The Strengths of Acids and Bases

The Strengths of Acids and Bases References: Chemistry, Nelson, pages 503-551 Heath Chemistry, pages 570-590 McGraw-Hill Ryerson, Chapters 14,15

Acids and Bases outcomes • Refer to your outcomes handout • #11 Compare strong and weak acids and bases using the concept of equilibrium • #12 Understand that acid and base systems are quantitatively described using pH, pOH, [H3O+], [OH-], Kw, Ka, Kb and concentration • #13 perform calculations from any of the above from empirical (lab) data.

What properties can be used to determine if a solution is acid or base? • Tasks • 1. Refer to table 14.4, page 503 to refresh your memory about properties!! • 2. From the lab data, put the household acids in order form least acidic to most acidic. • 3. Do Lab Exercise 14D, Strengths of Acids,page 503.

Strong and Weak Acids • Acidic solutions of different substances at the same concentration do not possess acid properties to the same degree. • Recall Rates of Reaction Lab

What did the demonstration show ? • Refer to Figure 14.4, page 504 for a recap. • Reflection: Think back to the Chemical Reactions lab. Which reacted faster with marble, 6M acetic acid or 6M HCl? • Weak Acid characteristics: If characteristic properties are less than those of a common strong acid like HCl; weak electrolyte; reacts at a slower rate, pH closer to 7 • Most common acids fit this category. (Fig. 14.15)

Strong Acids • According to the revised Arrhenius theory, • HCl(aq) + H2O(l) ---> H3O+(aq) + Cl- (aq) • The H+ ion is a tiny proton that does not remain in solution, but immediately unites with a water molecule to make H3O+. • As a strong acid, when HCl reacts with water, no HCl molecules are left in solution. HCl ionizes completely. • From the table of Relative strengths of Acids and Bases on page 611, list the acids that ionize 100%

Weak Acids • When acetic acid reacts with water it only partially ionizes (1.3% see page 505 and 611) This reaction can be described as an equilibrium system • CH3COOH(aq) + H2O(l) ---> H3O+(aq) + CH3COO- (aq) • At equlibrium, a considerable amount of CH3COOH molecules remain. There is a small proportion of hydronium and acetate ions. • Weak conductor ---> weak acid.

The Table of Relative Strengths of Acids and Bases • Page 611. Also last page of booklet. • To determine the ranking: • The pH of 0.10 mol/L solutions are measured and the [H3O+] is calculated, the % ionization is determined and the solutions are ranked in order of decreasing strengths • See page 505 for example calculations

Comparing Strengths of Acids • To use the table on pg. 611, or any other like it, remember: • 1. A base is a proton acceptor, an acid is a proton donor • 2. A strong base readily accepts protons • 3. A weak base does not accept protons readily

An example. • When sodium hydrogen carbonate and hydrofluoric acid are combined in aqueous solution, what reaction is likely to occur? • Sodium ions are spectators…… • HCO3- + HF • HCO3- is a stronger base than HF and HF is the stronger acid, so the proton transfer occurs from HF to HCO3- • HCO3- + HF H2CO3 + F-

Weak Acid Equilibrium and Ka • Refer pages 506-507 • CH3COOH ionizes in water to the extent of 1.3% in a 1.0 mol/L solution. The equilibrium constant can also be used to express the extent of ion formation. • Equilibrium Law can be used to calculate the equilibrium constant, known as the acid ionization constant Ka

Ka= [H3O+] [CH3COO-] [CH3COOH] The table on page 611 shows Ka for Acetic acid to be 1.8 x 10-5 mol/L The Ka value can be used over a range of concentrations of an acid to predict the [H3O+]and the pH.

CH3COOH(aq) + H2O(l) ---> H3O+(aq) + CH3COO- (aq) • According to the balanced equation, • [CH3COO-] = [H3O+] 1:1 ratio of moles x = x Ka = x2 [CH3COOH] Perform the calculation and report the value of [H3O+]

pH calculation • Recall, pH = - log [H3O+] • pH = - log 4.2 X 10-3 • pH = 2.37 • Examples: What pH results when 0.25 mol acetic acid dissolves in water to make one litre of solution? • A 0.24 M solution of carbonic acid has a pH of 3.49. Determine the Ka of the weak acid.

According to Arrhenius,a base is a substance that increases the concentration of the hydroxide ion in solution. Ionic hydroxides are all strong bases - they dissociate100% into the metallic cation and the hydroxide ion. Which is more basic, sodium hydroxide or barium hydroxide? Explain Strong Bases (pg. 507)

Qualitative Analysis and reviewing the concepts!! • Do Lab Exercise 14 E on page 507 • Solution 1 is _______ • Solution 2 is _______ • Solution 3 is _______ • Solution 4 is ________ • Solution 5 is ________ • Solution 6 is _________ • Solution 7 is _________

Weak Bases(pg. 510) • Arrhenius - bases are soluble ionic hydroxides • LeChatelier - hydroxide ions added to water cause a shift in the water ionization equilibrium decreasing the hydronium ion concentration and producing a pH greater the 7. • Some molecular and ionic compounds other than hydroxides, also dissolve in water to produce basic solutions. These are not as basic as the ionic hydroxides and are called weak bases.

Kb, base ionization constant • Weak bases react non-quantitatively with water to form an equilibrium that includes aqueous hydroxide ions • Page 510 • Base + H2O(l) OH-(aq) + conjugate acid • Ammonia is an example. Write the ionization equation for ammonia. • NH3(aq) + H2O(l) OH-(aq) + NH4(aq)

Example calculations • The format is very similar to the calculations involving Ka • The pH of a 0.100 mol/L sodium carbonate solution is 11.66. What is the Kb for the carbonate ion? • The algorithm for solving this is as follows:

The solution! • 1. Write a balanced equation. • 2. Given the pH, calculate the pOH. • 3. Calculate the [OH-] • 4. Solve for Kb • 5. Read about calculation error on pg. 510

Ka and Kb are related • Well, that’s not surprizing! • To review………. • pH + pOH = 14 • [H3O+][OH-] = 1.00 x 10-14 = Kw • Ka x Kb= Kw • This applies to conjugate acid-base pairs. • See page 511 for examples.

The Strengths of Acids and Bases