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Questions. What is a strong acid? What is a weak acid? Write expression for pH HX ( aq ) + H 2 O (l)  X - ( aq ) + H 3 O + For a strong acid like one above, what can you say about the concentration of [HX] and [ H 3 O + ]?. Water dissociation constant, K w. K w = [H 3 O + ] x [OH - ]

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Questions
Questions

  • What is a strong acid?

  • What is a weak acid?

  • Write expression for pH

  • HX(aq) + H2O(l) X-(aq) + H3O+

    For a strong acid like one above, what can you say about the concentration of [HX] and [H3O+]?


Water dissociation constant k w
Water dissociation constant, Kw

Kw = [H3O+] x [OH-]

Increases with increased temperature

At 25 deg Kw = 1X10-14


Weak acids
Weak Acids

Do not fully dissociate so:

  • Are in equilibrium biased to left

  • Have a low concentration of ions

  • Are poor conductors of electricity

  • React slowly as [H3O+] is low

    CH3COOH + H2O n CH3COO- + H3O+

    As they are in equilibrium, when they are diluted water concentration is increased and there is a shift to the right to compensate.

    This means predicting pH increase is harder.


Acidity constant k a
Acidity Constant, Ka

Equilibrium constant

Kc =

But water is a solvent so

Kc =

So Ka =

[H3O+] [A-]

[HA] [H2O]

[H3O+] [A-]

[HA]

[H3O+] [A-]

[HA]


Weak acid ph
Weak acid pH

Because

So Ka =

Rearrange

and pH = -log[H3O+]

Now try Exercise 13C

[H3O+] = [A-]

[H3O+]2

[HA]

[H3O+]=

Ka x [HA]


pKa

pKa= -logKa

As the strength of acid increase

Ka gets bigger

pKagets smaller

Conjugate base gets weaker


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