8 Reactivity trends Flashcards
Why are Group 2 elements called reducing agents?
1) When group 2 elements are oxidised, they lose 2 electrons from their outer shell.
2) Another species will gain these two electrons and be reduced.
3) The group 2 element is called a reducing agent because it has reduced another species.
What is the trend of hydroxides in water down the group?
- Solubility increases
- pH increases
- Alkalinity increases
What is the solubility of Mg(OH)2 in water?
Only slightly soluble, as it has a low OH- concentration and pH of 10
What is the solubility of Ba(OH)2 in water?
Much more soluble, as it has a greater OH- concentration and a pH of 13.
What are some uses of Group 2 compounds in agriculture?
Calcium hydroxide is added to fields as lime by farmers to increase pH in acidic soils. (CaOH2)
What are some uses of Group 2 compounds in medicine?
Group 2 bases are often used as antacids for treating acid indigestion. Mg(OH)2
Why does boiling point increase down group 7 (halogen group)?
- There are more electrons as you go down and so stronger London forces.
- More energy is required to break the intermolecular forces.
- Therefore boiling point increases.
Why are group 7 elements called oxidising agents?
1) Each halogen atom is reduced, gaining one electron to form a 1- halide ion.
2) Another species loses electrons to halogen atoms.
3) The halogen is called an oxidising agent because it has oxidised another species.
What colour is fluorine at RTP?
Pale yellow gas
What colour is chlorine at RTP?
Pale green gas
What colour is bromine at RTP?
Red-brown liquid
What colour is iodine at RTP?
Shiny grey-black solid
Reactivity of halogens…
decreases down the group.
What happens in a halogen-halide displacement reaction?
A solution of each halogen is added to aqueous solutions of the other halides.
E.g a solution of chlorine is added to two aqueous solutions containing bromine and iodine ions
What are the results of the displacement reactions?
- Chlorine displaces bromine (orange colour appears from Br2 formation)
- Chlorine displaces iodine (violet colour appears from I2 formation)
- Bromine displaces I- (violet colour from I2 formation)
- Iodine has not reacted at all.
Why is cyclohexane added?
- An organic non-polar solvent such as cyclohexane is added and the mixture is shaken.
- The non-polar halogens dissolve more readily in cyclohexane than in water.
- In cyclohexane the colours are easier to tell apart.
CHLORINE: PALE GREEN
BROMINE: ORANGE
IODINE: VIOLET
Why does reactivity decrease down group 7?
- Atomic radius increases
- More inner shells so shielding increases
- Less nuclear attracton to capture an electron from another species
- Reactivity decreases
What is a disproportionation reaction?
A redox reaction in which the same element is both oxidised and reduced.
What are the benefits and risks of chlorine use?
BENEFITS: Ensures water is fit to drink and that bacteria is killed
RISKS: Extremely toxic gas and is a respiratory irritant in small concentrations
What is qualitative analysis?
It relies on simple observations rather than measurements.
What is the carbonate test?
1) In a test tube, add dilute nitric acid to the solid and solution to be tested.
2) If you see bubbles, the unknown compound could be a carbonate.
3) To prove that the gas is carbon dioxide, bubble the gas through lime water (calcium hydroxide). Carbon dioxide should react to form a fine white precipitate of calcium hydroxide which turns lime water cloudy.
What is the sulfate test?
- Aqueous barium ions are added to a solution of an unknown compound.
- The formation of a white precipitate of barium sulfate is the basis for the test as it is very insoluble.
What is the halide test?
1) Add aqueous silver nitrate, AgNO3, to an aqueous solution of a halide.
2) The silver halide precipitates are different colours:
CHLORIDE: WHITE
BROMIDE: CREAM
FLUORINE: YELLOW
3) Add aqueous ammonia to test solubility of precipitates.
CHLORIDE: SOLUBLE IN DILUTE NH3
BROMIDE: SOLUBLE IN CONC NH3
IODIDE: INSOLUBLE IN CONC NH3
