SCH 4U - Acids & Bases

Dissociation of Water

H₂O(l)  ↔  H⁺(aq)  +  OH^-(aq)     very small K

 

This process is called the autoionization of water (1:10⁸ ionized: unionized)

K_W = [H⁺][OH^-] = 1.00 x 10^-14 at 25°C

 

The equilibrium constant, K_W, is called the ion product constant.  This equation is valid for solutions as well as, pure water.

• If [H⁺] > [OH^-], solution is acidic
• if [H⁺] < [OH^-], solution is basic
• if [H⁺] = [OH^-], solution is neutral

The Proton in Water

Water can dissociate to form the hydrogen ion (which is equivalent to a single proton*).  But, when H⁺ is in solution, its positive charge is attracted to the negative end of the polar water molecule to form a hydrated hydrogen ion – the hydronium ion (H₃O⁺).  

 

 

Since a hydronium ion is simply a hydrated hydrogen ion, we can use the hydronium ion and the hydrogen ion interchangeably in reaction equations.

* A hydrogen atom, H, has 1 proton, 1 electron and 0 neutrons.  Thus, the hydrogen ion, H⁺, has 1 proton, 0 electrons and 0 neutrons.

Arrhenius Acids and Bases

One the first acid-base definitions (Svante Arrhenius 1859-1927)

“Acids are substances that, when dissolved in water, increase the hydrogen ion concentration.  Likewise, bases are substances, that when dissolved in water, increase the hydroxide ion concentration.”

Acid Properties

• electrolyte (a solute that produces ions in solution – conducts electricity)
• reacts with most metals to form hydrogen gas
• indicators: litmus turns red; bromthymol blue turns yellow; phenolphthalein stays colourless
• tastes sour

Base Properties

• electrolyte (a solute that produces ions in solution – conducts electricity)
• indicators: litmus turns blue; bromthymol blue stays blue; phenolphthalein turns pink
• tastes bitter and feels soapy

Brønsted-Lowry Acids and Bases

"An acid is a proton donor and a base is a proton acceptor."

Compared to the Arrhenius definition, the Brønsted-Lowry definition allows for a broader range of substances which can be acidic or basic. 

HCl(g)  +  H₂O(l)  ↔  H₃O⁺(aq)  +  Cl^-(aq)

   A              B

HNO₃(aq)  +  NH₃(g)  ↔  NH₄⁺(aq)  +  NO₃^-(aq)

   A                   B

Conjugate Acid-Base Pairs

In any acid-base equilibrium, both the forward and reverse reactions involve proton transfer.

HNO₃(aq)  +  H₂O(l)  ↔  NO₃^-(aq)  +  H₃O⁺(aq)

     A                  B               CB                CA

      ↑_________________↑

                          ↑_________________↑

NH₃(aq)   +   H₂O(l)   ↔   NH₄⁺(aq)  +  OH^-(aq)

     B                  A                CA                 CB

      ↑_________________↑

                        ↑_________________↑

 

Notice in these two examples that the acid (A) and its conjugate base (CB) differ by a single proton.  The same can be said for the base (B) and its conjugate acid (CA).

TryIt!:

Finish the reaction below.  Label the acid, base, conjugate acid and conjugate base.  Link the related substances across the reaction, like in the example above.

 

HClO₄(aq)  +  NH₃(aq)  ↔  ________  +  ________

Conjugate Acid-Base Strengths

The stronger an acid, the weaker its conjugate base; the weaker the acid, the stronger its conjugate base.  This will be important to remember in the coming lessons.

Strong Electrolytes

A strong electrolyte is a substance which dissolves to produce exclusively ions.  

• Some common strong acids: hydrochloric acid, hydrobromic acid, nitric acid, perchloric acid.
• Some common strong bases: all group 1 & 2 metal hydroxides, like sodium hydroxide, barium hydroxide.
  

HCl(aq)  +  H₂O(l)  →  H₃O⁺(aq)  +  Cl^-(aq) 

The above equation can be written in a simplified form as well:

HCl(aq)  →  H⁺(aq)  +  Cl^-(aq)

Regardless of whether we write the simplified version of the equation or not, notice that the arrow we use only points in one direction.  This is because the acid dissociates 100% in solution.  Since the acid dissociates completely, for every 1 acid molecule that dissociates, 1 hydrogen (or hydronium) ion ends up in solution.

Weak Electrolytes

A weak electrolyte is a substance that dissolves to produce ions only to a limited extent.

• Some common weak acids: acetic acid, lactic acid, benzoic acid, hydrocyanic acid. 
• Some common weak bases: all non-group 1 & 2 metal hydroxides, like ammonia.

CH₃COOH(aq)  +  H₂O(l)  ↔  H₃O⁺(aq)  +  CH₃COO^-(aq)

The above equation can be written in a simplified form as well:

CH₃COOH(aq)  ↔  H⁺(aq)  +  CH₃COO^-(aq)

Regardless of whether we write the simplified version of the equation or not, notice that the arrow we use points in two directions.  This is because the acid dissociates less than 100% in solution (in the case of the acetic acid, the amount of dissociation is about 1.6%). 

 

TryIt! Answer:

 

Homework #1, 4, 5 
Answer Keys can be found here.