An introduction to acids and bases
The grade 10 definitions:
Acid: sour, conductor, red in litmus
Base: bitter, conductor, slippery, blue in litmus
Arrhenius’ definitions:
Acid: produce H+ ions in aqueous solutions
Bases: produce OH- ions in aqueous solutions
**these two definitions can explain most acids and bases but not
those like NH3 **
Bronsted-Lowry theory:
Acids: donate protons
Bases: accept protons
Examples :
1.
2.
ALSO, some substances can donate OR accept a proton. These are
called amphoteric. (they can act like acids OR bases)
Example:
Reversible acid-base reactions:
Example:
- 2 acids:
-2 bases:
-substances whose formulas differ by a single proton are called
CONJUGATE acid-base pairs:
A general term for this:
Which simplifies to:
The autoionization of water…takes place in all water samples
Looks like:
-this is why water has a slight conductivity
We can simplify to:
And make an equilibrium equation:
And then simplify again because the concentration of water molecules
is always 55.6mol/L:
Measurements show that the concentrations of H+ and OH- are both
1.0 x 10-7 at SATP so:
We use calculate Kw to calculate the H+ and OH- concentrations in
aqueous solution of a strong or a weak base at SATP if the
concentration of the other is known:
Strong acids
-ionize quantitatively (completely) in water to make H+
-assume 100% ionization
-include: HCl, HBr, H2SO4 , HNO3 and H3PO4
-one hydrogen is called monoprotic, two is diprotic, three is triprotic
Example:
Calculate the concentration of hydroxide ions in a 0.15 mol/L solution
of HCl.
Strong bases
-ionize completely in water to make OH-assume 100% ionization
-examples include group 1 and group 2 hydroxides (
Example: calculate the concentration of hydroxide in a 0.25 mol/L
solution of barium hydroxide, a strong base.
Hydrogen ion concentration and pH
pH = -log[H+(aq) ]
Therefore, we can calculate pH if we have [H+(aq) ]
Example: calculate the pH of a solution with a concentration of H+
ions of 4.7 x 10-11 mol/L
)
In pure water:
Therefore:
To calculate concentration of hydrogen ions from pH use this formula:
Example: convert pH 10.33 to concentration of hydrogen ions
pOH and pKw
pOH is a way to describe hydroxide ion concentrations
pOH = -log[OH-(aq) ]
and [OH-(aq) ] = 10-pOH
Example:
Calculate pOH of a solution with a hydroxide ion concentration of 3.0
x 10-6 mol/L
The relationship between pH and pOH:
pH + pOH = pKw
So… pH + pOH =
Example: What is the pOH of a solution whose pH is measured to be
6.4?
…just a reminder that you can measure pH using indicators (natural or
synthetic) or pH meters (some actually give pretty accurate
readings… others are annoying)
The pH of strong acids
Example: Calculate the pH, pOH and [OH-(aq) ] of a 0.04 mol/L solution
of HNO3
1.
write dissociation equation
2.
calculate pH
3.
calculate pOH
4.
use pOH to calculate [OH-(aq) ]
The pH of strong bases:
Example: calculate the pH of a 0.02mol/L NaOH solution
*note: you could also be asked to calculate hydrogen ion
concentrations, you could also be asked to start with grams and
convert to moles, find concentration and then go from there.*