Presentation on theme: "Title: Lesson 3 Chemical Properties Learning Objectives: –Understand the following trends in reactivity: Alkali metals with water Alkali metals with halogens."— Presentation transcript:
Title: Lesson 3 Chemical Properties Learning Objectives: –Understand the following trends in reactivity: Alkali metals with water Alkali metals with halogens Halogens with halide ions –Complete an experiment to investigate the above
Main Menu Refresh Which species has the largest radius? Do not use the data booklet…work it out! A. Cl – B. K C. Na + D. K +
Main Menu Chemical Trends Members of a group often have very similar reactivity. You probably know that carbon will react with hydrogen to form methane, CH 4 You probably did not know that silicon will also react with hydrogen to form silane, SiH 4 Watch this demonstration to see some silane being madethis demonstration Silane Demo Video
Main Menu Chemical Properties Determined by electron configuration of it’s atoms. Determined by the valence electrons hence, elements in the same group with have similar chemical properties. Group 18: Noble Gases – Colourless, monatomic, very unreactive Inability to lose or gain electrons (no cations as already highest energy ionisation energy, no anions as adding electron to an empty shell would experience negligible nuclear charge force) Stable octet of valence electrons (except Helium = 2) The reactivity of elements in other groups can be explained by their unstable incomplete electron energy levels. They gain or lose electrons to achieve the stable electron configuration of the nearest noble gas.
Main Menu Group 1: The Alkali Metals Silvery metals Too reactive to be found in nature. Stored in oil to prevent contact with air and water.
Observations when added to cold water Li Na K Rb Cs Reactions of Group 1 Elements with Water Floats. Gentle fizzing (H 2 ). Li gradually dissolves to give a colourless solution (LiOH). Final solution has pH12-14. Floats. Vigorous fizzing (H 2 ). Melts and Na rapidly dissolves to give a colourless solution (NaOH). Final solution has pH14. Floats. Very vigorous fizzing (H 2 ) which bursts into lilac- tinged flame and K very rapidly dissolves to give a colourless solution (KOH). Final solution has pH14. Sinks. Explosive release of gas (H 2 ) and Rb instantaneously dissolves to give a colourless solution (RbOH). Final solution has pH14. Sinks. Violently explosive release of gas (H 2 ) and Cs instantaneously dissolves to give a colourless solution (CsOH). Final solution has pH14.
2M(s) + 2H 2 O(l) 2M + + 2OH - (aq) + H 2 (g) Equations – Write showing the disassociated ions Reactivity INCREASES down group Outer electron becomes easier to remove Proton number increases Outer e - is further from the attracting nucleus More shielded from the attracting nucleus “M” represents any of the group 1 metals e.g.1 2Li(s) + 2H 2 O(g) 2Li + OH - (aq)) + H 2 (g) e.g.2 2K(s) + 2H 2 O(g) 2K + OH - (aq)) + H 2 (g) DECREASE IN FIRST IONISATION ENERGY!
Observations when burned in Oxygen Li Na K Rb Cs Reactions of Group 1 Elements with Oxygen Burns very vigorously with dark red flame white lithium oxide, Li 2 O solid Burns very vigorously with brilliant yellow flame white sodium oxide, Na 2 O and sodium peroxide, Na 2 O 2 solids in mixture Burns very vigorously with lilac flame Burns violently with red flame Burns violently with blue flame white potassium peroxide, Na 2 O 2 and potassium superoxide, KO 2 solids in mixture white rubidium superoxide, RbO 2 solid white caesium superoxide, RbO 2 solid Alkali Metals reacting with Oxygen Video 6.02 ONWARDS
4M(s) + O 2 (g) 2M 2 O(s ) General Equation Reactivity INCREASES down group Outer electron becomes easier to remove Proton number increases Outer e - is further from the attracting nucleus More shielded from the attracting nucleus. DECREASE IN FIRST IONISATION ENERGY!
TREND IN OXIDISING POWER OF HALOGENS (a) FURTHER FROM and (b) MORE SHIELDED FROM outer shell TAKES e- LESS readily halogens become LESS oxidising F 2 > Cl 2 > Br 2 > I 2 in oxidizing power (even though number p+ increases). the attracting nuclear charge. Down group VII, outer e- shell of halogen is :
Main Menu Group 17: The Halogens Diatomic molecules, X 2 Very exothermic electron affinities High effective charge of +7 strong pull on electrons from other atoms. Reactivity decreases down the group due to increasing atomic radius and outer electron attraction decreases.
Main Menu Reaction of Halogens with Group 1 Metals Form ionic HALIDES Halogen atoms gains one electrons to form a halide ion X - Resulting ions have the stable octet of the noble gases, e.g. 2Na(s) + Cl 2 2NaCl(s) Electrostatic force of attraction between the oppositely charged Na + and Cl - ions bonds them together.
How is a sodium ion formed? 2.8.1 (partially full outer shell) 11 protons=+11 11 electrons= -11 Total charge= 0 Sodium atom: 11 protons=+11 10 electrons= -10 Total charge= +1 Sodium ion: loses 1 electron + Na [2.8] (Full outer shell)
How is a fluoride ion formed? 2.7 (partially full outer shell) 9 protons=+9 9 electrons= -9 Total charge= 0 Fluorine atom: 9 protons=+9 10 electrons=-10 Total charge= -1 Fluoride ion: - FF gains 1 electron [2.8] (Full outer shell)
Ionic Bonding Between Group 1 and Group 7 Outer electrons moves like a harpoon from Sodium to Fluorine. Held together by the mutual attraction of their opposite charges Alkali Metals reacting with Chlorine Video
Main Menu Displacement Reactions The relative reactivity of the elements can be seen by placing them in direct competition for the extra electron. E.g. Solution changes from colourless to orange/red/brown (bromine has been produced) Chlorine has a stronger attraction for an electron than bromine (smaller atomic radius, greater electron affinity) Chlorine takes the electron from the bromine ion.
X 2 + 2Y - 2X - + Y 2 where X 2 is a general halogen and Y - is a general halide X 2 is REDUCED Y - is OXIDISED Half equation: X 2 + 2e - 2X - Half equation: 2Y - Y 2 + 2e - This trend can be demonstrated by comparing the abilities of the halogens to oxidise halides. OILRIG! OXIDATION IS LOSS, REDUCTION IS GAIN… Which has been reduced, which has been oxidised…?
Complete the Displacement Reactions Practical… Follow the method on your worksheet and fill out the results table shown below…
Complete the Displacement Reactions Practical… Expected Results…
Displacement (redox) reactions of halide ions with halogen molecules in aqueous solution. Water Colourless Cl - (aq) Colourless Br - (aq) Colourless I - (aq) + PALE GREEN aq Cl 2 + RED- BROWN aq Br 2 BROWN aq I 2 Paler green soln Red- brown soln Brown soln Paler red-brown soln Brown soln Paler brown soln positive reactions
The experimental results show: (1)Cl 2 oxidises both Br - and I - (2)Br 2 oxidises only I - (3)I 2 oxidises none of these halides Summary: Halogen (X 2 ) will oxidise halide (Y - ) Ionic equation: X 2 + 2Y - 2X - + Y 2 if X is ABOVE Y in group VII Cl 2 + 2Br - 2Cl - + Br 2 Cl 2 + 2I - 2Cl - + I 2 Br 2 + 2I - 2Br - + I 2
Similarly, the experimental results show: (1)I - reduces both Cl 2 and Br 2 (2)Br - reduces only Cl 2 (3)Cl - reduces none of these halogens Summary: Halide (Y - ) will reduce halogen (X 2 ) Ionic equation: X 2 + 2Y - 2X - + Y 2 if Y is BELOW X in group VII Will astatide (At - ) be more or less reducing than iodide? Will astatine(At 2 ) be more or less oxidising than iodine? MORELESS MORELESS
Main Menu The Halides Halogens form insoluble salts with Silver. Adding a Halide solution to a solution with Silver ions will produce a precipitate. The precipitate can be used to identify the halide ion. Left to right: AgCl, AgBr, AgI
Main Menu Three reactions to know The Group I (alkali) metals react with water as follows: Metal + Water Metal Hydroxide + Hydrogen The Group I (alkali) metals react with halogens (Group VII) as follows: Metal + Halogen Metal Halide Halogens can react with halide ions as follows (using the example of bromide and chlorine): Bromide + Chlorine Chloride + Bromine
Main Menu Investigating chemical trends In this experiment you will investigate trends in the reactions mentioned on the previous slide Follow the instructions hereinstructions here
Main Menu Key Points Alkali metals become more reactive down the group: Due to the outer shell electron becoming increasingly easy to remove Halogens become less reactive down the group: Due to the increased numbers of electron shells (and thus shielding) causing them to attract electrons less strongly