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CH10314 > Ionic Bonding and s-Block Chemistry- Group 2 > Flashcards

Ionic Bonding and s-Block Chemistry- Group 2 Flashcards

1
Q

Which of the group 2 elements are all radioactive

A
  1. Radium

2. Longest lived is 226Ra

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2
Q

What state are group 2 elements in at room temperature and pressure

A
  1. Metallic solids
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3
Q

Compare physical properties to group 1 elements

A
  1. Higher melting points, boiling points and enthalpy changes of atomisation than group 1
  2. Indicates stronger metallic bonding in the metal lattices due to presences of two rather than one delocalised electrons per atom
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4
Q

What is the chemistry of group 2 elements dominated by

A
  1. +2 oxidation state normally in the form of M2+ ion
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5
Q

What are group 1 compounds stabilised by

A
  1. Relatively low values of the first and second ionisation enthalpies of the metals and high values of lattice enthalpies for group 2 salts
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6
Q

Why is berylium exceptional

A
  1. It forms a number of covalent compounds
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7
Q

Why group 2 metal needs to be stored under oil

A
  1. Barium
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8
Q

Describe how reactive group 2 metals are

A
  1. Much less than group 1
  2. Most have a coating of oxide on their surfaces which makes them less reactive than might be expected
  3. Still sufficiently reactive that the free metals do not occur naturally
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9
Q

Hows is magnesium prepared and show equations for each step

A
  1. Obtained from sea water
  2. Aqueous calcium hydroxide solution is added to the sea water causing Mg(OH)2 to precipitate
  3. Mg2+ (aq) + Ca(OH)2 (s) –> Mg(OH)2 (s) + Ca2+ (aq)
  4. The solid magnesium hydroxide is treated with hydrochloric acid to produce MgCl2 and the metal is liberated from molten MgCl2 using electrolysis
  5. Mg2+ + 2e- –> Mg (l) reduction at cathode
  6. 2Cl- –> Cl2 (g) + 2e- oxidation at the anode
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10
Q

How is calcium metal obtained

A
  1. From minerals such as limestone CaCO3
  2. First carbonate is converted into the oxide by heating to 900-1100 degrees
  3. CaCO3 (s) –> CaO (s) + CO2 (g)
  4. Calcium metal is obtained from reduction of CaO (quicklime) with aluminium
  5. 6CaO (s) + 2Al (s) –> 3CaO.Al2O3 (s) + 3Ca (g)
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11
Q

What happens when group 2 metals burn in air

A
  1. React with oxygen to give oxides

2. 2M(s) + O2 (g) –> 2MO (s)

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12
Q

What happens in formation of MgO

A
  1. Magnesium burns with a bright white flame

2. Some of the nitride Mg3N2 produce alongside MgO

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13
Q

What happens to the reaction of group 2 metals burning in the air as you go down the group

A
  1. Becomes more exothermic down the group

2. Barium can ignite in moist air

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14
Q

What else can barium form with excess oxygen

A
  1. The peroxide

2. Ba (s) + O2 –> BaO2 (s)

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15
Q

Describe group 2 oxides

A
  1. Basic
  2. With the exception of BeO, they dissolve in water to give alkaline solutions of the hydroxides
  3. This is a very exothermic reaction for CaO but less so for MgO which only reacts slowly
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16
Q

What happens when Group 2 metals react with water

A
  1. Form a hydroxide and liberates Hydrogen gas in the process
  2. Except beryllium
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17
Q

Show equation for reaction of Ba with H2O

A
  1. Ba (s) + 2H2O (l) –> Ba(OH)2 (aq) + H2 (g)
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18
Q

How does the reaction of group 2 metals with water vary down the group

A
  1. Reaction becomes more vigorous down the group as with group 1
  2. Magnesium requires hot water or steam to react
  3. But calcium will react with cold water
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19
Q

Describe Group 2 hydroxides

A
  1. Generally basic in solution

2. Decompose on heating to give oxides

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20
Q

Show the equation for the decomposition of group 2 hydroxide

A
  1. M(OH)2 (s) –> MO (s) + H2O (g)
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21
Q

Describe properties of Be(OH)2

A
  1. Amphoteric
  2. Reacts with acids as expected
  3. Be(OH)2 (s) + 2H3O+ –> Be2+ (aq) + 4H2O (l)
  4. Also dissolves in excess alkali to form a tetrahedral complex anion
  5. Be(OH)2 (s) + 2OH- –> [Be(OH)4]2- (aq)
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22
Q

What is produced when group 2 metals react with halogens and show equation for Mg + Cl

A
  1. Form the halides

2. Mg (s) + Cl2 (g) –> MgCl2 (s)

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23
Q

Describe group 2 halides

A
  1. Most are ionic

2. But BeX2 compounds have polymeric structures with covalent bonds and bridging halides

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24
Q

Compare BeCl2 vs BeH2

A
  1. Both structures contain bridging atoms but differ with respect to bonding
  2. BeH2 is electron deficient, and the Be-H-Be bridges involve 3-centre 2-electron bonding
  3. BeCl2 is not electron deficient because the bridging chloride donates 3 electrons- 3-centre 4- electron
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25
Q

Why doesn’t MgCl exist

A
  1. It is stable with respect to the elements
  2. Unstable with respect to disproportionation- compound is oxidised and reduced simultaneously
  3. Very favourable to 2MgCl (s) –> MgCl2 (s) + Mg (s)
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26
Q

What compounds do group 2 metals form with carbons

A
  1. Form binary compounds with carbon

2. Called ethynides since they contain C2 2- anion

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27
Q

Show what happens when group 2 ethynides react with water e.g. CaC2 (S)

A
  1. React with water to give ethyne
  2. CaC2 (s) + 2H2O (l) –> Ca(OH)2 (aq) + C2H2 (g)
  3. Ethyne can be burned to provide light - used for caving + mining etc
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28
Q

How do group 2 metals with nitrogen e.g. with Mg

A
  1. React on heating to give nitrides

2. 3Mg (s) + N2 (g) –> Mg3n2 (s)

29
Q

What happens to group 2 nitrides in water e.g. Mg3N2 (s)

A
  1. Decompose in water to form metal hydroxide and ammonia

2. Mg3N2 (s) + 6H2O (l) –> 3Mg(OH)2 (aq) + 2NH3 (g)

30
Q

Describe properties of group 2 carbonates and how they decompose

A
  1. Not soluble in water
  2. Decompose on heating to the oxides and CO2
  3. MCO3 (s) –> MO (s) + CO2 (g)
31
Q

How does the decomposition temperature change down the group

A
  1. Increases down the group

2. Large cation to stabilise the larger anion

32
Q

What is the decomposition temperature related to

A
  1. The temperature at which a reaction becomes spontaneous
  2. DrG = DrH- TDrS
  3. For decomposition DrS is positive and largely independent of M
  4. So any change in DG are caused by changes in DrH
  5. DrH is related to differences between the lattice enthalpies of MCO3 and MO
33
Q

Show equation for decomposition of 2M(NO3)2

A

2M(NO3)2 (s) –> 2MO (s) + 4NO (g) + 3O2 (g)

34
Q

Show equation for decomposition of MSO4 (s)

A

MSO4(s) –> MO(s) + SO3(g)

35
Q

What group 2 compound is used as construction materials

A
  1. Calcium compounds- Limestone (CaCO3)
36
Q

What happens when limestone is heated

A
  1. Heated to 900-1100 it loses CO2 forming CaO (quicklime)

2. CaCO3 (s) –> CaO(s) + CO2 (g)

37
Q

What is a common component of cement and how does it work

A
  1. Ca3SiO5
  2. Hardens by absorbing water Ca3SiO.2H2O
  3. Long needle-like crystals of this hydrated calcium silicate are formed, which bond the cement particles together
  4. Adding gypsum (CaSO4.2H2O) slows solidification- mixture is called Portland cement
  5. Concretes typically contain a cement together with an aggregate
38
Q

Are compounds from group 1 or 2 generally more soluble in water and why

A
  1. Less soluble

2. Lattice enthalpies are higher

39
Q

What are the general rules for compounds solubility

A
  1. Compounds with large cations and large anions tend to be insoluble e.g. BaSO4
  2. Similar solubility trends as in group 1
40
Q

How is the insolubility of BaSO4 used

A
  1. Used for the test for sulfate ions
  2. Presence gives a dense white precipitate
  3. Ba2+ (aq) + SO42- (aq) –> BaSO4 (s)
  4. It is used as an x-ray contrast agent in images of the digestive tract
  5. The high atomic number of barium means it strongly absorbs X-rays so its location is easy to see- x-ray absorbed by electrons
41
Q

Describe why there is difference in the coordination chemistry between group 1 and 2

A
  1. Group 2 M2+ ions are smaller than the group 1 M+ ions and have twice the charge
  2. As a result of the higher charge densities on the group 2 cations, group 2 coordination chemistry is more extensive than that of group 1
42
Q

What effect does the increasing size of M2+ ions down the group

A
  1. It influences the coordination number- number of atoms that are coordinated to the metal ion
  2. Be2+ typically has a coordination number of 4
  3. Mg2+ - 6
  4. Ca2+, Sr2+, Ba2+ can have higher numbers
43
Q

How does the size of the ions dictate the nature of complex formed with aqua ligands

A
  1. The high charge density of Be2+ leads to hydrolysis so aqueous solutions of Be2+ are acidic
  2. In contrast, solutions of the other group 2 ions are generally neutral- as bigger than Be
44
Q

Show equation of hydrolysis [Be(H2O)4]2+

A
  1. [Be(H2O)4]2+ (aq) + H2O (l) [Be(H2O)3(OH)]+(aq) + H3O+ (aq)
45
Q

What happens in the presence of ions such as acetate to [Be(H2O)4]2+

A
  1. Hydrolysis eventually leads to compounds such as [Be4O(O2CMe)6]
46
Q

Describe the coordination present in Chlorophylls

A
  1. Magnesium coordination complexes

2. Contain Mg2+ ions lying in the centre of a chlorin ring

47
Q

What are exceptions from the expected chemistry for both group 1 and 2 to

A
  1. Li, Be

2. The first element in groups 1 and 2 is different from that of the other elements

48
Q

How is the chemistry of Li different to the rest of the group 1 elements

A
  1. Only group 1 metal that reacts with N2 to give a nitride
  2. On burning in the air, lithium gives Li2O whereas the other group 1 elements give peroxides or superoxides
  3. Li2CO3 and LiOH both decompose on heating to give Li2O, whereas the other group 1 carbonates and hydroxides are generally stable on heating
  4. LiNO3 decomposes on heating to form Li2O whereas the other group 1 nitrates decompose to nitrites MNO2
  5. The carbonate, fluoride and hydroxide are all far less soluble in water for lithium than for other group 1 metals
49
Q

Why is Lithium different

A
  1. The behaviour fits the expected pattern of changes that occur with a decrease in the size of the cation and consequent increase in the charge density on the ion
  2. Small cation size of Li+ leads directly to increases in lattice enthalpies and Gibbs energy changes of hydration
  3. These are key factors in determining the thermal stabilities and solubilities of the group 1 compounds
50
Q

How is the chemistry of Be different to the rest of the elements in group 2

A
  1. Does not react with water in contrast to the others
  2. Amphoteric- reacts with acids as the others do but also with alkalis forming [Be(OH)4]2-
  3. Be compounds have a much greater covalent character than those of the other group 2 metals- BeCl2 has a polymeric covalently-bonded structure, other chlorides are ionic
  4. Be2+ is acidic in aqueous solution unlike the other group 2 cations which are neutral
  5. BeO does not react with water in contrast to other group 2 oxides
51
Q

Why is Be different

A
  1. High charge density makes the lattice enthalpies of its compounds very high
  2. This high charge density means that the free Be2+ ion is strongly polarising
  3. It draws the electrons of neighbouring ions towards itself, which gives these bonds a much higher degree of covalent character than equivalent bonds of other group 2 elements
  4. The increase in electronegativity of the group 2 element as the group is descended also contributes to the decrease in covalent character
52
Q

What is an organometallic compound

A
  1. Compounds containing one or more bonds between a metal and carbon are known as organometallic
53
Q

Which metals are the most common in organometallic compounds

A
  1. Can be found in all group 1 and group 2 metals

2. Lithium and magnesium are the most common and most importantly synthetically

54
Q

Describe the bonding of organometallic compounds

A
  1. Since the ions have high charge density there is a large degree of covalent character in the bonding of the organometallic compounds
  2. The carbon atoms are polarised delta-, so the compounds are nucleophilic and used extensively in organic synthesis
55
Q

What is the general formula for an organolithium compound

A
  1. RLi, R= alkyl or aryl
56
Q

How are organolithium compounds formed and show example with RI

A
  1. Reacting lithium with a halogenoalkane
  2. RI + 2Li –> RLi + LiI
  3. Both air and water sensitive so must be prepared under an inert atmosphere (argon) using a solvent that has been rigorously dried
57
Q

What are two of the most important organolithium compounds

A
  1. Methylithium MeLi

2. n-butylithium BuLi

58
Q

How do organolithium compounds exist and give an example

A
  1. As oligomers both in solution and the solid state

2. Solid methylithium contains tetrameric [MeLi]3 units

59
Q

Describe bonding in methylithium

A
  1. Mainly ionic though with significant covalent character
60
Q

Describe dimethylmagnesium, MgMe2

A
  1. A polymeric solid containing bridging methyl groups

2. The structure contains 3-centre 2-electron bonding

61
Q

What are Grignard

A
  1. Organomagnesium compounds with general formula- RMgX, where X is a halide
62
Q

How are Grignard reagents formed e.g. EtMgBr

A
  1. From the reaction of a halogenoalkane with magnesium using ethoxyethane (diethyl ether) or a tetrahydrofuran (THF) as a solvent.
  2. EtBr + Mg –> EtMgBr
  3. Air and mositure sensitive so must be prepared adn used under an inert atmosphere using a dry solvent
63
Q

What is used to activate magnesium before Grignard reagent is formed and why

A
  1. Mg normally has a coating of insoluble oxide on its surface which acts as a kinetic barrier to reaction
  2. This problem is overcome by adding a trace amount of iodine to activate magnesium
64
Q

Describe the structure of Grignard reagents

A
  1. In the solid, Mg centres are normally tetrahedral, with solvent molecules occupying some of the coordination sites
  2. The compounds that are present in solution depend on a number of factors- nature of solvent and concentration
65
Q

What is the Schlenk redistribution reaction

A
  1. 2MeMgBr MgMe2 + MgBr2
66
Q

What is the diagonal relationship

A
  1. As you move diagonally- one element down and one across to the right the elements are many properties in common
  2. Atomic and ionic radii increase down group 1 due to increasing size of the outermost occupied atomic orbital, but decrease across the period due to ineffective shielding and the consequent increase in effective nuclear charge.
67
Q

Give an example of two elements who share a diagonal relationship

A
  1. Lithium and Magnesium
68
Q

What properties do Li and Mg have in common

A
  1. Both react with N2 to give nitrides
  2. Both burn in air to form the normal oxides and not peroxides
  3. Carbonates and nitrates both decompose to oxides on heating
  4. Both ions are more heavily hydrated than others in their respective groups
  5. Both form an extensive number of organometallic compounds that have a large degree of covalent character in the M-C bonds

CH10314 (4 decks)

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