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An Arrhenius acid is a substance that releases hydrogen ions in solution and a base is a substance that releases hydroxide ions in solution. Example: HCl - when dissolved in water it dissociates (ionizes) into hydrogen ions and chloride ions.
This dissolution increases the hydrogen ion concentration in the water. Since a hydrogen ion has a plus one charge and is seeking electrons, this makes the solution more reactive than a neutral solution.
A substance like NaOH dissolved in water dissociates into sodium ions and hydroxide ions.
This increase in the hydroxide ion concentration makes the solution favor a reaction in which the hydroxide ion will donate or share its electrons.
This makes this solution more reactive than a neutral solution.
As we saw with neutralization reactions when an acid reacts with a base the hydrogen ion (+1) combines with the hydroxide ion (-1) to form water.
Another definition of an acid and base we use in Chem. I is the Brønsted-Lowry definition in which both the acid and base are defined in terms of the hydrogen ion.
Remember that a hydrogen ion is just a proton. An acid is a hydrogen ion (proton) donor and a base is a hydrogen ion (proton) acceptor. Example: Ammonia (NH3) acts as a Brønsted-Lowry because it will accept a hydrogen ion to form an ammonium ion (NH4+).
If this reaction occurs in water it is also accompanied by and increase in hydroxide ion concentration because the hydrogen ion come from the dissociation of water.
In this instance water acts as the Brønsted-Lowry acid. NH3 + H2O → NH4+ + OH- A reaction like this is called a hydrolysis reaction because water is being split (lysed) into a hydrogen ion and a hydroxide ion.
Water normally ionizes to a very small degree (1 x 10-7 moles/liter) H2O H+ + OH- This means that (1 x 10-7 moles/liter) of hydrogen ion and (1 x 10-7 moles/liter) of hydroxide ion are produced in equal molar ratios.
This is why pure water is said to be neutral (equal amount of hydrogen and hydroxide ions). Any substance that increases the hydrogen ion concentration will make the solution acidic. Any substance that increases the hydroxide ion concentration will make the solution basic.
This leads us to the concept of pH. A more convenient way to represent acidity than moles per liter is pH.
pH is defined as the negative log of the hydrogen ion concentration. The log10 of (1 x 10-7 ) is -7, there for the negative log is 7.
This is why a pH of 7 is said to be neutral. That is; the concentration of hydrogen ion in pure water is (1 x 10-7 moles/liter) and the negative log of (1 x 10-7 moles/liter) is 7.
This also means that the hydroxide ion concentration is (1 x 10-7 moles/liter). We can also assign a “p” value to the hydroxide ion concentration: pOH.
Therefore the pOH of water is 7.
Remember the pH scale runs from 0-14 with pH values less than 7 being acidic and pH values greater than 7 being basic.
Note that pH is a logarithmic scale and each unit change in pH represents a 10 fold change in hydrogen ion concentration.
Example: Hydrochloric acid is a strong acid that completely dissociates in water, this means that for every mole of hydrochloric acid one mole of hydrogen ions and one mole of chloride ions are produced. HCl→H+ + Cl- This means a 0.1 M solution of HCl produces 0.1 moles/liter of H+.
The negative log of 0.1 (pH) is 1.0 (quite acidic) A 0.01 M solution has a pH of 2.0 (less acidic). Note: Negative log of 1x10-2 = 2.0 A 1.0 M solution has a pH of -1.0.
How can this be if the pH scale goes from 0-14? The pH scale was originally based on biological pH’s.
Very concentrated inorganic acids can have negative pH’s and very concentrated inorganic bases have pH’s greater that 14.
There is a relationship between pH and pOH that will be given here but will be explained in AP Chemistry. 14-pH=pOH and 14-pOH =pH
This means if we can calculate the pH of a solution then we can determine the pOH from this relationship.
Example: if we have a solution of nitric acid (a strong mono-protic acid) with a concentration 0.03M the pH=-log(0.003) = 2.52; therefore the pOH = (14-2.52)=11.58. From the pOH we can calculate the hydroxide ion concentration as the antilog10 of -11.58 or 10-11.58=2.63 x10-12.