3.2 Reactivity Trends Flashcards
What are group 2 metals ?
Are reducing agents that are reactive and do not occur in their element form naturally. They have 2 valence shell electrons, which are In the s-sub shell.
Redox reactions with group 2
Each metal is oxidised, losing 2 electrons to form a 2+ ion.
Eg., Ca = Ca2+ + 2e-
Redox reaction with group 2 and oxygen
Eg., Mg and O
React to form a metal oxide.
Eg., 2Mg + O2 = 2MgO
-Mg is oxidised from 0 to +2
-O is reduced from 0 to -2
Redox reaction with group 2 and water
Eg., Sr and H20
React to form alkaline hydroxide and hydrogen gas.
Reacts slowly but gets more vigorous.
Eg., Sr + 2H2O = Sr(OH)2 + H2
-Sr is oxidised from 0 to +2.
-H is reduced from +1 to 0.
Redox reactions with group 2 and dilute acids
Eg., Mg and HCl
Forms salt and hydrogen gas and the reactivity increases as you go down the group.
Eg., Mg +2HCl = MgCl2 + H2
-Mg is oxidised from 0 to +2.
-H is reduced from +1 to 0.
Reactivity trend in group 2
Reactivity increases as you go down the group as they react by loosing electrons. Attraction between nuclei and valence shell decreases so is more easily lost.
What does formation of 2+ ion require ?
Requires first and second ionisation energy.
Solubility of hydroxides as you go down group 2 ?
Solubility in water increases as you go down the group so solutions contain more OH- ions.
How are redox reactions with group 2 metals used in agriculture ?
Neutralises acidic soil and lakes.
Eg., Ca(OH)2
How are redox reactions with group 2 metals used in medicine ?
Antacids used to aid digestion.
Eg., Mg(OH)2, CaCO3
What are halogens?
Halogens are the most reactive non-metallic group. They do not occur in their natural elemental form but as stable halide ions dissolved in the sea or in salt mines.
Trends in boiling point of halogens
At RTP, all halogens exist as diatomic molecules. They form lattices with simple molecular structures.
BP increases as you go down the group as there are more electrons and London forces so more energy required to break.
Redox reactions with halogens
Each halogen atom is reduced (+1) and another species is oxidised (-1). Halogens are oxidising agents.
Trends in reactivity of halogens
Reactivity decreases down the group.
-Atomic radius increases.
-More inner shells, so more shielding.
-Less nuclear attraction to capture electrons from another species.
Disproportionation reaction
Eg., Cl and H2O
Same element is oxidised AND reduced.
Eg., Cl2 + H2O = HClO + HCl
0 = -1
0 = +1
Why is chlorine toxic ?
Purifies water but is a toxic gas that is a respiratory irritant. Forms chlorinated hydrocarbons which can cause cancer.
Test for halide ions
Precipitation reaction with aqueous silver ions.
Ag+ + X- = AgX
What is qualitative analysis ?
Relies on simple observations rather than measurements.
What is the test for carbonate ?
React it with acid and see if effervescence is produced (CO2)
Eg., Na2CO3 + 2HNO3 = 2NaNO3 + CO2 + H2O
(To prove its CO2, bubble through limewater)
What is the test for sulphates ?
Most are soluble in water but BaSO4 is insoluble.
Eg., Ba2+ +SO4 2- = BaSO4
What’s the test for halide ions ?
Most are soluble in water but silver halides are insoluble. They react to form precipitates.
Eg., Ag+ + X- = AgX
Results for halide ion test
Cl- = white ppt and soluble in dilute NH3.
Br- = cream ppt and soluble in conc NH3.
I- = yellow ppt and soluble in conc NH3.
What is the test for ammonium ions ?
When heated, ammonium ions and hydroxide ions react to form ammonium gas.
Test with damp indicator paper, which turns it blue if ammonia gas is present.
