Timing
About 3 hours will be neededSuggested treatment
This section gives students the opportunity to use moles and molarities in the context of neutralisation reactions. Concentrations of solutions are often
conveniently expressed in units of moles of specified entities (ions, molecules etc.) per cubic decimetre of solution. Dissolving a mole of substance in sufficient water to make I cubic decimetre of solution gives a solution concentration of I mole per cubic decimetre, sometimes known as a molar solution. The abbreviation 'M'is sometimes used to indicate this concentration.
Teachers should note that for calculations the concentration of a solution is expressed more helpfully as moles per cubic decimetre (abbreviated mol dm-3). In laboratories the convenient system of labelling solutions as 'M', '2 M' is usual;
students should be familiar with both practices.
As a preliminary to the practical work in Experiment 4.2, students are introduced to calculations involving concentrations of solutions. A number of examples involving the calculation of concentration in mol dm-3 (from a knowledge of the mass of dissolved substance in a given volume) and the calculation of volume of solution required to neutralise a solution of known concentration should be given.
Students should be able to use the relationship:
. (' Id -3) amount of substance (in mol)
concentratIOn III mo m = 3
volume (in dm ) or
. (' Id -3) amount of substance (in mol)
concentratIOn III mo m
=
3volume (in cm )/1000
-1
ANSWERS TO THE BACKGROUND QUESTIONS1---Questions, page 82
1
a
0.0382
a
36.5g3
a
0.125b 0.03 b 2.72g b 0.1
cid-base reactions and the alkaline earth elements
Each group of students will need 24hours in advance:
Eye protection
Conical flask, 250 cm3, with stopper Calcium hydroxide IRRITANT
For the titration:
Conical flask
Filter funnel and filter paper 0-110 °C thermometer
Titration apparatus with 10 cm3 pipette and filler 0.050Mhydrochloric acid
Indicator, methyl orange or bromophenol blue FLAMMABLE
Procedure
The solubility of calcium hydroxide in water can be found by titrating a saturated solution with a standard acid. Details are given in the Students' Book. Depending on the experience of the students, it may be necessary to demonstrate the correct use of titration technique. As an alternative to a pipette and pipette filler, a second burette may be used.
Students should obtain values for the solubility of calcium hydroxide of about 1.65gdm-3 at 20°C.
4.3 The properties of the alkaline earth elements
Timing
About 2 hours will be required plus homework timeSuggested treatment
The students are asked to draw up a number of tables in their notebooks, and then to look up or observe the required information and record it in the tables. Much of the information can be observed using a Periodic Table database on CD-ROM or the Web. The tables are:
• the physical appearance of the elements,
• the radii of the atoms and ions of the elements,
• the reactions of the metals with oxygen and water.
Class time can be spent helping students to begin to collect the numerical data and in demonstrating the reactions of some of the elements with oxygen and water.
TEACHER
DEMONSTRATION 4.3
For the reactions with oxygen the teacher will need:
1 strip of cleaned magnesium ribbon, 10 cm long A few calcium granules, fresh and shiny 2 gas-jars of oxygen
1 pair of tongs 1 combustion spoon
Eye protection and safety screen
4.3 The properties of the
HAZARD
Magnesium and calcium are highly flammable. Barium and petroleum ether are
flammable. Barium is harmful
Procedure
Hold the piece of magnesium by one end in a pair of tongs. Ignite the other end in a Bunsen burner flame. Immediately transfer the burning magnesium into a gas-jar of oxygen. The flame is very bright indeed and students and teacher should not look directly at the flame but concentrate on a point about a metre to one side.
Burning calcium in oxygen is very capricious and carries a high failure rate.
The calcium should be fresh and have a shiny appearance but, even so, when hot it tends to acquire a coating of calcium oxide which prevents attack by further oxygen. Sputtering of hot particles can occur - use a safety screen.
Put the calcium granules in a combustion spoon and get them as hot as possible as quickly as possible. Watch for signs of a glow or spark. Transfer into the oxygen. When it does burn there is a very quick reaction with a red tinge to a very bright flame.
It is not recommended that strontium or barium be burned in oxygen.
For the reactions with water the teacher will need:
1 piece of cleaned magnesium ribbon, 4-5 cm long A few calcium granules
2-3 small (3 mm) pieces of barium (if available) stored under oil 1 small beaker (5 cm3)containing petroleum ether (80-100 or 100-120) 1 spatula
3 glass Petri dishes 1 piece of filter paper Overhead projector Universal indicator solution Eye protection
Procedure
Put water in the first Petri dish and put it on the overhead projector. Put the magnesium in the water and focus the projector on the magnesium. No reaction is visible initially but a few bubbles of gas may appear around the magnesium after a time.
Put a second Petri dish of water on the overhead projector and add the calcium granules. Bubbles of gas are immediately seen and a white precipitate of calcium hydroxide appears. This obscures the light from the projector so that the image on the screen appears black.
Using the spatula, remove the barium pieces from their protective oil and transfer them into the small beaker of petroleum ether. This washes off the oil.
Remove the pieces from the petroleum ether and blot them dry using filter paper.
Put a third Petri dish of water on the overhead projector and add the barium pieces.
The reaction is comparable in vigour to the reactions involving alkali metals.
Add a few drops of universal indicator to each Petri dish. There will be no alkaline indications in the case of the magnesium - there is very little reaction and in any case magnesium hydroxide is virtually insoluble - but the calcium and barium hydroxides will show a blue colour.
Interpretation
Each table is followed by a question or questions about the data. The intention is that students should write the answers in the form of sentences which stand on their own. For example, the answer to the question: 'What generalisation can be made about the sizes of the ions of these metals, relative to the sizes of their atoms?' might be: 'The table shows that the ions of these elements are much smaller than the corresponding atoms.'
Throughout this and later topics, students should be encouraged to make sure that their notebooks are used to collect information logically and to record the outcome of discussions. At this stage it is not intended to do more than draw attention to the similarities and trends in the properties. Several of the points raised are taken up in later topics, or may be discussed in the light of later concepts. At this stage, it would clearly not be a good idea to try, for example, to teach possible
cid~base
reactions and the alkaline earth elements4.4
reasons for differences between atomic and ionic radii. The fact that ionic radii do show a trend is used in Experiments 4.4c and 4.4d, which is why attention is drawn to the trend now.
For the teacher's convenience, the numerical data required in the various tables are collected below.