Acid-Base Reactions

 

            When acids an base react they neutralize each other.  If the reaction is allowed to run to completion, the end pH should be seven.  There are some factor to consider when looking at a an acid-base reaction.  The strength of the acid or base is very important to determining the overall process that is occurring.  A strong acid or a strong base will ionize completely, where a weak acid or weak base will only ionize partially.  The ionization constants for selected weak acids and weak bases are in the chart below.

            The reaction between a strong acid and a strong base will always result in the formation of water and an ionic salt (remember that all ionic substances are salts in their solid form, not just NaCl).  The reaction between a strong acid and a strong base is written as follows: Acid + Base à water + salt for strong acid + strong base.  An example is shown below:

 

HCl + KOH à H2O + KCl

HCl is the strong acid

KOH is the strong base

H2O is water

KCl is a salt

 

 

 

A weak acid and weak base generally result in a reversible reaction and the formation of a conjugate acid and a conjugate base as products.  The reaction between a weak acid and a weak base can be written as follows: Acid + Base à Conjugate Base + Conjugate Acid.  An example is shown below:

 

H2O + NH3 à NH4+ + OH-

 

H2O is the acid

NH3 is the base

NH4+ is the conjugate acid

OH- is the conjugate base

 

Strong acids will neutralize strong bases of equal concentrations in equal amounts.  More volume of a weak acid is needed to neutralize a strong base if the concentrations are equal and vise versa for weak bases and strong acids.  Weak acids and weak bases will neutralize each other if their if their ionization constants are equal.  If not, more of the less ionized solution will be required to neutralize the more ionized solution.

When an acid and base find equilibrium (the reaction is proceeding forward as fast as it is proceeding backward) the system is vulnerable to stress caused by either reactants or products ending up in the majority (by rapid shift in temperature, adding chemicals to one side or the other, or any other factor that causes the reaction to shift one direction or the other).  The stress on the system is relieved by a phenomenon known as LeChatelier’s Principle.  LeChatlier’s Principle states that a system at equilibrium when placed under stress will adjust to attempt to balance the system again.

If a chemical is added to an acid-base system that is at equilibrium, and that chemical has an anion similar to either of the ones in the reaction, the system will shift away from that side.  This process is known as the Common Ion Effect.  LeChatlier’s Principle and the Common Ion Effect are used to create chemicals known as buffers.  A buffer is a solution that contains a weak acid and salt with the same anion as the acid.  This solution will not change pH significantly when placed under stress.  Buffers are important to chemical reactions that have to remain in a narrow pH range to work properly.

            The process of making acid rain is actually a sequence of a variety of chemical reactions.  Elements like carbon, nitrogen and sulfur, found in most fossil fuels to some degree, combine with oxygen during combustion to form the oxides CO2, NOx & SOx, where the variable x represents a series of different oxygen compounds.  The oxides find their way to the upper atmosphere and combine with water to form acidic compounds including H2CO3, H2SO4 , H2SO3, HNO2, HNO3).  Acid rain has a pH around 5.6 and is responsible for much of the chemical damage to statues, roads, paint, etc.