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Types of reactions: Acid and Bases

AIM:

To observe the neutralization reaction between hydrochloric acid and sodium hydroxide by using Bromothymol blue as indicator.

HOW DOES IT WORK?

One of the products in an acid-base reaction is always a salt – i.e. an ionic compound.  In reactions of acids with metal hydroxides and oxides, water is always formed with the salt.  An acid and a metal carbonate produces a salt, water and carbon dioxide.

When an acid reacts with a metal the products are a salt and hydrogen gas.  The type of salt that forms depends on the acid used in the experiment:

Hydrochloric acid produces a chloride e.g. magnesium chloride (MgCℓ2).

Nitric acid produces a nitrate e.g. potassium nitrate (KNO3)

Sulphuric acid produces a sulphate e.g. copper(II)sulphate (CuSO4)

Acetic acid (vinegar) produces an acetate e.g. sodium acetate (CH3COONa).

Acids are proton donors. The hydrochloric acid HCℓ(ℓ) ionises in water and form protons, that can be transferred to the base, as well as Chloride (Cℓ-ions).

The base sodium hydroxide (NaOH(s)) dissociates in water to form NaOH(s) and hydroxide Ions (OH– ions). Ion exchange takes place to form the salt, NaCℓ(aq) and H2O(ℓ).

The indicator used to determine the end point of the titration is Bromothymol blue. At the end point the acid has neutralised the properties of the base completely. The reaction is also known as a   neutralisation reaction.

The strong acid HCℓ, is used to fill the burette to the zero-mark. The strong base, NaOH, is transferred to the Erlenmeyer flask by using the pipette. A few drops of Bromothymol blue indicator solution is added to the base. The colour of the indicator will be blue. The acid is then carefully added to the base until the solution in the Erlenmeyer flask just becomes yellow. The properties of the base are neutralised by the acid and the products formed are sodium chloride (NaCℓ(aq)) and water. By evaporating the water in the flask, learners will be able to observe the salt crystals that remain. 

APPARATUS:

  • Dilute 100ml hydrochloric acid 0.1M
  • Dilute 100ml solution of sodium hydroxide1M
  • 25 ml Bromothymol blue solution in dropper bottle
  • 25 ml burette acrylic
  • Funnel 50mm
  • 50ml beaker
  • 50ml Measuring Cylinder
  • 2 x 100 ml beaker
  • Retort stand and clamp
  • Bunsen burner with tripod stand and wired gauze
  • 100 ml Distilled water
  • White tile
  • Wash Bottle 125ml
  • Matches or gas lighter

METHOD:

  1. Rinse the 100ml beaker with distilled water placed inside the wash bottle.
  2. Transfer 50 ml Diluted solution of sodium hydroxide (NaOH(aq)) with a 50 ml measuring cylinder to the 100ml beaker.
  3. Squeeze at least 10-20 drops of Bromothymol blue to the sodium hydroxide NaOH(aq) and observe the colour.
  4. Place 30ml hydrochloric acid into the other 100ml beaker.
  5. Take the beaker with the funnel and rinse the burette with around 5ml of Diluted hydrochloric acid (HCℓ(aq)) open the tap and allow 5ml to pour out into the 50ml waste beaker
  6. Making sure the tap is closed and using the 100ml beaker with funnel placed into the top of the burette, fill HCl to the 25 ml mark of the burette.
  7. Place the beaker with sodium hydroxide NaOH(aq) under the Burette on the white tile.
  8. Using the burette, carefully add hydrochloric acid HCℓ(aq) by opening the tap slowly into the sodium hydroxide NaOH(aq) until the colour of the solution in the beaker has just changed.
  9. Note the colour in the neutralised solution.
  10. Light the burner, place the beaker on the wired gauze and allow the water to evaporate.
  11. Study the solid that remains in the beaker.

PRECAUTIONS:

  • Hydrochloric acid used in the experiment is diluted, but must be handled with care. It can still cause harm to the eyes or in a cut.
  • Sodium hydroxide can irritate the eyes and skin. Handle with great care.
  • Where safety goggles and gloves when using the Bunsen burner.
  • Make sure that a fire extinguisher is at hand in the laboratory.
  • Emergency actions
    • Hydrochloric acid:
      • In the eye – flood the eye with gently-running tap water for at least 10 minutes. See a doctor.
      • Swallowed – Wash out the mouth with water. Do not induce vomiting. See a doctor.
      • Spilt on floor, bench, etc. – cover with mineral absorbent (e.g. cat litter) and scoop into a bucket. Neutralise with sodium carbonate and rinse with water.
    • Sodium hydroxide
      • In the eye – flood the eye with gentle running tap water for at least 20 minutes. See a doctor.
      • Swallowed – Wash out the mouth with water. Do not induce vomiting. See a doctor.
      • Spilt on the skin – remove contaminated clothing and drench skin with plenty of water. If blisters occur, see a doctor.
      • Spilt on floor, bench etc. – cover with mineral absorbent (cat litter) and scoop into a bucket. Neutralise with citric acid and rinse with water.