbrev_ intro to acids and bases_alg

General Chemistry II CHEM 152 Unit 2 Week 7

Week 7 Reading Assignment Chapter 15 – Sections 15.2 (acids/bases), 15.3 (acids/bases), 15.4 (K a ), 15.5 (pH)

Equilibrium in Aqueous Solutions Acids and Bases The ideas we have discussed about chemical equilibrium are very useful to understand the behavior of acids and bases in water.

Water’s Role as Acid or Base Water acting as a Base HA + H 2 O  H 3 O + + A - base acid Water acting as an Acid B + H 2 O  BH + + OH - acid base Water behaves like a base when an acid is present. Water behaves like an acid when a base is present.

More About Water H 2 O can function as both an ACID and a BASE. In pure water there can be AUTOIONIZATION Equilibrium constant for autoionization = K w K w = [H 3 O + ] [OH - ] = 1 x 10 -14 (exact) at 25 o C H 2 O(l) + H 2 O(l)  H 3 O + (aq) + OH - (aq) H +

More About Water If K w = [H 3 O + ] [OH - ] = 1.00 x 10 -14 at 25 o C, what would be the concentration of H 3 O + and OH - in pure water? In a neutral solution [H 3 O + ] = [OH - ] and so [H 3 O + ] = [OH - ] = 1.00 x 10 -7 M Auto-ionization

Calculating [H 3 O + ] and [OH - ] Now, imagine that you add an acid or a base to pure water. What are the new concentrations of H 3 O + and OH - in the system? What happens if you add a strong acid? What if you add a strong base? The answer depends on the type or acid or base that you add, but in all cases: K w = [H 3 O + ] [OH - ] = 1 x 10 -14 (exact) at 25 o C

Calculating [H 3 O + ] and [OH - ] Imagine that you add 0.0010 moles of NaOH to 1.0 L of pure water. Calculate [H 3 O + ] and [OH - ] K w = [H 3 O + ] [OH - ] = 1 x 10 -14 [H 3 O + ] [0.0010 M] = 1 x 10 -14 NaOH = strong electrolyte (and base!) 0.0010 M NaOH  0.0010 M OH ¯ [H 3 O + ] = 1.0 x 10 -11 M

Calculating [H 3 O + ] and [OH - ] Now, do it yourself: Calculate the concentration of OH - and H 3 O + in a 6.0 M solution of nitric acid ( HNO 3 ). Analyze the results ( How acidic is this solution? How do you know )

Acidity and Basicity A solution is considered acidic when [H 3 O + ] > [OH - ] A solution is considered basic when [H 3 O + ] < [OH - ] A solution is neutral when [H 3 O + ] = [OH - ]

A common way to express acidity and basicity is with pH pH = - log [H 3 O + ] or [H 3 O + ]=10 -pH Acidity and Basicity The # of sig figs in the concentration = the number of decimal places in pH In an acidic solution, [H 3 O + ]= 1.25 x 10 -4 M at 25 o C pH =-log (1.25 x 10 -4 )= -(-3.903) = 3.903

What is the pH of Black Coffee, [H 3 O + ]= 1.0 x 10 -5 M?

What is the H 3 O + concentration in sea water if pH=8.30?

What is the OH ¯ concentration in acid rain if pH=5.25?

[H 3 O + ], [OH - ] and pH General conclusion @25ºC Basic solution pH > 7 Neutral pH = 7 Acidic solution pH < 7

Other pX Scales In general pX = -log X and so pOH = - log [OH - ] Which solution is more basic, one that has a pH of 5.5 or one with a pOH of 8.5? If K w = [H 3 O + ] [OH - ] = 1 x 10 -14 Taking the log of both sides -log (10 -14 ) = - log [H 3 O + ] + (-log [OH - ]) pK w = 14 = pH + pOH

Methods for Measuring the pH of an Aqueous Solution pH meter pH (indicator) paper

According with this theory: ACIDS DONATE H + IONS (hydrogen ion donors) HA(aq) + H 2 O(l)  A - (aq) + H 3 O + (aq) BASES ACCEPT H + IONS (hydrogen ion acceptors) A - (aq) + H 2 O(l)  HA(aq) + OH - (aq) Br Ø nsted – Lowry Theory H + H +

Strong or Weak Generally, we divide acids and bases into STRONG or WEAK ones STRONG ACID: HNO 3 (aq) + H 2 O(l)  H 3 O + (aq) + NO 3 - (aq) HNO 3 (nitric acid) is about 100% dissociated in water. H + The equilibrium constant  is much larger than 1.

Strong Acid? HA(aq) + H 2 O(l)  H 3 O + (aq) + A - (aq) Write K a for this acid and comment on its value?

HNO 3 , HCl, HBr, HI, H 2 SO 4 and HClO 4 are strong acids Strong Acids The H 3 O + ion forms strong hydrogen bonds with surrounding water molecules

Weak Acids Weak acids donate a small fraction of their H + ’s most of the weak acid molecules do not donate H + to water Roughly 1% ionized in water [H 3 O + ] << [HF] HF  H + + F - HF + H 2 O  H 3 O + + F -

Weak acids are much less than 100% ionized in water. Weak Acids HA + H 2 O  H 3 O + + A - H +

Strong Base: 100% dissociated in water NaOH(aq)  Na + (aq) + OH - (aq) Strong Bases Solubility rules – all soluble hydroxides. Other common strong bases include KOH and Ca(OH) 2 . Ionic compounds that that add so much OH ¯ as to overwhelm the balance of H 3 O + and OH ¯

Weak Bases Weak bases are molecules that have an affinity to ACCEPT an H + Weak base: only a small percentage ionized in water One of the best known weak bases is ammonia NH 3 (aq) + H 2 O(liq)  NH 4 + (aq) + OH - (aq) H + How would you express K b for this reaction?

Conjugate Acid-Base Pairs NH 4 + / NH 3 is a conjugate acid-base pair — related by the gain or loss of H + Every acid has a conjugate base and vice-versa. Consider the acid-base reaction: H + H + Acid

The Conjugate Pairs in Some Acid-Base Reactions Conjugate Pair Conjugate Pair Conjugate Acid-Base Pairs HPO 4 2- SO 3 2- + PO 4 3- HSO 3 - + Identify the acid, the base, and the conjugate acid/base pairs: Base Acid + Acid Base +

Identify the acid and its conjugate base in: HF + H 2 O  F ¯ + H 3 O + HF, F ¯ H 2 O, H 3 O + HF, H 3 O + H 2 O, F ¯ HF, H 2 O

Identify the base and its conjugate acid in: CN ¯ + HCOOH  HCOO ¯ + HCN CN ¯ , HCOO ¯ HCOOH, HCN CN ¯ , HCN HCOOH, CN ¯ HCOOH, HCOO ¯

Identify the conjugate acid-base pairs: H 2 PO 4 ¯ + OH ¯  H 2 O + HPO 4 2- OH ¯ , H 2 O OH ¯ , HPO 4 2- H 2 PO 4 ¯ , OH ¯ H 2 PO 4 ¯ , H 2 O H 2 PO 4 ¯ , HPO 4 2-

Identify the conjugate acid-base pairs: N 2 H 5 + + H 2 SO 4  HSO 4 ¯ + N 2 H 6 2+ N 2 H 5 + , H 2 SO 4 H 2 SO 4 , HSO 4 ¯ H 2 SO 4 , N 2 H 6 2+ N 2 H 5 + , HSO 4 ¯ N 2 H 5 + , N 2 H 6 2+

Relative Strengths of Acid and Bases As acid strength decreases, base strength increases; the weaker the acid, the stronger its conjugate base. As base strength decreases, acid strength increases; the weaker the base, the stronger its conjugate acid.

Relative Strengths of Acid and Bases The stronger acid and the stronger base will always react to form a weaker conjugate base and a weaker conjugate acid. Would you expect these acids and bases to react? What would be the outcome? CN - (aq) + HCl(aq)  HF(aq) + Cl - (aq)  H 2 O(l) + S 2- (aq) 

For the reaction: CN ¯ + HCl  Cl ¯ + HCN K is large K is ~1 K is small

For the reaction: Cl ¯ + HF  F ¯ + HCl K is large K is ~1 K is small

For the reaction: S 2- + H 2 O  HS ¯ + OH ¯ K is large K is ~1 K is small

Summary Activity Label the following as strong acids, weak acids, strong bases, weak bases: HCl HNO 2 (CH 3 ) 2 NH HCOOH Ca(OH) 2

2. What is the [H 3 O + ] of a 0.0050 M Ca(OH) 2 solution?

3. What is the pOH of a 0.0028 M HNO 3 solution?

Lect w7 152_abbrev_ intro to acids and bases_alg

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