Acid
When acids dissolve in water, they produce protons (H+ ions). These protons make the solution acidic.
Example)
HCl(aq) → H+(aq) + Cl-(aq)
H2SO4(aq) → 2H+(aq) + SO42-(aq)
Alkali
When alkalis dissolve in water, they produce hydroxide ions (OH-). These hydroxide ions make the solution alkaline.
Example)
NaOH(aq) → Na+(aq) + OH-(aq)
Ca(OH)2(aq) → Ca2+(aq) + 2OH-(aq)
Tests for Acids and Alkalis
Litmus
- Litmus indicator solution turns red in acidic solutions, and turns blue in alkaline solutions.
- Litmus papers are also used.
Phenolphthalein and Methyl Orange
Universal Indicator and pH scale
pH scale (0 to 14) shows the strength of acidic or alkaline solutions and can be measured by a universal indicator.
- acidic solution: pH is smaller than 7. The smaller the pH, the more acidic the solution.
- neutral solution: pH is 7.
- alkaline solution: pH is larger than 7. The bigger the pH, the more alkaline the solution.
Neutralization Reaction
Neutralization reaction between an acid and an alkali produces a salt and water.
Acid + Base → Salt + Water
This is because hydrogen ion from the acid and hydroxide ion from alkali react to form water.
H+(aq) + OH-(aq) → H2O (l)
Example of neutralization reaction:
H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)
HCl(aq) + NaOH (aq) → NaCl(aq) + H2O(l)
Acid-Alkali Titration
Titration is a method where a solution of known concentration is used to determine the concentration of another solution. The solution is added slowly to another solution until neutralization happens.
Example
A student carries out a titration to find the concentration of some dilute sulfuric acid. She is given
- a supply of phosphoric acid (H3PO4)
- sodium hydroxide (NaOH) solution of concentration 0.180 mol/dm3
- apparatus suitable for carrying out a titration
- phenolphthalein indicator
She uses this method to do the titration.
- Using a pipette, add 25.0 cm3 of the sodium hydroxide solution to a conical flask.
- Add drops of phenolphthalein indicator to the conical flask.
- Fill a burette with the phosphoric acid.
- Slowly add phosphric acid to the conical flask until the phenolphthalein indicator just changes color from pink to colorless. At this point, the solution has just become neutral.
After several repeats, the student calculated the average volume of phosphoric acid added to be 28.30 cm3.
(a) Find the balanced chemical equation for the neutralization reaction.
3NaOH + H3PO4 → Na3PO4 + 3H2O
(b) Calculate the amount, in moles, of NaOH in 25.0 cm3 of the sodium hydroxide solution.
0.180 x 25.0 / 1000 = 0.00450 moles
(c) Calculate the amount, in moles, of H3PO4 in the phosphoric acid solution.
0.00450 / 3 = 0.00150 moles
(d) Calculate the concentration, in mol/dm3 , of the phosphoric acid.
0.00150 x 1000 / 28.30 = 0.0530 mol/dm3