Acid Base Chemistry

We live in an aqueous world. From the rivers and lakes that make up our landscape, the paints, soaps and many commercial products we use, to the cells that make up our very bodies, we are immersed in water. The quality of that very water, determines whether chemical reactions occur. One way to measure that quality is to assess the acid nature of the solution in question.

Changes in pH change the way the materials dissolved in the water interrelate. For instance, milkweeds, dandelions, and rubber trees all have a latex sap running in their veins. If the plant is injured, and the latex sap is exposed to to the air, the latex sap coagulates becoming a gooey, sticky substance. Why? The pH of the sap changes from a basic solution to an acidic one, thus changing the way the molecules in the sap interact with one another.

That is just one example of how pH changes impact substances in our physical world. We can find literally hundreds of such examples. From making jelly, to protein formation, from the absorption of minerals by plants, to perming hair, pH mediated responses are everywhere we look. It is for that reason that understanding the acidic/basic nature of water solutions is so important.

Acids are materials that release hydrogen ions when in a water solution . We can easily determine if a substance is an acid by noting whether the compound has hydrogen as the first expressed atom in the compound. (H₂O, although not usually thought of as an acid, could be considered one using this definition.) Bases are substances that when in a water solution release hydroxide ions. (Be aware that an alcohol group is not a hydroxide group.) But these two groups are not the only substances that can behave as acids or bases.

One group that can have acidic or basic properties is the oxides. Metal oxides have basic properties and are classified as basic anhydrides or bases without water. Lime, CaO is an example.

Nonmetal oxides have acidic properties and are classified as acidic anhydrides- acids without water. CO₂ is an example.

Substances that can act as both an acid or a base are said to be amphoteric. Al(OH)₃, Zn(OH)₂, and H₂O are examples of amphoteric substances. Water for example, has both hydrogen and hydroxide ions present and thus can act as either an acid or base.

A look at the periodic table gives a quick look at the nature of the oxides. The implication is also that materials that have loose electrons behave as bases and those with tight electrons behave as acids.

There is another group too that can behave acidic/basic/or neutral. This group is the salts. While we may think of sodium chloride as salt, technically a salt is any ionic compound except for oxides and hydroxides. CaCl₂, CuSO₄, Na₂S, Ba₃(PO₄)₂ are all examples of salts. Some salts will be neutral. Some will be acidic. Others will be basic.

There are several ways to explain how salts can have varying acidic/basic natures. Let's look at two different explanations.

The first is a simplified look at the nature of salts assuming a particular method of production. It can be assumed that the salt water was prepared by reacting an acid and a base resulting in the formation of the salt and water.

Using this assumption, the positive ion of the salt, it is assumed, had its origin in the base. The negative ion of the salt originated from the acid. For any salt then it is possible to conclude from what acid and what base the salt could have originated. For example:

Using this reasoning, the nature of the salt can be determined from the nature of the acid and base used in the preparation. Sodium chloride's precursors - HCl and NaOH -are a strong acid and base, respectfully. It would be reasonable to say that the resulting salt, the product of a strong acid and a strong base, would be a neutral salt. Ammonium chloride, on the other hand, can be thought to be derived from ammonia and hydrochloric acid, a weak base and a strong acid. The resulting salt - it should make sense - is an acidic salt. The charts below indicate to us the various combinations of different strengths of acids and base and the resulting salt's nature. There are also a couple of charts detailing differences in strengths among acids and bases.

The other way that a salt's nature can be interpreted is an academically more accurate way of looking at the nature of the compound. Instead of matching up presumed precursors to the salt, the salt's ion effects are analyzed in the water solution. For example, in a beaker of some NaHCO₃ dissolved in a water solution, there are Na⁺and HCO₃^- ions present. But because water is present too, there are H⁺, OH^- ions also present. Since H⁺ and HCO₃^-prefer to join up to form H₂CO₃, the equilibrium constant for H₂CO₃ ⇄ H⁺ + HC0₃^-is Ka = 4.3 x 10^-7, the [H⁺] goes down. Since the [H⁺] goes down, this causes a shift to the right in the water equilibrium (H₂O ⇄ H⁺ + OH^-) thus producing more OH^-. Thus instead of having 1x 10^-7 moles per liter of OH- as water does, NaHCO₃ (aq) has a higher number of [OH^-] and thus is basic.

A Baking Soda Solution

NaHCO₃(aq)

An ammonium chloride solution

Analyzing one more example using this more sophisticated approach, let's analyze NH₄Cl. NH₄Cl dissolves into NH₄⁺and Cl^- in solution. The NH₄⁺ions combine with the hydroxide ions to form ammonia and water due the ammonium equilibrium. NH₃ + H₂O ⇄ NH₄⁺+ OH^- Kb = 1.8 x 10^-5 at 25 °C. This means that the [OH^-] in solution is lowering. This shifts the water equilibrium (H₂O ⇄ H⁺+ OH^-) thus producing more H⁺. Thus instead of having 1 x 10^-7 moles per liter of H⁺ as water does, the NH₄Cl(aq) has a higher concentration of [H⁺] and thus is acidic.

To see an example of how different materials can be acidic, basic, or neutral, see the litmus solutions page.