Group 2 Metals - Twig Learning Center

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Group 2 Metals

In the study of Group 2 elements, beryllium is separated from the rest because it has properties different from the others. Magnesium to barium have similar properties and they are studied as a group. Their reactions and compounds would be the main focus of this topic. Radium is radioactive and its chemistry would not be discussed.


Be resembles Al in many reactions (Be2+ and Al3+ have similar charge densities)

1. BeCl2 has a simple molecular structure due to the high charge density of Be2+ that is able to polarize the Cl- ion to a great extent.

1. The other Group 2 chlorides are ionic.

2. The other Group 2 elements form M(H2O)62+2. Be forms [Be(H2O)4]2+ in solution.

3. BeO and Be(OH)2 are amphoteric.

Be(OH)2 + H2SO4 → BeSO4 + 2H2O

Be(OH)2 + 2NaOH → Na2Be(OH)4

or Be(OH)2 + 2OH- → Be(OH)42-

3. The other Group II oxides and hydroxides are all basic.

M(OH)2 + H2SO4 → MSO4 + 2H2O

Beryllium, an element that is different from the other Group 2 elements

Charge density of M2+ α q2+ / r2+

Be2+

64.5 30.8 20.2 17.7 14.8 60

Mg2+ Ca2+ Ba2+Sr2+ Al3+

- The outermost electron of alkaline earth elements is always ___.- To reach octet structure, they have to lose their outermost ____ electrons.- The _____ ionisation energy is too high.

Due to the _____ charge density of Be2+, BeCl2 is a _____________ compound whereas MgCl2 is an ______ compound. Thus ______ energy is needed to break the ____________forces of attraction between BeCl2 compared to the _____________________ in MgCl2.

Due to the _______ charge density of Be2+, Be2+ is able to ___________ the oxide ion to a ________ extent, hence BeO is an _____ compound with _________ character. Thus it has both ______ and ______ properties. On the other hand, the oxides of all the other alkaline earth metal oxides have __________________. Thus these oxides are _______.

Explain each of the following observations in terms of structure and bonding.

(a) The alkaline earth metals (Be, Mg, Ca, Sr and Ba) all have fixed oxidation number in their compounds.

(b) BeCl2 (boiling point 547oC) is much more volatile than MgCl2 (boiling point 1418oC)(c) Beryllium oxide is amphoteric but the oxides of all the other alkaline earth metals are basic..

(a)

(b)

(c)

Try!


Trenddownthegroup Reasons

Atomicradius/Ionicradius ↑

Downthegroup,eachsucceedingelementhasonemoreshellofelectrons.Hence,theouterelectronsareprogressivelyfurtherawayfromthenucleus.

Ionisa=onenergy ↓Asatomicsizeincreases,theouterelectronsarefurtheroutandareprogressivelybe@ershieldedfromthenucleus.Hence,lessenergyisrequiredtoremoveanelectron.

Electronega=vity ↓

Asatomicsizeincreases,theouterelectronsbecomeprogressivelylessstronglyheldbythenucleus.Hence,thereislessa@rac=onforaddi=onalbondingelectrons.

Group II elements: Be, Mg, Ca, Sr, Ba, Ra

Physical properties of Group 2 elements

Down a group,● Nuclear charge increases (increase in number of protons)● Screening effect increases due to additional (filled) inner shell of electrons● Increase in screening effect outweighs the increase in nuclear charge● Outer electrons are less strongly attracted to the nucleus and are further away● Atomic radius increases

Reducing Power

Group II metals have very negative standard electrode potentials, indicating that they are powerful reducing agents.

Mg2+ (aq) + 2e Mg (s) E = −2.38 V

Ca2+ (aq) + 2e Ca (s) E = −2.87 V

Sr2+ (aq) + 2e Sr (s) E = −2.89 V

Ba2+ (aq) + 2e Ba (s) E = −2.90 V

Φ

reducing power increases down the group

Why does the reducing power (or reactivity) of these metals increase down the group?

The atomic radius ____________ down the group, outer electrons are ______________ and _____________________ the nucleus, thus the tendency for the atom to lose valence electrons to form cations ______________.

Chemical properties of Group 2 elements (Mg to Ba)

+•• Mg +

•• Sr+

•• Ba

⇌⇌⇌⇌

ΦΦ

Φ


In general, Mg to Ba undergo similar reactions with common reagents such as water and oxygen but reactivity increases from Mg to Ba.

Reaction with Water

(i) Magnesium metal reacts very slowly with cold water but rapidly with steam to give magnesium oxide:

Mg (s) + H2O (g) → MgO (s) + H2 (g)

MgO dissolves slightly in water to give a weakly alkaline solution.

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

(ii) From Ca to Ba, reaction with cold water is rapid though less vigorous than that of Group I metals. Hydrogen is given off and the resulting solution is alkaline due to formation of the respective hydroxides.

Ca gives bubbles of H2 and leaves a white precipitate of Ca(OH)2

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

Reactions of Group 2 elements

Solubility of group II hydroxides increases down the group

⇌

Reaction with Oxygen

(i) With the exception of Be and Mg, the rest of Group 2 elements are stored under liquid paraffin. Be and Mg are less reactive than the other Group 2 elements and form a protective oxide layer so that they tarnish more slowly.

(ii) When heated, - Mg burns in air with an intense white flame to form MgO- Ca burns in air with an orange red flame to form CaO- Sr burns in air with a red flame to form SrO- Ba burns in air with a pale green flame to form BaO

e.g. 2Ba (s) + O2 (g) → 2BaO (s)

Reactions of Group 2 elements

Element With Water With Air

Mg slowly with cold water but readily with steam ignites only above melting point

Ca readily with cold water ignites on heating

Sr readily with cold water

Ba vigourously with cold water inflames spontaneously


Magnesium oxide dissolves sparingly in water to give a slightly alkaline solution.

MgO (s) + H2O (l) Mg(OH)2 (s) Mg2+ (aq) + 2OH- (aq)

Calcium, strontium and barium oxide dissolve in water to give alkaline solutions. (pH 10 - 13) and their enthalpies of solution are exothermic.

CaO (s) + H2O (l) → Ca(OH)2 (aq)

SrO (s) + H2O (l) → Sr(OH)2 (aq)

BaO (s) + H2O (l) → Ba(OH)2 (aq)

Note: Solubility of Group 2 hydroxides increases down the group.Base strength increases down the group. Ba(OH)2 is a strong base.

However, solubility of Group 2 sulfates decreases down the group. MgSO4 is soluble but SrSO4 and BaSO4 are insoluble in water.

BeO is insoluble in water due to its high lattice energy.

Reactions of Group 2 oxides with water

⇌→

Driving force for decomposition

(a) This is due to the stability of the oxide formed compared to the original carbonate.

(b) Group 2 compounds (with the exception of Be compounds) are predominantly ionic so that relative stability of solid reactants or products is dependent on their lattice energy. In general, the more exothermic the lattice energy, the more stable the compound.

(c) Lattice energy

Stability of an ionic salt is dependent on both charge (q) and ionic radius (r).The greater the charge and the smaller the ions, the stronger will be the attraction between the ions and the more stable the ionic compound. Hence one would expect an ionic compound to become more stable as ionic charge increases or as ionic radius decreases.

Hence the group II oxide is more stable than its respective carbonate.

The carbonates of Group 2 elements MCO3, decompose on heating to a metal oxide and carbon dioxide gas.

MCO3 (s) → MO (s) + CO2 (g)

e.g. SrCO3 (s) → SrO (s) + CO2 (g)

Thermal stability of the Group 2 Carbonates


Why does thermal stability of Group 2 carbonates increase down the group?

(a) The decomposition temperature increases down the group showing that the thermal stability of the carbonates increases down the group.

(b) This trend may be explained by considering the variation in charge density of the cation and the polarisability of the anion.

(i) The positively charged M2+ attracts the electron density of the anion towards itself. There is thus a drift of electron cloud towards M2+ and this weakens the C−O bond in CO3

2−. When the electron cloud of the carbonate anion is distorted by a cation, the anion is said to be polarised by the cation.

(ii) The greater the distortion, the more easily it is to break the C−O bond. Hence the thermal stability of a Group 2 CO3

2− is dependent on the ability of M2+ to polarise the anion, i.e the polarising power of M2+. CO3

2− is easily polarised as it is a large anion.

The delocalised electrons are pulled towards the positive ion

The oxygen atom is well on the way to becoming an oxide ion

The end of the ion is on its way to breaking away and becoming carbon dioxide

As we go down Group 2,

(i) cationic size increases, decreasing the charge density on M2+ and hence polarising power of cation decreases.

(ii) electron cloud of CO32− is distorted to a

lesser extent so that the weakening effect on the C−O bond is less.

(iii) thermal stability of group 2 carbonates increases down the group.

Mg2+

Ba2+

Ca2+

Sr2+

−OO

O−

C

Why does thermal stability of Group 2 carbonates increase down the group?

MCO3 (s) → MO (s) + CO2 (g)

As we go down Group 2,

(i) cationic size increases, decreasing the charge density on M2+ and hence polarising power of cation decreases.

(ii) electron cloud of CO32- is distorted to a lesser extent so

that the weakening effect on the C−O bond is less.

(iii) thermal stability of group 2 carbonates increases down the group.

−OO

O−

C

−OO

O−

C

−OO

O−

C

Good point to note:

Li exhibits a higher charge density than the rest of the Group 1 metals. As a result, Li does have some properties similar to group II metals. The other Group 1 carbonates do not decompose on heating.

Li2CO3 decomposes on heating to form Li2O.

Li2CO3 → Li2O (s) + CO2 (g)


(a) group II carbonate → group II oxide + carbon dioxide

e.g. CaCO3 (s) → CaO (s) + CO2 (g)

(b) group II nitrate → group II oxide + nitrogen dioxide + oxygen

e.g. Sr(NO3)2 (s) → SrO (s) + 2NO2 (g) + ½O2 (g)

(c) group II hydroxide → group II oxide + steam

e.g. Ba(OH)2 (s) → BaO (s) + H2O (g)

(i) Barium peroxide BaO2 decompose on heating to oxide and oxygen.

Construct balanced equations, with state symbols, for the above.

Thermal stability of the Group 2 carbonates, nitrates and hydroxides

Carbonates, nitrates and hydroxides decompose upon heating.What do they have in common?

Try! 2012 Nov A Level Paper 3

(a) When heated, the carbonates of Group II elements can lose carbon dioxide.

MCO3 (s) → MO (s) + CO2 (g)

Under the influence of the metal cation, the carbonate ion is thought to break up as follows:

The nitrates of Group II elements also decompose on heating.

(i) Write an equation for the decomposition of Mg(NO3)2.

(b) Dinitrogen pentoxide, N2O5, exists as covalent molecules O2N-O-NO2 in the gas phase, but as the ionic [NO2

+NO3− ] in the solid or in solution. N2O5 has been suggested as a

possible intermediate in the thermal decomposition of Group II nitrates M(NO3)2.

(i) Using this information, and that provided in (a), suggest a mechanism for the formation of N2O5 from a Group II nitrate, M(NO3)2.


The following table lists the decomposition temperature of Group 2 carbonates and the melting points of the Group 2 oxides.

MgCO3 350 MgO 2852CaCO3 840 CaO 2614SrCO3 1100 SrO 2430BaCO3 1360 BaO 1918

(i) Suggest explanations for the different trends.

Try!

Carbonate Decomposition temperature / oC Oxide Melting point/oC

Samples of anhydrous magnesium ethanoate, Mg(CH3COO)2, and barium ethanoate, Ba(CH3COO)2, were subjected to heating at 600 °C. White residues were formed in both decomposition reactions.A common gaseous product, V, with the molecular formula of C3H6O, was also formed in both decomposition reactions. V gave a yellow precipitate upon warming with alkaline aqueous iodine.In addition, decomposition of magnesium ethanoate also produced a second gaseous product which formed a white precipitate with calcium hydroxide solution.

(a) Identify the common gaseous product V.

(b) Write a balanced equation for each decomposition reaction, including state symbols.

(c) Account for the difference in decomposition products formed.

Try! RJC Prelim 2012


X, Y and Z are Group II metals (Mg to Ba). X(IO3)2, Y(IO3)2 and Z(IO3)2 are Group II iodates(V). Upon heating, each of these iodates(V) decomposes to yield a metal oxide a purple gas and a. colourless gas which rekindles glowing splints..

(a) Draw a dot-and-cross diagram of magnesium iodate(V).

(b) Write a balanced equation for the decomposition of Mg(IO3)2.

(c) Two graphs given below show the change in mass when 2.00 g each of X(IO3)2 and Y(IO3)2 were heated separately. When 2.00 g Z(IO3)2 was heated 0.29 g of residue remained. .All heating was carried out at the same temperature. .

(i) With the aid of information given, identify the three iodates(V).

(ii) Arrange the three iodates(V) in order of increasing thermal stability.

(iii) Sketch on the same graph as X(IO3)2, the decomposition profile for Z(IO3)2.

Try!


Barium peroxide, BaO2, is formed when barium is heated with oxygen. However, magnesium peroxide MgO2 readily decomposes into magnesium oxide.

(a) Explain why magnesium peroxide is unstable but barium peroxide is stable.

(b) When barium peroxide is dissolved in water, effervescence is observed and the solution turns red litmus blue. (i) Identify the products formed and write an equation to show the reaction between

barium peroxide and water. (ii) Name the type of reaction taking place.

Try!

A student was given a sample of calcium carbonate that was mixed with calcium nitrate. He carried out two separate experiments using the same mass of the solid mixture.

Experiment 1: The sample was heated strongly to constant mass and the volume of gas evolved was found to be 200 cm3.

Experiment 2: Excess dilute sulfuric acid was added to the sample and volume of gas .evolved was found to be 75 cm3.

All volumes were measured at the same temperature and pressure.

What is the mole ratio of calcium carbonate to calcium nitrate in the sample?

A 3:2 B 3:5 C 6:5 D 9:5

Try!

When a mineral was heated in a Bunsen flame to a constant mass, a colourless gas that gave a white precipitate with lime water was evolved. The remaining solid was cooled and then added to aqueous hydrochloric acid. Vigorous effervescence was seen. What was the mineral?

A aragonite, CaCO3 B dolomite, CaCO3.MgCO3C artinite, MgCO3.Mg(OH)2.3H2O D barytocalcite, BaCO3.CaCO3


Which one of the following pairs of 0.1 mol dm-3 aqueous solutions is most likely to give a precipitate when added together?

A KBr and MgSO4 B NaNO3 and CaC/2C NH3 and BaC/2 D MgSO4 and SrC/2

X is a mixture of white solids. When X is treated with an excess of dilute hydrochloric acid, a colourless gas is evolved and some, but not all of the mixture dissolves.Which one of the following mixtures could X be?

A Ba(NO3)2 and Ca(OH)2 B BaSO4 and CaCO3C CaCO3 and MgSO4 D Ca(OH)2 and MgCO3

Dilute sulphuric acid was added to aqueous barium hydroxide until the acid was in excess.Which graph best represents the variation in the total number of ions in solution?

Try! A Level MCQ

A is a compound of a Group II element M. When A was treated with dilute hydrochloric acid, a colourless gas, B was evolved, which turned moist litmus paper red. On bubbling B through lime water, a precipitate, C was formed, but passage of further gas resulted in a clear hydrogencarbonate solution, D. On strong heating, 1.9735 g of A decomposed, giving E its oxide, a white solid. The sample of E was dissolved in water and made up to 250 cm3 in a standard flask. 25.0 cm3 aliquots were titrated with acid and required 20.30 cm3 of 0.0985 mol dm-3 hydrochloric acid.

(i) Identify the compounds B, C and D and write the equation for formation of D from C.

(ii) Calculate the relative molecular mass of A.

(iii) Consider the identity of A and calculate the relative atomic mass of the Group II element M in A. Hence identify M.

Try! NYJC P3 Prelim 2011 Adapted


Explain why the decrease in the magnitude of the lattice energy of oxides is greater than that of nitrates. [2]

Lead(II) carbonate and zinc carbonate decompose on heating the same way as calcium carbonate. Use relevant data from the Data Booklet, rank the order of decomposition temperatures, starting from the carbonate with the lowest temperature. Explain your answer.

Try!

When do we use charge density to explain chemistry concepts?

Charge Density

∆Hhydration of ions

Higher charge density of cation or anion⇒ more exothermic ∆Hhyd

Lattice energy of ionic compounds and hence the melting point of the compound.Higher charge density of cation or anion⇒ greater electrostatic forces

of attraction⇒ higher melting point

Ability of metal ions to form complexes. Will study more of this under transition metals

Polarising Power of Cation

Thermal Stability of Group II compounds (e.g. nitrates, carbonates, hydroxides)

Extent of hydrolysis of NaCl, MgCl2, AlCl3 etc.

e.g. [Mg(H2O)6]2+(aq) ⇌ [Mg(H2O)5(OH)]+ (aq) + H+ (aq)

[Al(H2O)6]3+ (aq) ⇌ [Al(H2O)5(OH)]2+ (aq) + H+(aq)

Degree of covalency of ionic chlorides and oxidesHigher charge density of cation ⇒ more covalent character

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Group 2 Metals - Twig Learning Center

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