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
what tests do i need to know
carbonate
sulfate
halide
ammonium
carbonate test
add dilute nitric acid to the solution
- carbonate will react with the acid to form CO2 gas
iff bubbles are formed, it could be CO2
Na2CO3 + 2HNO3 -> 2NaNO3 + CO2 + H2O
- to test if the gas is CO2
-> bubble through lime water - Ca(OH)2(aq)
-> CO2 react to form a white precipitate of CaCO3
CO2 + Ca(OH)2 -> CaCO3 + H2O
sulfate test
- most sulfates are soluble but barium sulfate isn’t BaSO4
- use aqueous barium chloride or barium nitrate (if u are carrying out a halide test use barium nitrate)
Ba2+(aq) + SO4 2- (aq) -> BaSO4 (s)
halide test
most halides are soluble but silver halides arent
1) add aqueous silver nitrate to aqueous solution of the halide
Ag+ + X- -> AgX(s)
2) the silver halide precipitates are different colours. Cl= white Br=cream I= yellow
3) to distinguish between them add aqueous ammonia to test the solubility. AgI is insoluble in NH3
sequence of tests
1) carbonate
2) sulfates
3) halides
CaseOh
1) neither sulfate of halide ions produce bubbles with dilute acid so it can be carried out without the possibility of an incorrect conclusion
2) barium carbonate is also white and insoluble in water so you carry it out after carbonate to ensure there are no carbonate present and don’t get a false positive
3) Ag2CO3 and AG2SO4 are both insoluble in water and form a precipitate. so carry out this test last to ensure there is no carbonate or sulphate to make a false positive
test for ammonium ions
NH4 +
1) added aqueous
sodium hydroxide to a solution of an ammonium ion
2) ammonia gas is produced- no bubbles form as ammonia is very soluble in water
3) warm the mixture and ammonia gas is released
4) test for the ammonia gas by using moist pH indicator paper - it turns paper blue as ammonia is alkaline
