2013-2014
Time allowed: Up to 2 hours (practical)
- Listen carefully to instructions from the staff members and you should stop immediately if asked to do so.
- You should work safely at all the time and wear appropriate clothing.
- If you need help or any additional equipment raise your hand up.
- Scientific Calculators are allowed.
- Dictionaries (except electronic ones) are allowed.
- Before you do your practical your teacher will explain what you are going to do and might carry out a demo- you are advised to listen carefully and take notes.
Information to candidates
Only use this paper to prepare your lab-report which should be done electronically. Graphs or diagrams can be done by hand and then scanned for submission.
Total marks for this paper: 20
Mark Scheme (This is a simplified version of the rubric calculator)
Qualitative Practical Task – 20 Marks
Qualitative Analysis-Inorganic Chemistry
Aim: to compare the reactivity of halogens by studying their displacement by one another.
Prediction: (What you think you will find out from the experiment. I predict that..)
I predict that the smallest halogens will be the most reactive and will be able to displace the larger halogens. Chlorine, the smallest halogen, will be the most reactive. Iodine, the largest halogen, will be the least reactive.
Materials: (A list of all the materials you will use)
Apparatus: (*Draw a well labelled, simple apparatus diagram. You can also use suitable information technology programmes. Any apparatus diagrams taken from the internet should be referenced properly).
Safety Precautions:
- Chlorine water and bromine water are both toxic and corrosive so I’m going to use them in a ___fume__ _cupboard________. I’m also going to use protective clothing such as ____lab coat, gloves_______ and eye protection like goggles. I should avoid inhaling any gas fumes.
- Cyclohexane is highly flammable and harmful. I should ___keep away from heat and sources of ignition _ and ___wear protective clothing_
Method:
- In test-tubes, mix 2 cm3 of a halogen solution with 2 cm3 of a solution containing halide ions.
- Add 2 cm3 of cyclohexane, cork the tube and shake it vigorously.
- When layers have separated, note the appearance of the organic layer (top layer).
- Repeat the procedure with different halogen and halide ion combinations.
Results Table:
Evaluation:
Include answers of the following questions in your evaluation:
- Why did the organic layer (cyclohexane) settle on top and the halide ion solution settle at the bottom in each of the experiments?
Many organic solvents, such as cyclohexane, are less dense than water and thus will settle on top of the aqueous solution(water). The halide ion solution settles at the bottom because halides are soluble in water. The halogen is nonpolar and thus remains in the organic solvent cyclohexane.
?
- What was the purpose of cyclohexane in this practical?
The cyclohexane is a nonpolar solvent and the halogens dissolve in it. When a halogen reacts and forms a halide ion, it can then be detected in the aqueous layer by a colour change. The change in colour depends on what halide ion has been displaced. The cyclohexane also assists in this reaction.
The second row of the table illustrates the effect of each halogen when displaced into halide ions in the cyclohexane/water mixture and when in competition for displacement with other halogens. Chlorine becomes displaced to chloride ions, which are colourless or light yellow. When chlorine is mixed with bromine, the chlorine outcompetes bromine to release bromide ions and create an orange colour. When chlorine and iodine are mixed together, chlorine outcompetes iodine to release iodide ions and create a purple colour.
The third row illustrates a competition between bromine and the other halogens. When bromine is mixed with chlorine, there is a change in colour to reddish brown as the chlorine outcompetes the bromine to result in the release of bromide ions. The results are the same when bromine is added to bromide, as bromide ions are displaced. When bromine is mixed with iodine, the bromine outcompetes iodine to release iodide ions and thus creates a purple colour.
The fourth row illustrates the competition between iodine and other halogens. When iodine is mixed with chloride, the chloride outcompetes the iodine and iodide ions are released, resulting in a purple colour. When iodine is mixed with bromide, the bromide outcompetes the iodine and this displacement releases iodide ions, resulting again in a purple colour. Iodine and iodide together produce a purple colour as a result of displaced iodide ions.
Conclusions: (What did the experiment show you? Check your Aim and make sure that each Conclusion is clear)
The conclusion of this experiment is that chlorine is the most reactive halogen of the three tested, followed by bromine. Iodine is the least reactive halogen, as illustrated by the displacement assay with cyclohexane. Therefore, chlorine displaced bromine, and bromine displaced iodine.
References:
- Royal Society of Chemistry 2013, http://practicalchemistry.org/experiments/reactions-of-aqueous-solutions-of-the-halogens,136,EX.html
- Anderson CP, Spears KG, Wilson KR, Sension RJ. Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI. J Chem Phys. 2013 Nov 21;139(19):194307.
- Warnhoff, E. W. (1996). "The Curiously Intertwined Histories of Benzene and Cyclohexane". J. Chem. Ed. 73 (6): 494.
- Kenneth Barbalace (1995-10-22). "Chemical Database: Chlorine
- Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p.4.121
- Windholz, Martha; Budavari, Susan; Stroumtsos, Lorraine Y. and Fertig, Margaret Noether, ed. (1976). Merck Index of Chemicals and Drugs, 9th ed. J A Majors Company.
NOTES FOR THIS EXPERIMENT (Use these notes and other sources to prepare for your lab-practical and lab-report)
- Produce chlorine water by bubbling chlorine through distilled water, in a fume cupboard.
*Another viable method is by reacting potassium permanganate (KMnO4) with
concentrated hydrochloric acid.
- Halogens are soluble in non-polar organic solvents whereas halide ions are not. Separating the halogens from the halide ions by solvent extraction with cyclohexane makes their identification easier.
- Halogens react in the following reactivity order: F2>Cl2 > Br2 > I2.
- The halide ion being displaced becomes oxidised by transferring an electron to the halogen displacing it e.g.:
Cl2 + 2Br- Br2 + 2Cl-
- The smaller the atom, the more readily the electron is accepted because the atom’s nucleus attracts strongly the bond electrons, there is also less shielding. Therefore reactivity of halogens decreases down Group 7.
- Do not use rubber bungs because cyclohexane reacts with rubber.
- The colour intensity of halogens increases down the group from pale yellow (F2) to dark purple (I2).
