Ch8- Reactivity Trends Flashcards
What are Group 2 metals called in redox reaction?
Reducing Agent
What happens to a Gr2 metal in a redox reaction?
Gr2 metal is oxidised (loses 2 e-) to form a 2+ ion
What happens when Gr 2 metal reacts with oxygen?
Gr 2 metal + oxygen = metal oxide. General formula MO (Metal Oxide- 1 to 1 ratio, M2+ and O2-)
2 Mg (s) + O2 (g) —> 2 MgO (s)
Oxidation state Mg- from 0 in Mg to +2 in MgO (oxidised)
Oxidation state O- from 0 in O2 to -2 in MgO (reduced)
What happens when Gr 2 metals react with water?
Gr 2 metal + water = alkaline hydroxide M (OH)2 and H2 gas.
Reaction becomes more vigorous with metals further down the group- reactivity increases down Gr2.
What happens when Gr2 metals react with dilute acids?
Gr 2 metal + dilute acid = salt and hydrogen gas.
Reactivity increases down Gr2.
Why does reactivity increase down Gr 2?
Atoms of Gr2 elements react by losing 2 e- to form 2+ ions. The formation of 2+ ions from gaseous atoms requires input of 2 ionisation energies.
Ionisation energies decreases down group because the attraction between nucleus and outer electrons decreases as result of increasing atomic radius, increasing shielding and decreasing nuclear attraction.
Total energy input from ionisation energies to form 2+ ions decreases down group.
Gr 2 metals become more reactive and stronger reducing agents down group.
What happens when Gr 2 oxides react with water?
Gr2 oxide + water = metal 2+ ion and OH- ion.
Forms an alkaline solution of metal hydroxide.
Gr 2 hydroxides are only slightly soluble in water- when solution becomes saturated, any further metal and hydroxide ions will form solid precipitate.
Ca2+(aq) + 2OH- —> Ca(OH)2 (s)
Describe the solubility of Gr2 metal hydroxides down the group?
Solubility of hydroxides in water increases down Gr 2, so resulting solutions contain more OH-(aq) ions and are more alkaline.
Solubility, pH and alkalinity increases.
Describe uses of Gr 2 compounds:
Gr 2 metal oxides, hydroxides and carbonates all have ability to neutralise acids = salt + water.
- Agriculture: Ca(OH)2 added to fields as lime by farmers to increase pH of acidic soils. Calcium hydroxide neutralises acid in soil and forms neutral water.
- Ca(OH)2 (s) + 2H+ (aq) –> Ca2+ (aq) + 2H2O
- Medicine: MgCO3 and CaCO3 used in indigestion tablets to neutralise excess hydrochloric acid in stomach.
- Also known as milk of magnesia.
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What is qualitative analysis?
Simple tests where an observation can be used to identify a species.
Obsevations include identification of gases, colour changes, formation of precipitates.
Usually carried out quickly on test tube scale.
Describe the Carbonate Test:
- Add dilute nitric acid to suspected carbonate
- Carbonate ions react with acids to produce CO2 gas so bubbles would be seen
- Bubbling has through lime water would turn it ‘cloudy’
CO3 2- (aq) + 2H+ –> H2O (l) + CO2 (g)
Describe the Sulfate Test when testing for anions:
- Add aqueous barium nitrate to suspected sulfate.
- Barium ions react with sulfate ions to produce white precipitate of barium sulfate.
Ba2+ (aq) + SO42- (aq) –> BaSO4 (s)
Describe the Halide Test:
- Add aqueous silver nitrate to suspected halide.
- Silver ions react with halide ions to produce silver halide precipitate.
Ag+ (aq) + X- —> AgX
Silver Chloride- white precipitate
Silver Bromide- cream precipitate
Silver Iodide- yellow precipitate
adding aqueous ammonia (dilute then concn) helps distingush between white, cream + yellow:
Silver chloride- white precipitate, soluble in dilute NH3
Silver Bromide- cream precipitate, insoluble in dilute NH3 but soluble in concn NH3
Silver Iodide- yellow precipitate, insoluble in both dilute and conc NH3.
Halide test must be carried out after carbonate and sulfuric test as silver carbonate and silver sulfate would produce precipitates.
Describe the test for Ammonium Cations:
- Add aqueous sodium hydroxide to suspected ammonium compound
- Warm gently
- Ammonia is alkaline gas, so test with pH paper and hover above test tube- should turn blue.
NH4+ (aq) + OH- (aq) —> NH3 (g) + H20 (l)
