Acids bases and salts
Acids bases and salts
The following text is used only for teaching, research, scholarship, educational use and informative purpose following the fair use principles.
We thank the authors of the texts and the source web site that give us the opportunity to share their knowledge
Chemistry
Acids bases and salts
Section 19.1- Acid-Base Theories
KEY CONCEPTS
- What are the properties of acids and bases?
- How did Arrhenius define an acid and a base?
- What distinguishes an acid from a base in the Brönsted-Lowry theory?
- How did Lewis define an acid and a base?
--Many everyday materials contain acids or bases
--Acids
1. taste sour
2. change the color of an acid-base indicator
3. are electrolytes
--Bases
1. taste bitter
2. feel slippery
3. change the color of an acid-base indicator
4. are electrolytes
--An ARRHENIUS ACID is a hydrogen-containing compound that ionizes in aqueous solution to produce hydrogen ions (H+)
--An ARRHENIUS BASE is a compound that produces hydroxide ions (OH-) in aqueous solution
--A MONOPROTIC ACID is one that contains only one ionizable hydrogen
Ex. HCl, HNO3
--A POLYPROTIC ACID is one that contains 2 or more ionizable hydrogens
Ex. H2SO4- DIPROTIC, H3PO4- TRIPROTIC
--Note that only hydrogens in very polar bonds are ionizable
Ex. CH3COOH
--Random facts about bases:
1. NaOH (lye) and KOH (found in ashes) are very soluble bases and extremely caustic
2. Ca(OH)2 (lime) and Mg(OH)2 (in milk of magnesia) are not very soluble
--What do the formulas of the bases above have in common?
--A BRÖNSTED-LOWRY ACID is a proton/hydrogen ion (H+) donor
--A BRÖNSTED-LOWRY BASE is a proton/hydrogen ion (H+) acceptor
Ex. NH3(aq) + H2O(l) £ NH4+(aq) + OH-(aq)
Which reactant is a proton donor? Proton acceptor?
B-L base? B-L acid?
--A CONJUGATE ACID is what the base becomes after it accepts a proton
--A CONJUGATE BASE is what the acid becomes after it donates a proton
Ex. What is the conjugate acid and base in the following reactions
HCl(g) + H2O(l) £ H3O+(aq) + Cl-(aq)
NH3(aq) + H2O(l) £ NH4+(aq) + OH-(aq)
Ex. What is the conjugate acid of each of the following:
NH3, H2O, OH-, HCO3-, CO32-, NH2-
Ex. What is the conjugate base of each of the following:
NH3, H2O, HCO3-, H2CO3, H2SO4, HSO4-
--A HYDRONIUM ION (H3O+) is the ion that results when a water molecule gains a hydrogen ion
--An AMPHOTERIC substance is one that can behave like an acid or base
--Which substances in the examples above are amphoteric?
--A LEWIS ACID is an electron pair acceptor; a LEWIS BASE is an electron pair donor
Ex. H+ + OH-
--An ACIDIC ANHYDRIDE is a nonmetallic oxide that, when dissolved in water, produces an acid
Ex. CO2(g) + H2O(l) £ H2CO3(aq)
--A BASIC ANHYDRIDE is a metallic oxide that, when dissolved in water, produces a base
Ex. Na2O(s) + H2O(l) ß 2NaOH(aq)
--Are the following acidic or basic anhydrides?
CaO, SO2, NO2, K2O, BaO
SEE 19.1 SECTION ASSESSMENT, P. 593
Section 19.2-Hydrogen Ions and Acidity
KEY CONCEPTS
- How are [H+] and [OH-] related in an aqueous solution?
- How is the hydrogen-ion concentration used to classify a solution as neutral, acidic, or basic?
- What is the most important characteristic of an acid-base indicator?
--Water is amphoteric (meaning what?) and can undergo SELF (AUTO)-IONIZATION
H2O(l) + H2O(l) £ H3O+(aq) + OH-(aq)
or
H2O(l) £ H+(aq) + OH-(aq)
--In pure water, [H3O+] = [OH-] = 1 X 10-7 M
--Since an equilibrium exists between water molecules, hydronium ions, and hydroxide ions
Keq = [H3O+][OH-]/[H2O]2
As [H2O] is much larger than [H3O+] and [OH-] and is essentially constant, one can move [H2O]2 to the other side of the equation to obtain
[H2O]2 Keq = [H3O+][OH-]
--The ION PRODUCT CONSTANT FOR WATER (Kw) is the product of the concentrations of H3O+ (or H+) and OH-
Kw = [H3O+][OH-] = (1 X 10-7 M)2 = 1 X 10-14
--What happens to the concentration of H3O+ (H+) if an acid is added to water? OH- concentration? If a base is added?
Ex. SEE SAMPLE PROBLEM 19.1, PRACTICE PROBLEMS 9, 10, P. 596
--The pH of a solution is the negative logarithm of [H3O+] (or [H+])
pH = -log[H3O+]
Ex. What is the pH of a neutral solution?
Ex. What is the pH of a solution with [H+] = 6 X 10-10 M
Ex. What is the [H3O+] ([H+]) of a solution with
pH = 4.8?
--To summarize
1. An ACIDIC SOLUTION is one in which
[H3O+] > [OH-]
pH < 7
[H3O+] > 1 X 10-7M
2. A BASIC (ALKALINE) SOLUTION is one in which
[H3O+] < [OH-]
pH > 7
[H3O+] < 1 X 10-7M
3. A NEUTRAL SOLUTION is one in which
[H3O+] = [OH-] = 1 X 10-7M
pH = 7
--pOH is the negative logarithm of the [OH-]
pOH = -log[OH-]
--If one takes the negative logarithm of both sides of the Kw equation- Kw = [H3O+][OH-] = 1 X 10-14
the result is another useful equation
pH + pOH = 14
--Given one of the following
[H3O+] [OH-] pH pOH
one should be able to calculate the other three values for an aqueous solution
Ex. A solution has a [H3O+] = 4.2 X 10-10 M. What is the [OH-], pH, and pOH?
Ex. A solution has a pH = 9.2. What is the [H3O+], pOH, and [OH-]?
Ex. A solution has a [OH-] = 3.5 X 10-3 M. What is the [H3O+], pH, and pOH?
Ex. A solution has a pOH = 4.5. What is the [OH-], pH, and [H3O+]?
--An INDICATOR (HIn) is a weak acid or base that undergoes a color change at a specific pH when the molecule gains or loses a proton
HIn £ H+ + In-
SEE FIG. 19.12, P. 602
Ex. Universal indicator, pH paper, red/blue litmus paper
--A pH meter is an electrical instrument used to make rapid, accurate pH measurements
SEE FIG. 19.15, P. 603
SEE 19.2 SECTION ASSESSMENT, P. 604
Section 19.3-Strengths of Acids and Bases
KEY CONCEPTS
- How does the value of an acid dissociation constant relate to the strength of an acid?
- How can you calculate an acid dissociation constant (Ka) of a weak acid?
--A STRONG ACID is one that ionizes completely in aqueous solution
Ex. HCl(g) + H2O(l) ß H3O+(aq) + Cl-(aq)
--A WEAK ACID is one that only partially ionizes in aqueous solution
Ex. CH3COOH(aq) + H2O(l) £ CH3COO-(aq) + H3O+(aq)
--The strong binary acids are HCl, HBr, HI
--A rule-of-thumb for strong ternary acids is
“If the number of oxygens exceeds the number of hydrogens by two or more, the acid is strong”
Ex. H2SO4-strong H2SO3-weak
Ex. What is the concentration of H3O+ in a 0.3 M solution of HNO3? What is the pH?
Ex. What is the pH of a 1.3 M solution of HCl?
--Since an equilibrium exists between a weak acid and its conjugate base, an equilibrium expression can be written; for a weak acid HA
HA(aq) + H2O(l) £ A-(aq) + H3O+(aq)
Keq = [A-][H3O+] / [HA][H2O]
[H2O] is essentially a constant, and can be moved to the other side of the equation to form a new constant- the ACID IONIZATION (DISSOCIATION) CONSTANT (Ka)
Ka = [A-][H3O+] / [HA]
--The larger Ka is for an acid, the stronger the acid
SEE TABLE 19.7, P. 607
--Write the Ka expression for each of the following
H2SO4, HSO4-, H2CO3, HCO3-, CH3COOH
--Note that the conjugate base of a polyprotic acid is a weaker acid than the original acid
Ex. H3PO4
--A STRONG BASE is one that dissociates completely in aqueous solution
--All soluble metallic hydroxides are strong bases, in particular Group 1 and heavy Group 2 hydroxides
--A WEAK BASE is one that reacts slightly with water to produce OH-
Ex. NH3(aq) + H2O(l) £ NH4+(aq) + OH-(aq)
--Note that the reaction in the example above lies to the left at equilibrium
Ex. What is the [OH-] of a 0.75 M NaOH solution? What is the pH?
--The BASE DISSOCIATION CONSTANT (Kb) for a weak base is analogous to Ka for a weak acid; for the weak base B
B + H2O £ HB+ + OH-
Kb = [HB+][OH-] / [B]
--Write the Kb expression for each of the following
NH3, NH2-, SO42-
--Is there a difference between concentration and strength of an acid/base? Can one have a weak acid/base solution with a high concentration?
--The Ka for a weak acid or Kb for a weak base can be calculated if one knows the initial concentration of the acid/base and the pH
Ex. SEE SAMPLE PROBLEM 19.5, P. 610
SEE 19.3 SECTION ASSESSMENT
Section 19.4-Neutralization Reactions
KEY CONCEPTS
- What are the products of the reaction of an acid with a base?
- What occurs at the endpoint of a titration?
--A SALT is a compound consisting of the anion from an acid and a cation from a base
Ex. HCl + NaOH ß NaCl + HOH
--The reaction in the example above is a NEUTRALIZATION REACTION-a reaction between an acid and a base that produces a salt and water
--Acid-base reactions are called neutralization reactions because if the reaction involves a strong acid and a strong base, and they are in stoichiometrically equivalent amounts, the resulting solution will be neutral
Ex. What acid/base combination could form the following salts (cation from base, anion from acid):
KCl, NaNO3, CaCO3, KBr, (NH4)2SO4
--TITRATION is a method which uses a standard solution to measure the concentration of another solution
--A STANDARD SOLUTION is a solution of known concentration
--In a titration, the solutes in the standard and unknown solution react. The standard solution is added to a given volume of unknown solution until the reaction is complete and there are stoichiometrically equivalent amounts of the two solutes-called the ENDPOINT. From the volumetric data obtained, the concentration of the unknown solution can be calculated
--In a strong acid-strong base titration (a neutralization reaction), the endpoint is reached when the pH is 7 (solution of a strong acid-strong base salt).
--Generally a titration would proceed as follows:
1. A volume of unknown solution is measured and placed in an appropriate container (Erlenmeyer flask)
2. Indicator (or a pH meter probe) is added to the unknown solution. An indicator commonly used in student laboratories is phenolphthalein (see Fig. 19.12, p. 602)
3. A BURET-graduated instrument used to measure out small volumes of liquid-is filled with the standard solution and an initial volume reading taken
4. Standard solution from the buret is released into the unknown solution until the slightest color change is detected
5. A final standard solution volume reading is taken
6. Calculation of unknown solution concentration is performed
Ex. The following volumetric data were obtained from the titration of an unknown HCl solution with a 0.75 M NaOH solution:
Volume of HCl solution - 15.50 mL
Initial buret reading - 0.52 mL
Final buret reading - 27.87 mL
Calculate the concentration of the unknown solution.
Ex. If 23.4 cm3 of 0.551M NaOH is used to titrate 50.0 cm3 of HCl to the endpoint, what is the concentration of the HCl solution?
Ex. How many cm3 of 0.0947M NaOH are needed to neutralize 21.4 cm3 of 0.106M HCl?
Ex. How many cm3 of 0.200M KOH will completely neutralize 15.0 cm3 of 0.400M H2SO4?
Section 19.5-Salts in Solution
KEY CONCEPTS
- When is the solution of a salt acidic or basic?
- What are the components of a buffer?
--While salts formed from reactions of strong acids with strong bases produce neutral solutions, this is not the case for salts of weak acids or bases
--In SALT HYDROLYSIS, ions from a salt react with water to produce an excess of H3O+ (H+) or OH-, which changes the pH of the solution
--Consider salts formed from the following acid/base neutralization reactions
1. STRONG ACID + WEAK BASE
Ex. Al(NO3)3 (from Al(OH)3 and HNO3)
Al3+ reacts with water to re-form the weak base
Al3+ + 3HOH £ Al(OH)3 + 3H+
Since this reaction lies to the right, and produces excess H+ (H3O+), the pH is lowered and an acidic solution is the result.
Will NO3- react with water to re-form HNO3? Why or why not?
2. WEAK ACID + STRONG BASE
Ex. NaCH3COO (from CH3COOH + NaOH)
CH3COO- reacts with water to re-form the weak acid
CH3COO- + HOH £ CH3COOH + OH-
Since this reaction lies to the right, and produces excess OH-, the pH is raised and a basic solution is the result.
Will Na+ react with water to re-form NaOH? Why or why not?
--Why will the salt from a strong acid/base reaction produce a neutral solution?
--What type of solution will a weak base/weak acid salt produce?
--To summarize
strong acid + strong base salt – neutral solution
strong acid + weak base salt – acidic solution
weak acid + strong base salt – basic solution
weak acid + weak base salt - ?
Ex. Will the following salts produce an acidic, basic, or neutral solution?
NaBr, NaSO3, CaSO4, NH4Cl, NH4NO3, KClO2
--A BUFFER is a solution of
1. a weak acid and one of its salts
or
2. a weak base and one of its salts
that resists changes in pH when small amounts of acid or base are added
--In a buffer system, an equilibrium is established between the weak acid and its conjugate base or the weak base and its conjugate acid. These provide species for added acid or base to react with instead of reacting with water and changing pH
Ex. CH3COOH and NaCH3COO buffer
The species present in this buffer system are
CH3COOH Na+ CH3COO- H2O
An equilibrium between acid and conjugate base is established
CH3COOH + H2O £ CH3COO- + H3O+
If acid is added to the system, the proton is donated to the strongest base present-acetate
If base is added to the system, the strongest acid donates a proton-acetic acid
In either case, the concentration of H3O+ doesn’t change significantly, nor does the pH
--The BUFFER CAPACITY of a buffer system is the amount of acid or base that can be added to a buffer solution before a significant change in pH occurs
--What would determine buffer capacity of a buffered system?
Source : http://www.duplinschools.net/22032033115139260/lib/22032033115139260/_files/Chemistry-Notes_Chapter_19_(Acids,_Bases,_and_Salts).doc
Web site link: http://www.duplinschools.net/
Author : not indicated on the source document of the above text
If you are the author of the text above and you not agree to share your knowledge for teaching, research, scholarship (for fair use as indicated in the United States copyrigh low) please send us an e-mail and we will remove your text quickly.
Acids bases and salts
Acids bases and salts
Acids bases and salts
This is the right place where find the answers to your questions like :
What is Acids bases and salts? What does Acids bases and salts mean ? Which is the meaning of Acids bases and salts?
Acids bases and salts chemistry notes
Alanpedia.com from 1998 year by year new sites and innovations
Main page - Disclaimer - Contact us