8 - Reactivity trends Flashcards
are group 2 metals reducing or oxidising agents
Reducing
This is because they have a tendency to lose 2 electrons (from their outer shell) to form positive ions. In chemical reactions, they donate electrons to other species, causing reduction by themselves undergoing oxidation.
Ca-> Ca2+ + 2e Ca is oxidised
another species will gain the 2e-
group 2 and oxygen
undergo a redox reaction with O2 to form a metal oxide - MO
2Mg + O2 –> 2MgO
0 —> +2
0—–> -2
Metal is oxidised
Oxygen is reduced
group 2 metals with water
undergo a redox reaction and form a alkaline hydroxide - M(OH)2 and hydrogen gas
-> Mg and water react slowly but reaction becomes more vigorous down the group as reactivity increases
Sr + 2H2O -> Sr(OH)2 + H2
0 -> +2
+1 -> 0
group 2 and dilute acids
a redox reaction
metal + acid -> salt + hydrogen
- reactivity increases down the go[
Mg + HCl -> MgCl2 + H2
0 -> +2
+1 -> 0
trend in reactivity and ionisation energy
As you move down the group, the atomic radius increases due to additional electron shells.
- the shielding effect from inner electron shells increases, reducing the effective nuclear charge felt by outer electrons.
- the outer electrons experience less nuclear attraction
- leading to a decrease in ionization energy as less energy is needed to remove the e-
- enhanced reactivity as the outer electrons are more easily lost in chemical reactions.
Group 2 oxides and water
CaO + H2O –> Ca2+(aq) + OH-(aq)
- releases oh- and forms an alkaline solution of the metal hydroxide
- group 2 hydroxides are only slightly soluble so any when the solution become saturated, any additional metal and OH ions form a solid precipitate
trend in solubility of Group 2 hydroxides
- solubility of hydroxides in water increases down the group, so the solution contains more OH- and is more alkaline
- Mg(OH)2 is slightly soluble so pH=10
- Ba(OH)2 is more soluble so pH=13
experiment to show trend in solubility of group 2 hydroxides
1- add MO to water
2- shake mixture and a saturated solution should form so a solid precipitate form
3- measure the pH
group 2 compounds in agriculture
Ca(OH)s is added to fields to increase the pH of acidic soils
- in the form of white lime
Ca(OH)2 + 2H+ -> Ca2+ 2H2O
group 2 compounds in medicine
- often used as antacids to treat indigestion
- contain magnesium and calcium carbonates
- and milk of magnesia a suspension of white magnesium hydroxide
show the reactions that take place of group 2 compounds in indigestion tablets
Mg(OH)2 + 2HCl -> MgCl2 + 2H2O
CaCO3 + 2HCl -> CaCl + H2O + CO2
trend in Tb of halogens
- more electrons
- stronger induced dipole-dipole interactions
- more energy needed to break IF
- boiling point increases down the group
are halogens reducing or oxidising agents
oxidising agents
-> 7 3- in outer shells so in reactions they gain one electron to form 1- ions
Cl2 + 2e- -> 2Cl-
halogen halide displacement reactions
- displacement reactions sow that reactivity of halogens decreases down the group
- a solution of each halide is added to aqueos solutions of the other halides
- if a more reactive halogen is added to a less reactive halide then a reaction takes place- the halogen displaces the halide from solution and it changes colour
colours of Cl2, Br2 and I2 in water and cyclohexane
water
Cl2 = pale green
Br2 = orange
I2 = brown
Cyclohexane (top layer)
Cl2 = pale green
Br2 = orange
I2 - violet
what solutions of group 7 compounds are hard to tell apart in displacement reactions
-> how do you get over this
solutions of iodine and bromine in water are hard to tell apart as they can appear a similar orange-brown colour
- add a non-polar organic solvent like cyclohexane and the mixture is shaken
- the non-polar halogens dissolve more readily in cyclohexane than water - so colours are more easier to tell apart
Cl2 added Br-(aq)
pale green -> orange
Cl2 added to I- (aq)
pale green -> violet
Br2 added to I-(aq)
orange -> violet
trend in reactivity of the halogens
- atomic radius increases
- more inner shells so shielding increases
- less nuclear charge so to capture an e-
- so reactivity decreases down a group
what is disproportionation
a redox reaction where the same element is oxidised and reduced
e.e reation of water and cold dilute sodium hydroxide
reaction of chlorine and water
disproportion reaction takes place
Cl2(aq) + H2O -> HClO(aq) + HCl(aq)
0 -> +1 -1
- chlorine is used as a disinfectant as a water treatment
- ClO- kills the bacteria
reaction of chlorine and cold, dilute aqueous sodium hydroxide
disproportiojnation reaction
Cl2 + 2NaOH -> NaClO + NaCl + H2O
0 -> +1 -1
- reaction of water and chlorine is limited by its low solubility so if the water contains dissolved NaOH much more chlorine is dissolved
- so there is a large number of ClO- formed which is useful for household bleach
:) and :( of using chlorine
:)
- water is fit to drink as bacteria is killed
-
:(
- chlorine is a toxic gas#- a respiratory irritant where large [] can be fatal
- Cl in drinking water can react with organic hydrocarbons (from decaying vegetation). it forms chlorinated hydrocarbons which are carcinogenic
