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The Equilibrium of Weak Acids and Bases

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The Equilibrium ofWeak Acids and Bases

- The dissociation of an acidic or basic compound in aqueous solution produces ions that interact with water (REVIEW!)
- Pure water contains a few ions, produced by the dissociation of water molecules:

- At 25°C, only about two water molecules in one billion dissociate.
- In neutral water, at 25°C, the concentration of hydronium ions is the same as the concentration of hydroxide ions: 1.0 × 10−7 mol/L.
- These concentrations must be the same because the dissociation of water produces equal numbers of hydronium and hydroxide ions:
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- [H3O+] is not 1.0 × 10−7 mol/L at other temperatures. (Le Chatelier?)
- The same is true of [OH−] .

What would the equilibrium constant, Kc, be for the dissociation of water?

- So few ions form that the concentration of water is essentially constant.
- The product Kc[H2O]2 is equal to the product of the concentrations of hydronium ions and hydroxide ions (work it out!)
- The equilibrium value of the concentration ion product [H3O+][OH−] at 25°C is called the ion product constant for water. It is given the symbol Kw.

The units are commonly dropped, as in other equilibrium expressions you have encountered.

- Acidity of an aqueous solution can be stated quantitatively by the concentration of the hydronium ions that are present.
- pH scale - devised by Danish biochemist named Søren Sørensen to represent acidity (and, by extension, basicity).
- We use logarithms to define pH because [H3O+] is often a very small number.

- Write the exponential equation, 43 = 64 as a logarithm.
- Write 25 as a logarithm.

- Write 5-2 as a logarithm.

- Since [H3O+] is so small, pH is defined by using the logarithmic equation:
pH = - log10 [H3O+]

In neutral water, [H3O+] is:

pH = -log10 [H3O+]

In neutral water, [H3O+] is:

pH = -log10 [H3O+]

= -log10 (1.0 X 10-7 )

What exponent do we need to apply to the base10 to equal 1.0 X 10-7 ?

In neutral water, [H3O+] is:

pH = -log10 [H3O+] = - (-7)

= -log10 (1.0 X 10-7 ) = 7

What exponent do we need to apply to the base10 to equal 1.0 X 10-7 ?

- You can calculate the pOH(the power of hydroxide ions) of a solution from the [OH−].
- pOH = −log[OH−]
- Kw = [H3O+][OH−] = 1.0 × 10−14 at 25°C
- ∴pH + pOH = 14

- You can calculate [H3O+] or [OH−] by finding the antilog of the pH or pOH.
- [H3O+] = 10−pH
- [OH−] = 10−pOH

- When a weak acid dissolves in water, it does not completely dissociate.

- You can represent any weak monoprotic acid with the general formula HA. The equilibrium of a weak monoprotic acid in aqueous solution can be expressed as follows:

- This new constant is called the acid dissociation constant, Ka, (aka the acid ionization constant. With either name, the symbol is Ka.)

- The smaller the value of Ka, the less the acid ionizes in aqueous solution.

- The percent dissociation of a weak acid is the fraction of acid molecules that dissociate compared with the initial concentration of the acid, expressed as a percent.
- The percent dissociation depends on the value of Ka for the acid, as well as the initial concentration of the weak acid.

- polyprotic acids have more than one hydrogen atom that dissociates.
- Each dissociation has a corresponding acid dissociation
- constant.
- Problems that involve polyprotic acids can be divided into as many sub-problems as there are hydrogen atoms that dissociate.
- The ion concentrations that are calculated for the first dissociation are substituted as initial ion concentrations for the second dissociation, and so on.

PPs 13-18 (Pg. 392)

PPs 19-25 (Pg. 400)

PPs 25-28 (Pg. 402)

SR, Pg. 403:

1-5.