Chapter 5 The Periodic Table: Group2 5.1 Redox Reactions 5.2 Using Oxidation Numbers 5.3 Group2 5.4...

Chapter 5 The Periodic Table: Group2

5.1 Redox Reactions

5.2 Using Oxidation Numbers

5.3 Group2

5.4 Compounds of Group 2 Element

5.1 & 5.2 What is the outcome from syllabus?

Candidates should be able to:

Describe and explain redox processes in terms of electron transfer and/or of changes in oxidation number (oxidation state)

5.1 Redox Reactions

Reduction – oxidation happens everywhere!

Because both reduction and oxidation are going on side-by-side, this is known as a redox reaction.

5.1 Redox Reactions

♣ Oxidation and reduction in terms of oxygen transfer

Definitions:• Oxidation is gain of oxygen• Reduction is loss of oxygen

(oxidising agent)

(reducing agent )

5.1 Redox Reactions

♣ Oxidation and reduction in terms of hydrogen transfer

Definitions:• Oxidation is loss of hydrogen• Reduction is gain of hydrogen

Oxidising agent : acidified potassium dichromate(VI)

Reducing agent : sodium tetrahydridoborate, NaBH4

5.1 Redox Reactions

♣ Oxidation and reduction in terms of electron transfer

Definitions:• Oxidation is loss of electrons.• Reduction is gain of electrons.

5.1 Redox Reactions

Oxidation Number/State (Ox/ON):The number of electrons lost or gained by an atom in a compound compared to the uncombined atom. It forms the basis of a way of keeping track of redox (electron transfer) reactions.

♣ Every element in its uncombined state has an oxidation number of zero.

♣ A positive number shows that the element has lost electrons and has therefore been oxidised.

♣ A negative number shows that the element has gained electrons and has therefore been reduced.

♣ The more positive the number, the more the element has been oxidised. The more negative, the more it has been reduced.

♣ The numbes are written in Roman numberals and awlways have a sign unless they are zero, e.g. +II.

•Some elements almost always have the same oxidation states in their compounds:

elementusual oxidation state

exceptions

Group 1 metals

always +I  

Group 2 metals

always +II  

Oxygen usually -IIexcept in peroxides (H2O2) , superoxide (NaO2) and F2O

Hydrogen usually +Iexcept in metal hydrides where it is -1 (NaH)

Fluorine always -I  

Chlorine usually -I except in compounds with O or F (HClO)

5.1 Redox Reactions

5.2 Using Oxidation Numbers

Oxidation numbers are used:

■ In the systematic naming system of inorganic compounds (ones not based on carbon chain)

■ To work out in a redox reaction which elements have been oxidised and which reduced.

5.2 Using Oxidation Numbers

Naming inorganic Compounds (the sum of ON = 0):

♦ Compounds with two elements

① Ionic compounds: “-ide”

CuO Copper (II) oxide

Cu2O Copper (I) oxide

FeCl2 Iron (II) chloride

FeCl3 Iron (III) chloride

NaH Sodium Hydride

PCl3 Phosphorus trichloride

Phosphorus (III) chloride

PCl5 Phosphorus pentachloride

Phosphorus (V) chloride

CO2 Carbon dioxide

② Molecular compounds:

5.2 Using Oxidation Numbers

♦ Compounds with three elements

K2SO4 Potassium sulphate

Potassium sulphate (VI)

K2SO3 Potassium sulphite

Potassium sulphate (IV)

SO42- sulphate

NO3- nitrate

NH4+ ammonium

PO43- phosphate

MnO4- manganate

CO32- carbonate

5.2 Using Oxidation Numbers

Oxidation involves an increase in oxidation numberReduction involves a decrease in oxidation number

5.2 Using Oxidation Numbers

The chlorine is the only thing to have changed oxidation state.

This is a good example of a disproportionation (歧化 )reaction. A disproportionation reaction is one in which the same element is both oxidised and reduced.

5.3 & 5.4 Group 2

What is the outcome from syllabus?

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) describe the thermal decomposition of the nitrates and carbonates

(d) interpret, and make predictions from, the trends in physical and chemical properties of the elements and their compounds

(e) explain the use of magnesium oxide as a refractory lining material and calcium carbonate as a building material

(f) describe the use of lime in agriculture

5.3 Group 2

Sr

Ba

Ra

Ca

Be

Mg

Alkaline metal: ns2

12+

Mg

BERYLLIUM

MAGNESIUM

CALCIUM

STRONTIUM

BARIUM

RADIUM

5.3 Group 2

5.3 Group 2

Element Color Element Color

Li Scarlet Be －Na Yellow Mg －K Lilac Ca Brick-red

Rb Red Sr Crimson

Cs Blue Ba Apple-green

The Flame Color:

5.3 Group 2

Li Be B C N O F NeThe second period:

It is generally true that elements in the same group of the Periodic Table have similar properties. However, the elements in the second period (Li to Ne) show many properties which are not typically of their groups.

The reasons for these anomalies are:

♦ These atoms are particularly small

Unusually high ionisation energy

Unusually electronegatitive

High charge density to polarise anions

♦ The elements of Period 2 can form bonds only through their 2s and 2p orbitals.

5.3 Group 2

nitron

Ene

rgy

2900 kJ mol-1 required to promote a paired electron to the next empty orbital (3s)

Nitrogen only forms NCl3

1370 kJ mol-1 required to promote a paired electron to the next empty orbital (3d)

Phosphorus can forms PCl3 and PCl5

phosphorus

Ene

rgy

5.3 Group 2

I II III IV

Li Be B C Period 2

Na Mg Al Si Period 3

Increasing electronegativity

Increasing electronegativity

The untypically large electronegativities of Period 2 elements (caused by their small size) mean that they are in some ways more typical of elements one group to the right than to elements of their own group.

Diagonal relationships ( 对角线规则 )

For example: Be(OH)2 + 2H+ → Be2+ + 2H2O Be(OH)2 + 2OHˉ→ [Be(OH)4]2ˉ

5.3 Group 2

Electron arrangement

Metallic radius/nm

First + Second IE/ kJ mol-1

Tm/K Tb/K Density/g cm-3

Mg

Magnisum

[Ne]3s2 0.160 2189 922 1380 1.74

Ca

Calcium

[Ar]4s2 0.197 1735 1112 1757 1.54

Sr

Strontium

[Kr]5s2 0.215 1614 1042 1657 2.60

Ba

Barium

[Xe]6s2 0.224 1468 998 1913 3.51

The physical properties of Group 2:

Magnesium oxide

2Mg (s) + O2 (g) 2MgO (s)

① burns very vigorously ② bright white flame ③ white solid produced

a. in the air b. in oxygen

5.3 Group 2

5.3 Group 2

The reason that there are different types of oxides is related to the sizes of the ions:

O2ˉ > O2

2ˉ > O2ˉ

If the cation is too small, it is not easy for enough peroxide or superoxide ions to cluster round it to form a stable crystal lattice. For example, Lithium can only forms the ‘normal’ oxide.

..¨OO.. ..

oxygen ion

¨ ¨O—OO—O¨.. ....

superoxide ionperoxide ion

¨ ¨O—OO—O¨ ¨.... ....

22 ..22....

5.3 Group 2

The ‘normal’ oxide, MO(M2+ + O2ˉ), is formed when the metals are heated in oxygen. Strontium and Barium also form peroxides. As the M2+ ions are smaller than the M+ ions in Group I, peroxides do not form until lower down the group II than in Group I.

The closer the anions with cations, the more stable the ionic compounds crystal.

5.3 Group 2

Mg (s) + 2H2O (l) Mg(OH)2 (aq) + H2 (g)

♦ Reaction with water

Mg (s) + H2O (g)

steam

MgO (s) + H2 (g)

slowly

rapidly

Beryllium does not react directly with water all. The rest of the Group II metals react with increasing rapidity on descending the group.

5.3 Group 2

MgO (s) + H2O (l)

♦ Oxide reaction with water

Partially soluble

The rest of the Group II oxides react with increasing rapidity on descending the group.

Mg(OH)2 (aq)

In the saturated solution, pH(Mg(OH)2) = 10

♦ Reaction with acids

5.3 Group 2

Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

Mg (s) + H2SO4(aq) MgSO4 (aq) + H2 (g)

The reaction is more vigorous as we go down the group.

5.3 Group 2

Carbonates, CO3

2ˉ

Mg Ca SrBa

MgCO3 → MgO + CO2

Group IIGroup II

Same pattern but higher temperatures needed for decomposition

Nitrates, NO3

ˉ

MgCaSrBa

M(NO3)2 → MO + 2NO2 + 1/2O2

Thermal stability describes how easily or otherwise a compound will decompose on heating. Increased thermal stability means a higher temperature is needed to decompose the compound.

The charge density (Z/r) of the cations (polarization) will affect the decomposition temperature.

The larger value of Z/r,The easier breaking up of distorted anions:

CO32- → CO2 + O2-

NO32- → NO2 + O2-

Same pattern but higher temperatures needed for decomposition

5.3 Group 2

Which one of the following equations represents the reaction that occurs when calcium nitrate is heated strongly?

A. Ca(NO3)2 → Ca(NO2)2 + O2

B. 2Ca(NO3)2 → 2CaO + 4NO2 + O2

C. Ca(NO3)2 → CaO + N2O + 2O2

D. 3Ca(NO3)2 → Ca3N2 + 4NO2 + 5O2

E. Ca(NO3)2 → CaO2 + 2NO2

√

5.3 Group 2

Which one of the following elements is likely to have an electronegativity similar to that of aluminium?

A. Barium

B. Beryllium

C. Calcium

D. Magnesium

E. Strontium

√ diagonal relationship

5.3 Group 2

Which one of the following statements is true?

A. All nitrates of Group II metals are decomposed by heat to give the oxide NO2

B. Aqueous sodium nitrate in acidic to litmus.

C. Aqueous ammonium nitrate is alkaline to litmus.

D. The alkali metal nitrites are insoluble in water.

E. Metals dissolve in concentrated nitric acid to give hydrogen.

√

5.4 Compounds of Group II Elements

coins cosmetics

pipeShip

5.4 Compounds of Group II Elements

Oxide Melting point/℃

MgO 2852

CaO 2614

SrO 2430

BaO 1918

Table 1: the melting points of the oxides of the Group II elements.

As M2+ cationic size increases down the Group, the ionic bonds become weaker, hence, less energy is needed to break the bonds and a low melting point is expected.

refractory material

Magnesium oxide is used to line industrial furnaces because it has a very high melting point. Which type of bond needs to be broken for magnesium oxide to melt?

A. co-ordinate

B. covalent

C. ionic

D. metallic

5.4 Compounds of Group II Elements

√

5.4 Compounds of Group II Elements

CaCO3(limestone)

CaO(lime)Ca(OH)2(slaked lime)

Δ

+ H2O

+ CO2

5.4 Compounds of Group II Elements

Acid + Base → Salt + Water

Ca(OH)2 (s) + 2HNO3(aq) → Ca(NO3)2(aq) + 2H2O(l)

This is a base and is used in agriculture to treat acidic soil.

A farmer spreads lime on land which has already been treated with a nitrogenous fertilizer. Which reactions will occur over a period of time?

1. Ca(OH)2 + CO2 → CaCO3 + H2O

2. Ca(OH)2 + 2H+ (aq) → Ca2+(aq) + 2H2O

3. Ca(OH)2 + 2NH4+(aq) → Ca2+(aq) + 2NH3 + 2H2O

5.4 Compounds of Group II Elements

√

√

√

5.4 Compounds of Group II Elements

When decomposing in water, organic refuse is oxidised to form carboxylic acids. The water becomes acidic and aquatic life is destroyed.

Which additives are suitable to remove this acid pollution?

1. calcium carbonate

2. calcium hydroxide

3. potassium nitrate

√

√

5.4 Compounds of Group II Elements

Hard water: Ca2+, Mg2+, SO42-, Cl-

Soft water: Ca2+, Mg2+, HCO32-

Ca2+(aq) + SO42-(aq) → CaSO4(s)

“temporary hardness”

“permanent hardness”

Ca2+ (aq) + 2HCO3ˉ(aq) → CaCO3(s) + CO2(g) + H2O(l)

        Mg2+ (aq) + 2HCO3ˉ(aq) → MgCO3(s) + CO2(g) + H2O(l)

Δ

Δ

5.4 Compounds of Group II Elements

scum

     Ca2+ (aq) + 2C17H35COOˉ(aq) → Ca(C17H35COO)2 (s)

        Mg2+ (aq) + 2C17H35COOˉ(aq) → Mg(C17H35COO)2 (s)

calcium stearate

magnesium stearate

stearate

stearate

5.4 Compounds of Group II Elements

A number of methods can be used for softening water:

♦ Boiling removes temporary hardness, but is expensive.

♦ Calcium hydroxide is cheap and can be added to precipitate out temporary hardness as calcium carbonate. Ca(HCO3)2(aq) + Ca(OH)2(s) → 2CaCO3(s) + 2H2O(l)

♦ Sodium carbonate may be added to precipitate out calcium or magnesium ions. Mg2+ (aq) + Na2CO3(aq) → MgCO3(s) + 2Na+(aq)

♦ Use ion exchange resins: plastic beads which contain sodium ions.

River water in a chalky agricultural area may contain Ca2+, Mg2+, CO3

2-, HCO3-, Cl- and NO3

- ions. In a waterworks, such water is treated by adding a calculated quantity of calcium hydroxide.

Which will be precipitated following the addition of calcium hydroxide?

A. CaCl2

B. CaCO3

C. MgCO3

D. Mg(NO3)2

5.4 Compounds of Group II Elements

√

Table 2: Active Ingredients in Commercial Antacid Tablets

Chemical NameChemical Formula

Chemical Reaction

Magnesium Hydroxide

Mg(OH)2

Mg(OH)2 + 2H+ →

Mg2+ + 2H2O

Calcium Carbonate CaCO3 CaCO3 + 2H+ →

Ca2+ + H2O + CO2 (g)

Sodium Bicarbonate NaHCO3 NaHCO3 +  H+ →

Na+ + H2O + CO2 (g)

Aluminum Hydroxide

Al(OH)3 Al(OH)3 + 3H+ →

Al3+ + 3H2O

Dihydroxyaluminum Sodium Carbonate

NaAl(OH)2CO3

NaAl(OH)2CO3 + 4H+

→ Na++ Al3++ 3H2O +

CO2(g)

5.4 Compounds of Group II Elements

5.4 Compounds of Group II Elements

The metals of Group II react readily with oxygen to from compounds of general formula MO. When each of these oxides is added to water, which forms the most alkaline solution?

A. MgO

B. CaO

C. SrO

D. BaO√

5.4 Compounds of Group II Elements

The solubility of some Group II metal compounds in mmol·dm-3

Mg2+

Ca2+

Sr2+

Ba2+

CO32- SO4

2- CrO42- C2O4

2-

1.5 1830 8500 5.7

0.13 47 870 0.05

0.07 0.71 5.9 0.29

0.09 0.009 0.01 0.52

decreasesdown the

group

NaCl(s)(g)Clˉ+(g)Na+

781 kJ•mol-1－latt

H

定义 : 由无限远离的气态正负离子 , 在标准状态下形成1mol 离子晶体时的焓变 , 叫该种晶体 的晶格能 Hlatt 。

The enthalpy change when 1 mole of an ionic compound is

formed from its gaseous ions under standard conditions

(298K ， 100 kPa)

5.4 Compounds of Group II Elements

5.4 Compounds of Group II Elements

Hydration Enthalpy(Hhyd , 水合热 ): The amount of

energy relaeased when one mole of aqueous ions is

formed from its gaseous ions.

Na+ (aq)(g) + aqNa+

Hhyd = － 406 kJ·mol-1

5.4 Compounds of Group II Elements

When an ionic solid is dissolved in water, two processes are taking place. They are the breakdown of the ionic solid, and subsequent stabilization of the ions by water molecules (hydration).

Na+(g) + Clˉ(g)

Hlatt =

－ 776 kJ·mol-1

Hhyd = － 772 kJ·mol-1

Hsolu = Hhyd － Hlatt

NaCl (s) Na+(aq) + Clˉ(aq)Hsolu

5.4 Compounds of Group II Elements

For MSO4, SO42- is quite large compared with M2+.Going

down the group II, the increase in size of the cations Hlatt does not cause a significant change in the but the Hhyd become less and less negative down the group. As a result, the dissolution process becomes less and less exothermic and the solubility of the sulphates(VI) of Group II metals decreases down the group.For M(OH)2, OH- and M2+ are of the same order of magnitude, Going down the group II, the increase in size of the cations Hhyd does not cause a significant change in the but the Hlatt

become less and less negative down the group. As a result, the dissolution process becomes less and less exothermic and the solubility of the hydroxides of Group II metals increases down the group.