Design – Potassium Dichromate Essay
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The reaction that is carried out in this design practical is between acidified Potassium dichromate and Iron wool. The reaction is a redox reaction. A Redox reaction (a short way of saying “reduction-oxidation reaction”) is the simultaneous transfer of electrons between multiple substances. The oxidation numbers or the oxidation state of atoms is changed through the process of oxidation or reduction. Reduction is where electrons are gained to the reaction and oxidation is when electrons are lost in a reaction between substances.
In this experiment Iron (or Fe) is losing two electrons meaning it undergoing oxidation. Potassium Dichromate is gaining electrons meaning it is undergoing reduction. The Iron is the Reducing agent because it gave electrons to the Potassium Dichromate. The Potassium Dichromate is the oxidising agent because it took electrons from the iron.
This experiment will attempt to prove the redox reaction through experimentation.
Redox half equations:
x3 ( Fe ï¿½ Fe2+ + 2e- )
Cr2072- + 14H+ + 6e- ï¿½ 2Cr3+ + 7H2O
Cr2072- + 14H+ + 3 Fe ï¿½ 2Cr3+ 7H2O 3Fe2+
The aim of this experiment is discover the relationship between the temperature (20ï¿½C,25ï¿½C,30ï¿½C,35ï¿½C,40ï¿½C) of the reaction between acidified Potassium Dichromate and Iron using a water bath on the rate in which the acidified Potassium Dichromate and Iron react and change colour from yellow to dark purple.
The independent variable in this experiment is the temperature. The reaction will be carried out at a range of temperatures. (20ï¿½C,25ï¿½C,30ï¿½C,35ï¿½C,40ï¿½C) . The reaction will be carried out in a water bath. The temperature will be measured with an electronic thermometer and will be measured every 10 seconds to make sure the water bath is at a constant temperature in each trial.
The dependent variable is the time it takes for the reaction between Potassium Dichromate and Iron wool to come to completion or to turn from yellow to green. Time will be measured with a digital stopwatch.
Range (ï¿½C ï¿½0.1ï¿½C)
How it is measured
How they are changed
Changing temperature of water bath
How it is measured
Stop watch controlled by human
How they are kept constant /monitored
Amount of H2SO4 used
The same measuring cylinder was used
Amount of Potassium Dichromate used
The same measuring cylinder was used
Slow but constant
Magnetic stirrer wasn’t able to be used in water bath, so I manually stirred the solution slow and constantly
Mass of wool
0.295g – 0.305g ï¿½0.01
Mass of iron wool was kept as close as possible in each test
Surface area of wool
Wool was squeezed into cube shape 1cmx1cmx1cm
Type/brand of wool
Superior manufactures ltd
Same box of Iron wool is used
Tests are carried out on same day
Concentration of sulphuric acid
Same bottle is used in each test
Concentration of potassium dichromate
Same solution is used in each test
Yellow- dark green
A pilot with dark green colour is used to compare to the colour change in the trials
How to ensure reliability of results:
To ensure the reliability of results the temperature of the water bath and the reaction are measured very frequently (once a min). The chemicals came from the same containers as to keep concentration consistent. I did a pilot test first to see what colour I am looking for. This beaker of dark green liquid is then used to compare with the colours in the trials so I know when the reaction has come to completion.
1) 5x 20ml Sulphuric acid-2.0mol
2) 5x 20ml Potassium Dichromate- 0.1mol
3) 5x 100ml Beakers
4) 1x Digital stop watch ï¿½ 0.5 sec
5) 1x Electronic thermometer ï¿½0.1ï¿½C
6) 1x large water bath ï¿½0.1ï¿½C
7) Approximately 2g Iron wool- split into 5x 0.300g -1cm3 -cubes
1) Gather all the equipment that is listed in the equipment list provided above and prepare a table like the one shown on the following page.
2) Secondly set your water bath at 20ï¿½C. Weigh the iron with electronic scales (ï¿½0.001g) wool making and modify its mass to make it 0.300g or as close as you can get. Squeeze the iron wool so that the wool is approximately 1cmx1cmx1cm. This makes the surface area constant over the trials.
3) Next measure 20ml with a 50ml ï¿½1ml measuring cylinder of 0.1mol Potassium dichromate. Pour it into a 100ml beaker making sure to pour all drops into the beaker. Measure 20ml of 2.0mol sulphuric acid with a 50ml ï¿½1ml measuring cylinder. Pour it into the same 100ml beaker as the potassium dichromate.
4) Measure the temperature of the water bath the an electric thermometer making sure it is 20ï¿½C. When certain the temperature is constant place the beaker with the potassium dichromate and sulphuric acid into the water bath. Make sure as much of the beaker is under the water as possible but keeping enough above not to let any of the water in. Get someone else to hold the beaker down if required.
5) Measure the temperature of the potassium dichromate and sulphuric acid making sure the temperature is exactly 20ï¿½C. When satisfied this is accurate hold the stop watch in one hand and the 0.300g ï¿½0.001g iron wool in the other hand. Place the iron wool in the beaker and simultaneously push the start button on the digital stop watch. The colour should change from yellow to a dark green colour. As soon as you are satisfied the colour has changed to the dark green colour push the stop button on the digital stop watch. Record this time in your pre prepared data table.
6) Next repeat steeps (2-5) 5 times with the same chemicals. These will be the five trials.
7) Repeat steps (2-6) 5 times but now change the water bath temperature. The range of temperatures are ). Record all the data in the table you have prepared.
Raw data table showing the effect of a change on temperature on the rate in which the reaction between acidified Potassium Dichromate and Iron wool occurs.