Group 2: The Alkaline Earth Metals

Explanation of this trend

Remember the definition: The first ionisation energy is the enthalpy change when one mole of gaseous atoms loses one mole of electrons to form one mole of gaseous mono-positive ions

Going down Group 2:

• Nuclear charge increases

• The radius of the atom increases, so the distance between the nucleus and the outer electron increases

• There are more filled energy levels between the nucleus and the outer electron these shield the outer electron from the attraction of the nucleus.

The first factor is less significant than the other two, therefore the force of attraction between the nucleus and outer electron is reduced, and less energy is needed to remove the outer electron.

3600 965.1 502.8 Ba

4210 1064.2 549.5 Sr

4910 1145 589.7 Ca

7732.6 1450.7 737.7 Mg

14848 1757.1 899.4 Be

3rd 2nd 1st

IEs (kJmol-1)

With regard to successive IEs, the “big jump” here is between IE(2) and IE(3).

Why?

Reactivity - use the data on first and second ionisation energy to suggest

- how Gr2 metals will react in terms of electron loss/gain

- how the reactivity changes down the group

Reaction of Group 2 Metals

With O2

#Except Be, whose oxide is amphoteric

2M (s) + O2 (g) 2MO (s)

White solid

The compounds formed are almost completely ionic, except with Be.

The ions formed correspond to the +2 oxidation state, simply because of energetics – more stable compounds are formed with a larger net energy release*.

Group 2 (except Ba) react to form the expected simple, basic oxide.#

*The energy required to overcome IEs is recovered in the EAs and the lattice enthalpies.

The colour of the flame on combustion is dealt with later but can you remember what Mg looks like on combustion?

Ba

Ba (s) + O2 (g) BaO2 (s) White solid

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

White solid

Both the simple oxide and the peroxide are formed.

REACTIONS OF THE GROUP 2 ELEMENTS WITH WATER

Beryllium has no reaction with water or steam even at red heat.

Magnesium burns in steam to produce magnesium oxide and hydrogen.

Very clean magnesium has a very slight reaction with cold water. The reaction soon stops because the magnesium hydroxide formed is almost insoluble in water and forms a barrier on the magnesium preventing further reaction.

Note: As a general rule, if a metal reacts with cold water, you get the metal hydroxide. If it reacts with steam, the metal oxide is formed. This is because the metal hydroxides thermally decompose (split up on heating) to give the oxide and water.

Calcium, strontium and barium

These all react with cold water with increasing vigour to give the metal hydroxide and hydrogen. Strontium and barium have reactivities similar to lithium in Group 1 of the Periodic Table.

The equation for the reactions of any of these metals would be:

Summary of the trend in reactivity

The reactions become easier as the energy needed to form positive ions falls. This is mainly due to a decrease in ionisation energy as you go down the Group. This leads to lower activation energies, and therefore faster reactions.

Reactions with Cl2

All react on heating to form predominantly ionic chlorides (except Be):

Ca(s) + Cl2(g) CaCl2(s)

Na(s) + Cl2(g) NaCl(s)

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/021_SODIUMCHLOR.MOV

MgCl2 shows some covalent character, due to the high charge density of the Mg2+ ion. BeCl2 is covalent, and forms a solid polymer via dative covalent bonds to give the Be an octet of electrons:

Reactions of Gr2 Oxides and Hydroxides with Water and Acids

These basic oxides react with water to produce the hydroxide - to what extent this happens will depend on the solubility of the hydroxide. Colourless solutions form…

General Eqn:

Demo: Test the pH

What happens to oxide ions in water?

Reactions with acids

All oxides and hydroxides react to form salt and water

Ba

Mg

Sr

Solubility of Group 2 Sulphates and Hydroxides

Ba

Sr

Ca

Mg

Be

HydroxidesSulphates

More soluble

Sulphates (and carbonates) become less soluble as you go down the Group; hydroxides become more soluble.

Easy to remember, as you know that BaSO4 is a white ppt. used to identify sulphates,

NB1. The pH of magnesium hydroxide NB2. Limewater

Thermal stability of the s-block carbonates and nitrates

The carbonates of Li, and the Gp2 metals decompose according to the general equation:

MCO3 MO (s) + CO2 (g)

You try the Lithium eqn.

The other Gp1 metal carbonates (and barium carbonate) do not decompose at bunsen temperatures.

You will need to know: - the trends - the reactions - the explanation

In all cases, for Gp1 or Gp2 carbonates and nitrates, the thermal stability increases down the group as the ionic radius of the cation increases, and so its polarising power decreases. For the same reason, the thermal stability decreases from Gp1 to Gp2 across a period.

The nitrates of Li, and the Gp2 metals decompose according to the general equation:

2M(NO3)2 2MO (s) + 4NO2 (g) + O2(g)

Brown gas is evolved Eg… Again, you try the Lithium eqn.

The other Gp1 metal nitrates decompose to form the nitrites:

2MNO3(s) 2MNO2(s) + O2(g)

Eg…

The same pattern is observed for the s-block nitrates.

The nitrates are white solids, the nitrites yellow.

Explanation of the thermal stability trends

One view is that thermal stability increases as polarising power of the cation decreases.

Basically, large polarisable anions (CO32-, NO3-) will be more stable with non-polarising cations, but small cations with a large polarising power (high charge density) will pull the oxygen in the anion towards them, thus favouring the decomposition of the anion to form the oxide.

The flame test

crimson redstrontium

carmine redlithium

bright orangesodium

lilacpotassium

blue/greencopper

brick redcalcium

light (apple) greenbarium

flame colourmetal

The distinctive colours that appear when we heat metals or their compounds can be used to identify them. You need to learn them…

The energy from the bunsen flame causes electrons to jump up to higher energy levels. When they return to their original “ground” state, they release a certain characteristic amount of energy.

Expt: 1.6.2 Flame tests

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/039_FlameTestsMet.MOV

Sodium and sodium compounds emit almost monochromatic light used for distinguishing optical isomers. Other uses: Analysing body fluids, astronomy.

P179-187 Questions

Observation can be qualitative or via a spectrometer. The latter method gives a series of lines of frequencies that correspond to the differences in the energy levels for a particular atom (ion). See p180. These are called line spectra.