Acid-base reactions

Measuring acidity

Equipment:
  • 2x 100 ml beaker
    • 50 ml of water in each one
    • 3 drops of BTB in each one
  • 1x straw
  • 1 bottle of [[$\text{HCl(aq)}$]]
  • 1 bottle of [[$\text{NaOH(aq)}$]]
  • 10 slips of pH paper
  • 1 tissue
Instructions:
  1. Pick either beaker.
  2. Blow air into the liquid with a straw.
    • Caution! Do not inhale, just exhale.
    • Caution! Blow gently to prevent spills.
  3. Observe the change.
  4. Write the reaction as an equation.
    • What is the substance you exhale?
    • What is the substance you exhale into?
    • What is the substance the reaction yields?
  5. Switch to the unused beaker
  6. Add 15 drops of [[$\text{HCl}$]] into the solution, one drop at a time.
  7. Measure the pH of the solution with a slip of pH paper.
  8. Place the slip on the tissue.
  9. Add 3 drops of [[$\text{NaOH}$]] into the solution.
  10. Repeat from step 7 onwards until the pH slips are exhausted.
  11. Plot the pH changes as a diagram.
    • horizontal: drops of [[$\text{NaOH}$]]
    • vertical: pH of solution
    • With a trendline, find the amount of [[$\text{NaOH}$]] drops needed for pH 7.

Measuring buffering ability

Equipment:
  • 100 ml beaker
    • 20 ml of milk
    • 3 drops of BTB
  • 1 bottle of [[$\text{HCl(aq)}$]]
  • 1 bottle of [[$\text{NaOH(aq)}$]]
  • 10 slips of pH paper
  • 1 tissue
Instructions:
  1. Add 15 drops of [[$\text{HCl}$]] into the solution, one drop at a time.
  2. Measure the pH of the solution with a slip of pH paper.
  3. Place the slip on the tissue.
  4. Add 3 drops of [[$\text{NaOH}$]] into the solution.
  5. Repeat from step 1 onwards until the pH slips are exhausted.
  6. Plot the pH changes as a diagram.
    • horizontal: drops of [[$\text{NaOH}$]]
    • vertical: pH of solution
    • With a trendline, find the amount of [[$\text{NaOH}$]] drops needed for pH 7.

Examples

Example. When mixed with water, hydrochloric acid [[$\text{HCl}$]] dissociates into hydrogen ions [[$\text{H}^+$]] and chloride ions [[$\text{Cl}^-$]]:
[[$$\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$$]]
In this context, water acts as a base for the hydrochloric acid. Water molecules [[$\text{H}_2\text{O}$]] absorb the dissociated hydrogen ions [[$\text{H}^+$]] to yield hydronium ions [[$\text{H}_3\text{O}^+$]]:
[[$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$]]
Example. 
When mixing water solutions of hydrochloric acid [[$\text{HCl}$]] and sodium hydroxide  [[$\text{NaOH}$]], both ionic compounds dissociate into positive and negative ions:
[[$$\text{HCl} + \text{NaOH} \rightarrow \text{H}^+ + \text{Cl}^- + \text{Na}^+ +\text{OH}^-$$]]
In this context, the hydroxide in sodium hydroxide acts as the base for the hydrochloric acid. Each hydrochloric acid particle donates a hydrogen ion [[$\text{H}^+$]] to one sodium hydroxide particle. In simpler words, the acidic [[$\text{H}^+$]] and the basic [[$\text{OH}^-$]] combine to make water:
[[$$\text{H}^+ + \text{Cl}^- + \text{Na}^+ + \text{OH}^- \rightarrow \text{NaCl} + \text{H}_2\text{O}$$]]
As ions of opposite charge, the remaining sodium ions [[$\text{Na}^+$]] and chloride ions [[$\text{Cl}^-$]] associate to form sodium chloride [[$\text{NaCl}$]].

Acids and their salts

hydrochloric acid [[$\text{HCl}$]] -chloride
sulphuric acid [[$\text{H}_2\text{SO}_4$]] -sulphate
nitric acid [[$\text{HNO}_3$]] -nitrate
carbonic acid [[$\text{H}_2\text{CO}_3$]] -carbonate
phosphoric acid [[$\text{H}_3\text{PO}_4$]] -phosphate

Example.
a. hydrochloric acid + lithium hydroxide [[$\rightarrow$]] lithium chloride + water
[[$$\text{HCl} + \text{LiOH} \rightarrow \text{LiCl} + \text{H}_2$$]]
b. sulphuric acid + calcium hydroxide [[$\rightarrow$]] calcium sulphate + water
[[$$\text{H}_2\text{SO}_4 + \text{Ca(OH)}_2 \rightarrow \text{CaSO}_4 + 2\text{H}_2\text{O}$$]]