Key unit competence: compare and contrast the properties of the group 2 elements and their compounds in relation to their position in the periodic table
Learning objectives
By the end of this unit, I will be able to:
•Describe the physical properties of group 2 elements.
•Describe the properties of group 2 oxides and hydroxides.
•Explain the trends in the thermal decomposition of group 2 carbonates and nitrates.
•Explain the trends in the solubility of group 2 compounds.
•State the uses of group 2 elements and their compounds.
•Describe the industrial manufacture of cement.
•Discuss the environmental and health issues associated with the manufacturing of the cement.
•Perform experiments to compare and contrast the reactivity of group2 elements.
•Write balanced equations of the reactions of group 2 elements, different elements and the compounds.
•Illustrate practically the trends in solubility and thermal decomposition of group 2 compounds.
•Test the alkaline character of group 2 hydroxides.
•Be aware that the compounds of beryllium are different from the compounds of the other group elements.
•Perform chemical test for the presence of group 2 cations in solution.
•Suggest preventive measures for environmental and health issues associated with the manufacture of the cement.
•Appreciate the logic underlying the position of elements in the periodic table ,their electronic structure and the properties.
•Appreciate the application of the chemistry of group 2 elements and their compounds in the social economic development.
•Develop the team work approach while performing experiment and writing field –study reports.
•Develop the attitude of sustainable exploitation of natural resources .
•Stimulate the culture of entrepreneurship in the area of chemistry.
Group 2 elements are located at the left section of the Periodic Table of Chemical Element, second column; they are: beryllium (4Be), magnesium (12Mg), calcium (20Ca), strontium (38Sr), barium (56Ba) and radium (88Ra). All Group 2 elements are metals, solid at room temperature.
They are characterized by the valence electronic structure of ns2. The whole electronic structure can be represented as follows:[Noble gas]ns2, where Noble gas is the noble gas that precedes the given element in the periodic table, and n represents the valence shell.
Group 2 elements have 2 outer electrons in s orbital and when those 2 electrons are lost, they form cations with the charge + 2.
7.1. Occurrence and physical properties of group 2 elements
Activities
Name 1 or 2 elements of group 2 or their compounds that we commonly find in Rwanda
7.1.1. Occurrence
Group 2 elements are active metals and are found in nature in form of compounds or minerals such as: Limestone and marble for calcium, dolomite and magnesite for magnesium etc... Hence Group 2 metals must be produced from the minerals they are found in.
7.1.2. Physical properties
Group 2 elements are all metals, solid at room temperature; they are good conductors of electricity. They have a silvery luster that soon disappears upon exposure to air. They are malleable and ductile but less than alkali metals of Group 1. Their atomic radius and their volume are smaller than those of Group 1 elements in the same period. The Table 7.1 below shows some other physical properties of Group 2 elements.
Atomic radius increases down the group due to increasing of electronic levels.
Melting and boiling temperature decreases down the group due to increasing of atomic radius resulting in weakening of the metallic bond.
The increasing of atomic radius explains also the decreasing of first ionization of the elements down the group. This also explains that the metallic character of the elements increases down the group.
Table 7.1: Physical properties of Group 2 elements
Checking up 7.1
Question1: Metals are reducing agents because they lose easily electrons. You are given 3 elements of Group 2: Be, Ca and Ba. Which one are you going to choose as the best reducing agent, and explain why?
Question 2: How does each of the following properties of the elements in Group 2 changes down the group and why?
i) Atomic radius
ii) Ionisation energy
iii) Electropositivity
7.2. Reactivity of group 2 elements
Activities 7.2 (a)
Activity 1:
•Pour 200cm3 of water in two different beakers
•To the first beaker, add a small piece of magnesium ribbon. To the second beaker, add a very small piece of sodium.
•Record your observation.
•Put a piece of blue and red litmus paper in both beakers
•Record your observations.
Activity 2:
•Pour 200cm3 of water in pyrex beaker or borosilicate beaker
•Heat until water boils•Using crucible tongs, hold a large piece of magnesium in the steam•Record your observations
Activities 7.2 (b)
Reaction of magnesium and hydrochloric acid
Experimental procedures
(1) Reaction of magnesium and hydrochloric acid
•Put 100 cm3 of 0.2M dilute HCl(aq) in a beaker. Add a small piece of magnesium ribbon.
•Record your observations
Do not test the gas given off, it can cause the explosion.
Group 2 metals react by losing 2 electrons; they areless reactive than the corresponding alkaline metals. They mainly form ionic compounds.
Checking up 7.2
1. With equations of chemical reactions, show how the following group 2 metals (Be, Ca and Mg) react with:
i) HCl(aq)
ii) N2
iii) H2OGive the reason why beryllium shows different properties from the rest
7.3. Properties of group 2 compounds
7.3.1. Ionic and covalent character of oxides and halides
Activity 7.3.1 (a)
•Pour 50 ml of paraffin in a beaker
•Put 1g of calcium chloride and try to make a solution
.•Pour 50 ml of water in another beaker
•Put 1g of calcium chloride and try to make aqueous solution
•Write down your observations and comments.
Activity 7.3.1 (b)
•Place a beaker on a table•Cut 15cm of magnesium ribbon
•Using crucible tongs, hold and burn the magnesium ribbon over the beaker.
•What do you observe?•Add some water to the ash in the beaker.
•Shake the mixture and add 2 drops of phenolphthalein or touch the mixture with a red litmus paper•Record all your observations.
a. oxides
Group2 (like group 1), react with oxygen to form normal oxides (containing O2-ion) and peroxide (containing O22-ion). The oxides are ionic and dissolve in water to give solutions of hydroxide ions except BeO. Group 2 oxides are basic oxides; they react with water to produce metal hydroxides:
Checking up 7.3.1
1. Write three factors that cause chlorides of group 2 to dissolve with or without hydrolysis.
2. What’s a basic oxide?
3. When limestone is heated at high temperature, (i) it form calcium oxide or quicklime; (ii) when water is added to quicklime, slaked lime (ishwagara) is formed. Write the chemical equations involved in
(i) and (ii).
7.3.2. Trends in thermal stability of carbonates, hydroxides and nitrates
Activity 7.3.2
Procedure
•Put 2g of Ca(NO3)2 in a pyrex test tube•Heat strongly
•Use a wet blue litmus paper to test the gas given off.
•Note your observations
•Replace Ca(NO3)2 with CaCO3,CaSO4 and Na2CO3
Question:Write balanced equations for reactions in the above activities.
Caution: avoid the use of barium nitrate and strontium nitrate which can explode.
Thermal stability refers to the resistance to decomposition of the compound on heating. Increased thermal stability means a higher temperature is needed to decompose the compound.
Group 2 metal carbonates, nitrates and hydroxides decompose to heat to give the corresponding metal oxide and release CO2, NO2 and O2, and H2O respectively.
The thermal stability increases down the group as theionic character of the compounds increases down the group. Compounds of metals higher in the group (smaller ionic radius) decompose more easily than compounds of metals lower in the group, due to increase in covalent character upward in the group that results from high polarizing power of small ions. Compared to group 1 metals, compounds of group 2 metals are less stable to heat.
Note: The hydrogen carbonates, M(HCO3)2, only exist in solution and are very unstable. When their aqueous solutions are heated they decompose to carbonate, water and carbon dioxide:
This is the reaction that takes place when hard water (presence of Mg(HCO3)2(aq) and Ca(HCO3)2(aq)) is heated.
Checking up 7.3.2
Question:1. You have two carbonates A and B. Heated at high temperature, B decomposes and a gas evolves; on the contrary, A seems not to change. Which one corre-sponds to sodium carbonate and which one corresponds to calcium carbonate? Justify your answer.
2. Write the chemical equation to represent the thermal decomposition of lime-stone (CaCO3(s)) to produce carbon dioxide (quicklime)
7.3.3. Trends in solubility of hydroxides, carbonates, hydrogencarbonates and sulphates
Activity 7.3.3
You are provided with 8 test tubes .
•Put a half spatula end of each alkaline earth salt into 3ml of water,in labeled test tubes.
•No1= Magnesium hydroxide; No2= Magnesium sulphate; No3= Calcium hydroxide, No4= Calcium sulphate; No5= Strontium hydroxide; No6 = Strontium sulphate; No7= Barium hydroxide; No8= Barium sulphate
•Shake the test tubes and try to make aqueous solution.
•Write down your observations and related comments.
The solubility of salts depends on two main opposite factors:
The energy of dissociation of the crystal: The energy needed to dissociate the solid crystal into its ions. This process requires energy; the process is endothermic.
The energy of hydration of the ions produced: the amount of energy released when ions undergo hydration or are surrounded by water molecules; this process is exothermic.When the combination of the two processes above is in favor of the hydration of ions, the salt is soluble; otherwise the salt is not soluble.
The solubility will increase when the hydration process predominates more and more the dissociation process and vice-versa.
a. Sulphates
Their solubility decreases down the group BeSO4 and MgSO4 are soluble
CaSO4 is slightly soluble
SrSO4 and BaSO4 are insoluble
b. Hydroxides
Their solubility increases down the group.
Be(OH)2 and Mg(OH)2 are insoluble
Ca(OH)2 and Sr(OH)2 are slightly soluble, and Ba(OH)2 is fairly soluble
c. Carbonates
They are sparingly soluble in water.The carbonates of group2 elements tend to be less soluble as you go down the group.
d. Hydrogen carbonates
The metals of group 2, magnesium, calcium, strontium, and barium form
hydrogen carbonates compounds that contain the hydrogen carbonate anion, HCO3-, also known as the bicarbonate anion. Hydrogen carbonates of the alkaline earth metals exist only in solution; they are soluble in water.
Due to the high charge of their cations that attract water molecules, group 2 salts are more hydrated than group 1 salts. That is why they are often used as drying agents.
Checking up 7.3.3
1.
An aqueous solution may contain barium ion, Ba2+, how are you going to
organize to test if that ion is or not present in your sample?
Illustrate your explanation by chemical equations.
2. You are given two
white solid samples A and B. The only information you have is that one
of them is Be(OH)2 and the other is Ba(OH)2. What are you going to do to
try to identify those samples?
7.4. Anomalous properties of beryllium compounds
Activity 7.4
1. Give 1 or 2 examples showing an abnormal behavior of beryllium compared to the other members of the group?
2. What do you think may cause that anomaly? Beryllium shows the uniqueness in its group. Beryllium has higher tendency of forming complexes compared to other group 2 elements.
•Beryllium ion, Be2+, is very small compared to other group 2 metal ions.
•Beryllium has a high electronegativity compared with the rest of Group 2, hence it forms compounds with more covalent characters.
•For an ionic bond to form, beryllium must give up its electrons, but it is too electronegative to do so.The uniqueness of Beryllium makes its compounds to show anomalous properties.
For example:
1. Beryllium hydroxide, Be(OH)2is amphoteric whereas the other group 2 hydroxides are basic. An amphoteric substance behaves as an acid when it reacts with a base, and as a base when it reacts with an acid:
2. Beryllium chloride, BeCl2, is soluble in non-polar organic solvents, whereas the other group 2 chlorides are ionic and dissolve in water (a polar solvent). This fact illustrates the covalent character of beryllium compounds.
3. Beryllium complexes. It forms complexes ions such as [Be(OH)4]2- and [BeCl4]2-.
Checking up 7.4
Complete the table concerning the compounds of beryllium.
7.5. Identification test for Ba2+ ions in aqueous solutions
Activity 7.5
Task 1:
•Pour 100ml of distilled water into the beaker 1
•Add 1g of Beryllium nitrate
•Stir to make the aqueous solution
•Repeat the procedure, by replacing Beryllium nitrate by nitrates of other group 2 elements(Mg;Ca;Sr;Ba).
•Use a nichrome wire or magnesia rod to put each compound in a Bunsen flame.
•Write down your observations and related comments.
Task 2
•Put 3ml of barium nitrate (Prepared also for task 1) in a test tube
•Add 10 drops of dilute HNO3(aq)
•Using a dropper, add 10 drops of sodium sulphate solution.
•Write down your observation and the ionic equation.
a. Formation of precipitate
The insolubility of barium sulphate is the basis of a test for barium ions in solution. If a few drops of a solution containing sulphate ions, SO42- (CaSO4, Na2SO4, etc.) are added to a solution containing barium ions, a white precipitate of barium sulphateinsoluble in acidic solution will form.
b. Flame test
Like the compounds of group 1 metals, calcium, strontium and barium produce coloured flames when their compounds are put in a Bunsen flame.
Checking up 7.5
Question: By accident, a child drinks a solution of barium chloride.
•How can a physician proceed in order to precipitate Ba2+ into the patient’ stomach?
•What is the needed ion for this treatment?
•You have two unknown sample containing Ca2+ and Ba2+ ions respectively. You need to identify them, Suggest two different methods you can use and explain.
7.6. Uses of group 2 elements and their compounds
Activity 7.6
Describe the following compounds and show how each compound can be used to prepare another if possible.
a. Limestone
b) Quicklime
c)Slaked limeb. Have you heard about soil amendment in Rwanda? What is it?c. In groups, the students do research to find out how chalk used on blackboard is produced.
1. Beryllium
Because beryllium is relatively light and has a wide temperature range, it can be used in the manufacture of aircrafts’ components.
2. Magnesium
•Chlorophyll, the pigment that absorbs light in plants, is a complex of magnesiumand is necessary for photosynthesis.
•Magnesium hydroxide is used as Anti-acid medicine
•Magnesium is used in making Grignard reagents, the organomagnesium compounds.
•Magnesium is used as sacrificial anode to prevent iron sheet from rusting.
•Salts of magnesium and calcium are used in chemistry laboratory as drying agents.
3. Calcium
•Calcium is the major component of Limestone, raw material for manufacturing cement and slaked lime.
•One of the important material involving calcium is plaster of Paris, which gets its name because a major ingredient (gypsum), CaSO4.nH2O, wasmined near Paris. Plaster of Paris is used in making casts for broken bones, but it is also used for sculptures and plasterboard walls. Gypsum is used in the manufacture of cement.
•Calcium carbonate is used in the extraction of iron, manufacture of tiles, plates, laboratory mortar and pestle, in agriculture to reduce the acidity of soil.
4. The process of making slaked lime from limestone
Limestone is a naturally occurring material composed mainly by calcium carbonate.Limestone is used to produce quicklime and slaked lime (ishwagara):
a. Steps
First step: Decomposition of limestone by heating, a process called calcinations, giving quicklime.
Quicklime
Calcination is carried out in a high-temperature kiln (about 1000oC) with continuous removal of CO2 (g)to promote the forward reaction. The product formed, CaO(s), is called lime or quicklime.QuicklimeCalcination is carried out in a high-temperature kiln (about 1000oC) with continuous removal of CO2 (g)to promote the forward reaction. The product formed, CaO(s), is called lime or quicklime.
Second step: Hydration of quicklime giving Slaked limeIn the process of hydration, the reaction of quicklime with water produces Ca(OH)2(s) known as slaked lime.
Slaked lime
Uses of limestone and slaked lime
•Some limestone isused as building stones.
•However, most limestone is used to manufacture other building materials.
•Limestone is the raw material for the manufacture of cement (see Production of Cement in Rwanda at Bugarama Cement Factory).
•Slaked lime, Ca(OH)2, is the cheapest commercial base and is used in all applications where high water solubility is not essential.
.•Quicklime is used in the manufacture of sodium carbonate from caustic soda.
•Calcium carbonate and slaked lime are used to decrease soil acidity and acidity in lakes which are acid polluted.
5. Manufacture of cement
Cement is an important building material. It was first introduced in England in 1824 by Joseph Aspdin. Some types of cement are called Portland cement because it resembles with the natural limestone quarried in the Isle of Portland, England.
Cement is a product obtained by combining a material rich in lime, CaO, such as limestone, with other material such as clay which contains silica, SiO2 along with the oxides of aluminium, iron and magnesium.
The raw materials for the manufacture of cement are limestone and clay. When clay and limestone are strongly heated together they fuse and react to form ‘cement clinker’. This clinker is mixed with 2-3% by weight of gypsum (CaSO4·2H2O) to form cement.
Environmental risks: during the process of producing cement and quicklime, gases, mainly carbon dioxide, and solid particles are sent in air and constitute an environmental risk if their concentration increases in the atmosphere. All measures must be taken to avoid that those gases and solid particles become a real environmental problem.
6. Strontium
•It is used in fireworks as it produces a scarlet flame color.
•Strontium chloride (SrCl2) is used to make toothpaste for sensitive teeth. 7. Group 2 and Group 1 elements are used in fireworks to produce spectacular colours of the fireworks
Checking up 7.6
1. Name any two compounds of calcium and in each case give two uses of each.
2. Describe how slaked lime can be obtained from limestone.
3. The elements Be, Mg and Ba belong to Group 2 in the periodic table. How do they react with water and NaOH?
1. Which of the following is NOT an alkaline earth metal?
(a) Ba (b) K (c) Mg (d) Be (e) Ra2. True or False: group2 metals do not react vigorously with water.
3. Which alkaline earth metal is a main component in our bones?
4. Which group 2 metal has the highest ionization energy? And why?5. Which element has the lowest melting point? (a) Ba (b) Ra (c) Ca (d) Mg
6. Why is the term alkaline earth used to describe the Group 2 elements?7. Identify what is observed when the flame test is carried out for group 2 elements?.
8. Explain why all alkaline earth metals form ions by losing 2 electrons?
9. Which Group 2 element is radioactive?
10. Which substance is acting as the reducing agent and which one as oxidizing agent in the reaction below?
. Explain
11 .a) How do each of the following properties of the elements in group 2 change with increasing atomic number?
i) Atomic radiusii) Ionization energy
iii ) Strength as reducing agents
iv) Reactivity of reaction with chlorine
v) Electopositivity
b) In each case, explain why the property changes in the way you have suggested.
12. Group 2 contains the elements Be; Mg; Ca; Sr; Ba.
a) Are those elements metals or non metals? Give one reason for your answer.
b) What ion or ions are formed by calcium?
c) Explain why it is not possible to find a compound with Mg+
d) Group 2 elements form hydrated salts, while the corresponding compounds of group 1 elements are anhydrous. Suggest reasons for this difference.
13) The elements in group 2 of the periodic table are barium, beryllium, calcium, magnesium and strontium.
a ) Why do all the metals in group 2 have an oxidation number of +2 ?
b) Why do the elements in group 2 not have an oxidation number of +1 and +3
c) Write an equation to illustrate the process occurring when the second ionization energy of magnesium is measured.
Group 2 elements are located at the left section of the Periodic Table of Chemical Element, second column; they are: beryllium (4Be), magnesium (12Mg), calcium (20Ca), strontium (38Sr), barium (56Ba) and radium (88Ra). All Group 2 elements are metals, solid at room temperature.They are characterized by the valence electronic structure of ns2. The whole electronic structure can be represented as follows:[Noble gas]ns2, where Noble gas is the noble gas that precedes the given element in the periodic table, and n represents the valence shell.Group 2 elements have 2 outer electrons in s orbital and when those 2 electrons are lost, they form cations with the charge + 2.7.1. Occurrence and physical properties of group 2 elementsActivitiesName 1 or 2 elements of group 2 or their compounds that we commonly find in Rwanda7.1.1. OccurrenceGroup 2 elements are active metals and are found in nature in form of compounds or minerals such as: Limestone and marble for calcium, dolomite and magnesite for magnesium etc... Hence Group 2 metals must be produced from the minerals they are found in.
7.1.2. Physical propertiesGroup 2 elements are all metals, solid at room temperature; they are good conductors of electricity. They have a silvery luster that soon disappears upon exposure to air. They are malleable and ductile but less than alkali metals of Group 1. Their atomic radius and their volume are smaller than those of Group 1 elements in the same period. The Table 7.1 below shows some other physical properties of Group 2 elements.Atomic radius increases down the group due to increasing of electronic levels.Melting and boiling temperature decreases down the group due to increasing of atomic radius resulting in weakening of the metallic bond.The increasing of atomic radius explains also the decreasing of first ionization of the elements down the group. This also explains that the metallic character of the elements increases down the group.Table 7.1: Physical properties of Group 2 elements
Checking up 7.1Question1: Metals are reducing agents because they lose easily electrons. You are given 3 elements of Group 2: Be, Ca and Ba. Which one are you going to choose as the best reducing agent, and explain why?Question 2: How does each of the following properties of the elements in Group 2 changes down the group and why?i) Atomic radiusii) Ionisation energyiii) Electropositivity7.2. Reactivity of group 2 elementsActivities 7.2 (a)Activity 1:•Pour 200cm3 of water in two different beakers•To the first beaker, add a small piece of magnesium ribbon. To the second beaker, add a very small piece of sodium.•Record your observation.•Put a piece of blue and red litmus paper in both beakers•Record your observations.Activity 2:•Pour 200cm3 of water in pyrex beaker or borosilicate beaker•Heat until water boils•Using crucible tongs, hold a large piece of magnesium in the steam•Record your observationsActivities 7.2 (b)Reaction of magnesium and hydrochloric acidExperimental procedures(1) Reaction of magnesium and hydrochloric acid•Put 100 cm3 of 0.2M dilute HCl(aq) in a beaker. Add a small piece of magnesium ribbon.•Record your observationsDo not test the gas given off, it can cause the explosion.Group 2 metals react by losing 2 electrons; they areless reactive than the corresponding alkaline metals. They mainly form ionic compounds.
Checking up 7.21. With equations of chemical reactions, show how the following group 2 metals (Be, Ca and Mg) react with:i) HCl(aq)ii) N2iii) H2OGive the reason why beryllium shows different properties from the rest7.3. Properties of group 2 compounds7.3.1. Ionic and covalent character of oxides and halidesActivity 7.3.1 (a)•Pour 50 ml of paraffin in a beaker•Put 1g of calcium chloride and try to make a solution.•Pour 50 ml of water in another beaker•Put 1g of calcium chloride and try to make aqueous solution•Write down your observations and comments.Activity 7.3.1 (b)•Place a beaker on a table•Cut 15cm of magnesium ribbon•Using crucible tongs, hold and burn the magnesium ribbon over the beaker.•What do you observe?•Add some water to the ash in the beaker.•Shake the mixture and add 2 drops of phenolphthalein or touch the mixture with a red litmus paper•Record all your observations.a. oxidesGroup2 (like group 1), react with oxygen to form normal oxides (containing O2-ion) and peroxide (containing O22-ion). The oxides are ionic and dissolve in water to give solutions of hydroxide ions except BeO. Group 2 oxides are basic oxides; they react with water to produce metal hydroxides:
Checking up 7.3.11. Write three factors that cause chlorides of group 2 to dissolve with or without hydrolysis.2. What’s a basic oxide?3. When limestone is heated at high temperature, (i) it form calcium oxide or quicklime; (ii) when water is added to quicklime, slaked lime (ishwagara) is formed. Write the chemical equations involved in(i) and (ii).7.3.2. Trends in thermal stability of carbonates, hydroxides and nitratesActivity 7.3.2Procedure•Put 2g of Ca(NO3)2 in a pyrex test tube•Heat strongly•Use a wet blue litmus paper to test the gas given off.•Note your observations•Replace Ca(NO3)2 with CaCO3,CaSO4 and Na2CO3Question:Write balanced equations for reactions in the above activities.Caution: avoid the use of barium nitrate and strontium nitrate which can explode.Thermal stability refers to the resistance to decomposition of the compound on heating. Increased thermal stability means a higher temperature is needed to decompose the compound.Group 2 metal carbonates, nitrates and hydroxides decompose to heat to give the corresponding metal oxide and release CO2, NO2 and O2, and H2O respectively.
The thermal stability increases down the group as theionic character of the compounds increases down the group. Compounds of metals higher in the group (smaller ionic radius) decompose more easily than compounds of metals lower in the group, due to increase in covalent character upward in the group that results from high polarizing power of small ions. Compared to group 1 metals, compounds of group 2 metals are less stable to heat.Note: The hydrogen carbonates, M(HCO3)2, only exist in solution and are very unstable. When their aqueous solutions are heated they decompose to carbonate, water and carbon dioxide:
This is the reaction that takes place when hard water (presence of Mg(HCO3)2(aq) and Ca(HCO3)2(aq)) is heated.Checking up 7.3.2Question:1. You have two carbonates A and B. Heated at high temperature, B decomposes and a gas evolves; on the contrary, A seems not to change. Which one corre-sponds to sodium carbonate and which one corresponds to calcium carbonate? Justify your answer.2. Write the chemical equation to represent the thermal decomposition of lime-stone (CaCO3(s)) to produce carbon dioxide (quicklime)7.3.3. Trends in solubility of hydroxides, carbonates, hydrogencarbonates and sulphatesActivity 7.3.3You are provided with 8 test tubes .•Put a half spatula end of each alkaline earth salt into 3ml of water,in labeled test tubes.•No1= Magnesium hydroxide; No2= Magnesium sulphate; No3= Calcium hydroxide, No4= Calcium sulphate; No5= Strontium hydroxide; No6 = Strontium sulphate; No7= Barium hydroxide; No8= Barium sulphate•Shake the test tubes and try to make aqueous solution.•Write down your observations and related comments.The solubility of salts depends on two main opposite factors:The energy of dissociation of the crystal: The energy needed to dissociate the solid crystal into its ions. This process requires energy; the process is endothermic.The energy of hydration of the ions produced: the amount of energy released when ions undergo hydration or are surrounded by water molecules; this process is exothermic.When the combination of the two processes above is in favor of the hydration of ions, the salt is soluble; otherwise the salt is not soluble.The solubility will increase when the hydration process predominates more and more the dissociation process and vice-versa.a. SulphatesTheir solubility decreases down the group BeSO4 and MgSO4 are solubleCaSO4 is slightly solubleSrSO4 and BaSO4 are insolubleb. HydroxidesTheir solubility increases down the group.Be(OH)2 and Mg(OH)2 are insolubleCa(OH)2 and Sr(OH)2 are slightly soluble, and Ba(OH)2 is fairly solublec. CarbonatesThey are sparingly soluble in water.The carbonates of group2 elements tend to be less soluble as you go down the group.d. Hydrogen carbonatesThe metals of group 2, magnesium, calcium, strontium, and barium formhydrogen carbonates compounds that contain the hydrogen carbonate anion, HCO3-, also known as the bicarbonate anion. Hydrogen carbonates of the alkaline earth metals exist only in solution; they are soluble in water.Due to the high charge of their cations that attract water molecules, group 2 salts are more hydrated than group 1 salts. That is why they are often used as drying agents.Checking up 7.3.31. An aqueous solution may contain barium ion, Ba2+, how are you going to organize to test if that ion is or not present in your sample? Illustrate your explanation by chemical equations.2. You are given two white solid samples A and B. The only information you have is that one of them is Be(OH)2 and the other is Ba(OH)2. What are you going to do to try to identify those samples?7.4. Anomalous properties of beryllium compoundsActivity 7.41. Give 1 or 2 examples showing an abnormal behavior of beryllium compared to the other members of the group?2. What do you think may cause that anomaly? Beryllium shows the uniqueness in its group. Beryllium has higher tendency of forming complexes compared to other group 2 elements.•Beryllium ion, Be2+, is very small compared to other group 2 metal ions.•Beryllium has a high electronegativity compared with the rest of Group 2, hence it forms compounds with more covalent characters.•For an ionic bond to form, beryllium must give up its electrons, but it is too electronegative to do so.The uniqueness of Beryllium makes its compounds to show anomalous properties.For example:1. Beryllium hydroxide, Be(OH)2is amphoteric whereas the other group 2 hydroxides are basic. An amphoteric substance behaves as an acid when it reacts with a base, and as a base when it reacts with an acid:
2. Beryllium chloride, BeCl2, is soluble in non-polar organic solvents, whereas the other group 2 chlorides are ionic and dissolve in water (a polar solvent). This fact illustrates the covalent character of beryllium compounds.3. Beryllium complexes. It forms complexes ions such as [Be(OH)4]2- and [BeCl4]2-.Checking up 7.4Complete the table concerning the compounds of beryllium.
7.5. Identification test for Ba2+ ions in aqueous solutionsActivity 7.5Task 1:•Pour 100ml of distilled water into the beaker 1•Add 1g of Beryllium nitrate•Stir to make the aqueous solution•Repeat the procedure, by replacing Beryllium nitrate by nitrates of other group 2 elements(Mg;Ca;Sr;Ba).•Use a nichrome wire or magnesia rod to put each compound in a Bunsen flame.•Write down your observations and related comments.Task 2•Put 3ml of barium nitrate (Prepared also for task 1) in a test tube•Add 10 drops of dilute HNO3(aq)•Using a dropper, add 10 drops of sodium sulphate solution.•Write down your observation and the ionic equation.a. Formation of precipitateThe insolubility of barium sulphate is the basis of a test for barium ions in solution. If a few drops of a solution containing sulphate ions, SO42- (CaSO4, Na2SO4, etc.) are added to a solution containing barium ions, a white precipitate of barium sulphateinsoluble in acidic solution will form.
b. Flame testLike the compounds of group 1 metals, calcium, strontium and barium produce coloured flames when their compounds are put in a Bunsen flame.
QuicklimeCalcination is carried out in a high-temperature kiln (about 1000oC) with continuous removal of CO2 (g)to promote the forward reaction. The product formed, CaO(s), is called lime or quicklime.QuicklimeCalcination is carried out in a high-temperature kiln (about 1000oC) with continuous removal of CO2 (g)to promote the forward reaction. The product formed, CaO(s), is called lime or quicklime.Second step: Hydration of quicklime giving Slaked limeIn the process of hydration, the reaction of quicklime with water produces Ca(OH)2(s) known as slaked lime.
Slaked limeUses of limestone and slaked lime•Some limestone isused as building stones.•However, most limestone is used to manufacture other building materials.•Limestone is the raw material for the manufacture of cement (see Production of Cement in Rwanda at Bugarama Cement Factory).•Slaked lime, Ca(OH)2, is the cheapest commercial base and is used in all applications where high water solubility is not essential..•Quicklime is used in the manufacture of sodium carbonate from caustic soda.•Calcium carbonate and slaked lime are used to decrease soil acidity and acidity in lakes which are acid polluted.5. Manufacture of cementCement is an important building material. It was first introduced in England in 1824 by Joseph Aspdin. Some types of cement are called Portland cement because it resembles with the natural limestone quarried in the Isle of Portland, England.Cement is a product obtained by combining a material rich in lime, CaO, such as limestone, with other material such as clay which contains silica, SiO2 along with the oxides of aluminium, iron and magnesium.The raw materials for the manufacture of cement are limestone and clay. When clay and limestone are strongly heated together they fuse and react to form ‘cement clinker’. This clinker is mixed with 2-3% by weight of gypsum (CaSO4·2H2O) to form cement.Environmental risks: during the process of producing cement and quicklime, gases, mainly carbon dioxide, and solid particles are sent in air and constitute an environmental risk if their concentration increases in the atmosphere. All measures must be taken to avoid that those gases and solid particles become a real environmental problem.6. Strontium•It is used in fireworks as it produces a scarlet flame color.•Strontium chloride (SrCl2) is used to make toothpaste for sensitive teeth. 7. Group 2 and Group 1 elements are used in fireworks to produce spectacular colours of the fireworksChecking up 7.61. Name any two compounds of calcium and in each case give two uses of each.2. Describe how slaked lime can be obtained from limestone.3. The elements Be, Mg and Ba belong to Group 2 in the periodic table. How do they react with water and NaOH?7.7. End unit assessment1. Which of the following is NOT an alkaline earth metal?(a) Ba (b) K (c) Mg (d) Be (e) Ra2. True or False: group2 metals do not react vigorously with water.3. Which alkaline earth metal is a main component in our bones?4. Which group 2 metal has the highest ionization energy? And why?5. Which element has the lowest melting point? (a) Ba (b) Ra (c) Ca (d) Mg6. Why is the term alkaline earth used to describe the Group 2 elements?7. Identify what is observed when the flame test is carried out for group 2 elements?.8. Explain why all alkaline earth metals form ions by losing 2 electrons?9. Which Group 2 element is radioactive?10. Which substance is acting as the reducing agent and which one as oxidizing agent in the reaction below?