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)
chlorine+ cold, dilute aqueous sodium hydroxide (disproportionation)
when water contains dissolved sodium hydroxide, much more chlorine dissolves & different reaction takes place
Cl2(aq) + 2NaOH(aq) –> NaClO(aq) + NaCl (aq) + H2O (l)
–> large concentration of chlorate(I) so household bleach
Benefits and risks of chlorine use
Chlorine is an extremely toxic gas/ respiratory irritant in small conc.
Chlorine in drinking water can react with organic hydrocarbons (these can cause cancer)
Without chlorine, quality would be compromised (typhoid and cholera might break out)
Benefits and risks of chlorine use
Chlorine is an extremely toxic gas/ respiratory irritant in small conc.
Chlorine in drinking water can react with organic hydrocarbons (these can cause cancer)
Without chlorine, quality would be compromised (typhoid and cholera might break out)
Testing for halide ions
Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides
Ag+(aq) + X-(aq) –> AgX(s)
Qualitative analysis
relies on simple observations rather than measurements
Test for anions
gas test: carbonate
precipitate test: sulfate / halide tests
Carbonate test
Na2CO3(aq) +2HNO3(aq) –> 2NaNO3(aq) + CO2(g) + H2O(l)
1. Add dilute nitric acid to solid/ solution
2. If bubbles form, unknown compound could be a carbonate
How to prove unknown compound is a carbonate?
- Add dilute nitric acid
- Collect sample gas produced
- Bubble gas through lime water (saturated aqueous solution of Ca(OH)2
CO2 reacts to form a fine, white precipitate of calcium carbonate which turns the lime water cloudy( milky)
CO2(g) +Ca(OH)2 (aq) –> CaCO3(s) + H2O(l)
Sulfate test
BaSO4 is insoluble in water (white precipitate)
–> Barium ions are added to a solution of an unknown compound
Ba2+ (aq) + SO42- (aq) –> BaSO4
–> usually added as barium chloride or barium nitrate (if doing halide test, use barium nitrate)
Halide tests
silver halides are insoluble in water
–> Silver ions react with aqueous halide ions to form precipitates
Ag+(aq) + X- (aq) –> AgX(s)
- Add aqueous silver nitrate AgNO3 to aqueous solution of halide
- Different coloured precipitates are formed
- Add Aqueous ammonia to test the solubility
Halide test results
Chloride: white // soluble in DILUTE NH3 (aq) => clear solution
Bromide: cream // soluble in CONC. NH3 (aq) => clear solution
Iodide: yellow // insoluble in CONC. NH3 (aq) => same colour
Sequence of tests
1.carbonate test
2. sulfate test
3. halide test
Why is the carbonate test first
dilute acid is added to look for effervescence from CO2
–> neither sulfate nor halide ions produce bubbles with dilute acid so test can be carried out without possibility of incorrect conclusion
Why is sulfate test second?
Barium ions added to look for white precipitate of BaSO4
–> Barium carbonate is white and insoluble in water so white precipitate is also formed when carrying out sulfate test
–> must make sure whether or not the substance is a carbonate or not
Why is the halide test last?
Silver nitrate is added to form a precipitate.
Silver carbonate (Ag2CO3) & Silver sulfate (Ag2SO4) are both insoluble in water and form precipitates
What to do if there is a mixture of ions
same sequence of test and on the SAME SOLUTION
Mixture of ions: carbonate test
If bubbling, continue to add dilute nitric acid until bubbling stops (all CO3^2-
–> dilute nitric acid HNO3 should be used (no chloride or sulfate ions)
Mixture of ions: sulfate test
Solution left from carbonate test, add excess of Ba(NO3)2 (aq)
Filter solution to remove barium sulfate
–> if doing halide test, do not use barium chloride (ruins test)
Mixture of ions: Halide test
Add AgNO3 to solution left
–> any precipitate formed must involve only halide ions
Add NH3 to confirm which halide you have
Ammonium cation test
When heated together, aqueous ammonium ions and aqueous hydroxide ions react to form ammonia gas NH3
NH4+ +OH- –> NH3 + H2O
- Aq NaOH is added to solution of ammonium ions
- Ammonia gas is produced (unlikely air bubbles as NH3 is soluble in water)
- Mixture is warmed & gas is released
- Test gas using moist pH indicator paper (alkaline so paper turns blue)
