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Ma. Adeliza F. Mortalla BSChE-4 March 6, 2012

Physical Chemistry 2 Dr. Dahlia H. Pescos

The Arrhenius Theory of Acids and Bases

The Theory

Acids are substances which produce hydrogen ions in solution. Bases are substances which produce hydroxide ions in solution.

Neutralization happens because hydrogen ions and hydroxide ions react to produce water.

Limitations of the theory

Hydrochloric acid is neutralized by both sodium hydroxide solution and ammonia solution. In

both cases, you get a colourless solution which you can crystallize to get a white salt - either

sodium chloride or ammonium chloride.

These are clearly very similar reactions. The full equations are:

In the sodium hydroxide case, hydrogen ions from the acid are reacting with hydroxide ions

from the sodium hydroxide - in line with the Arrhenius theory.

However, in the ammonia case, there don't appear to be any hydroxide ions!

You can get around this by saying that the ammonia reacts with the water it is dissolved in to

produce ammonium ions and hydroxide ions:

This is a reversible reaction, and in a typical dilute ammonia solution, about 99% of theammonia remains as ammonia molecules. Nevertheless, there are hydroxide ions there.

However, this same reaction also happens between ammonia gas and hydrogen chloride gas.

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In this case, there aren't any hydrogen ions or hydroxide ions in solution - because there isn't

any solution. The Arrhenius theory wouldn't count this as an acid-base reaction, despite the fact

that it is producing the same product as when the two substances were in solution.

Problems with Arrhenius' Theory

1) The solvent has no role to play in Arrhenius' theory. An acid is expected to be an acid in any

solvent. This was found to not be the case. For example, HCl is an acid in water, behaving in the

manner Arrhenius expected. However, if HCl is dissolved in benzene, there is no dissociation,

the HCl remaining as undissociated molecules. The nature of the solvent plays a critical role in

acid-base properties of substances.

2) All salts in Arrhenius' theory should produce solutions that are neither acidic or basic. This is

not the case. If equal amounts of HCl and ammonia react, the solution is slightly acidic. If equal

amounts of acetic acid and sodium hydroxide are reacted, the resulting solution is basic.

Arrhenius had no explanation for this.

3) The need for hydroxide as the base led Arrhenius to propose the formula NH4OH as the

formula for ammonia in water. This led to the misconception that NH4OH is the actual base, not

NH3.

In fact, by 1896, several years before Arrhenius announced his theory, it had been recognized

that characteristic base properties where just as evident in such solvents as aniline, where no

hydroxide ions were possible.

4) H+, a bare proton, does not exist for very long in water. The proton affinity of H2O is about

799 kJ/mol. Consequently, this reaction:

H2O + H+

---> H3O+

happens to a very great degree. The "concentration" of free protons in water has been

estimated to be 10130

M.

The Arrhenius theory of acids and bases will be fully supplanted by the theory proposed

independently by Johannes Brnsted and Thomas Lowry in 1923.

Debye-Hckel Theory

A solution is defined as a homogeneous mixture of two or more components existing in a single

phase. In this description, the focus will be on liquid solutions because within the realm of

biology and chemistry, liquid solutions play an important role in multiple processes. Without

the existence of solutions, a cell would not be able to carry out glycolysis and other signaling

cascades necessary for cell growth and development. Chemists, therefore, have studied the

http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Solutionshttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Solutions

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processes involved in solution chemistry in order to further the understanding of the solution

chemistry in nature.

The mixing of solutions is driven by entropy, opposed to being driven by enthalpy. While

an ideal gas by definition does not have interactions between particles, an ideal solution

assumes there are interactions. Without the interactions, the solution would not be in aliquid phase. Rather, ideal solutions are defined as having an enthalpy of mixing or enthalpy

of solution equal to zero (Hmixingor Hsolution = 0). This is because the interactions

between two liquids, A-B, is the average of the A-A interactions and the B-B interactions. In

an ideal solution the average A-A and B-B interactions are identical so there is no difference

between the average A-B interactions and the A-A/B-B interactions.

Since in biology and chemistry the average interactions between A and B are not always

equivalent to the interactions of A or B alone, the enthalpy of mixing is not zero.

Consequently, a new term is used to describe the concentration of molecules in solution.

Activity, 1, is the effective concentration that takes into account the deviation from idealbehavior, with the activity of an ideal solution equal to one.

An activity coefficient, 1, is utilized to convert from the solutes mole fraction,x1, (as a unit

of concentration, mole fraction can be calculated from other concentration units like

molarity, molality, or percent by weight) to activity, 1.

=

Debye-Hckel Formula

The Debye-Hckel formula is used to calculate the activity coefficient.

This form of the Debye-Hckel equation is used if the solvent is water at 298 K.

mean ionic activity coefficent catonic charge of the electrolyte for

anionic charge of the electrolyte forI ionic strength

relative dielectric constant for the solution

T temperature of the electrolyte solution

http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Entropyhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Enthalpyhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Gases/The_Ideal_Gas_Lawhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Solutions/Solution_Basics/Units_Of_Concentrationhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Solutions/Solution_Basics/Units_Of_Concentrationhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Gases/The_Ideal_Gas_Lawhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Enthalpyhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Entropy

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Example

Consider a solution of 0.01 M MgCl2(aq) with an ionic strength of 0.030 M. What is the mean

activity coefficient?

Reference:

http://www.chemguide.co.uk/physical/acidbaseeqia/theories.html http://www.chemteam.info/AcidBase/Arrhenius-AcidBase.html http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Soluti

ons/Nonideal_Ssolutions/Debye-H%C3%BCckel

http://www.chemguide.co.uk/physical/acidbaseeqia/theories.htmlhttp://www.chemteam.info/AcidBase/Arrhenius-AcidBase.htmlhttp://www.chemteam.info/AcidBase/Arrhenius-AcidBase.htmlhttp://www.chemguide.co.uk/physical/acidbaseeqia/theories.html