Chapter 8 Flashcards
Group two redox reactions and reactivity
( no compounds)
- reducing agents
- redox reactions with oxygen
- redox reactions with water
- redox reactions with dilute acids
Group 2 Reducing agents
2 outer-shell electrons in s sub-shell
–> metal atom is oxidised and loses two electrons to form a 2+ ion with elec. configuration of a noble gas
Ca –> Ca2+ + 2e-
Another species gains the two electrons and is reduced
Group two redox reactions with oxygen
forms a metal oxide with formula MO (M2+ & O2-)
–> Mg burns with brilliant white light to form Mg oxide
2Mg(s) + O2(g) –> 2MgO(s)
Oxidation no. change=
0 –> +2 in Mg (+4 oxidation)
0 —> -2 in O2 (-4 reduction)
Group 2 redox reactions with water
forms alkaline hydroxide, with general formula M(OH)2 & hydrogen gas
–> Mg+ H2O is slow but reactions get more vigorous as you go down group 2
Group 2 redox reactions with water EXAMPLE
Sr(s) + 2H2O(l) –> Sr(OH)2 (aq) + H2 (g)
Oxidation no. change=
O –> +2 in Sr (oxidation)
+1 –> 0 in H (reduction)
NOT ALL hydrogen is reduced, 2 decrease by 1 to form H2 but two do not change X(OH)2
Group 2 redox reactions with dilute acids
Metal + dilute acid –> salt + hydrogen gas (reactivity increases as you go down)
Mg(s) + HCl(aq) –> MgCl2(aq) + H2(g)
Oxidation no. change=
0 –> +2 in Mg (oxidation)
+1 –> 0 in H (reduction)
Trend in Group 2 reactivity and ionisation energy
reactivity increases as:
–> atoms in G2 must lose electrons to form +2 ions
–> this requires two ionisation energies which decreases as attraction between the nucleus and the outer electrons decreases (due to increasing atomic radius and increased shielding)
Reactions of Group 2 compounds
Group 2 oxides with water
Group 2 compounds as bases (agriculture)
Group 2 compounds in medicine
Group 2 oxides + water
releases OH- ions to form alkaline solutions of metal hydroxides
CaO(s) + H2O(l) –> Ca2+ (aq) + 2OH- (aq)
G2 hydroxides are only slightly soluble in water (once saturated, any further metal and hydroxide ions form a precipitate
Ca2+ (aq) + 2OH- (aq) –> Ca(OH)2 (s)
solubility of hydroxides
& test
solubility increases as you go down G2 (more alkaline solutions= higher pH)
- Add spatula of each G2 oxide to water in a test tube
- Shake mixture (not enough water to dissolve fully so precipitate forms: white solid)
- Measure pH of solution and alkalinity will increase down G2
Group 2 compounds in agriculture
Calcium hydroxide Ca(OH)2 is added to fields as lime to increase pH of acidic soil
Ca(OH)2 (s) +2H+ (aq) –> Ca2+ (aq) + 2H2O(l)
Group 2 compounds in medicine
antacids for treating acid indigestion (many contain Mg or Ca carbonates whilst ‘milk of magnesia’ isa suspension of white magnesium hydroxide in water
Mg(OH)2 (s) + 2HCl (aq) –> MgCl2(aq) +2H2O(l)
CaCO3(s) +2HCl (aq) –> CaCl2(aq) +H2O(l) + CO2(g)
Group 7 halogens
all found as stable halide ions dissolved in sea water/ combined with sodium or potassium as solid deposits
G7 Trends in boiling point
- why?
At RTP, all halogens exist as diatomic molecules X2
–> As you go down G7, state changes from gas to solid.
More electrons= stronger London forces= more energy required to break intermolecular forces= boiling point increases
G7 appearance and state at RTP
Fluorine: pale yellow gas
Chlorine: pale green gas
Bromine: red brown liquid
Iodine: shiny grey-black solid that sublimes into a purple vapour
Astatine: never seen
G7 Halogen redox reactions
n s2p5 (halogens need one electron to complete their outermost electron shell
–> each halogen atom is reduced, forming a 1- halide ion
Cl2 + 2e- –> 2Cl-
Halogen is oxidising agent
Halogen-halide displacement reactions
Reactivity of G7 decrease as you go down
1. Solution of each Halogen is added to aqueous solutions of other halides (Cl2 added to Br- & I- ions
–> reaction takes place as halogen displaces halide ion
Cl, Br, I in water (colour)
Cl2: pale green
Br2: orange
I2: brown
why is cyclohexane added to haldie solution
organic non-polar solvent that the halogens can dissolve into to show their colours better
Cl, Br, I in cyclohexane (colour)
Cl2: pale green
Br2: orange
I2: violet/lilac (depends on conc.)
Chlorine reacting with bromide ions
Cl2 (aq) + 2NaBr(aq) –> 2NaCl(aq) + Br2(aq)
ionic: Cl2 + 2Br- –> 2Cl- + Br2
chlorine is reduced to chloride whilst bromide is oxidised to bromine
Trend in reactivity for G7
As you go down G7,
the atomic radius increases= more inner shells so greater shielding= less nuclear attraction to capture an electron from another species= reactivity decreases
Fluorine is strongest
Disproportionation
redox reaction in which same element is both oxidised and reduced
1. chlorine + water
2. Chlorine + cold, dilute aqueous sodium hydroxide
Chlorine + water (disproportionation)
chlorine can be used to disinfect water
When small amounts of Chlorine is added, reaction takes place
Cl2(aq) + H2O(l) –> HClO(aq) + HCl(aq)
0 –> -1 Chlorine is reduced in HCl
0 –> +1 in HClO chlorine is oxidised
Bacteria is killed by chloric (I) acid and chlorate ions ClO- (can be weak bleach)