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Redox Reactions

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. Each of these reactions are known as a half-reaction. They combine to give a net oxidation-reduction reaction or redox reaction. Half reactions always occur simultaneously, one of the reactions can never occur without the other. Most common oxidation-reduction (redox) reactions are synthesis, decomposition, displacement, and combustion reactions.

 The aim of these experiments, is to do one of each of the following as an example of a redox reaction:

 

Synthesis reaction: two or more reactants bond with each other to form a new, more complicated product.

Decomposition reaction: a complex reactant decomposes into simpler compounds.

Displacement reaction: a less reactive element is replaced in a compound by a more reactive element.

 

Experiment 1:  Synthesis Reactions and combustion reactions                                  

Time Allocation: 45 Min

Observe the synthesis reaction of magnesium ribbon with oxygen and sulphur powder with oxygen.

Part 1:

Sulphur powder(s) is heated in the deflagration spoon and allowed to combust with oxygen (O2(g)) in the gas jar. The product Sulphur dioxide (SO2(g)) is dissolved in water (H2O (l)) and tested with litmus paper to determine the degree of acidity.

S(s) + O2 (g) → SO2 (g)

Part 2:

Magnesium ribbon Mg(s) is heated in the deflagrating spoon and allowed to combust with oxygen (O2(g)) in the gas jar. Learners observe the flame colour and dissolve magnesium oxide MgO(s), which is the product, in water. The solution is tested with litmus paper to determine the degree of acidity.

2Mg(s) + O2 (g) → 2MgO (g)

Experiment 2: Decomposition reaction                                                                                

Time Allocation: 45 Min

Observe the decomposition reaction between Hydrogen Peroxide and Potassium permanganate.

Decomposition of hydrogen peroxide by potassium permanganate. In this experiment, Potassium permanganate is reduced by hydrogen peroxide to manganese oxide (IV), which catalyses the decomposition of hydrogen peroxide. This produces a large amount of heat that causes the water to boil and emit a large amount of oxygen

H2O2 + KMnO4                                                     O2 + MnO2 + KOH + H2O

Experiment 3: Displacement Reactions

Time Allocation: 60 Min

Part 1: Metal Displacement- Observe the oxidation reactions reaction between zinc metal and a Copper(II)sulphate solution.

Part 2: Hydrogen Displacement- Observe the oxidation reduction reaction between sodium (Na(s)) and distilled water (H2O(l)) solution.

Part 3: Hydrogen Displacement- Observe the oxidation reduction reaction between chlorine (Cl2(aq)) and solutions of potassium bromide (KBr(aq)) and potassium iodide (KI(aq)).

Part 1: Metal Displacement

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)

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.

 

Part 2:  Hydrogen Displacement

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.

 

Part 3: Halogen Displacement

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

Experiment: Types of Redox Reactions

 

EXPERIMENT 1: SYNTHESIS REACTIONS AND COMBUSTION REACTIONS      

 

AIM                                                                                                   
Time allocation: 45 Min

Observe the synthesis reaction of magnesium ribbon with oxygen and sulphur powder with oxygen.

 

APPARATUS:
  • 5g Sulphur Powder
  • 5cm Magnesium ribbon (2g)
  • Gas jar filled with O2 (g)
  • Gas jar cover slip
  • 10ml Petroleum jelly
  • Deflagration spoon
  • Forceps
  • Lighter (Matches)
  • Bunsen burner
  • Water
  • Litmus paper (red and blue)
METHOD:
Part 1: Sulphur and Oxygen
  1. Fill a gas jar with oxygen by turning the gas jar upside down and releasing the gas in the gas jar.
  2. Seal the gas in the gas jar by placing petroleum jelly on the rim of the gas jar and cover with the cover slip.
  3. Working in a fume cupboard or outside.
  4. Place 5g sulphur Powder in the deflagration spoon.
  5. Ignite the Bunsen burner.
  6. Allow the sulphur to ignite and drop the deflagration spoon in the gas jar filled with oxygen.
  7. Observe the flame colour and the appearance of the new product.
  8. Dissolve the new product in water and test the solution with both litmus papers.
Part 2: Magnesium and Oxygen
  1. Fill a gas jar with oxygen by turning the gas jar upside down and releasing the gas in the gas jar.
  2. Seal the gas in the gas jar by placing petroleum jelly on the rim of the gas jar and cover with the cover slip.
  3. Place 5cm magnesium ribbon (2g) in the deflagration spoon.
  4. Ignite magnesium ribbon and drop the deflagration spoon in the gas jar filled with oxygen.
  5. Observe the flame colour and the appearance of the new product.
  6. Dissolve the new product in water and test the solution with both litmus papers.
  7. Record the observation in a suitable table.

EXPERIMENT 2: DECOMPOSITION REACTION         

AIM                                                                                                 
   Time allocation: 45 Min

Observe the decomposition reaction between Potassium Permanganate and Hydrogen Peroxide.

APPARATUS:
  • Spatula
  • 8g Potassium Permanganate
  • 50ml Hydrogen Peroxide
  • 1 x deflagrating spoon
  • 1 x 250ml Conical Flask wide neck

METHOD

  1. It is advisable that your teacher perform this experiment for the class, allow students to stand at least a meter away from the area where the experiment is being performed and wear appropriate safety clothing throughout the duration of the demonstration.
  2. Use the 250ml Conical Flask and place exactly 50ml of Hydrogen Peroxide into the flask.
  3. Take the pre-prepared vial containing 8g Potassium Permanganate and place it in the receptacle of the deflagrating spoon.
  4. Standing a little distance away, extend the deflagrating spoon toward the mouth of the conical flask.
  5. Turn the spoon over so that all the contents of the spoon falls into the flask
  6. Observe the rapid reaction that occurs.
Precaution:

The decomposition of Hydrogen Peroxide is an extremely exothermic reaction, all students to remain a meter and a half square radius away from the teacher performing the experiment. Refrain from touching the flask or contents after the decomposition reaction as they will be hot and will burn you.

 

EXPERIMENT 3: DISPLACEMENT REACTIONS                       

AIM                                                                                               
     Time allocation: 45 Min

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

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

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

 

APPARATUS:

 

  • Four Test Tubes
  • 50ml Copper(II)sulphate (CuSO4.5H2O(aq))
  • 25ml Potassium chloride Solution (KCℓ(aq))
  • 25ml Potassium bromide Solution (KBr(aq))
  • 25ml Potassium iodide Solution (KI(aq))
  • 50ml Chlorine water Cℓ2(aq)
  • 50ml Xylene (CH3)2C6H4(aq))
  • 25g Sodium (Na(s))
  • Lighter / matches
  • Thermometer
  • 25g Zinc powder(Zn(s))
  • Spatula
  • Forceps
  • 250ml Beaker
  • 250ml Distilled water (H2O(aq))

 

METHOD

Part 1: Metal Displacement

  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.
Part 2: Hydrogen Displacement
  1. Half fill a 250ml beaker with 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 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, remove the test tube from the beaker.
  7. Ignite the gas in the test tube with a lighter.
  8. Observe the burn rate and identify the gas.
Part 3: Halogen Reactions
  1. Fill test tube A one third full of Potassium chloride Solution (KCℓ(aq))
  2. Fill test tube B one third full of Potassium bromide Solution (KBr(aq))
  3. Fill test tube C one third full of Potassium iodide Solution (KI(aq))
  4. Add 10 mℓ of chlorine water to each test tube (A, B and C) and shake carefully. Note any colour changes.
  5. Add 20 mℓ of xylene to each test tube.
  6. Shake well and allow the layers to settle.
  7. 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.
  • The decomposition of Hydrogen Peroxide is an extremely exothermic reaction, all students to remain a meter and a half square radius away from the teacher performing the experiment.
  • Refrain from touching the flask or contents after the decomposition reaction as they will be hot and will burn you.
  • 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.
      • Spilled on the skin – remove any pieces of solid with forceps. Drench the skin with plenty of water. See doctor.
      • Spilled 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.
      • Vapour 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.
      • Spilled on the skin – remove contaminated clothing and drench skin with plenty of water.
      • Spilled on floor, bench etc. – open windows, mop up and rinse with plenty of water.
    • Potassium Permanganate
  • Vapour breathed in – remove casualty to fresh air. Call a doctor if breathing is slightly affected
  • Wash exposed areas if you have had direct skin contact.