11

Acids, Bases and Acids, Bases and Salts and Covalent Salts and Covalent SolutesSolutes Chapter 14Chapter 14

Acids, Bases and Acids, Bases and Salts and Covalent Salts and Covalent SolutesSolutes Chapter 14Chapter 14

Tro, 2Tro, 2ndnd ed. ed.

22

Acids and Bases: Acid Properties“Sour taste”Change the color of litmus from blue to

redReact with:

- metals such as zinc and magnesium to produce hydrogen gas

- strong bases to produce water and an ionic compound (salt)

- carbonates to produce carbon dioxide.

These properties are due to the release of hydrogen ions, H+, into water.

33

Acids and Bases: Base PropertiesBitter or caustic taste

Have a slippery, soapy feeling.

Change the color of litmus from red to blue

React with acids

These properties are due to the release of hydroxide ions, OH+, into water.

44

Acid-base TheoriesAn Arrhenius acid “is a hydrogen-containing

substance that dissociates to produce hydrogen ions.”

Strong: HA(aq) + H2O(l) → H3O+(aq) + A-

(aq)

Weak: HA(aq) + H2O(l) H3O+(aq) + A-

(aq)

(List your six acids again and sort them as being weak or strong.)

55

Acid-base Theories

An Arrhenius base is a substance that dissociates or reacts with water to produce hydroxide ions in aqueous solution.

Strong: MOH(aq) → M+(aq) + OH-

(aq)

Weak: BH(aq) + H2O(l) BH2+

(aq) + OH-(aq)

Strong bases are groups I and II metal hydroxides; weak bases are ammonia and amines.

66

Arrhenius Theory: Figure 14.9

HCl ionizes in water,producing H+ and Cl– ions

NaOH dissociates in water,producing Na+ and OH– ions

77

Acid-base TheoriesA Bronsted-Lowry acid is a proton (H+) donor.

A Bronsted-Lowry base is a proton (H+) acceptor.

HCl(aq) + H2O(l) → H3O+

(aq) + Cl-(aq)

proton acceptorBronsted-Lowry Baseproton donorBronsted-Lowry Acid

88

hydrogen ion does not exist

in water

hydrogen ion combines with water

a hydronium ion is formed

99

HCl → Cl-

Conj. baseacid

Conjugate acid-base pairs differ by a proton.

When an acid donates a proton it becomes the conjugate base.

When a base accepts a proton it becomes the conjugate acid.

H2O →Conj. acidbase

H3O+

1010

Conjugate acid-base pairs are identified in an acid-base reaction.

conjbase

acid

HCl(g) + → Cl-(aq) + H3O+(aq)H2O (l)

conjacid

base

1111

Bronsted-Lowry: acid-conj base and base-conj acid

Practice: You identify the conjugate acid-base pairs in each of the following reactions.

1. H2CO3(aq) + CN-(aq) HCN(aq) + HCO3

-(aq)

2. H2O(l) + HCl(aq) H3O+(aq) + Cl-(aq)

3. H2O(l) + CH3COOH(aq) H3O+(aq) + CH3COO-

(aq)

1212

Lewis AcidElectron Pair

Acceptor

Lewis BaseElectron Pair Donor

Electron pair donated to H+

Acid-base Theories:A Lewis acid is an electron-pair acceptor. A Lewis base is an electron-pair donor.

1313

Lewis AcidElectron Pair Acceptor

Lewis BaseElectron Pair Donor

Electron pair donated to B

1414

1515

DEFINITIONS:

Monoprotic acid: can donate only one proton: HF, HCl, HNO3

Polyprotic acid: can donate > 2 protons: H2SO4, H3PO4

(Apply to bases as well in accepting protons: CO3

2- can accept two protons.)

1616

Reactions of Acids

In aqueous solution, the H+ or H3O+ ions are responsible for the characteristic reactions of acids

Reaction with Metals: Acids react with metals that lie above hydrogen in the activity series of element to produce hydrogen and an ionic compound (salt)

acid + metal → hydrogen + ionic compound

Reaction with Metals Oxidizing acids react with metals to produce water instead of hydrogen

acid + metal → water + ionic compound

1717

Reactions of Acids

Reaction with Bases The reaction of an acid with a base is called a neutralization reaction. In an aqueous solution the products are a salt and water:

HBr(aq) + KOH(aq) → KBr(aq) + H2O(l)

2 HNO3(aq) + Ca(OH)2(aq) → Ca(NO3)2(aq) + 2 H2O(l)

1818

Reactions of Acids

Reaction with Metal Oxides In an aqueous solution the products are a salt and water. This type of reaction is closely related to that of an acid with a base:

2 HCl(aq) + Na2O(s) → 2 NaCl(aq) + H2O(l)

H2SO4(aq) + MgO(s) → MgSO4(aq) + H2O(l)

1919

Reactions of Acids

Reaction with Carbonates Most acids react with carbonates to produce carbon dioxide, water and an ionic compound:

H2SO4(aq) + MgCO3(s) →MgSO4(aq) +H2O(l) +CO2(g)

HCl(aq) + NaHCO3(aq) → NaCl(aq) + H2O(l) + CO2(g)

2020

SALTS

Salts can be considered compounds derived from acids and bases. They consist of positive metal or ammonium ions combined with nonmetal ions (OH- and O2- excluded).

Salts are crystalline solids and have very high MPs and BPs.

We use the terms salt and ionic compound interchangeably.

2121

Electrolytes

Electrolytes are solutes whose aqueous solutions can carry an electrical current because they are charged particles (ions).

Salts: water-soluble ionic compounds are all strong electrolytes.

Acids: form H+ ions in water along with their anions; strong acids are strong electrolytes; weak acids are weak electrolytes.

Bases: form OH- in water along with their cations; strong bases are strong electrolytes; weak bases are weak electrolytes.

Covalent solutes: do not form ions when they dissolve in water and therefore do not carry a current; they are NONelectrolytes.

2222

Ionization vs. Dissociation

Ionization is the formation of ions.Ionization occurs as the result of a

chemical reaction of certain substances with water.

Dissociation is when cations and anions separate from each other in the solvent.

2323

Ionization vs. Dissociation

Acetic acid ionizes and dissociates in water to form acetate ion and hydronium ion. It is a WEAK acid, so it is less than 10% dissociated.

HC2H3O2(aq) + H2O(l) ↔ H3O+(aq) + C2H3O2

-(aq)

HNO3, a strong acid, is 100 % dissociated.

HNO3(aq) + H2O(l) → H3O+(aq) + NO3

-(aq)

2424

HCl Solution

HC2H3O2 Solution

Strong Acid Weak Acid1% ionized100%

ionized

15.3

2525

STRONG ACID OR BASE VS. WEAK ACID OR BASE:

Strong acid or base dissociates completely.

Relative strengths: just memorize these

SA: HCl, HNO3, H2SO4, (HClO4, HBr, HI).

WA: your other three acids plus all others, including all organic acids; dissociate < 10%.

SB: any Group I or Group II hydroxide or oxide.

WB: all others, including NH3 and amines dissociate < 10%.

2626

Classify the following as strong electrolytes, weak electrolytes or nonelectrolytes AND as strong or weak acids, strong or weak bases, salts or covalent solutes:

H2SO4, C12H22O11, NH3, K2SO4, CH3CH2OH, HNO3, NaOH, NaNO3, HC2H3O2, CH3NH2

Practice with Electrolytes

2727

SELF-IONIZATION OF WATER:

H2O(l) + H2O(l) H3O+(aq) + OH-

(aq)

Kw = [H3O+][OH-] = 1.0 x 10-14 at 25oC

In pure water: [H3O+] = [OH-] = x

Then Kw = x*x = 1.0 x 10-14

and therefore, x = 1.0 x 10-7 M = [H3O+]

2828

THE pH SCALE: A LOGARITHM SCALE OF [H3O+]

pQ = - log QpH = - log [H3O+] pOH = - log [OH-][H3O+] = 10-pH [OH-] = 10-pOH

The number of decimal places of a logarithm is equal to the number of significant figures in the original number.

Using logs: log (a*b) = log a + log blog ([H3O+][OH-]) = log [H3O+] + log [OH-]log (1.0 x 10-14) = log [H3O+] + log [OH-]-14 = (-pH) + (-pOH)14 = pH + pOH

(What is –log(nutbutter)?

2929

THE pH SCALE: A LOGARITHM SCALE OF [H3O+]

The pH scale is from 0.00 to 14.00 because of this.Scale was developed for dilute solutions,

where pH = -log[H3O+] <---inc acidity----- neutral ------inc alkalinity--->|----|----|----|----|----|----|----|----|----|----|----|----|----|----|0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.1 .01 10-4 10-7 10-10

pH of pure water is 7, meaning [H3O+] is 1.0 x 10-7.Then [OH-] is 1.0 x 10-7 and pOH is 7 also.

3030

TO FIND pH OF A STRONG ACID OR STRONG BASE: DO A SOLUTION INVENTORY AND FIND [H3O+] OR [OH-]. THEN FIND pH OR pOH.

Find pH, pOH and [OH-] for 1.0 x 10-3 M HCl(aq).

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq),

therefore [H3O+] = 1.0 x 10-3 M

pH = - log (1.0 x 10-3 M) = 3.00

14.00 = 3.00 + pOH, pOH = 11.00

[OH-] = 10-pOH = 10-11.00 = 1.00 x 10-11 M

3131

TO FIND pH OF A STRONG ACID OR STRONG BASE: DO A SOLUTION INVENTORY AND FIND [H3O+] OR [OH-]. THEN FIND pH OR pOH.

Noninteger pH values:

If [HCl] = 6.0 x 10-4 M, then so is [H3O+]

pH = - log (6.0 x 10-4 M) = 3.22

What is pOH? 14.00 = 3.22 + pOH

pOH = 10.78

3232

TO FIND pH OF A STRONG ACID OR STRONG BASE: DO A SOLUTION INVENTORY AND FIND [H3O+] OR [OH-]. THEN FIND pH OR pOH.

A saturated solution of calcium hydroxide has [OH-] of 0.025 M. What is its pH?

pOH = - log(0.025) = 1.60

pH + 1.60 = 14.00

pH = 12.40

3333

The pH scale of Acidity and Basicity

15.4

3434

pH of Common Substances

Substance pH

1.0 M HCl 0.0

0.1 M HCl 1.0

stomach acid 1.0 to 3.0

lemons 2.2 to 2.4

soft drinks 2.0 to 4.0

plums 2.8 to 3.0

apples 2.9 to 3.3

cherries 3.2 to 4.0

unpolluted rainwater 5.6

human blood 7.3 to 7.4

egg whites 7.6 to 8.0

milk of magnesia (sat’d Mg(OH)2) 10.5

household ammonia 10.5 to 11.5

1.0 M NaOH 14

3535

Acid-Base Stoichiometry

Neutralization: acid-base reactions are analyzed by titration. This is volumetric analysis using a standardized solution to find mass or concentration of another substance.

Titration: the process of measuring the volume of one reagent required to react with a measured mass or volume of another reagent.

Burettes: contain the titrant, usually standardized.Flask: contains analyte, the substance we want to

analyze.Endpoint: when the indicator changes color to say

reaction is done.

3636

TitrationUse reaction stoichiometry to

determine the concentration of an unknown solution.

Titrant (unknown solution) added from a burette.

Indicators are chemicals added to help determine when a reaction is complete.

Endpoint of the titration occurs when the reaction is complete.

3737

Titration:

Actually, this is TOO PINK!!!

3838

Titration:The base solution is thetitrant in the buret.

As the base is added tothe acid, the H+ reacts withthe OH– to form water. But there is still excess acid present so the colordoes not change.

At the titration’s endpoint,just enough base has been added to neutralize all theacid. At this point the indicator changes color.

3939

If 42.00 mL of 0.150 M NaOH solution is required to neutralize 50.00 mL of hydrochloric acid solution, then what is the molarity of the acid solution?

The equation for the reaction is

molMolarity =

L

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) acid base salt

Convert mL of NaOH to liters of NaOH

The unit of volume when using molarity is liters.

Calculate the moles of NaOH that react.

0.150 mol NaOH0.004200 L = 0.00630 mol NaOH

1L

1 L42.00 mL = 0.04200 L

1000 mL

Calculate the liters of NaOH that react.

4040

The moles of NaOH that react equals the moles of HCl that react.

0.00630 mol NaOH react. 0.00630 mol HCl react.

molM = =

L0.0630 mol HCl

0.05000 L

0.126 M HCl

The molarity of the HCl solution is

4141

Buffers

Buffers are solutions that resist changing pH when small amounts of acid or base are added.

They resist changing pH by neutralizing added acid or base.

Buffers are made by mixing together a weak acid and its conjugate base, or weak base and it conjugate acid.

4242

How Buffers Work

The weak acid present in the buffer mixture can neutralize added base.

The conjugate base present in the buffer mixture can neutralize added acid.

The net result is little to no change in the solution pH.

4343

A Buffer made from Acetic acid and Sodium Acetate

A buffer solution with a pH of 4.75 can be made by mixing equal volumes of 1 M HC2H3O2 and 1 M NaC2H3O2.

Adding 10 mL of 0.1 M HCl to 1 L of this solution will give a solution with a pH of 4.75. Adding 10 mL of 0.1 M HCl to 1 L of distilled water will give a solution with pH of 3.0.

Adding 10 mL of 0.1 M NaOH to 1 L of this solution will give a solution with a pH of 4.75. Adding 10 mL of 0.1 M NaOH to 1 L of distilled water will give a solution with pH of 11.0.

4444

Acetic Acid/Acetate Buffer

4545