Group 2 elements presentation

CHAPTER 14"GROUP II ELEMENTS"

START

ALKALINE EARTH METALS

NOTES

• Beryllium does not follow the normal trend of the group II elements because it has a higher melting and boiling point than other members of its group.

• Radium is radioactive because it is created from the decay of the uranium atom so we do not look at its chemistry in analyzing group II

SECTION 01

PHSICAL PROPERTIES OF GROUP II ELEMENTS

Atomic radius

• NB: Atomic radius is the distance from the centre of the nucleus to the edge of the surrounding electron cloud.

• as the atomic number increases, the atomic radius increases.
• Going down Group II there are more filled shells between the nucleus and the outer electrons. This results in the outer electrons being further from the nucleus thus, increasing the size of the atom

Ionization energy

• Ionization energy is the energy needed for the complete removal of an electron from its outermost shell.

• It is an endothermic process of the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge. As a result, Ionization energy decreases down the group.

• Since the atomic radius of the atom increases down the group, the distance between the nucleus and the outer electrons also increases. Therefore, the force of attraction between the nucleus and outer electrons is reduced and less energy is needed to remove an outer electron.

Melting Point

• There is a general decrease in melting point going down group II in terms of atomic and ionic radii, with the exception of magnesium.

• Moving down the group, the atomic and ionic radii increase which results in less attraction between the positive ions and the delocalized electrons in the metallic structure. Thus, less energy is required to melt the structure.

Boiling Point

• With the exception of Magnesium, the boiling points decrease from beryllium to strontium, then increase to radium.

Electronegavity

• Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond

• Going down Group II, as the atomic number increases, the electronegativity of the elements decreases. This is because the outer electrons are more shielded from the attraction of the nucleus and thus, electrons are less strongly attracted to the nucleus.

SECTION 02

Chemical Reactions

Standard electrode values for group II elements

All the values for the reverse reactions are positive, therefore electrons are easily lost from these elements. These values tell us the ease with which each reaction occurs

Reactions with oxygen

• All group II elements react with oxygen to form its metallic oxide.
• They all burn readily in oxygen.
• Reactions are highly exothermic.
• Reactivity increases down the group.
• All of group II metals tarnish in the air. This is because a layer of oxide is formed on the surface of the metal

Reaction with Oxygen

Video

Mg(s) burns with a brilliant flame in oxygen to produce MgO(s)

Reactions with water

• Fact: There is an additional reason for the lack of reactivity of beryllium compared with the rest of the Group. Beryllium has a strong resistant layer of oxide on its surface which lowers its reactivity at ordinary temperatures

Reaction with Water

Video

When Ca(s) reacting with cold water it reacts vigourously to produce Ca(OH)2(aq)and H2(g).

Reactions with Acids

• All the group II metals react with acids to produce salts and hydrogen gas.

• Reactions with dilute hydrochloric acid

All the metals react with dilute hydrochloric acid to give bubbles of hydrogen and a colourless solution of the metal chloride. The reactions get more vigorous as you go down the Group.

• Calcium reacts vigorously with acids whereby barium reacts violently.

• Reactions with dilute sulphuric acid

These are more complicated, because of the formation of insoluble sulphates. For instance, when sulphuric acid reacts with barium, it forms insoluble barium sulphate, which coats the metal surface and prevents the inner barium from reacting.

Reaction with Hydrochloric acid

Video

• Mg(s) reacts with hydrochloric acid to produce MgCl2(aq) and H2(g)

SECTION 03

SOLUBILITY OF GROUP II ELEMENTS

NOTE: The energy required to break the lattice into ions and the energy released when the ions are hydrated determines the solubility of solid compounds. Endothermic energy is necessary to shatter the lattice, whereas exothermic energy is required to hydrate the ions. When the lattice energy exceeds the hydration energy, low solubility occurs.

The solubility of the sulphates

• The sulphates become less soluble down the group. Therefore, the solubility of the sulphates in group II decreases down the group.

• For example, magnesium and dilute sulfuric acid, produces hydrogen gas and a colorless solution of magnesium sulfate. On the other hand, barium sulfate exists as a white precipitate in solutions. The ready formation of a precipitate indicates that barium sulfate is insoluble.

• NB If the dissolving process is exothermic, then the sulphate will be soluble. Eg magnesium sulphate. However, if the dissolving sulphate is endothermic, it will be sparingly soluble in water. Eg calcium sulphate.

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

• Magnesium(s) + Sulphuric acid(aq) = MagnesiumSulphate(aq) +Hydrogen gas(g)

Main processes

Table showing some enthalpies of Group II sulphates

some enthalpies of group II elements

Solubility of Carbonates

• Solubility of the carbonates decreases down the group similarly to that of the sulphates.

• The trend to lower solubility is however broken at the bottom of the Group. This is because barium carbonate is slightly more soluble than strontium carbonate.

Solubility of Hydroxides

• The solubility of the hydroxides increases down the group.
• This is because the hydroxide ion is not as large as the sulphate or carbonate ions and thus, the lattice energy decreases significantly down the group

Variation in solubility

increase in solubility

decrease in solubility

SECTION 04

Thermal Stability

nitrates

All the nitrates in this Group undergo thermal decomposition to give the metal oxide, nitrogen dioxide and oxygen.

NB The decomposition of a nitrate involves breaking down the larger nitrate compound into a smaller more stable oxide compound.

• Decomposition of the nitrate involves breaking down the larger nitrate compound into a smaller more stable oxide compound.

• This is promoted by the polarizing effect of the cation that attracts the electron cloud of the nitrate ion.

• Smaller highly charged cations are more polarizing and tend to attract the

electron cloud of the nitrate ion enough to weaken the structure and break the bonds.

• On going down the group, the cations get larger and the charge remains

the same, so that there is less polarizing effect of the cation and the compound becomes more stable.

Thermal decomposition of magnesium nitrate

Magnesium and calcium nitrates normally have water of crystallisation, and the solid may dissolve in its own water of crystallisation to make a colourless solution before it starts to decompose. As you go down the Group, the nitrates also have to be heated more strongly before they will decompose.The nitrates also become more stable to heat as you go down the Group.

• (Mg(NO3)2

Carbonates

• All the carbonates in Group II undergo thermal decomposition to give the metal oxide and carbon dioxide gas

• As you go down the Group, the carbonates have to be heated more strongly before they will decompose. The carbonates become more stable to heat as you go down the Group.

All of these carbonates are white solids, and the oxides that are produced are also white solids.

• Magnesium Carbonate(s) = Magnesium Oxide(s) + Carbon Dioxide(g)

thermal decomposition of magnesium carbonate

SECTION 05

Uses of group II elements and their compounds

magnesium

raw solid

liquid

ribbon

The manufacturing of car parts and aircrafts- This is because magnesium can make lightweight alloys with high tensile strength

Magnesium oxide is used to make refractory lining in furnaces because of its high melting point and low reactivity.

Magnesium is also used as a medication to relieve heartburn and acid indigestion since it is basic. In addition, it can be used as a mild laxative.

Beryllium

Strontium

Calcium

Calcium carbonate is used in the construction industry as a building material such as marble.Calcium carbonate, calcium oxide and calcium hydroxide are used in agriculture to counteract soil acidity

Strontium is best known for the brilliant reds its salts give to fireworks and flares. It is also used in producing refining zinc.

Beryllium is used in gears and cogs. It is used in alloys with copper or nickel to make gyroscopes, springs, electrical contacts and non-sparking tools.

bARIUM

• Barium compounds are used by the oil and gas industries to make drilling mud.

• They are used to make paint, bricks, tiles, glass, and rubber.

• Barium nitrate, similarly to calcium and strontium, are used in fireworks and flares. It gives fireworks a green colour.

SALT of STRONTIUM AND BARIUM EMITTING BRILIANT FLAMES IN FIREWORKS

team

Shakonia Meade

Shania Thomas

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