Arrhenius Definition of Acids &
Bases
One the first acid-base definitions (Svante
Arrhenius 1859-1927).
“Acids are substances that, when dissolved in water, increase the
hydrogen ion concentration. Likewise,
bases are substances, that when dissolved in water, increase the hydroxide ion
concentration.”
- common acids are HCl(aq), HNO3(aq), H2SO4(aq), H3PO4(aq), HC2H3O2(aq), H2CO3(aq)
- common bases are group I & II metal hydroxides, like NaOH & Ca(OH)2
Ionization
Ionization is a reaction in which electrically
neutral molecules or atoms produce ions:
HCl(aq)
+ H2O(l) →
H3O+(aq)
+ Cl-(aq) ionization equation
As
seen above, the hydronium ion, H3O+,
forms from the combination of H2O and H+ and it is
responsible for the acidic properties. Since
the hydronium ion is simply a hydrated H+ ion, the above equation
can be simplified:
HCl(aq) →
H+(aq) + Cl-(aq) simplified ionization equation
Indicators
An indicator is a dye that shows a different colour
in acidic solution than it does in basic solution (ex. phenolphthalein,
bromthymol blue, litmus).
👉 Check out the cool way this high school student has used the colour changing properties of indicators to make a difference in the world. Wow!
Bronsted-Lowry Definition of Acids
& Bases
A hydrogen ion is a hydrogen atom (1 proton, 1 electron, 0 neutron) that has become positively charged by losing an electron. So a hydrogen ion and is just a proton.
"An acid is a proton donor and a base is a proton acceptor."
Compared to the Arrhenius definition, the Brønsted-Lowry definition allows for a broader range
of substances which can be considered acidic or basic.
HCl(g)
+ H2O(l) ↔ H3O+(aq) + Cl-(aq)
A B
HNO3(aq)
+ NH3(g) ↔ NH4+(aq)
+ NO3-(aq)
A B
Conjugate Acid-Base Pairs
In any acid-base equilibrium, both the forward and
reverse reactions involve proton transfer.
HNO3(aq)
+ H2O(l) ↔ NO3-(aq) + H3O+(aq)
A B CB CA
NH3(aq)
+ H2O(l) ↔ NH4+(aq) + OH-(aq)
B A CA CB
Notice in these two examples that the acid (A) its conjugate base (CB) differ by a single proton. The same can be said for the base (B) and its conjugate acid (CA).
Lab:
Fill in the following chart using the chemicals and equipment provided.
For the "conductivity" section, find out if acid and/or base solutions are conductive. For the "indicator" sections, find out the colour the indicator would display in an acid or a base. For the "reaction" sections, find out if a reaction occurs or not.
