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* PAGE 1 * PAGE 3 * PAGE 4 * PAGE 5 * PAGE 6 * PAGE 7 * PAGE 8 * page 9 from the Futures Language School, Cairo * EMAIL Dr Mahmoud Marsafy * back to Dr M's index * doc b email query?comment REMEMBER: you must do your own experiment risk assessment, ideas are illustrated but health and safety details are NOT included 9. The reaction of ammonia and hydrogen chloride
10. Halogen Displacement
Preparation
of Chlorine water by drops of reagents in each vial produces enough
chlorine to demonstrate clearly the reaction, minimising to a very great
extent the possibility of any significant amount to escape in the
laboratory atmosphere. Thus in a reasonably ventilated laboratory this
experiment can be very safely carried out simultaneously by many students
without need for a fume hood. §
Not
only hexane can be used to extract bromine and iodine liberated by
displacement, but also other denser organic solvents such as CCl4. §
I
am intending to apply this method to prepare Bromine water solution and
bromine in organic solvents and demonstrate experimentally the free
radical substitution in alkanes (HEXANE) and the electrophilic addition to
double bonds in Alkenes (I shall be using a drop of olive oil as a source
for unsaturation, since no alkene chemical is available to me here). I
hope to be able , using universal indicator to demonstrate also the
release of acid in the electrophilic
addition reaction. §
Bromine
is very difficult to obtain in many school laboratories
in
GCSE/IGCSE revision notes on the Group 7 Halogens 11. Oxidation of alcohols Few drops of Cr2O7 are added in the reaction vial followed by 2drops of Conc. H2SO4 on further addition of a few drops of Ethanol there is almost an immediate change of colour to blue. In this redox reaction Ethanol is oxidized to Ethanal (note distinct smell) and Cr2O7 (orange) is reduced to Cr3+ (blue).
§
It
is interesting that Ethanal can be detected by the reduction of Schiff`s
reagent by Ethanal . This is visualized by inserting a small twisted
tissue paper soaked with colorless Schiff`s reagent. The vial is stoppered
and the tissue paper colour changes to a reddish tint. This is a positive
test for Ethanal. §
The
oxidation of Ethanol by the acidified dichromate can also be well achieved
utilising drops on a plastic surface.
Chart
2. The oxidation reaction of Ethanol. §
Ethanol
is oxidized in two consecutive steps to Ethanal and then to Ethanoic acid.
The second step would require extended heat
refluxing . §
The
oxidation in each step requires the loss of two hydrogen ions. Bead models
well illustrates the changes occurring in these redox reactions. Chart
3. A flow Chart for the
various important Alcohol reactions.
§
Bead
models are included to visualize the bonding in the end products of each
reaction.
Advanced Level Chemistry Notes on Oxidation and Reduction in Organic Synthesis 12. Small scale enthalpy experiments
The
A2 students were required to carry out an enthalpy change for the reaction
between Zinc Powder and Copper sulphate solution.
They used 25 ml of the copper sulphate solution.
GCSE/IGCSE Notes on Exothermic and Endothermic Reactions 13. Some simple conductivity experiments The
three charts attached focus on comparative quantitative
determination of the conductivities of any electrolyte solution (Chart
1). Also application of this technique to investigate the changes in
conductivity during drop titration of HCl as well as Acetic acid with dilute
NaOH. The PH Change is simultaneously estimated utilizing Universal Indicator
(CHART 2). Chart 3 is a plot of the changes in conductivities and PH during
the drop titrations in Chart 2. Chart 1. Electrolyte Conductivities
§
The
conductivity tester circuit is assembled by utilizing the common AC/DC
converter, as the power supply and connecting a digital electric multi-meter
in series selecting the 200 mA measuring range. §
Metal
wire electrodes of various types are connected to the +ve and –ve terminals
of this circuit. §
The
electrolyte is transferred into a small plastic container. §
As
soon as both electrodes are immersed in the electrolyte the initial mA
reading of the meter is noted
ignoring any drifting of this measurement. §
To
obtain valid comparative mA measurements ( a function of the electrolyte
conductivity), the following parameters must be strictly adhered to : The
same volume of the electrolyte must be transferred to identical clean small
plastic containers. The
electrodes must be inserted at the same distance.
This is easily achieved with the circular
containers by carefully aligning each electrode to the periphery of the
container. The voltage of the power supply must be identical throughout the
measurements. Chart 2. Drop Titration of HCl and Ethanoic acid with simultaneous recording of changes in pH and conductivities during the titration
§
One
drop of Conc HCl or Acetic acid is added to 3 ml of water with 2 drops of
universal indicator (UI) in a suitable small
plastic container. §
Drops
of 1M NaOH are then added noting the colour of the universal indicator ( a
function of pH) and the mA meter reading after brief insertion of the metal
electrodes. §
About
20 drops of NaOH are added to locate the end point and proceed beyond it. §
It
will be noted that the initial Ma for HCl is about (90mA), while that for
Acetic acid is only about 2.6mA. The conductivity steadily decreases during
the titration course for HCl, reaching a value about 50 mA
after addition of about 14 drops of NaOH (the end point
titer). With further addition of drops of NaOH solution the
conductivities steadily increase recording a terminal value of 70 mA after
having added a total of 20 drops. The colour of the universal indicator
changes from red to pale blue on the addition of
a total of 14 drops of NaOH. The next drop
of NaOH added changes the colour to Violet. §
The
conductivities of acetic acid steadily significantly increase from the
addition of the first drop of NaOH till it reaches a mA reading of 25 after
the addition of a total of 14 Drops of NaOH. The colour of the UI sharply
changes from yellow at the stage of drop 14 to
a distinct green colour at the
added drop 15 proceeding to a violet colour at the drop 16. §
The
variation in the size of the drops of the various solutions from various
containers must be taken into consideration when trying to correlate the
experimental titer values with the predicted stoichiometric values. The drop
size of the NaOH solution in particular, due to its surface tension must
differ significantly from the acid drops. Chart 3. Titration Curves for HCl & Acetic Acid with NaOH
§
The
estimated pHs and mA conductivity measurements during the course of the drop
titrations as discussed in Chart 2 have been plotted and curves obtained
utilizing the EXCEL software of the Microsoft Office. §
These
curves are similar to the reference curves in the international literature.
Advanced Level chemistry Notes on acid-base equilibria, pH and the theory of acid-base titrations * PAGE 1 * PAGE 3 * PAGE 4 * PAGE 5 * PAGE 6 * PAGE 7 * PAGE 8 * page 9 from the Futures Language School, Cairo * EMAIL Dr Mahmoud Marsafy * back to Dr M's index * doc b email query?comment
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