2.7 The periodic table — Group 2

1. 2.7 The periodic table — Group 2 Revision Notes Crowe2012

2. Properties down group 2 a. explain the trend in the first ionization energy down group 2 Both the table and graph above also shows a specific trend in ionisation energies down the group. What is the trend and can you explain it?

3. Properties down group 2 a. explain the trend in the first ionization energy down group 2 Down the group: Ionisation energies decrease down the group There is an increasing effective nuclear charge down the group. However, the atomic radius increases too, so the outermost electrons experience this less strongly. Also, there is an increased shielding effect down the group too.

4. Properties down group 2 Here you can see that there is a massive jump between the second and third ionisation energies a. explain the trend in the first ionization energy down group 2 Successive ionisation energies: two electrons in the outer shell. Losing two electrons leave them with noble gas configurations. So readily form 2+ ions. However, removing the third would mean breaking into a new shell, and so requires great energy.

5. Properties down group 2 b. recall the reaction of the elements in group 2 with oxygen, chlorine and water Reactivity increases down the group. So Barium is the most reactive, and is often stored under oil like the alkali metals, as a result!

6. Properties down group 2 b. recall the reaction of the elements in group 2 with oxygen, chlorine and water Reactions with oxygen Metals burn in oxygen to form a simple metal oxide. CaO is called quick lime and is used in farming to counteract soil acidity! It is made commercially by the thermal decomposition of limestone!

7. Properties down group 2 b. recall the reaction of the elements in group 2 with oxygen, chlorine and water Reaction with Chlorine

8. Properties down group 2 b. recall the reaction of the elements in group 2 with oxygen, chlorine and water Reactions with Water Beryllium has no reaction with water or steam even at red heat – This is due to a thick oxide layer on the surface Magnesium has a very slight reaction with cold water. The reaction soon stops because magnesium hydroxide is almost insoluble in water and forms a barrier preventing further reaction. Magnesium will react with STEAM to form the metal oxide and hydrogen gas. Calcium, strontium and barium all react with cold water with increasing vigour to give the metal hydroxide and hydrogen.

9. Properties down group 2 c. recall the reactions of the oxides of group 2 elements with water and dilute acid, and their hydroxides with dilute acid

10. Properties down group 2 c. recall the reactions of the oxides of group 2 elements with water and dilute acid, and their hydroxides with dilute acid Reaction of Oxides with Hydrochloric/Nitric Acid

11. Properties down group 2 c. recall the reactions of the oxides of group 2 elements with water and dilute acid, and their hydroxides with dilute acid Reaction of Hydroxides with Hydrochloric/Nitric Acid

12. Properties down group 2 d. recall the trends in solubility of the hydroxides and sulfates of group 2 elements SULPHATE solubility DECREASES down the group HYDROXIDE solubility INCREASES down the group

13. Properties down group 2 d. recall the trends in solubility of the hydroxides and sulfates of group 2 elements Solubility is determined by two factors: Lattice dissociation enthalpy (energy needed to break up a crystal lattice) Hydration enthalpy (energy released when ions are hydrated) Ionic size has an effect on these factors. • Solubility of Hydroxides • There is a decrease in lattice dissociation enthalpy down the group. • This outweighs the change in enthalpy of hydration. • As a result, there is an INCREASE in SOLUBILITY down the group. insoluble slightly soluble soluble soluble

14. Properties down group 2 d. recall the trends in solubility of the hydroxides and sulfates of group 2 elements Solubility is determined by two factors: Lattice dissociation enthalpy (energy needed to break up a crystal lattice) Hydration enthalpy (energy released when ions are hydrated) Ionic size has an effect on these factors. Solubility of Sulphates Magnesium and calcium sulphates are soluble. Anion >> Cation: so lattice enthalpy does not vary much down group as cation size changes. However, hydration enthalpy decreases down the group. Therefore, solubility decreases down the group. soluble soluble insoluble insoluble

15. Properties down group 2 e. recall the trends in thermal stability of the nitrates and the carbonates of the elements in groups 1 and 2 and explain these in terms of size and charge of the cations involved

16. Properties down group 2 e. recall the trends in thermal stability of the nitrates and the carbonates of the elements in groups 1 and 2 and explain these in terms of size and charge of the cations involved Thermal Stability of Carbonates Carbonates of Group 1 are thermally stable: Exception is lithium carbonate which decomposes to give the oxide: All group 2 carbonates decompose to form stable oxides. E.g.

17. Properties down group 2 e. recall the trends in thermal stability of the nitrates and the carbonates of the elements in groups 1 and 2 and explain these in terms of size and charge of the cations involved Explanation of stability of carbonates Polarising the carbonate ion • The positive ion attracts the delocalised electrons in the carbonate ion towards itself. • The carbonate ion becomes polarised. • If this is heated, the carbon dioxide breaks free to leave the metal oxide. • The smaller the positive ion is, the higher the charge density, and the greater effect it will have on the carbonate ion. • As the positive ions get bigger as you go down the Group, they have less effect on the carbonate ions near them. • To compensate for that, you have to heat the compound more in order to persuade the carbon dioxide to break free and leave the metal oxide. • In other words, as you go down the Group, the carbonates become more thermally stable. (A similar explanation can be used for the stability of nitrates.)

18. Properties down group 2 f. recall the characteristic flame colours formed by group 1 and 2 compounds and explain their origin in terms of electron transitions • Electrons occupy certain discrete energy levels. • When an electron is promoted from its usual energy level to a higher one, the atom becomes excited. • When the electron drops back down to its ground state (usual energy level), the atoms emits a photon of light in the visible range • Meaning a certain amount of energy that has a wavelength corresponding to a certain colour.

19. Properties down group 2 h. demonstrate an understanding of how to minimise the sources of measurement uncertainty in volumetric analysis and estimate the overall uncertainty in the calculated result. Accuracy Unless an activity instructs students differently, they should assume that readings from equipment and apparatus should be made with the following precision.

20. Properties down group 2 h. demonstrate an understanding of how to minimise the sources of measurement uncertainty in volumetric analysis and estimate the overall uncertainty in the calculated result. Errors Students should appreciate that any piece of equipment (burette, pipette, thermometer, balance) used in a quantitative exercise has an uncertainty associated with its use. Even if the equipment is used carefully, the uncertainty leads to an error in the reading and in the final result.