Redox Reactions

Experiment 19:

Aim:

To prepare potassium permanganate solution

Equipment:

Refer to STAWA book page 148

Procedure:

Refer to STAWA book page 148

Processing of Results and Questions:

The solution was first boiled for 10 minutes to speed up the rate of reaction. This is because at room temperature, the solution has a very slow rate of reaction.

Experiment 20:

Aim:

To prepare standard oxalic acid solution

Equipment:

Refer to STAWA book page 149

Procedure:

Refer to STAWA book page 149

Processing of Results and Questions:

m(Oxalic Acid) = 1.58 g
M(Oxalic Acid) = 126.068 gmol ^-1
n(Oxalic Acid) = 0.0125 mol
V = 0.25 L
c(Oxalic Acid) = n ÷ V
c = 0.0125 ÷ 0.25
c(Oxalic Acid) = 0.05013 mol L  ^-1

Experiment 21:

Aim:

Standardising potassium permanganate solution

Equipment:

Refer to STAWA book page 150

Procedure:

Refer to STAWA book page 151

Results:

	Rough Estimate	Accurate 1	Accurate 2	Accurate 3
Final Reading (mL)	40.5	40.5	40.7	40.6
Initial Reading (mL)	20	20	20	20
Titration Volume (mL)	20.5	20.5	20.7	20.6

Processing of Results and Questions:

1.       c(Oxalic Acid) = 0.05013
V(Oxalic Acid) = 0.02
n(Oxalic Acid) = 0.001 mol
n(Permanganate) = [n(Oxalic Acid) ÷ 5] x 2
n(Permanganate) = 0.000401 mol
V(Permanganate) = 0.0206 L
c(Permanganate) = n(Permanganate) ÷ V(Permanganate)
c(Permanganate) = 0.000401 ÷ 0.0206
c(Permanganate) = 0.0195 mol L ^-1

2a)    Condy’s Crystals, a name given to potassium permanganate has a pinkish, purple colour.

2b)  Manganese oxide has a pale yellowish brown colour

2c)   MnO₄^- + 8H ⁺ + 5e ^-  --> Mn²⁺ + 4H₂O
H₂S --> S + 2H⁺ + 2e ^-
2MnO₄^- + 6H ⁺ + 5H₂S --> 2Mn²⁺ + 8H₂O + 5S

2d)  A very horrendous smell of skunk scent and other smells such as garlic.

3.       Yes oxalic acid can be used as a primary standard to standardise a basic solution such as sodium hydroxide. This is because oxalic acid is pure and can be accurately weighed out on a scale.

Experiment 22:

Aim:

Analysis of iron content

Equipment:

Refer to STAWA book page 152

Procedure:

Refer to STAWA book page 152-153

Results:

	Rough Estimate	Accurate 1	Accurate 2	Accurate 3
Final Reading (mL)	43.0	43.7	43.5	43.6
Initial Reading (mL)	30	30	30	30
Titration Volume (mL)	13.0	13.7	13.5	13.6

Processing of Results and Questions:

1.       MnO₄^- + 8H ⁺ + 5e ^-  --> Mn²⁺ + 4H₂O
Fe²⁺ --> Fe³⁺ + e ^-
MnO₄^- + 8H ⁺ + 5Fe²⁺   --> Mn²⁺ + 4H₂O + 5Fe³⁺

2.       c(Permanganate) = 0.0195 mol L ^-1
V(Permanganate) = 0.0136 L
n(Permanganate) = c(Permanganate) x V(Permanganate)
n(Permanganate) = 0.000265 mol

3.       n(Iron) = n(Permanganate) x 5
n(Iron) = 0.001324 mol in 20 ml

4.       n(Iron) = 0.016548 mol in 250 ml        ==           ([0.001324 ÷ 20] x 250)
M(Iron) = 55.85 gmol ^-1
m(Iron) = n(Iron) x M(Iron)
m(Iron) = 0.924224 g

5.       0.924224 ÷ 1 = 0.924224
0.924224 x 100 = 92.4%

6.       Iron can be oxidised from its surroundings. Oxygen is a main source of this impurity.

7.       Fe + H₂SO₄ --> FeSO + H₂
Fe + 2H⁺ --> Fe²⁺ + H₂

Investigation 12:

Aim:

To determine the concentration of hydrogen peroxide

Equipment:

·         Beakers

·         Conical Flask

·         Funnel

·         Volumetric Flask (500ml)

·         Pipette

·         Pipette Filler

·         Burette and Stand

·         Sulfuric Acid

·         Potassium Permanganate

·         Hydrogen Peroxide

·         Distilled Water

·         Safety Equipment

Procedure:

1.       Put on safety equipment

2.       Take out 2 ml of concentrated hydrogen peroxide

3.       Mix 20 ml of sulphuric acid with 30 ml of potassium permanganate solution

4.       Add 2 ml intervals of the solution to the hydrogen peroxide

5.       Do this until you see the first undissolved purple colour in the hydrogen peroxide solution

6.       Place 5 ml of hydrogen peroxide in volumetric flask and dilute it (should be by a factor of 100), we diluted ours by a factor of 102

7.       Titrate with sulphuric acid and potassium permanganate solution

Results:

	Rough Estimate	Accurate 1	Accurate 2	Accurate 3
Final Reading (mL)	44.0	43.7	43.7	43.8
Initial Reading (mL)	20	20	20	20
Titration Volume (mL)	24	23.7	23.7	23.8

 

 

Processing the Data:

1.       MnO₄^- + 8H ⁺ + 5e ^-  --> Mn²⁺ + 4H₂O
H₂O₂ --> O₂ + 2H⁺ + 2e ^-
2MnO₄^- + 6H ⁺ + 5H₂O₂  --> 2Mn²⁺ + 8H₂O + 5O₂

2.       c(Permanganate) = 0.0195 mol L ^-1
V(Permanganate) = 0.023733 L
n(Permanganate) = c(Permanganate) x V(Permanganate)
n(Permanganate) = 0.000462 mol
n(Hydrogen Peroxide) = [n(Permanganate) ÷ 2] x 5
n(Hydrogen Peroxide) = 0.001155 mol in 20 ml
n(Hydrogen Peroxide) = 0.029456 mol in 510 ml         ([0.001155 ÷ 20] x 510)
c(Hydrogen Peroxide) = n(Hydrogen Peroxide) ÷ V(Hydrogen Peroxide)
V(Hydrogen Peroxide) = 0.51 L
c(Hydrogen Peroxide) = 0.058912 mol L ^-1 (dilute)
c(Hydrogen Peroxide) = 6.01 mol L ^-1               (concentrated)                                ([0.057757 ÷ 5] x 510)

3.       c(Hydrogen Peroxide) x M(Hydrogen Peroxide) = g mol ^-1
M(Hydrogen Peroxide) = 34.016 g mol ^-1
c(Hydrogen Peroxide) = 204.4 gL ^-1

Evaluating the Investigation:

1.       Based on our results and the time it took us to finish the investigation, I would say that we worked fairly efficiently. Therefore, I do not see any places to make improvements. The only minor ‘problem’ we hit was the fact that we diluted the hydrogen peroxide too much. After filling the volumetric flask with distilled water, we found out that we had 510 ml of standard solution instead of 500 ml. This only changed our calculation number.

2.       [c(Hydrogen Peroxide) ÷ 1000] x 100 = % by mass
% by mass = 20.44%
The bottle stated a 30%, however, we got 20%. This can easily be explained through chemistry. As soon as the seal cap is opened, gases start to escape the bottle. Therefore the percentage drops over time since the gases escape. This is the reason why our results are lower than the one stated on the bottle.

3.       We were the first group to finish this experiment.  Compared to the other groups, our results were more accurate since we used the hydrogen peroxide sooner; therefore, fewer gases had escaped. Overall, the only error that may have occurred in our experiment would be from the hydrogen peroxide solution; since it wasn’t seal closed when we used the solution.