Types of reaction: Electron Transfer

AIM

Investigate the difference between the different types of displacement reactions namely, Metal displacement, Hydrogen displacement and Halogen displacement.

HOW DOES IT WORK?

The driving force in redox reactions is a transfer of electrons.  The atom or ion that loses one or more electrons is oxidised and is known as the reducing agent.  The atom or ion that gains one or more electrons is reduced and is known as the oxidising agent.

Oxidation and reduction always occur together, because if one substance is oxidised there must be another one that is reduced.  It implies that the reducing agent is responsible for reduction of the oxidising agent and the oxidising agent is responsible for oxidation of the reducing agent.  The reducing agent loses electron(s) and the oxidising agent gains electron(s) and is reduced.

The aim of the experiments is to investigate the different types of displacement reactions as listed below.

Experiment 1: Metal Displacement (Time Allocation: 20 Min)

Observe the oxidation reactions reaction between zinc metal and a copper(II)sulphate solution.

In this experiment, the more reactive zinc metal (Zn(s)) displaces the less reactive copper metal (Cu(s)) from the copper(II)sulphate CuSO4 ­solution. The blue colour of the solution disappears, because Cu²⁺ – ions are reduced. The zinc metal (Zn(s)) is oxidised and replaced by copper metal (Cu(s)).

Zn(s) + CuSO₄(aq)  ZnSO₄(aq) + Cu(s)

APPARATUS:

• One Test Tube
• 50ml Copper(II)sulphate CuSO₄.5H₂O (aq) ­solution
• Thermometer
• 25g Zinc powder(Zn(s))
• Spatula

METHOD

1. Half fill a test tube with a solution of Copper(II)sulphate pentahydrate CuSO₄.5H₂O (aq).
2. Measure the temperature of the solution.
3. Add a spatula full of Zinc powder(Zn(s)) to the solution.
4. Stir with the thermometer and take the temperature again.
5. Note all the observations.

Experiment 2:  Hydrogen Displacement (Time Allocation: 20 Min)

Observe the oxidation reduction reaction between sodium (Na(s)) and distilled water (H₂O(l)) solution.

Alkali metals and some of the alkali earth metals can displace hydrogen from cold water. Sodium metal reacts with water to form sodium hydroxide and hydrogen gas.

Oxidation: Na-atoms loose electrons.

Na(s) ⟶ Na+(aq) + e-

Reduction: H+ – ions gain electrons.

2H+(aq) + 2e- ⟶ H2(g)

The OH- – ions act as spectator ions. Balanced chemical equation, is:

Na(s) + H2O(ℓ) ⟶ NaOH(aq) + H2(g)

Note: this reaction can also be classified as a gas forming reaction.

APPARATUS:

• One Test Tube
• 25g Sodium (Na(s))
• Lighter / matches
• Spatula
• Water
• 250ml Beaker
• Test tube tong wire

METHOD

1. Half fill a 250ml beaker with distilled water.
2. Using the spatula remove the sodium from the oil.
3. Use the spatula to cut a 1cm² cube of sodium.
4. Place the sodium in the beaker with water, by using the spatula or forceps.
5. Hold a test tube with tongs over the sodium as it reacts with the water and collect the gas.
6. While holding the test tube so that the opening faces down with the tong, remove the test tube from the beaker.
7. Ignite the gas in the test tube with a match that is ignited.
8. Observe the burn rate and identify the gas.

Experiment 3: Halogen Reactions (Time Allocation: 20 Min)

Observe the oxidation reduction reaction between chlorine (Cl₂(aq)) and solutions of potassium bromide (KBr(aq)) and potassium iodide (KI(aQq)).

Chlorine (Cℓ₂) is used to displace Barium (Br₂) and iodine (I₂) from the halide salt-solutions. The non-polar substances are soluble in Xylene. Bromine is reddish brown and iodine pink in Xylene.

Cℓ₂(aq) + 2KBr(aq)  Br₂(aq) + 2KC (aq)

Cℓ₂(aq) + 2KI(aq)  I₂(aq) + 2KC (aq)

Cℓ₂(aq) + KCℓ(aq)  no reaction

APPARATUS:

• Three Test Tubes
• Test Tube rack Polypropylene
• 25ml 0.1M Potassium chloride Solution (KCℓ(aq))
• 25ml 0.1M Potassium bromide Solution (KBr(aq))
• 25ml 0.1M Potassium iodide Solution (KI(aq))
• 2 x 25ml Measuring Cylinder
• 50ml Chlorine water Cℓ₂(aq)
• 50ml Xylene (CH₃)₂C₆H₄(aq))

METHOD

1. Label 3 test tubes A,B and C
2. Fill test tube A one third full of Potassium chloride Solution (KCℓ(aq))
3. Fill test tube B one third full of Potassium bromide Solution (KBr(aq))
4. Fill test tube C one third full of Potassium iodide Solution (KI(aq))
5. Add 10 mℓ of chlorine water a measuring cylinder to each test tube (A, B and C) and shake carefully. Note any colour changes.
6. Add 20 mℓ xylene into the second measuring cylinder and into each test tube.
7. Shake well and place in the test tube rack, allow the layers to settle.
8. Record all observations

PRECAUTIONS:

• Alkali metals are highly flammable and corrosive.
• It reacts violently with water to produce H₂(g) that is extremely flammable. The gas is difficult to ignite but once lit, it burns readily in air and is difficult to extinguish.
• Alkali metals react violently with many substances.
• Handle sample using forceps, wear eye protection and safety screens.
• Conduct the investigation on small scale.
• Make sure learners understand the hazards and take steps to prevent theft.
• Chlorine water is low hazard, but be careful of toxic gas given off.
• Emergency actions
  • Sodium:
    • 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. See a doctor.
    • Spilt on the skin – remove any pieces of solid with forceps. Drench the skin with plenty of water. See doctor.
    • Spilt on floor, bench, etc. – cover with dry sand or anhydrous sodium carbonate.
  • Chlorine water
    • In the eye – flood the eye with gentle running tap water for 10 minutes. See a doctor.
    • Vapor breathed in – remove casualty to fresh air. Call a doctor if breathing is even slightly affected.
    • 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.
    • Spilt on floor, bench etc. – open windows, mop up and rinse with plenty of water.