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Decomposition and Synthesis Reactions

AIM:

Will introduced to new concepts like catalysts and decomposition reactions as well as decompose hydrogen peroxide into water and oxygen gas and synthesize water in a reaction between hydrogen gas and oxygen gas.

HOW DOES IT WORK?

Time Allocation: 45 min
Experiment 1: To decompose hydrogen peroxide into water and oxygen gas.
Time Allocation: 45 min

Manganese dioxide is carefully added to hydrogen peroxide H2O2(ℓ) which is decomposed into water H2O(ℓ)  and oxygen O2(g). Covalent bonds between the Hydrogen (H)-atoms and Oxygen (O)-atoms in the Hydrogen Peroxide (H2O2) -molecules are broken and the atoms rearrange to form H2O and O2-molecules.

The balanced chemical equation, is: 2H2O2(ℓ)  2H2O(ℓ) + O2(g)

The Oxygen (O2(g)) changes the glowing splint into a flaming splint. It is interesting to know that the experiment can also be done without collecting the O2(g). It is commonly known as the elephant’s toothpaste reaction.

Experiment 2: To synthesise water in a reaction between hydrogen gas and oxygen gas.
Time Allocation: 45 min

Hydrogen (H2(g)) and Oxygen (O2(g)) reacts to form water. The covalent bonds between Hydrogen (H)-atoms and Oxygen (O)-atoms are broken and they rearrange to form H2O. This is a synthesis reaction.

The balanced chemical equation, is: 2H2(g) + O2(g)  2H2O(ℓ)

A mixture of hydrogen and oxygen gas is used as a fuel to get rockets into space

 

EXPERIMENT 1: DECOMPOSITION OF HYDROGEN PEROXIDE

Time Allocation: 45 min

AIM: 

To decompose hydrogen peroxide into water and oxygen gas.

APPARATUS:

  • Hydrogen peroxide (H2O2(ℓ)) – 30% solution
  • Manganese dioxide (MnO2(s)) – grinded into powder form (use a mortar and pestle)
  • Conical Flask
  • Spatula
  • Rubber stopper with one hole
  • Delivery tubes
  • Matches or gas lighter
  • Two gas jars
  • Two gas jar covers
  • Bee Hive Shelve
  • Matches or gas lighter
  • Wooden splint
  • Water
  • Gratnell Tray (Water Trough)

METHOD:

  1. Add 25g of manganese dioxide (MnO2(s)) in the conical flask.
  2. Fill the gas jar with water and carefully put the gas jar upside down in the water bath on a bee hive – see diagram.
  3. Add 50 ml of hydrogen peroxide (H2O2(ℓ)) to the conical flask.
  4. Fit the rubber stopper into the conical flask with the delivery tube below the gas jar.
  5. Prep the retort stand and Rod and attach the clamp but do not use it yet.
  6. Collect the gas with downward displacement of water in the second gas jar. Displace at least one third of the water in each gas jar.
  7. Gently lift the gas jar until there is enough space between the gas jar and bee hive to cover the gas jar with the cover slip. Carefully seal one gas jar with petroleum jelly and a coverslip for use in experiment 2.
  8. Clamp the jar, cover slip facing downward with the retort rod clamp.
  9. Light the wooden splint, let it burn for a few seconds.
  10. Put the flame out.
  11. Open the gas jar quickly and insert the glowing wooden splint into the gas jar.
  12. Insure that the wooden splint does not make contact with the water.
  13. Note all the observations.

EXPERIMENT 2: SYNTHESIS OF WATER

Time Allocation: 45 min

AIM: 

To synthesise water in a reaction between hydrogen gas and oxygen gas.

APPARATUS:

  • Zinc granules 50g
  • Diluted sulphuric acid 30%
  • Anhydrous Copper(II)Sulphate 25g
  • Matches or gas lighter
  • Ice / Cold Water
  • Conical Flask
  • Spatula
  • Rubber stopper with one hole
  • Delivery tubes
  • Gas jars with Oxygen (O) from Experiment 1
  • Gas jar covers
  • Bee Hive
  • Wooden splint
  • Beaker
  • Retort Stand, Retort Rod & Clamp
  • Water
  • Gratnell Tray (Water Trough)

METHOD:

Part 1: Preparation of Hydrogen
  1. Using the second, unopened gas jar from the previous experiment, containing two thirds (2/3) water and one third (1/3)Oxygen (O). Place the gas jar under the surface of the water in the water trough and remove the cover slip.
  2. Place the gas jar on the bee hive and inset the delivery tube under the gas jar, through the bee hive.
  3. Place 25g Zinc granules in the Conical flask, using the spatula.
  4. Add 50ml Diluted sulphuric acid 30% to the conical flask.
  5. Fit the rubber stopper on the conical flask with the delivery tube below the gas jar.
  6. Collect the gas with downward displacement of water in the gas jar. Displace the remaining two third of the water in each gas jar, if possible.
  7. Gently lift the gas jar until there is enough space between the gas jar and bee hive to cover the gas jar opening with the gas jar cover.
  8. Tilt the gas jar upright, whilst keeping it sealed with both gasses from the previous and current experiment. Place the gas jar aside for part two of the experiment.
  9. Remove the bee hive from the trough.
  10. Using a match or lighter. Carefully ignite the gas that is still bubbling fom the water and observe the burn rate and sound in order to identify the gas.

 Part 2: Synthesis of Water

  1. Secure a beaker filled with ice (or cold water) on a retort stand.
  2. Place the gas jar upright in the water trough under the beaker filled with water.
  3. Light the wooden splint.
  4. Open the gas jar by removing the cove slip and ignite the gas. Collect the liquid that forms in the gas jar and on the beaker, in the bottom of the as gas jar.
  5. Using the spatula add a spatula full of anhydrous copper (II) sulphate CuSO4(s) to the liquid.
  6. Note all observations.

 

PRECAUTIONS:

  • Safety glasses must be worn when the experiments are carried out.
  • It is best to demonstrate all the experiments and tests.
  • All chemicals and open flames must be considered as potentially dangerous.
  • Hydrogen peroxide can cause chemical burns. Work carefully with it and use strengthened glassware.
  • The amount of manganese dioxide MnO2(s) added must be small.
  • Make sure that the long legged funnel does not become clogged at any stage during experimental procedure.
  • Hydrochloric acid is a strong acid. Handle concentrated solutions with care.
  • Emergency actions
  • Hydrochloric acid:
  • In the eye – flood the eye with gently-running tap water for at least 20 minutes. See a doctor.
  • Swallowed – Wash out the mouth with water. Do not induce vomiting.
  • Spilt on skin or clothing – Remove contaminated clothing. Drench skin with plenty of water. If blisters occur, see a doctor.
  • Spilt on floor, bench, etc. – cover with mineral absorbent (e.g. cat litter) and scoop into a bucket. Neutralize with sodium carbonate and rinse with water.
  • The composition of water, is an exothermic reaction. It must be conducted with great care. Use safety glasses and do not allow learners to stand to close to where the experiment is conducted