Module 3: Reactivity Trends (Chapter 8) Flashcards
Why are group 2 known as reducing agents?
Have 2 outer electrons, which fill the outer s2 sub shell. In a reaction the metal atom is oxidised losing 2 electrons to form 2+ ions. Another species gains these 2 electrons, and is said to be reduced.
What is the most common type of reaction with group 2?
Redox reactions
Redox with group 2 metal and oxygen
Form a metal oxide with general formula MO.
Reactivity increases down the group due to increased atomic radius and shielding, so reduced nuclear attraction, meaning outer electrons can be lost more easily, and so form oxides faster
Redox with group 2 and water
Form a metal hydroxide with general formula M(OH)2 and H2
Reactivity increases down the group.
Redox with group 2 and dilute acids
Forms a salt and H2 gas
Reactivity increases down the group
What happens to the ionisation energy for group 2? What does this indicate about reactivity?
Ionisation energy decreases down the group
Reactivity increases down the group
Increased atomic radius and shielding
Decreased nuclear attraction
Less energy required to remove the outer electrons
Group 2 oxides and water reaction
Releases hydroxide ions
Forms alkaline solutions of metal hydroxides
Only slightly soluble in water
Saturated solution leads to the formation of precipitates
What happens to alkalinity down the group, and therefore solubility?
Solubility increases down the group, because they are more strongly alkaline as the metal hydroxides become more stable
High OH- concentration down the group
pH increases and therefore so does alkalinity
Group 2 in agriculture
Ca(OH)2 used as lime powder on acidic soil by farmers, to neutralise the soil.
Ca(OH)2 + 2H+ = Ca2+ + 2H2O
Group 2 in medicine
Used as antacids for treating acid indigestion
Mainly use Mg and Ca carbonates
‘Milk of Magnesia’ is a suspension of white Mg(OH)2 in water
Issue with Mg solubility
Low solubility in water
Neutralisation equations for Mg(OH)2 and CaCO3
Mg(OH)2 + 2HCl = MgCl2 + 2H2O
CaCO3 + 2HCl = CaCl2 + H2O + CO2
What are the halogens known as?
Diatomic molecules
Boiling point trend in the halogens
More electrons down the group
Stronger London forces
More energy needed to overcome strong intermolecular forces
Boiling point increases down the group
What outer shell electronic configuration do the halogens have?
s2p5
What is the most common reaction for halogens?
Redox - each halogen is reduced, gaining 1 electron to form a 1- halide.
Equation for reduction of halogen (Cl as example)
Cl2 + 2e- = 2Cl-
What are halogens known as?
Oxidising agents because they oxidise another species
Trend in reactivity of halogens
Reactivity decreases down the group
Atomic radius increases
Shielding increases
Less nuclear attraction to capture an electron from another species
Colour of halides in water
Cl2 = pale green Br2 = orange I2 = brown
Colour of halides in cyclohexane
Cl2 = pale green Br2 = orange I2 = violet / purple
What proves reactivity decreases down the group?
Displacement reactions
How are bromine and iodine distinguished between when added tp water?
Cyclohexane used or another non-polar solvent. Non-polar halogens dissolve more readily than in water.
Cl and Br colour
Orange - Cl more reactive
Cl and I colour
Purple - Cl more reactive than I
Br in I colour
Purple - Br more reactive than I
Define disproportionation
Redox reaction where the same element is both oxidised and reduced
Chlorine and water reaction
Purifies water
Cl2 + H20 = HClO +HCl
Disproportionation of Cl
Bacteria killed by chloric(I) acid and chlorate ions (ClO-)
What else in chloric(I) acid sued for?
Weak bleach - indicator paper shows this
Chlorine with NaOH
Cold, dilute aqueous sodium hydroxide More chlorine dissolves - not limited by low solubility of chlorine in water Disproportionation Cl2 + 2NaOH = NaClO + NaCl + H2O ClO- ions in high concentration Used as household bleach
Benefits of chlorine
Water treatment to ensure it is safe to drink and bacteria are killed
Prevents compromised drinking against cholera and typhoid
Controlled process
Risks of chlorine use
Extremely toxic gas - respiratory irritant in small concentrations and fatal in high concen.
Can react with organic hydrocarbons like methane from decaying vegetation - chlorinated hydrocarbons form which are carcinogenic
Describe the carbonate test
Add dilute nitric acid to a test tube containing the solution being tested
Effervesence can mean a carbonate
Bubble this through limewater
Equation for carbonate test
Na2CO3 + 2HNO3 = 2NaCO3 + CO2 + H2O
Equation for reaction of limewater with CO2
CO2 + Ca(OH)2 = CaCO3 + H2O
Describe the sulfate test
Most sulfates are soluble in water but BaSO4 is very insoluble. White precipitate of barium sulfate is basis. Barium ions are added to solution.
Barium ions added as barium chloride or barium nitrate.
Barium nitrate if hakide test being completed after
Equation for barium sulfate formation
Ba 2+ + SO4 2- = BaSO4
Describe the halide test
Add aqueous silver nitrate to aqueous solution of halide
Silver halide precipitates at different colours
Add aqueous ammonia to trst solubility of precipitate and better distinguish between colours of halides
Chloride precipitate colour
White
Bromide precipitate colour
Cream
Iodide precipitate colour
Yellow
General equation for silver halide formation
Ag+ + X- = AgX
Chloride and ammonia reaction
Dilute ammonia - soluble colourless solution
Bromide and ammonia reaction
Concentrated ammonia - soluble colourless solution
Iodide and ammonia reaction
Concentrated ammonia - insoluble yellow solution remains
What is the seqeunce of tests?
Carbonate
Sulfate
Halide
Why carbonate test first?
Only one to produce effervesence
Why sulfate test second?
Barium carbonate is white and insoluble as well so the same result will emerge giving a false positive sulfate test. Only do if no carbonate present
Why halide test last?
Silver carbonate and silver sulfate are both insoluble in water and will form precipitates. Rule out possibility of carbonate or sulfate present
Describe the test for ammonia
Warm aqueous sodium hydroxide addded to solution of ammonium ion
Ammonia gas is produce - soluble in water so effervesence not seen
Heat mixture and gas is released
Test using damp indicator paper - alkaline so turns blue
