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Acids and Bases

 

Acids, Bases and Salts
 
I.                   Operational Definitions: Those that are observable in the lab.
A.   Acids:
1.    Aqueous solutions of acids conduct electricity (because ions form H+, H3O+)   
2.    Acids react with certain metals to  form hydrogen gas     ex: 2HCl(aq) + Zn(s) = ZnCl2 (aq) + H2(g)
*Table J* Any metal higher on the list will replace H
3.    Acids cause acid-base indicators to change color
a.           Litmus paper  turns red                                              
b.          Phenolphthalein  remains colorless                           
4.    Acids react with bases to  form salt and water             ex: HCl + NaOH = NaCl + H2O
5.    Dilute aqueous solutions of acids taste sour. Responsible for the sour taste of fruit and vinegar.
 
B.   Bases:
1.    As with acids, aqueous solutions of bases  conduct electricity (because ions form OH
2.    Bases cause acid-base indicators to change color
a.     Litmus paper  turns blue                                             
b.    Phenolphthalein  hot pink                                            
3.    Bases are  caustic/corrosive        , they can cause chemical burns to skin.
4.    Bases  emulsify oils. This means they  can dissolve oils and can dissolve in water and as a result they make good  soaps.
5.    Bases taste  bitter     
6.    Bases feel        slippery     
 
II.                Conceptual Definitions of Acids and Bases: Those that cannot be directly observed. These are concepts that are based on other observations. 
A.   Arrhenius theory of acids and bases:
1. Acids produce H+  as the only positive ion in solution. 
In water  H+ forms a coordinate covalent bond with the water forming H3O+ (hydronium ion)                       ex: HCl → H+ + Cl-      H2SO4 → 2H+ + SO4-2                               
a. The Nature of the Hydrogen Ion
Hydrogen consists of one electron and one proton. When hydrogen becomes a positive ion it has lost its electron and is just a proton. *A positive hydrogen ion is a proton.* This proton cannot exist by itself, so it bonds with water to form a hydronium ion.
*Acids that contain one H are called monoprotic; acids that contain two H are called diprotic; acids that contain three H are called triprotic. Polyprotic is another term used to describe acids that contain more than one H.
Example: HCl – monoprotic H2SO4 - diprotic          H3PO4 - triprotic
2. Bases produce  OH-        as the only          negative     ion in solution.
a. The Nature of the Hydroxide Ion
The presence of the hydroxide ion makes that base an electrolyte. It also gives the base its properties of being slippery and bitter to the taste. 
Ammonia is a base because when dissolved in water produces an OH ion.
 
Bases that contain one OH are called monohydroxy; acids that contain two OH are called dihydroxy.
Example: NaOH – monohydroxy   
             Ca(OH)2 - dihydroxy
 
B.   An alternative to Arrhenius Theory:
Hydrogen ion donors and acceptors (Bronsted-Lowry Theory)
1.    Acids  donate hydrogen ion (donate a proton)       
2.    Bases  accept hydrogen ion (accept a proton)      
*More general than Arrhenius’ definition and they do not require aqueous solutions, so water is not necessarily a product. 
 
Example: 1. NH3   +   H2O   →   NH4   +   OH-
 
2.     HCO3-   +   H2O   →   CO3-2   +   H3O+
 
*Water is one of several “special” substances. In certain reactions it reacts like an acid (H+ donor) while in other reactions it reacts like a base (H+ acceptor). 
These substances are known as  amphiprotic        or amphoteric substances.
 
*Ionization/Dissociation:
Ionization: the reaction between solute and solvent resulting in the formation of ions. All acids and bases will ionize in solution. The degree to which they ionize will determine the strength of the acid or base. This will also determine how good or poor an electrolyte the substance is. 
       Salts will also ionize when placed in water because they are ionic substances in which H+ is not the only + ion in solution and OH- is not the only – ion in solution.
                 Example: KCl → K+ + Cl-
 
Dissociation: Results in almost complete separation of ions in which very little molecule is left as a result. Very strong acids and bases dissociate in water.
                 Example: HCl → H+ + Cl-
 
Ionization Constants (k) are used to determine the strength of an acid or base based on its ability to produce H+ or OH-. The larger the k, the more ions produced, the stronger the acid (base) and better the electrolytic abilities of the solution.
 
III.             Neutralization Reactions:
The reaction between       an acid and a base to produce a salt and water
Example: nitric acid + potassium hydroxide = salt + water
Now, put it into a formula: HNO3 + KOH = H2O + KNO3
 
1. Titration a process in which the strength of an unknown acid (base) is determined by dropping in a known base (acid) until an endpoint (near neutral) is reached.        
 
 
2.    Reaction: Acid + Base = Salt + Water
*For this reaction to become completely neutral, the number of moles of acid ion (H+) must be equal to the number of moles of base ion (OH-).
*Formula (found on Table T)
     MAVA = MBVB
              *MA = molarity of H+ in the acid
               MB = molarity of OH- in the base
               VA= volume of the acid
              VB = volume of the base
3.    Examples:
a.     What is the molarity of NaOH if 5mL are needed to exactly neutralize 10mL of 0.1M HCl?
MAVA = MBVÞ (.1)(10) = (x)(5) Þ x = .2M NaOH
 
b.    What is the molarity of 25mL of H2SO4 if 25mL of 2M KOH completely neutralize the solution?                  
*(2)MAVA = MBVB                  *2(H) vs 1(OH)
(2)(x)(25) = (2)(25) Þ 50x = 50 Þ x = 1M H2SO4
 
IV.            Salts:
A.   Salts are ionic substances that result from the reaction between an acid and a base.
Some common examples of salts are NaCl, KNO3, Ca(ClO3)2
 
B.   Salts can be slightly acidic, slightly basic, or neutral in nature depending upon the acids and bases that are used in producing them.
·        Strong Acid + Strong Base = Neutral salt + water
HCl + NaOH = NaCl + HOH
Hydrochloric acid + sodium hydroxide = sodium chloride + water
·        Strong Acid + Weak Base = Acidic salt + water
H2SO4 + 2NH4OH = (NH4)2SO4 + HOH
Sulfuric acid + ammonium hydroxide = ammonium sulfate + water
 
·        Weak Acid + Strong Base = Alkaline salt + water
HC2H3O2 + KOH = KC2H3O2 + HOH
Acetic acid + potassium hydroxide = potassium acetate + water
 
·        Weak Acid + Weak Base = salt that may be neutral/acidic/alkaline depending upon how well it hydrolyzes + water
 
C.   Hydrolysis of a Salt: the reaction of salt with water
1.    Salts that are slightly acidic or alkaline will hydrolyze in water. That means the H+ from the water will react with the negative ion (anion) of the salt if the parent base was weak, whereas the OH- from the water will react with the positive ion (cation) if the parent acid was weak. This will result in unequal amounts of H+ and OH- will be in the resulting solution. More H+ ions result in an acidic solution, more OH- results in an alkaline solution.
2.    Salts that are neutral, as those that are produced from a strong acid and strong base, will not hydrolyze in water and so excess H+ or OH- are produced in equal amounts. The resulting solution is neutral.
 
V.               pH and the pH scale:
*This scale involves negative logarithms and powers of 10*
A. pH is the  percentage of  hydrogen ion concentration in an aqueous solution.
·        pH can be considered percent  H+ or H3O+                           
·        Excess H+ in an aqueous solution will produce  H3O+ which is a result of a  coordinate covalent      bond between the       H+ and H2O
·        pOH determines the  percentage of OH-    
 
B.   the pH scale is usually shown as numbers between 1 and 14
 
1                                       7                                    14
                                            Acid                                  Base       
                   1. Acids will yield         H+     or  H3O+ in solution.
                   2. Bases will yield        OH-   in solution.
 
 
 
C.   To determine the pH (or pOH) of a solution, the following formulas may be used:
[H+] = 1 x 10-pH
[H+][OH-] = 1 x 10 –14
 
                   Questions:
1.    What is the hydroxide ion concentration of a solution with a pH of 3?
pH = 3 Þ [H+] = 1x10-3 Þ [OH-] = 1x10-11
 
2.    What is the pH of a solution with a hydroxide ion concentration of 1 x 10-13?
[OH-] = 1x10-13 Þ [H+] = 1x10-1 Þ pH=1
 
3.    Given a .001M solution of HCl, what is the pH?
                                      [H+] = 1x10-3 Þ  pH=3
 
4.    Given a .01M NaOH solution, what is the pH?
 [OH-] = 1x10-2 so [H+} = 1x10-12 Þ pH=12


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What is the concentration of H+ in a solution that has a pH of 4.0? A) 4 M B) –0.6 M C) 0.6 M D) 1 × 10–4 M
July 22, 2013

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July 24, 2013 -  Replied By Expert

 
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