Equilibrium Constants of Acidic and Basic Solutions

Acid Strength

Acid strength is easy to determine as you simply look at the table in the data book. Anything above hydronium ion (H3O+) is a STRONG ACID and anything below is a WEAK ACID With a strong acid, the concentration of the acid is THE EXCAT SAME as the concentration of hydronium ([H30+])

Base Strength (Strong Bases)

A strong base is a base the dissociates completely into a cation(+) and the hydroxide ion (OH). Most of the time it will be a Group 1 or 2 metal ion and hydroxide. Example: LiOH is a strong base.

 

HOWEVER, if the base has a Group 2 metal ion, these bases are ONLY strong when the concentration is 0.010 M or less. Example: Ca(OH)2 is a strong base only when concentration is 0.010 M or less.

 

With a strong base, the concentration of the base is THE EXCAT SAME as the concentration of hydroxide ([OH])

Base Strength (Weak Bases)

The second column on the table in the data book is the base conjugate of an acid. Anything above hydroxide (OH) is a weak base. Anything above hydroxide (H3O+) does not count as a weak base as it is too weak.

Showing Acidic Reactions

1.) Write the given acid. Locate it on the table and identify whether it is a strong or weak acid.

 

2.) Add water to acid. All acidic and basic reactions have water as a reactant.

 

3.) Write the conjugate (immediately right to it on the table) and hydronium.

Showing Basic Reactions

1.) Write down the base. If it’s aqueous, write the ions and cross out the base (since it is aqueous it no longer exists, only the ions do.)

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2.) In most cases, it will be a metal cation and another ion. We only want to work with what’s on our table. IGNORE the metal cation and find the other ion on the table.

 

3.) Add the ion to water. All basic and acidic reactions involve water. The products will be the conjugate acid (to the left of the ion) and hydroxide.

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1.) We have NaClO(aq)

 

2.) Realize aqueous compounds no longer exist. Only their ions exist. This leaves Na+ and ClO- (in this case it CANNOT be Cl- and O- because Cl- is above hydronium (H30+) on the table so it cannot exist as a base because it is too weak.

 

3.) Add water to ClO-. The products will be hydroxide (OH-) and the conjugate acid for ClO-

Water Ionization Constant (Kw)

To a very small degree, water will ionize (break into it’s ions, OH and H3O+) The amount of OH and H3O+ is equal when water ionizes.

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  • Kw is the equilibrium constant for the ionization of water.
  • Kw = [H3O+][OH-] (it follows the formula of Kc)
  • Kw = 1.00 x 10-14

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Kw can also be used to find Kb (base ionization with water) and Ka (acid ionization with water) Kw = (Ka)(Kb)

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Ka is the equilibrium constant for the reaction of an acid with water. Ka values are listed on the table in the data book.

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Kb is the equilibrium constant for the reaction of a base with water.

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It can be found by using, Kw, Ka from the table of the base’s conjugate acid, and the relation Kw = (Ka)(Kb).

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