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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.

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Redox half equations:

x3 ( Fe � Fe2+ + 2e- )

Cr2072- + 14H+ + 6e- � 2Cr3+ + 7H2O

Cr2072- + 14H+ + 3 Fe � 2Cr3+ 7H2O 3Fe2+


The aim of this experiment is discover the relationship between the temperature (20�C,25�C,30�C,35�C,40�C) 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. (20�C,25�C,30�C,35�C,40�C) . 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.

Independent variable:

Independent variable

Range (�C �0.1�C)

How it is measured

How they are changed



Electronic thermometer

Changing temperature of water bath

Dependent variable:

Dependent Variable

How it is measured



Stop watch controlled by human

Reaction time

Constant variables:

Constant variables


How they are kept constant /monitored

Amount of H2SO4 used

20ml �0.1

The same measuring cylinder was used

Amount of Potassium Dichromate used

20ml �0.1

The same measuring cylinder was used

Stirring speed

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 �0.01

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

Air temperature

19.0�C �0.1�C

Tests are carried out on same day

Concentration of sulphuric acid

2.0 mol

Same bottle is used in each test

Concentration of potassium dichromate

0.1 mol

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 � 0.5 sec

5) 1x Electronic thermometer �0.1�C

6) 1x large water bath �0.1�C

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 20�C. Weigh the iron with electronic scales (�0.001g) 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 �1ml 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 �1ml 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 20�C. 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 20�C. When satisfied this is accurate hold the stop watch in one hand and the 0.300g �0.001g 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.

Data Table:

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.

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