Patterns in the Chemical World

Transcription

1 Topic12 Patterns in the Chemical World Unit 41 Periodic trends in elements and their compounds Unit 42 The transition metals

2 Key C o ncepts Periodic trends in elements and their compounds Periodic variations in bonding and structures across periods 2 and 3 Variations in melting point and electrical conductivity across a period Bonding, composition and acid-base behaviour of oxides of elements across the third period Patterns in the Chemical World The transition metals Coloured compounds of transition metals Variable oxidation states of transition metals Transition metals and their compounds as catalysts

3 Topic 12 Patterns in the Chemical World Unit 41 Periodic trends in elements and their compounds Unit 41 Periodic trends in elements and their compounds 41.1 The modern periodic table 41.2 Categories of elements in the modern periodic table 41.3 Periodic variations in bonding and structures of elements across periods 2 and Variation in atomic radius of elements across a period 41.5 Variation in melting point of elements across the third period 41.6 Variation in electrical conductivity of elements across the third period 41.7 Variation of density of elements across a period 41.8 Reaction of elements in Period 3 with water 41.9 Composition of the oxides of Period 3 elements Acid-base nature of oxides of Period 3 elements Summary 1 The following table summarizes the variation in nature of bonding, structure and physical properties of elements across periods 2 and 3. Group I II III IV V VI VII 0 Elements in Period 2 element structure Nature of bonding Melting point ( C) Electrical conductivity Li Be B C (diamond) metals metalloid non-metals N O F Ne giant metallic giant covalent simple molecular metallic covalent weak van der Waals forces between molecules; strong covalent bonds holding atoms together within the molecule (except Ne) good conductors semiconductor non-conductors Atomic radius Elements in Period 3 element structure decrease Na Mg Al Si P S Cl Ar metals metalloid non-metals giant metallic giant covalent simple molecular Nature of bonding metallic covalent weak van der Waals forces between molecules; strong covalent bonds holding atoms together within the molecule (except Ar) Melting point ( C) Electrical conductivity Atomic radius good conductors semiconductor non-conductors decrease 2 The main trends across a period of elements: a change from metals through metalloid to non-metals; a decrease in atomic radius; a maximum in melting point is shown in the middle of the period; a change from conductors to non-conductors.

4 Topic 12 Patterns in the Chemical World Unit 41 Periodic trends in elements and their compounds 3 The following table summarizes the nature of bonding and the acid-base nature of oxides of Period 3 elements. Group I II III IV V VI VII Element Na Mg Al Si P S Cl Oxide(s) formed Nature of bonding of oxide(s) Acid-base nature of oxide(s) Na 2O MgO Al 2O 3 SiO 2 P 4O 6 P 4O 10 SO 2 SO 3 ionic mainly ionic mainly covalent strongly basic basic amphoteric acidic Cl 2O Cl 2O 7 strongly acidic DO NOT confuse the concepts of effective nuclear charge and screening effect. e.g. Moving across Period 3 of the periodic table (from sodium to argon), protons are added to the nucleus and electrons are added to the outermost shell of the atoms of the elements. The shielding effect due to the inner shell electrons remains almost the same. Hence the effective nuclear charge felt by the outermost shell electrons increases across the period. Outermost shell electrons are pulled closer to the nucleus. So, the atomic radii of elements decrease. Exam tips From lithium to fluorine in the second period of the periodic table, the carbon in the middle has the highest melting point. The elements do NOT show a gradual decrease in melting point. Students should be able to state and explain the trend of melting points of non-metals in Period 3. Phosphorus (white phosphorus) exists as P 4 molecules and sulphur exists as S 8 molecules in their solid states. Atomic radius Melting point (K) Atomic number Na 2 O 2 reacts with water to give H 2 O 2. Cl 2 O 7 reacts with water to give HClO 4. 0 Na Mg Al Si P S Cl Ar Element Example Account for each of the following: a) At room temperature and pressure, Na 2 O is a solid with a very high melting point whereas Cl 2 O is a gas. (2 marks) b) Al 2 O 3 (s) is soluble in both aqueous acids and aqueous alkalis. (3 marks) Answer a) Na 2 O is an ionic solid. A lot of heat is needed to overcome the strong attractions between the cations and anions in the solid. (1) Cl 2 O exists as simple molecules. Weak van der Waals forces exist between the molecules. Little heat is needed to separate the molecules. (1) b) Al 2 O 3 (s) is amphoteric. (1) Al 2 O 3 (s) + 6H + (aq) 2Al 3+ (aq) + 3H 2 O(l) (1) Al 2 O 3 (s) + 2OH (aq) + 3H 2 O(l) 2[Al(OH) 4 ] (aq) (1)

5 Topic 12 Patterns in the Chemical World Unit 42 The transition metals Remarks* Questions often ask about the difference in the reactions of different oxides with water. e.g. Na 2 O reacts with water to give an alkaline solution; SiO 2 has no reaction with water; and SO 2 reacts with water to give an acidic solution. Questions often ask about the amphoteric nature of Al 2 O 3. Unit 42 The transition metals 42.1 Introduction to transition metals 42.2 Coloured compounds of transition metals 42.3 Variable oxidation states of transition metals 42.4 Variable oxidation states of vanadium 42.5 Variable oxidation states of manganese 42.6 Variable oxidation states of iron 42.7 Transition metals and their compounds as catalysts 42.8 Uses of transition metals in medicine

6 10 Topic 12 Patterns in the Chemical World Unit 42 The transition metals Summary 1 Typical chemical properties of transition metals include: formation of coloured compounds; existence in more than one oxidation state in their compounds; and catalytic properties of the elements and their compounds. 2 The following reductions occur when a solution containing dioxovanadium(v) ions (VO 2 + (aq)) is shaken with zinc powder under acidic conditions VO + 2 (aq) VO 2+ (aq) V 3+ (aq) V 2+ (aq) name dioxovanadium(v) ions oxovanadium(iv) ions vanadium(iii) ions vanadium(ii) ions colour yellow blue green violet 3 Permanganate ion is a powerful oxidizing agent. In the presence of dilute sulphuric acid and a reducing agent, permanganate ion is reduced to manganese(ii) ion MnO 4 (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) purple Exam tips colourless (or pale pink) Question may ask students to compare the chemistry between main group metals and transition metals. Similarities Both main group metals and transition metals are metals. They are both lustrous, good conductors of electricity and heat, malleable and ductile. Both main group metals and transition metals form oxides upon standing in air. Differences Compared with metals in the main groups of the same period, the transition metals have higher melting points, boiling points, and density. Moreover, their atomic radii are smaller. Main group metals exhibit only one oxidation state while transition metals exhibit variable oxidation states. Main group metals form colourless compounds while transition metals form coloured compounds. Main group metals do not demonstrate catalytic activity while transition metals do. Example Consider the following ionic half-equations in the electrochemical series: higher in electrochemical series V 3+ (aq) + e V 2+ (aq) VO 2+ (aq) + 2H + (aq) + e VO 2 + (aq) + 2H + (aq) + e O 2 (g) + 4H + (aq) + 4e 2H 2 O(l) lower in electrochemical series Co 3+ (aq) + e Co 2+ (aq) V 3+ (aq) +H 2 O(l) VO 2+ (aq) + H 2 O(l) What would happen if an aqueous solution containing the species in each of the following cases is exposed to air? Explain your answer in each case. a) V 3+ (aq) ions (3 marks) b) Co 2+ (aq) and Co 3+ (aq) ions (3 marks) Answer a) V 3+ (aq) ions will be oxidized to VO 2+ (aq) ions. (1) O 2 (g) is a stronger oxidizing agent than VO 2+ (aq) ion. (1) The following reaction will occur: 4V 3+ (aq) + O 2 (g) + 2H 2 O(l) 4VO 2+ (aq) + 4H + (aq) (1) b) Co 3+ (aq) ions will be reduced to Co 2+ (aq) ions. (1) Co 3+ (aq) ion is a stronger oxidizing agent than O 2 (g). (1) The following reaction will occur: 4Co 3+ (aq) + 2H 2 O(l) 4Co 2+ (aq) + 4H + (aq) + O 2 (g) (1) Remarks* This method of predicting redox reactions that may occur has been discussed in Topic 5 Redox Reactions, Chemical Cells and Electrolysis.