Download Acid and Bases Notes

300 Chemistry
Acids and Bases
Intro to Acids and Bases
Definitions:
Acids…
Have a low pH
Taste sour
React with metals to produce a salt and
hydrogen gas
React with bases to produce a salt and
water (neutralization reaction)
Are electrolytes, conduct electricity
Turn blue litmus paper red
---
Bases…
Have a high pH
Taste bitter
--React with acids to produce a salt and
water (neutralization reaction)
Are electrolytes, conduct electricity
Turn red litmus paper blue
Feel slippery
Different scientists have come up with a number of definitions regarding acids and bases due to the
observation they made in their research. We will study the following: Arrhenius and Bronstead-Lowery
Theories.
=============================================================================
Arrhenius Theory
Around 1884, a Swedish scientist, Svante Arrhenius developed the theory that acids, when placed in water, acids
form H+1ions and bases form OH-1 ions. This was the first modern definition of acids and bases and he won the
Nobel Prize for it in 1903.
Arrhenius Acid = a substance that produces H+ ions in aqueous solution
Ex: HCl → H+1 + Cl-1
Note: Remember, a hydrogen atom only contains one electron and one proton and NO neutrons in its nucleus. As a
result, when it forms and H+1 ion, it loses its only electron, and all that is left is the proton in the nucleus. As result,
H+1 is called a “proton” as well as a “hydrogen ion.” A monoprotic (mono = 1) acid only has 1 acidic proton (H+);
a diprotic (di = 2) has 2… etc.
Arrhenius Base = a substance that produces OH-1 ions in aqueous solution
Ex: NaOH → Na+1 + OH-1
Svante Arrhenius
====================================================================================================================
Bronsted-Lowry Theory
The Bronsted-Lowry definition is named for Johannes Bronsted and Thomas Lowry, who independently
proposed it in 1923. A Bronsted-Lowry (BL) acid is defined as any substance that can donate a hydrogen ion
(proton) and a Bronsted-Lowry base is any substance that can accept a hydrogen ion (proton). Thus, according
to the BL definition, acids and bases must come in what is called conjugate pairs.
(http://www.nyu.edu/classes/tuckerman/honors.chem/lectures/lecture_21/node3.html)
BL Acid = proton (H+) donor
BL Base = proton (H+) acceptor
Conjugate acid = what the base becomes after it accepts a proton
Conjugate base = what the acid becomes after it donates a proton
Ex: NH3 + H2O → NH4+1 + OH -1
Ammonia is the Bronsted-Lowry base (it will accept a proton)
Water is the Bronsted-Lowry acid (it will donate a proton)
Ammonium ion is the conjugate acid of ammonia (it has accepted a proton)
Hydroxide ion is the conjugate base of water (it has donated a proton)
Great link for further reading and many examples:
http://www.mpcfaculty.net/mark_bishop/Bronsted_Lowry.pdf
Fun website – easier to understand:
http://www.blobs.org/science/article.php?article=3
===================================================================================
What are considered acids and bases?
Acids
The six strong acids are: HCl (hydrochloric), HBr (hydrobromic), HI (hydroiodic), HNO3 (nitric), H2SO4 (sulfuric),
and HClO4 (perchloric)
Remember: Strong acids and bases are strong electrolytes, weak acids and bases are weak electrolytes (can break
into ions and carry electric current. No ions = no current!) If a substance is not one of the strong acid listed
above, it’s a weak acid!
The weak acids are: HF (hydrofluoric), HC2H3O2 (acetic), H2CO3 (carbonic), and H3PO4 (phosphoric)
------------------Bases
The strong bases are: any alkali metal hydroxides (LiOH, NaOH, KOH, RbOH, etc.), Ca(OH)2 (calcium hydroxide),
Ba(OH)2 (barium hydroxide), and Sr(OH)2 (strontium hydroxide) If a substance is not a strong base, it’s a weak
base!
The most common weak one you’ll see if NH3 (ammonia)
What is pH?
pH (potential of hydrogen – or – how much hydrogen is in a substance) is very important biologically (pH of blood,
pH of the ocean or lakes, pH of rain, etc). It is a measure of acidity (how much H+1) and basicity (how much OH-1)
there is in a water solution.
If:
pH = 7
pH > 7
pH < 7
the solution is neutral
the solution is basic
the solution is acidic
If we use Universal Paper to determine pH, the following colors will appear:
If litmus paper is used:
Red litmus:
Base: turns paper blue
Acid: paper stays red (looks wet)
Blue litmus:
Base: paper stays blue (looks wet)
Acid: paper turns red
Determining pH
For strong acids and bases, it is easy to determine their strength, using pH
For weak acids and bases, it is a bit more complex to determine their strength and pH (we’re not going to worry
about those)
Acid and Base Strength for Strong Acids and Bases
Strong acids and bases dissociate 100% (equilibrium position is far to the right, reaction goes nearly completely to
the products (ions))
For all aqueous solutions water also dissociates a little: H2O ↔ H+1 + OH-1
We can write Keq = Kw = [H+1] x [OH-1] = 1.0 x 10-14
[It sounds strange… but this means even in a basic solution, there are hydrogen ions, and even in an acidic solution,
there are hydroxide ions… they come from the water]
pH below 7 indicates an acidic solution ([H+1] > [OH-1])
pH of 7 indicates a neutral solution (not an acid or a base) ([H+1] = [OH-1] = 1.0x10-7)
pH above 7 indicates a basic (alkaline) solution ([OH-1] > [H+1])
Neutralization
In a neutralization reaction, an acid and base react to produce some salt and water… at what is called the
“equivalence point”, we can say that:
Moles of H+ = Moles of OHFor a monoprotic acid and a base with one hydroxide, use the following equation:
Ma x Va = Mb x Vb
Titrations and Indicators
Titration = a process used to determine the amount (concentration) of acid or base in a solution
Indicator = a substance that marks the equivalence point of a titration (or identifies a pH range) by changing color
Common indicators: litmus paper (red in acid, blue in base), phenolphthalein (clear in acid, pink in base), and
universal indicator (a rainbow of colors depending on the pH)
pH meter = device used to make rapid, accurate pH measurements
Titration curve = a graph used to show how the pH of the unknown solution changes as various amounts of titrant
are added
How they are used:
-A solution of known concentration (the titrant) is delivered from a buret into an unknown solution containing an
indicator
-At the endpoint of the titration, when the unknown substance is just consumed, the stoichiometric (equivalence)
point is reached, and the indicator changes color
-Then, the amount of titrant added can be used to determine the concentration of the unknown