Acids & Bases

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 Creating the Context


A Bronsted-Lowry acid is defined as a substance that donates a proton and a Bronsted-Lowry base is a substance that accepts a proton in water. One example of a strong acid is HCl. Strong acids are defined as those that donate all their protons to water. A more accurate definition, discussed later, relies on the magnitude of the equilibrium constant for the reaction of the acid with water. In the following reaction HCl is the Bronsted-Lowry acid and H2O is the Bronsted-Lowry base.

HCl(aq) + H2O(l) <=> H3O+(aq) + Cl-(aq)

A very important function in acid-base chemistry is the "p" function, which indicates the negative log. Thus p(x) means take the negative logarithm (base 10) of x. Our working definition of the pH of a solution is thus given by the equation below. In this module, we will use the terms H+ and H3O+ interchangeably. The proton in water is heavily hydrated and thus is probably best described as H+(H2O)x where x may vary between 4 and 10.

pH = -log [H3O+] = -log[H+]

Thus, the pH of a 0.10 M HCl solution is 1.00. What is the pH of a 0.01 M solution of HCl?

Strong bases are defined as those that totally dissociate in water. An example of a strong base is NaOH in water.

NaOH(aq) <=> Na+(aq) + OH-(aq)

The pOH of a 0.10 M NaOH solution is 1.00. What is the pOH of a 0.01 M solution of NaOH?



Gathering Information


Your instructor will add one or more acid-base indicators to the following solutions:

0.1 M HNO3

0.01 M HNO3

0.1 M HBr

0.1 M NaOH

0.01 M NaOH

0.1 M KOH

0.1 M CH3COOH

water

 

Gather in a small group and explain the differences. After you finish the Working with Information section, calculate the pH of each solution.

 

Working with Information

All of the acid-base reactions that you will study are in aqueous solution. Water can act as both an acid and a base. One molecule of water acts as a base and another acts as an acid. Though the reaction is not thermodynamically favored in the forward direction (+DG), it is a very important reaction because this acid-base equilibrium in water determines the pH of the solution. This particular reaction is so important in acid-base chemistry that it is given a unique symbol, Kw, the dissociation constant for water. Its value at 25C = 1.00x10-14.

H2O(l) + H2O(l) <=> H3O+(aq) + OH-(aq)

1. Write the equilibrium expression corresponding to Kw.

2. How do the concentration of H3O+(aq) and OH-(aq) relate to one another in a pure water solution?

3. Calculate the concentration of H3O+(aq) and OH-(aq) in pure water.

4. What is the pH of the pure water solution?

5. What is the pOH of the solution?

6. Show that the following relation must always be true: pH + pOH = 14

 

 

Making the Link

1. Make a list of all the strong acids and bases that you will be responsible for learning in your class.

 

2. Write a few sentences answering the following questions:
What is the difference between an acid and a base?
What is the difference between a weak acid and a strong acid?
What is the difference between a weak base and a strong base?

 

3. List two experimental methods for determining whether a substance is an acid, base or neither.

Last Updated April 26, 2003
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