Weak Acids and Bases

 

Weak Acids and Bases

The relative strength of an acid or base is the extent to which it _____________ when dissolved in water. If the ionization reaction is essentially complete, the acid or base is termed _____________; if relatively little ionization occurs, the acid or base is _____________. Most acids are _____________ acids. Since the ionization is not complete, weak acids and bases are _____________ systems and the degree of ionization can be determined using equilibrium constants.

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The ionization constants Ka and Kb are defined as the equilibrium constants for the ionization of a weak acid or weak base when it is dissolved in _____________. Ka values can be compared to determine which of two acids is _____________. A _____________ value of Ka indicates that the reaction is more favorable toward the products and will have a _____________ concentration of H3O+ and A relative to the concentration of the nonionized acid, HA, in an equilibrium mixture. The ionization constant expression for the ionization of an acid or base does not include a term for the concentration of _____________ because pure liquids are never included in equilibrium-constant expressions.

Another measure of the strength of an acid is its _____________. In most cases of weak acid or weak base ionization, the extent of ionization is so small that the concentration of acid or base lost to ionization is a negligible amount (less than about 5% of the initial concentration). A comparison is made between the amount of _____________ ions produced at equilibrium and the maximum amount that could be produced if the acid ionized completely.

As equilibrium systems, determination of the pH of a weak acid or base will require an ICE table to write expressions for the _____________ concentrations. For an acid the equilibrium concentration of _____ will be ‘x’ and pH = -log x. If the ionization of the weak acid is sufficiently small, you will be able to make the ‘x is small’ approximation, otherwise you will need to solve a quadratic equation.

 

Weak bases are approached the same way. A _____________ Kb value indicates a stronger base, meaning it is better able to take on _____________ ions.

 

 

 

For a base the equilibrium concentration of _____ will be ‘x’ and pOH = -log x.

It is not necessary to memorize any Ka values. These are found in tables of acid ionization constants. Sometimes pKa is reported instead of Ka, where . If comparing pKa values, _____________ pKa value means a stronger acid.

 

The strength of an acid and its conjugate base are easily related using the ionization constants, Ka x Kb = Kw.   The inverse proportional relation between Ka and Kb means the stronger the acid or base, the _____________ its conjugate partner. This leads to the conjugate base of a strong acid not behaving as a base at all, because it is not strong enough to ionize with water. Hydronium ion is the strongest acid that may exist in water; any stronger acid will react completely with water to generate _____________ ions.

 

 

Practice Problems

 

  1. Calculate [H3O+] and pH for a 0.317 M solution of HF; Ka= 6.8 × 10−4.

 

 

 

  1. A 0.200 M solution of a weak base, represented as B, has a hydroxide ion concentration of 6.74 × 10−5 M. Calculate the value of Kb for B.

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