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Acids and Bases




The nature of acids and bases

Acids are generally a class of substances that taste sour, such as vinegar, which is a dilute solution of acetic acid. Bases, or alkaline substances, are characterized by their bitter taste and slippery feel. The first precise definition of an an acid and base was given by Svante Arrhenius, and is referred to as Arrhenius Theory.  

Acid Strength

The strength of an acid has to do with the percentage of the initial number of acid molecules that are ionized. If a higher percentage of the original acid molecules are ionized, and therefore, donated as hydrated protons (hydronium ions) then the acid will be stonger. Strong acids are Hydrochloric (HCl(aq)), Hydrobromic (HBr(aq)), Nitric (HNO3), Sulfuric (H2SO4), and Perchloric (HClO4) acids. In each of these molecular acids the percentage of ionization is almost 100%.

On the other hand, there are certain acids that will not ionize very easily when added to water. These acids will only give up there protons to water with difficulty. For example, acetic acid (HC2H3O2), Hydrofluoric acid (HF(aq)), HydroCyanic (HCN), Carbonic acid (H2CO3), Sulfurous acid (H2SO3), and Nitrous acid (HNO2) are only a few of the many "weak" acids. They will only allow 1-5% ionization. In other words, if 5% ionizatiuon takes place only 5 molecules out of 100 will ionize. The other 95 molecules go into solution as "molecules" and not ions. Weak acids can exhibit low pH readings just as strong acids. Acid strength of weak acids has more to do with the value of the ionization constant of the acid.

The pH Scale

The pH scale measures how acidic or basic a substance is. It ranges from 0 to 14. If a pH is lower than 7 it indicates the solution is an acid. If it is above 7 it is a base or Alkaline. If a pH is a 7 it is neutral. Strong acids have lower pHs than weak acids and strong bases have lower pHs than weak bases. A neutral solution is neither an acid or base. It has a pH of 7. Scientists use a pH number to show the strength of an acid or base. A pH is measured by dipping litmus into solution such as water or other substances. Chemicals that are very basic or acidic are called "reactive." Automobile batteries have acids which are reactive and contain a stronger form of some of the same acid that is in acid rain. Also household drain cleaners often contain lye, a very alkaline that is reactive. Vinegar and lemon juice are acidic substances and laundry detergents and ammonia are basic. Pure water is neutral witch means that it has a pH of 7.0 . It is not reactive.

Above is an example of the pH scale, provided with examples of a substance that is of that corresponding pH factor.



Calculating the pH of strong and weak acids

If you have an IONIC compound and you put it in water it will break apart into two ions. If one of those ions is H+... The solution is acidic. If one of the ions is OH-... The solution is basic. There are other ions which make acidic and basic solutions, but we won't be talking about them here.

That pH scale we talked about is actually a measure of the number of H+ ions in a solution. If there are a lot of H+ ions, the pH is very low. If there are a lot of OH- ions, that means the number of H+ ions is very low, so the pH is high.


A base is often described as a compound when they are in water or other solvents. Most bases contain atoms of metal. Bases often taste bitter and feels slippery. Bases will turn litmus paper blue.  A strong base has a pH of 10-14.  A weak base only partially ionizes in an aqueous solution.  They usually have a pH close to 7 (8-10). 

Baking soda is an example of a base.

Polyprotic acids

Polyprotic acids contain more than one mole ionizable hydronium ions per mole of acids. They ionize to give more than one H+ ions per molecule. Possible forms of three polyprotic acids are given below after their dissociation into H+ ions.

The effect of structure on acid-base properties

The effect of structure on acid-base properties helps determine relative acidity or basicity of species in the absence of information regarding K.  

The Lewis Acid-Base Model

In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor. A Lewis base is any substance, such as the OH- ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor.