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UNIT IV Strong and Weak Acids and Bases. W HAT IS A S TRONG A CID ? Strong Acid- An acid which is 100% ionized in a water solution. Ex.) HCl (g) + H 2.

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Presentation on theme: "UNIT IV Strong and Weak Acids and Bases. W HAT IS A S TRONG A CID ? Strong Acid- An acid which is 100% ionized in a water solution. Ex.) HCl (g) + H 2."— Presentation transcript:

1 UNIT IV Strong and Weak Acids and Bases

2 W HAT IS A S TRONG A CID ? Strong Acid- An acid which is 100% ionized in a water solution. Ex.) HCl (g) + H 2 O (l) H 3 O + (aq) + Cl - (aq) Single arrow (goes to completion) = Strong acid Ex.) What is the [HCl (g) ] in 1 M HCl? Ex.) What is [H 3 O + ] in 0.20 M HCl

3 W HAT IS A S TRONG A CID ? In a Strong Acid: [H 3 O + ] = [Acid] Ex.) What is [H 3 O + ] in 0.60 M HNO 3

4 W HAT IS A W EAK A CID ? Weak Acid: An acid which is less than 100% ionized in solution. (In Chem 12 WAs are usually significantly less than 100% ionized…usually < 5% ionized) In a solution of a weak acid, most of the molecules dont ionize. Ex.) HF (g) + H 2 O (l) H 3 O + (aq) + F - (aq) (Molecules) (Double arrow!) (Ions) id13.swf

5 S TRENGTH V ERSUS C ONCENTRATION Any acid (weak or strong) could have high or low concentration. Weak & Strong refers to % ionization Concentration the moles of acid dissolved per litre Examples: 10.0 M HCl conc. and strong [H 3 O + ] = 10.0 M M HCl dilute and strong [H 3 O + ] = M 10.0 M HF conc. and weak [H 3 O + ] = low M HF dilute and weak [H 3 O + ] = very low *Concentration of IONS determines CONDUCTIVITY!

6 T HE S TRONG A CIDS The strong acids are the top six acids listed on the Relative Strengths of Bronsted-Lowry Acids and Bases, namely: NameFormula Perchloric acidHClO 4 Hydriodic acidHI Hydrobromic acidHBr Hydrochloric acidHCl Nitric acidHNO 3 Sulphuric acidH 2 SO 4 (Note: H 2 SO 4 is only strong for the first dissociation: H 2 SO 4 H + + HSO 4 - )

7 T HE W EAK A CIDS All the acids listed on the LEFT side of the table in the white section are weak acids. H 3 O + H + + H 2 O net result of adding any strong acid to water HIO 3 H + + IO 3 - THE WEAK ACIDS H 2 O H + + OH - Note: The bottom two species on the left NEVER act as acids....SINGLE ARROW GOING BACKWARDS!

8 W HAT IS A S TRONG B ASE ? Strong Base- A base which is 100% ionized in a water solution. Ex. Ba(OH) 2(s) Ba 2+ (aq) + 2OH - (aq) Ex.) What is the [OH­ ­ - ] in 0.10 M NaOH? Ex.) What is the [OH­ - ] in 0.10 M Ba(OH) 2 ?

9 W HAT IS A S TRONG B ASE ? For a strong base: [OH - ] = [Base] x # of OHs in Formula

10 W HAT IS A W EAK B ASE ? Weak Base: A base which is less than 100% ionized in solution. Ex.) NH 3(aq) + H 2 O (l) NH 4 + (aq) + OH - (aq)

11 T HE S TRONG B ASES Any substance which dissociates completely to produce OH -, O 2- or NH 2 - is a Strong Base: Alkali Metal Hydroxides (Group 1) Ex. LiOH, NaOH, KOH, RbOH, CsOH Alkaline Earth Metal Hydroxides (Group 2) Ex. Ca(OH) 2, Ba(OH) 2, Sr(OH) 2 The hydroxide ion, OH -, found on the lower right side of the table, is the net result of adding any strong base to water.

12 T HE W EAK B ASES All the bases listed on the RIGHT side of the table in the white section are weak bases. H 2 O IO 3 -. THE WEAK BASES PO 4 3- H 2 O H + + OH - net result of adding any strong base to water

13 S PECTATORS The CONJUGATE of a STRONG ACID or a STRONG BASE is a SPECTATOR. These are the top 5 (not 6) bases on the right. CIO 4 - I - Br - They are so weak that they cannot react with H 2 O to form OH - Cl - NO 3 -

14 S PECTATORS H----Cl + H 2 O H 3 O + (aq) + Cl - (aq) weak So weak, it cannot take an H + from H 2 O or even H 3 O + Also, the alkali metal ions and alkaline earth metal ions are considered spectators since they are the conjugates of strong bases.

15 A MPHIPROTIC S PECIES are found on both sides of the table e.g.) HSO 4 - can act as acids (donate H + s) or as bases (accept H + s) to look at an amphiprotic species as an acid, you must find it on the left side: Example HCO 3 - is a ________er acid than C 6 H 5 OH HCO 3 - is a ________er acid than H 2 O 2

16 A MPHIPROTIC S PECIES to look at an amphiprotic species as a base, you must find it on the right side: Example HCO 3 - is a _________er base than C 6 H 5 O 7 3- HCO 3 - is a _________er base than Al(H 2 O) 5 (OH) 2+ HSO 4 - in shaded region on top right will not act as a base in water….but it is not a spectator (like NO 3 - is). Why not? (HSO 4 is also found on the left side quite a way up, it is a relatively strong weak acid.)

17 T HE L EVELLING E FFECT What is [H 3 O + ] in 1.0 M H 3 O + ? ______ What is [H 3 O + ] in 1.0 M HNO 3 ?______ What is [H 3 O + ] in 1.0 M HCl ?______ Acids from HClO 4 to H 2 SO 4 are 100% ionized in water So, even though HClO 4 is above HCl on the chart, it is no more acidic in a water solution.

18 T HE L EVELLING E FFECT H 3 O + is the strongest acid that can exist in undissociated form in water solution. All stronger acids ionize to form H 3 O +. The only way you can tell which strong acid is stronger is to react them in a non-aqueous (not H 2 O) solvent. Eg) HClO 4 + H 2 SO 4 H 3 SO ClO 4 - It is found that HClO 4 donates a proton to H 2 SO 4, not the other way around, so HClO 4 is a stronger acid than H 2 SO 4. This is not important in Chemistry 12. This would not happen in a water solution…in H 2 O, they would both form H 3 O +.

19 T HE L EVELLING E FFECT The strongest base which can exist in high concentrations in water solution is OH - The two stronger bases below it will react with water completely to form OH -. Eg) O 2- + H 2 O OH - + OH - OR…O 2- + H 2 O 2OH - Ex. What is the final [O 2- ] in 1.0 M Na 2 O ?

20 T HE L EVELLING E FFECT Write the equation for NH 2 - reacting with H 2 O. Hebden Textbook Pages Questions #21-27

21 A CID -B ASE E QUILIBRIA & R ELATIVE S TRENGTHS OF A CIDS & B ASES Example Mix some H 2 PO 4 - and some CO 3 2- H 2 PO CO 3 2- HCO HPO 4 2- (A) (B) (A) (B) At equilibrium, which will be favoured, reactants or products?

22 A CID -B ASE E QUILIBRIA & R ELATIVE S TRENGTHS OF A CIDS & B ASES Equilibrium favours the side with the weaker acid. Example Will HSO HCO 3 - H 2 CO 3 + SO 3 2- favour reactants or products?

23 A CID -B ASE E QUILIBRIA & R ELATIVE S TRENGTHS OF A CIDS & B ASES Mixing Two Amphiprotic Ions (products not given) Example Complete the reaction and determine which is favoured, reactants or products? (Also, compare the relative strengths of the bases.) HSO H 2 PO 4 -

24 A CID -B ASE E QUILIBRIA & R ELATIVE S TRENGTHS OF A CIDS & B ASES NOTICE: The STRONGER ACID is on the same side as the STRONGER BASE. (The stronger acid has the weaker conjugate base.) The WEAKER ACID is on the same side as the WEAKER BASE. Equilibrium favours the side with the weaker conjugate acid and the weaker conjugate base.

25 A CID -B ASE E QUILIBRIA & R ELATIVE S TRENGTHS OF A CIDS & B ASES Starting with Salts The amphiprotic ions are often products of the dissociation of salts. Spectator ions must be discarded. Example Complete the net ionic reaction and state whether equilibrium favours reactants or products NaHSO 3 + K 2 HPO 4

26 K EQ AND A CID S TRENGTH If products are favored Keq is large (>1) If reactants are favored Keq is small (<1) Ex) Given: HA + B - HB + A - Keq = Which acid is stronger, HA or HB? Hebden Textbook Page 133 Questions # 38-46


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