2013 Group II Lecture Notes (Student).Unlocked

Dunman High School Year 5 H2 Chemistry 2013

Lecture Outline 1. Introduction 2. Atomic and Physical Properties

2.1 Atomic and Ionic Radii 2.2 Ionisation Energy 2.3 Electronegativity 2.4 Melting and Boiling Points 2.5 Hardness and Density 3. Chemical Properties of Group II Metals 3.1 Good Reducing Agents 3.2 Reactivity 3.3 Reaction of Metal with Oxygen 3.4 Reaction of Metal with Water 4. Properties of Group II Compounds 4.1 Reaction of Group II Oxides with Water 4.2 Thermal Decomposition of Group II Nitrates and Carbonates 4.3 Trend in Thermal Stability of Nitrates and Carbonates 5. Diagonal Relationship 6. Uses of Group II Compounds

Learning Outcomes Candidates should be able to: (a) describe the reactions of the elements with oxygen and water (b) describe the behaviour of the oxides with water (c) interpret and explain qualitatively the trend in the thermal stability of the nitrates in terms of the

charge density of the cation and the polarisability of the large anion (d) interpret, and make predictions from, the trends in physical and chemical properties of the

elements and their compounds References Chemistry for Advanced level, Peter Cann & Peter Hughes, 1st Edition, John McMurray (Publishers) Ltd Chemistry, 3rd Edition, McGraw Hill Useful Websites http://www.chemguide.co.uk/inorganic/group2menu.html http://sockkhim.edu.glogster.com/properties-of-grp-ii-compounds/ http://sockkhim.edu.glogster.com/group-ii-physical-properties/ http://sockkhim.edu.glogster.com/chemistry-properties-of-group-ii-metals/

Students Copy

Group II Elements 2013 DHS Year 5 H2 Chemistry

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1. INTRODUCTION

The Group II elements are also known as the alkaline earth metals for two reasons:

a) Their oxides (white solids) form alkaline solutions when dissolved in water.

CaO(s) + H2O(l) Ca(OH)2(s) lime slaked lime

Ca(OH)2(s) + H2O(l) Ca(OH)2(aq) slaked lime limewater (alkali)

b) In ancient times, alchemists attempted to obtain pure metals from their ores by heating.They often encountered the Group II oxides in their furnaces. As these Group II oxidesmelt at very high temperatures, they remained as solids in these alchemists fires.Hence, these solids were referred to as earth which to the alchemists were materialswhich did not melt just like the sand and soil due to their extremely high melting points.

Electronic ConfigurationElement Electronic Configuration Short

Form 1st IE 2nd IE 3rd IE

Be 1s2 2s2 [He] 2s2 900 1760 14800

Mg 1s2 2s2 2p6 3s2 [Ne] 3s2 736 1450 7740

Ca 1s2 2s2 2p6 3s2 3p6 4s2 [Ar] 4s2 590 1150 4940

Sr 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p65s2 [Kr] 5s2 548 1060 4120

Ba 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d105s2 5p6 6s2 [Xe] 6s2 502 966 3390

Table 1: Electronic configuration and ionisation energies of Group II elements

Their outermost shell electronic configuration is ns2. These elements form ions with +2 charge by losing their 2 outermost electrons.

M M2+ + 2e

Eg. Be Be2+ + 2e Mg Mg2+ + 2e

They exist as silvery grey metals which tarnish readily in air.


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Question: Why should the Group II metals have a fixed oxidation state of +2?

The +2 oxidation state:

The outermost 2 electrons in the ns shell are easily removed as they are shielded fromthe nucleus by the inner core electrons. Hence +2 oxidation state is easily and mainlyformed.


It is known that MX compounds will disproportionate readily to give MX2 and M.2MX MX2 + M

As a result of greater lattice energy (|I.E|. rr

qq ), MX2 compounds are more stable

than MX compounds.


The +3 oxidation state is not possible as the 3rd IE is significantly greater than either 1stor 2nd IE. As the 3rd electron would be removed from an inner quantum shell, thisprocess requires too much energy which cannot be compensated by the higher latticeenergy of MX3 compounds.

2. ATOMIC AND PHYSICAL PROPERTIES

Element Boiling Point/ OC

Melting Point/ OC

Atomic Radius/ nm

Ionic Radius/ nm

Density/ g cm3

Be 2477 1280 0.112 0.031 1.84Mg 1100 650 0.160 0.065 1.73Ca 1487 850 0.197 0.099 1.55Sr 1380 768 0.215 0.113 2.63Ba 1640 714 0.217 0.135 3.51

Table 2: Physical properties of group II elements

2.1 Atomic and Ionic radii

Atomic radius/nm 0.11 0.16 0.20 0.21 0.22 Ionic radius/nm 0.031 0.065 0.099 0.11 0.14

Trend: Atomic and ionic radii increase down the group.

B Mg Ca Sr Ba


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Reason: Down the group,

Electrons are added up to a higher level shell. Significant increase in screening effect outweighs increase in nuclear charge due

to a greater number of inner core electrons.

Hence, effective nuclear charge decreases down the group.

This results in weaker electrostatic attraction between nucleus and outermost electrons.

Thus, valence electrons are progressively further away from the nucleus, leading to larger atomic and ionic radii down the group.

2.2 Ionisation Energies

(Also see Table 1 on page 2 for ionisation energy values).

Trend: Ionisation energies decrease down the group.

Reason: Down the group, Electrons are added up to a higher level shell.

Significant increase in screening effect outweighs increase in nuclear charge due to a greater number of inner core electrons.

Hence, effective nuclear charge decreases down the group. This results in weaker electrostatic attraction between nucleus and outermost

electrons.

Thus, valence electrons are progressively further away from the nucleus and lesser amount of energy needed to remove valence electrons.

Same as explanation for decrease in atomic radius down the group.


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2.3 Electronegativity

It is the ability of an atom in a molecule to attract electrons to itself.

Be Mg Ca Sr Ba Electronegativity: 1.5 1.2 1.0 1.0 0.9

Trend: Electronegativity decreases down the group.


Electrons are added up to a higher level shell. Significant increase in screening effect outweighs increase in nuclear charge. Effective nuclear charge decreases down the group. Atomic radius increases down the group. Electronattracting power of atom progressively decreases.

Quickcheck 1: State and explain why the atomic radius of Ba is greater than Mg.Reason: Ba is below Mg in group II , down the group:

Electrons are added up to a higher level shell, hence significant increase in screening

effect outweighs increase in nuclear charge.

Effective nuclear charge: Ba < Mg

Electrostatic attraction between nucleus and outer electrons: Ba < Mg

In Ba, the valence electrons are further away from nucleus than Mg. Atomic radius: Ba > Mg

Same as explanation for decrease in atomic radius down the group.


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2.4 Melting and Boiling Points

(Also see Table 2 on page 3 for melting and boiling points values).

a) Trend:

Reason:

Group II metals have giant metallic structures. Large amount of energy required to overcome strong electrostatic attraction

between cations and the sea of delocalised electrons.

b) Trend:


Ionic radius increases. Same number of valence electrons (i.e. 2) are contributed per atom for metallic

bonding.

Weaker electrostatic attraction between cations and sea of delocalised electrons. Hence, metallic bond strength progressively decreases.

Melting and boiling points decrease down the group

High melting and boiling points


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c) Trend:

Transition Elements: Fe Co Ni Cu Melting Point/oC: 1550 1492 1453 1083 Atomic Radius/nm: 0.116 0.116 0.115 0.117

Group II Elements: Be Mg Ca Sr Melting Point /oC: 1280 650 850 768

Atomic Radius/nm: 0.112 0.160 0.197 0.215

Reason:

Group II elements: Contribute only 2 valence electrons per atom for metallic bonding Bigger ionic radii

Transition Elements: Contribute more than 2 valence electrons (4s and 3d electrons)

per atom for metallic bonding

Smaller ionic radii Relatively weaker electrostatic attraction between cations and sea of delocalised

electrons in Group II elements.

Hence, weaker metallic bond strength in Group II elements. (Also to be discussed in The Introduction to the Chemistry of Transition Elements)

2.5 Hardness and Density

Hardness of metals decreases down the group due to weaker metallic bonds. Density of Group II metals generally increasing down the group. The reason is that

the mass increases faster than the volume on going down the group.

(See Table 2 on page 3 for density values).

Melting point of Group II elements are lower than those of transition elements


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3. CHEMICAL PROPERTIES OF GROUP II METALS

3.1 Reactivity

Trend:


Weaker electrostatic attraction exists between nucleus and valence electrons. Valence electrons are further away from the nucleus and easier to be removed

down the group.

Reactivity (in undergoing oxidation) progressively increases.

3.2 Good Reducing Agents

a)

The E [standard redox potential] value is a measure of the tendency of the species to undergo reduction.

Mn+ + ne M

Element Standard Electrode Potential, E/V Be2+ (aq) + 2e Be(s) 1.85Mg2+ (aq) + 2e Mg(s) 2.37Ca2+ (aq) + 2e Ca(s) 2.87Sr2+ (aq) + 2e Sr(s) 2.89Ba2+ (aq) + 2e Ba(s) 2.91

Table 3: Standard electrode potential values

The more negative the E value, the greater the tendency for oxidation (backward reaction) to occur.

The E values of Group II elements are very negative indicating that they are easily oxidised hence good reducing agents.

b) Trend:


Weaker electrostatic attraction exists between nucleus and valence electrons. Valence electrons are further away from the nucleus and easier to be removed

down the group.

Reducing power (tendency to undergo oxidation) progressively increases.

E values become more negative (See table 3 above).

Group II elements are good reducing agents

Reducing power of Group II elements increases down the group

Reactivity of Group II elements increases down the group


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3.3 Reaction of Metal with Oxygen

General equation: 2M (s) + O2 (g) 2MO (s)

Reactivity of metals with oxygen increases on going down the group, a consequenceof the increase in reducing strength of the metals.

All the oxides are white solids, basic and ionic in nature and have extremely highmelting points (except for BeO). BeO is amphoteric.

Examples: 2Be(s) + O2(g) 2BeO(s) 2Mg(s) + O2(g) 2MgO(s)

2Ca(s) + O2(g) 2CaO(s)

2Sr(s) + O2(g) 2SrO(s)

2Ba(s) + O2(g) 2BaO(s)

Note: Ba can also form barium peroxide, BaO2. Ba (s) + O2 (g) BaO2 (s) (In peroxide: oxidation state of O is 1)

All Group II elements burn in oxygen to form metal oxides


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3.4 Reaction of Metal with Water

General Equation: M (s) + 2H2O (l) M(OH)2 + H2 (g)

Reactivity of Group II metal increases down the group:

Reactivity Reaction with H2O

Be No reaction with either cold water or steam. Nil

Mg Reacts very slowly with cold water, but rapidly with steam to form the oxide.

Mg (s) + H2O (g) MgO (s) + H2 (g) (steam)

Ca, Sr, Ba

Reaction with cold water gets more vigorous

down the group to give the hydroxides and

hydrogen gas.

Ca (s) + 2H2O (l) Ca(OH)2(aq) + H2 (g) Sr (s) + 2H2O (l) Sr(OH)2 (aq) + H2 (g) Ba (s) + 2H2O (l) Ba(OH)2 (aq) + H2 (g)

The Group II hydroxides are not very soluble, but they get more soluble down the group. Calcium hydroxide (also known as slaked lime) is present mainly as a white

precipitate (although some does dissolve to form limewater).

All the hydroxides are basic except for Be(OH)2 which is amphoteric.

Quickcheck 2: Which of the following statements concerning Group II elements;strontium, calcium and barium are correct?

1 Their oxides are amphoteric. 2 Aqueous solutions of their hydroxides have a pH greater than 7. 3 The elements react with cold water liberating hydrogen.

Important!

Group II elements (except Be and Mg) reduces water to produce hydroxides and H2

2, 3 are correct.


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4. PROPERTIES OF GROUP II COMPOUNDSNature of oxides:

The oxides of Group II elements are all basic except for BeO which is amphoteric.

Ionic oxides are basic while covalent oxides are acidic. The amphoteric oxides possess both the ionic and covalent character.

BeO is amphoteric because the small Be2+ ion has high charge density and it polarises the O2 anion and result in some degree of covalent character in the bonding of BeO. [Compare this with what you know about Al2O3. Are they similar?]

4.1 Reaction of Group II Oxides with Water

General Equation: MO (s) + H2O (l) M(OH)2 (aq)

Reaction of group II oxides with water increases with vigor down the group:

Quickcheck 3: Group II metal, X, reacts only very slowly with cold water butvigorously with steam. Group II metal, Y, readily reacts with cold water. Suggest the identity of X and Y, and give a reason for your choice.

Oxides Reactivity Reaction with H2O

BeO Does not react with water

High lattice energy of BeO Nil

MgO

Reacts less vigorously to form basic solution

( pH 9 ) due to more stable lattice structure High lattice energy of MgO

MgO (s) + H2O (l) Mg(OH)2(s)

CaO, SrO, BaO

React vigorously with water to produce an alkaline solution ( pH 1013 ).

CaO (s) + H2O (l) Ca(OH)2(aq) SrO (s) + H2O (l) Sr(OH)2 (aq) BaO (s) + H2O (l) Ba(OH)2 (aq)

Important!

All Group II oxides (except BeO) react with water to form hydroxides.

Yes.

Answer:X = Mg; Y = Ca, Sr or Ba.Reactivity increases down the group.


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4.2 Thermal Decomposition of Group II Nitrates and Carbonates

The nitrates and carbonates of Group II elements are thermally unstable; they decompose on heating to form stable oxides. Nitrates decompose on heating to give oxides, nitrogen dioxide and oxygen:

General Equation: 2M(NO3)2 (s) 2MO (s) + 4NO2 (g) + O2 (g) brown gas

Carbonates decompose on heating to give oxides and carbon dioxide:

General Equation: MCO3 (s) MO (s) + CO2 (g)

M2+

C

O

O OM

2+O

2 + C OO

Thermal decomposition of these compounds occurs as a result of polarisation effect exerted by the cation (of high charge density) on the large anion:

Polarising Power refers to the ability of cation M2+ to distort the electron cloud ofthe anion

In general, the higher the charge density of M2+, the greater the polarising power ofthe cation.

Note : Group II salts are less stable than Group I salts because of the greater polarising power of the M2+ cation, which distorts the anion more.

Polarising Power of M2+ Charge Density

rq

High cationic charge (q+), small cationic radius (r+) high charge

density

Possible mechanism for decomposition of group II carbonates:


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N OO

O-

NO bond in the NO3 anion is polarised and weakened by the cation such that the NO bond break and MO, NO2 and O2 are formed.

2MO + 4NO2 + O2

Cleavage of the NO bond lead to decomposition

4.3 Trend in Thermal Stability of Nitrates and Carbonates

Compound DECOMPOSITION TEMPERATURE/ 0C

Nitrates Carbonates Mg 129 540 Ca 560 900 Sr 570 1290 Ba 592 1350

Group II nitrates and carbonates have to be heated more and more strongly downthe group before they will decompose.


Ionic radius of cation increases

Charge density of cation decreases since charge remains constant

Polarising power of cation decreases

Polarising effect on anion NO3 or CO32 decreases

Thermal stability of compounds increases hence decomposition temperature

progressively increases

M2+

Polarisation of the nitrate ion For your understanding

Note: Thermal Stability increase

Thermal decomposition temperature increase Ease of thermal decomposition decrease

Thermal stability of nitrates and carbonates increases down the group.

Very Important!

Recall:

Charge Density

rq


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Quickcheck 4: M is a Group II metal which can undergo reaction via two routes:Route 1

Route 2

Which set below contains 3 different compounds?

A. P Q U C. Q S U B. P R T D. Q S T

5. DIAGONAL RELATIONSHIP

Mg resembles Li in some of their reactions. Reason:

o Both Mg2+ and Li+ have similar charge density and polarising power.

Be resembles Al in some of their reactions. Reason:

o Both Be2+ and Al3+ have similar charge density and polarising power.

Beryllium shows quite different chemical behaviour when compared to the rest of Group II metals. In fact, it is closer to aluminum in its chemical properties.

o Both Be and Al dissolve in alkali to liberate hydrogen gas.o Both BeCl2 and AlCl3 are covalent compounds.o BeCO3 is unstable at room temperature and Al2(CO3)3 does not exist.o Both Be(OH)2 and Al(OH)3 are amphoteric.o Both BeO and Al2O3 are amphoteric.

GROUP I GROUP II GROUP III

Li Be B

Na Mg Al

K Ca Ga

(A)


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6. SOME USES OF GROUP II COMPOUNDS (NOT IN SYLLABUS)

Magnesium oxide (MgO) is used as a refractory lining in high temperature furnaces because of its high melting point and low reactivity.

Calcium carbonate (CaCO3), also known as limestone, is used in building

materials.

Calcium oxide (CaO), also known as quicklime, is used in agriculture to overcome the problem of acidity in soil:

o ability of crops to take in nutrients depends on pH of soil o optimum pH for most crops lies in the range of 67 o liming (i.e. process of adding basic calcium oxide to acidic soil) is employed to

remove acidity of soil for maximising crop yields.

Barium sulfate (BaSO4) is used in barium meal test in Xray diagnostic work.

Dunman High School Year 5 H2 Chemistry 2013

Tutorial: Group II Elements * Tutorial Discussion Questions

I Properties of Group II Metals

1. The element strontium is placed below calcium and above barium in Group II. (a) Rubidium (Rb) is next to strontium in the same period. Account for the difference in the

ionic radius Rb+ and Sr2+.

Ion Ionic radius Rb+ 0.148 nm Sr2+ 0.113 nm

(b) How should the ionisation energy of strontium compare with that of:

(i) rubidium (ii) calcium

Explain your answer. 2. Explain each of the following observations in terms of structure and bonding. (a) The alkaline earth metals (beryllium, magnesium, calcium, strontium and barium) all

have a fixed oxidation number in their compounds.

(b) The group II elements are strong reducing agents (E values are negative and the magnitude increases on going down the group). Give an explanation for this property and the trend in this property on going down the group.

3(a) Describe the reactions of Group II metals, magnesium to barium with

(i) Oxygen (ii) Water

Write equations where appropriate. (b) (i) Suggest reasons why magnesium gives the nitride, Mg3N2, in addition to its

oxide when burned in air.

(ii) A 1.00 g sample of the powder obtained from burning magnesium in air was boiled with water. The ammonia that was evolved neutralised 12.0 cm3 of 0.5 moldm3 hydrochloric acid.

Construct balanced equations for the production of magnesium nitride and its reaction with water. Calculate the percentage of magnesium nitride in the 1.00 g sample.

[Ans: 30.3%] II Properties of Group II Compounds *4 Predict the following properties of strontium: (a) Formula of its oxide and effect, if any on water. (b) Thermal stability of strontium nitrate relative to barium nitrate. (c) The effect of cold water on strontium metal.


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5 Explain each of the following observations in terms of structure and bonding. (a) Beryllium chloride (boiling point 547C) is much more volatile than magnesium chloride

(boiling point 1418C).

(b) Beryllium oxide is amphoteric, but the oxides of all the other alkaline earth metals are basic.

*6 Heating barium oxide with oxygen under pressure causes barium peroxide to be formed

in the following equilibrium reaction:

2BaO + O2 2BaO2

When 1.00 g of the mixture of oxides (BaO and Ba2O) was treated with dilute sulfuric acid, 1.49 g of a white solid was precipitated and a solution of hydrogen peroxide was produced. On the addition of an excess of acidified potassium iodide to the solution, 0.00118 mol of iodine, I2, was produced. Suggest an identity for the white solid and write an equation for the reaction of each of the oxides with dilute sulfuric acid. Use the above data to calculate the mass of each oxide present in the mixture. [Ans: BaO2 = 0.199 g ,BaO = 0.801 g]

7(a) State and explain the relative thermal stability of calcium nitrate and strontium nitrate. (b) (i) When a 1.01 g sample of potassium nitrate is heated above its melting point, a

colourless gas is evolved and the mass of the sample decreases by 0.16 g.

Suggests an identity for the gas and for the resulting solid, and hence construct an equation for the decomposition.

(ii) Unlike all other Group I nitrates, lithium nitrate decomposes on heating in the

same way as Group II nitrates. Suggest a reason for this difference in behaviour and give an equation for its decomposition.

*(c) (i) When water is added to the solid remaining after barium nitrate has been

heated, an alkaline solution is produced. The addition of sulfuric acid to this solution produces a white precipitate. Identify the products of these two reactions, and write equations for them.

(ii) When the procedure described in (a) is repeated using the solid remaining after

magnesium nitrate has been heated, several differences are observed. Describe clearly what these differences are, and explain why they occur.

8 The following table lists the melting points of the oxides of the Group II elements.

oxide melting point/ C MgO 2852 CaO 2614 SrO 2430 BaO 1918

(a) Suggest an explanation of this trend.

(b) Suggest a use for one of these oxides, based on its melting point.


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(c) The mineral dolomite is a double carbonate of magnesium and calcium, with the formulae CaMg(CO3)2. When 1.000 g of an impure sample of dolomite was completely dissolved in an excess of hydrochloric acid, 0.450 g of carbon dioxide was given off. Write a balanced equation for the reaction and calculate the purity of the dolomite.

[Ans: 94.3%]

*9 (a) Explain the following observation as fully as you can. Include any relevant equations.

Magnesium nitrate produces a brown gas on gentle heating whereas barium nitrate gives a brown gas only on prolonged heating.

(b) Why is barium peroxide more thermally unstable than barium monoxide? Past Year Prelim Question [NJC/Prelim 06/P3/5 either] *10 This question concerns Group II elements and their compounds (a) Group II elements are known to be reducing agents. Give an example of a chemical

reaction that illustrates the trend in the reducing ability of the Group II elements. (b) Magnesium reacts with ammonia to form a yellow binary salt, X, and hydrogen. In

another experiment, 0.198g of compound X upon complete reaction with water gives a white solid Y. Upon filtering to remove the white solid Y, the resultant solution which contains ammonia requires 20.0 cm3 of 0.1 mol dm3 sulfuric acid for reaction. Construct balanced equations for the two reactions.

(c) Beryllium carbonate must be kept in an atmosphere of carbon dioxide, whereas no

carbon dioxide is detected if barium carbonate is heated using a Bunsen burner. Explain these observations fully.

(d) The solubility products of some Group II compounds are shown in the following table:

Solubility products of Group II compounds Ba(OH)2 BaCO3 2.60 x 109 Ca(OH)2 7.88 x 106 CaCO3 4.95 x 109 Mg(OH)2 5.66 x 1012 MgCO3 1.15 x 105

(i) With reference to data given above, suggest a reason why when small quantities

of carbon dioxide are to be detected, barium hydroxide solution is occasionally used instead of limewater.

(ii) State one disadvantage of using barium hydroxide.

[3] [Total: 10]


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Multiple Choice Questions: 1. Lithium resembles magnesium in its chemical properties. Which property of lithium

compounds is unlikely to be correct? [N98/P3/Q14]

A. Lithium carbonate decomposes to give carbon dioxide on being heated. B. Lithium nitrate gives oxygen as the only gas on being heated. C. Lithium oxide in water produces a solution with pH greater than 7. D. Lithium sulfate is soluble in water.

2. On strong heating, CaSO4 decomposes into CaO and SO3. The compound CaCO3

decomposes at a lower temperature than CaSO4. Which factor best explains the greater thermal stability of CaSO4? [J99/P3/Q14]

A. CaCO3 has higher lattice energy than CaSO4. B. CO2 is a smaller molecule than SO3. C. CO32 ions are more easily polarised than SO42. D. The charge density of CO32 is greater than that of SO42.

3. The carbonates of Group II decompose according to the following equation.

MCO3 (s) MO (s) + CO2 (g) For this reaction, H increases on descending the group. [N00/P3/Q14]

Which property best explains this trend?

A. Ionic radius of the metal ion B. Ionisation energy of the metal C. Proton number of the metal D. Thermal stability of the oxide

4. A student made up a 0.10 mol dm3 solution of Ba(OH)2.8H2O which she found in the laboratory cupboard and left the solution in an open beaker. A week later, she returned to the laboratory, used the solution for titration with 0.10 mol dm3 HCl and was surprised to discover her titres were lower than expected.

What explains why the values were low? [N08/P1/Q15]

A. Some of the barium hydroxide had reacted with carbon dioxide in the air to form solid barium carbonate.

B. Some of the water had evaporated from the barium hydroxide solution. C. The concentration of HCl was less than the stated 0.10 mol dm3. D. The crystals had less water of crystallisation than stated.

Carbonates Mg Ca Sr Ba H / kJ mol1 +101 +178 +235 +269


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For Qns 59, choose from any of the following combination of statements.

A B C D E 1, 2 & 3 are

correct 1 and 2 only are

correct 2 and 3 only are

correct 1 only is correct

3 only is correct

5. Which of the following statements concerning the Group II elements, magnesium, calcium

and barium are correct? [N89/P3/Q35]

1. Their reactivity increases with increasing relative atomic mass. 2. The only oxidation number exhibited in their stable compounds is +2. 3. On strong heating their nitrates give off oxygen only.

6. Anhydrous barium nitrate and anhydrous magnesium nitrate both decompose on heating,

evolving nitrogen dioxide and oxygen and forming an oxide. Which of the following statements concerning these decompositions are correct? [J93/P4/Q36]

1. Nitrogen dioxide is evolved at a lower temperature from magnesium nitrate than from

barium nitrate. 2. For both nitrates the volume of nitrogen dioxide evolved is four times greater than the

volume of oxygen. 3. The numerical value of the lattice energy of magnesium nitrate is greater than that of

barium nitrate.

7. Soluble barium compounds are highly toxic. In hospitals, barium sulfate in suspension is safe for use in taking Xray photographs of the alimentary canal. It is mixed with food and eaten by the patient prior to the photographs being taken.

Why is sulfate used rather than other compounds of barium?

[N93/P4/Q35 modified] 1. Barium sulfate is not poisonous because it is insoluble in water. 2. Barium sulfate reacts with organic materials in the body. 3. Barium sulfate forms sulfuric acid with the acid in the stomach.

8. Which of the following statements about beryllium are true? [J85/P3/Q36]

1. Beryllium compounds tend to be covalent rather than ionic. 2. Beryllium shows a fixed oxidation number of +2 in its compounds. 3. Beryllium reacts rapidly with cold water.

9. Consider the energy changes in dissolving an ionic compound. Which of the following factors helps to explain why magnesium sulfate is soluble in water but not barium sulfate? [N95/P4/Q35 modified] 1. Barium sulfate has numerically larger lattice energy than magnesium sulfate. 2. The enthalpy change of hydration of magnesium ions is more exothermic than that of

barium ions. 3. The charge density of magnesium ions is greater than that of barium ions.

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2013 Group II Lecture Notes (Student).Unlocked