Acid is a solution that has an excess of H+ ions. It comes from the Latin word acere that means "sharp" or "sour". Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less acidic when mixed with bases.
Base is a solution that has an excess of OH- ions. Another word for base is alkali. Bases feel slippery, change litmus blue, and become less basic when mixed with acids.
Properties of Acids and Bases...
Strength of Acids and Bases:
Acids
Strong Acids:
· completely dissociate in water, forming H+ and an anion.
example: HN03 dissociates completely in water to form H+ and N031-.
The reaction is
HNO3(aq) → H+(aq) + N031-(aq)
A 0.01 M solution of nitric acid contains 0.01 M of H+ and 0.01 M N03- ions and almost no HN03 molecules. The pH of the solution would be 2.0.
Weak acids:
· a weak acid only partially dissociates in water to give H+ and the anion
for example, HF dissociates in water to give H+ and F-. It is a weak acid. with a dissociation equation that is
HF(aq) ↔ H+(aq) + F-(aq)
· Note the use of the double arrow with the weak acid. That is because an equilibrium exists between the dissociated ions and the undissociated molecule. In the case of a strong acid dissociating, only one arrow ( → ) is required since the reaction goes virtually to completion.
· An equilibrium expression can be written for this system:
Ka = [ H+][F-] / [HF]
· Which are the weak acids? Anything that dissociates in water to produce H+ and is not one of the 6 strong acids.
Bases
Strong Bases:
- They dissociate 100% into the cation and OH- (hydroxide ion).
example: NaOH(aq) → Na+(aq) + OH-(aq)
a. 0.010 M NaOH solution will contain 0.010 M OH- ions (as well as 0.010 M Na+ ions) and have a pH of 12.
- Which are the strong bases?
The hydroxides of Groups I and II.
- Note: the hydroxides of Group II metals produce 2 mol of OH- ions for every mole of base that dissociates. These hydroxides are not very soluble, but what amount that does dissolve completely dissociates into ions.
exampIe: Ba(OH)2(aq) → Ba2+(aq) + 2OH-(aq)
a. 0.000100 M Ba(OH)2 solution will be 0.000200 M in OH- ions (as well as 0.00100 M in Ba2+ ions) and will have a pH of 10.3.
Weak Bases:
What compounds are considered to be weak bases?
- Most weak bases are anions of weak acids.
- Weak bases do not furnish OH- ions by dissociation. They react with water to furnish the OH- ions.
Note that like weak acids, this reaction is shown to be at equilibrium, unlike the dissociation of a strong base which is shown to go to completion.
- When a weak base reacts with water the OH- comes from the water and the remaining H+ attaches itsef to the weak base, giving a weak acid as one of the products. You may think of it as a two-step reaction similar to the hydrolysis of water by cations to give acid solutions.
examples:
NH3(aq) + H2O(aq) → NH4+(aq) + OH-(aq)
methylamine: CH3NH2(aq) + H20(l) → CH3NH3+(aq) + OH-(aq)
acetate ion: C2H3O2-(aq) + H2O(aq) → HC2H302(aq) + OH-(aq)
Acid-Base Properties of Salt Solutions:
definition of a salt:
- an ionic compound made of a cation and an anion, other than hydroxide.
- the product besides water of a neutralization reaction
determining acidity or basicity of a salt solution:
- split the salt into cation and anion
- add OH- to the cation
a. if you obtain a strong base. the cation is neutral
b. if you get a weak base, the cation is acidic
b. if you get a weak base, the cation is acidic
- Add H+ to the anion
a. if you obtain a strong acid, the anion is neutral
b. if you obtain a weak acid. the anion is basic
b. if you obtain a weak acid. the anion is basic
- Salt solutions are neutral if both ions are neutral
- Salt solutions are acidic if one ion is neutral and the other is acidic
- Salt solutions are basic is one of the ions is basic and the other is neutral.
- The acidity or basicity of a salt made of one acidic ion and one basic ion cannot be determined without further information.
Acid-Base Theories
There are three theories that identify a singular characteristic which defines an acid and a base: the Arrhenius theory, for which the Swedish chemist Svante Arrhenius was awarded the 1903 Nobel Prize in chemistry; the Brönsted-Lowry, or proton donor, theory, advanced in 1923; and the Lewis, or electron-pair, theory, which was also presented in 1923. Each of the three theories has its own advantages and disadvantages; each is useful under certain conditions.
The Arrhenius Theory
 |
| Svante Arrhenius |
When an acid or base dissolves in water, a certain percentage of the acid or base particles will break up, or dissociate , into oppositely charged ions. The Arrhenius theory defines an acid as a compound that can dissociate in water to yield hydrogen ions, H+, and a base as a compound that can dissociate in water to yield hydroxide ions, OH− .
The Brönsted-Lowry Theory
 |
| Thomas Martin Lowry Johannes Brönsted |
Some substances act as acids or bases when they are dissolved in solvents other than water, such as liquid ammonia. The Brönsted-Lowry theory, named for the Danish chemist Johannes Brönsted and the British chemist Thomas Lowry, provides a more general definition of acids and bases that can be used to deal both with solutions that contain no water and solutions that contain water. It defines an acid as a proton donor and a base as a proton acceptor. In the Brönsted-Lowry theory, water, H2O, can be considered an acid or a base since it can lose a proton to form a hydroxide ion, OH−, or accept a proton to form a hydronium ion, H3O. When an acid loses a proton, the remaining species can be a proton acceptor and is called the conjugate base of the acid. Similarly when a base accepts a proton, the resulting species can be a proton donor and is called the conjugate acid of that base.
The Lewis Theory
 |
| Gilbert Newton Lewis |
Another theory that provides a very broad definition of acids and bases has been put forth by the American chemist Gilbert Lewis. The Lewis theory defines an acid as a compound that can accept a pair of electrons and a base as a compound that can donate a pair of electrons. Boron trifluoride, BF3, can be considered a Lewis acid and ethyl alcohol can be considered a Lewis base.
Titrations...
A titration is a method of analysis that will allow you to determine the precise endpoint of a reaction and therefore the precise quantity of reactant in the titration flask. A buret is used to deliver the second reactant to the flask and an indicator or pH Meter is used to detect the endpoint of the reaction.
What is pH???
A pH (potential of Hydrogen) measurement reveals if a solution is acidic or alkaline (also base or basic). If the solution has an equal amount of acidic and alkaline molecules, the pH is considered neutral. Very soft water is commonly acidic, while very hard water is commonly alkaline, though unusual circumstances can result in exceptions.
The pH scale is logarithmic and runs from 0.0 to 14.0 with 7.0 being neutral. Readings less than 7.0 indicate acidic solutions, while higher readings indicate alkaline or base solutions. Some extreme substances can score lower than 0 or greater than 14, but most fall within the scale.
Why is Litmus Paper used in Chemistry???
Litmus paper is a quick way to test a liquid to see whether it is acid or alkaline.
Dyes called indicators show very quickly if a substance dissolved in water is acid or alkaline. One of these dyes is litmus. If a piece of paper impregnated with litmus is dipped into a solution, it immediately turns red if the solution is acid. If the solution is alkaline, the litmus turns blue.
A similar dye to litmus is present in some red vegetables, such as red cabbage and beetroot and this dye changes colour in the same way during cooking. If your tap water is hard, or alkaline, the vegetables will be coloured a deep purplish-blue.
Sing the...
Chorus
Acids and bases have
two different faces,
Two different personalities,
And you gotta find out
what they're all about,
If you want to learn chemistry.
Now the first thing you gotta get
straight in your head,
Acids turn blue litmus paper to red.
They react with metals with awesome power,
They neutralize bases and taste very sour.
Bases are different from acids, it's true,
Bases turn red litmus paper to blue.
They neutralize acids, feel slippery on skin,
They taste sorta bitter, and are called alkaline.
Chorus.
Well, there's even more to this wonderful tale,
Something we call a pH scale.
It numbers from zero up to fourteen,
Those are powers of ten,
if you know what I mean.
When the pH is 7, then it's a case,
Where the sample is neither acid nor base.
But less than 7? . . . it's an acid we say,
More than 7? . . . it's a base all the way.
Chorus.
Acids and bases? . . .
Now why should you care?
'Cause acids and bases are everywhere.
You put them on salads,
and they get in the rain,
They settle your tummy,
and clean out your drain.
So the next time you're sitting
under some shade,
And you're sipping a glass full
of real lemonade,
You'll notice the sour taste
and think in your head,
I bet it turns blue litmus paper to red.
