periodicity Flashcards
across a period
- extra electrons are added to the same electron shell
- there’s a similar amount of electron shielding of the nuclear charge, as electrons are being added to the same electron shell
- the nuclear charge increases
- the atomic radii decreases as electrons are being added to the same shell & the nuclear charge increases which pulls the outer electrons closer to the nucleus
- therefore the overall the attraction between nucleus and outer electrons increase
down a group
- extra electrons shells are added that are further from the nucleus
- there’s more shells between the outer electrons and the nucleus leading to the greater shielding of the nuclear charge
- the nuclear charge increases
- the atomic radii increases as the nuclear charge increases the outer electron is in a new shell much further away
- therefore the overall the attraction between nucleus and outer electrons decrease
melting points across period 3
metals
silicon
simple covalent
metals - more outer shell electrons to delocalise gives stronger bonds
silicon - giant covalent so many strong bonds to break
simple covalent - van der wall forces so weak , slightly stronger forces when more electrons are in the molecule
period 3 - first ionisation energy
general trend - increase
(same reason as across the period)
group 3 & 6 - decrease, due to sub-shell filling
group 2 - trend in atomic radius
the radius of group 2 elements increased down the group as
- more shells
-more shielding
- less attraction of e- to nucleus
- (despite increase in nuclear charge)
group 2
first ionisation energy
the first ionisation energy of group 2 elements decreases down the group because
- more shells
-more shielding
- less attraction from e- to nucleus
group 2
melting points
melting points generally decreases because
- more shells
- large ionic size
- same number of electrons delocalised (2 from each atom)
- so attraction is reduced
group 2, what is the reactivity when you go down the group
the elements become more reactive because outer electrons are held less strongly
group 2 reacting with water equation
group 2 metal + water -> metal hydroxide + hydrogen
what happens when magnesium reacts with water
with steam, the oxide is formed instead as the heat makes the magnesium hydroxide unstable and decomposes to oxide
group 2 hydroxides solubility
solubility increase down a group. Ba(OH)2 is more soluble so more of it dissolves. therefore there are more OH- ions are released into the solution, making it a higher pH, (more alkaline)
solubility of sulphates
solubility of sulphates decrease down the group with barium sulphate being insoluble
testing for metal ions
adding sodium hydroxide to a solution continuing group 2 metal ions give a white precipitate
using barium salts as a test for a sulphate
method
to 1cm3 of the unknown solution
1) add 1cm3 of dilute HCL
2) add 1cm3 of aq barium chloride
3) white thick precipitate is formed then sulphate ions are present
why do you use BaSO4, to test for sulphates
its the least soluble so the precipitate would be the most visible
why is Hcl added during the BaSO4 experiment
it would remove any other ions e.g. carbonate ions which may affect the test
use of group 2 elements and their compounds:
magnesium
- used in the extraction of titanium
- aerospace
- marine
- motor vehicle industries ( extremely high corrosion resistance)
use of group 2 elements and their compounds:
magnesium hydorxide and barium sulphate
medicine
use of group 2 elements and their compounds:
calcium hydroxide
agriculture
use of group 2 elements and their compounds:
calcium oxide
remove SO2 from fule gases
use of group 2 elements and their compounds:
barium chloride
tests for sulphate ions
oxidation definition
gain of oxygen or loss of electrons or loss of hydrogen
reduction definition
loss of oxygen or gain of electrons
redox definition
reaction where oxidation and reduction occurs
oxidation state definition
number of electrons which are involved in bonding
metals oxidation state
- have positive values in compounds
- value is usually that of the group number
oxidation number rule
uncombined element (not in a compound)
0
oxidation number rule:
monoatomic ions
charge on ion
oxidation number rule
group 1 element
+1
oxidation number rule
group 2 element
+2
oxidation number rule
aluminium
+3
oxidation number rule
hydrogen
0
oxidation number rule
oxygen
-2
for F2O it will be +2
oxidation number rule
fluorine
-1
always
oxidation number rule
halogens except fluorine (Cl, Br, I)
-1
except when bonded to O, N, F - +1
oxidation number rule
polyatomic ions
sum of oxidation numbers in the ions charge
whats an oxidising agent
a reagent that oxidises (takes electrons from) another species
whats a reducing agent
a reagent that reduce (adds electrons to) another species
what is a disproportionation
when an elements has been oxidised and reduced
what is an ionic equation
without spectator ions (just ions/atoms reacting)
what is a half equation
only species being oxidised /reduced is shown
reaction of solid halides with H2SO4
F
NaF + H2SO4 -> NaHSO4 + HF
(HF - misty fumes)
reaction of solid halides with H2SO4
Cl
NaCl + H2SO4 -> NaHSO4 + HCl
(HCL - colourless gas)
reaction of solid halides with H2SO4
Br
NaBr + H2SO4 -> NaHSO4 + HBr
(HBr - acidic gas)
2HBr + H2SO4 -> Br2 + SO2 + 2H2O
reaction of solid halides with H2SO4
I
NaI + H2SO4 -> NaHSO4 + HI
(HI - acidic litmus)
2HI + H2SO4 -> I2 + SO2 + 2H2O
(I2 - brown aq solution when water is added, purple gas/ black solid)
6HI + H2SO4 -> 3I2 + S + 4H2O
(S - yellow)
8HI + H2SO4 -> 4I2 + H2S + 4H2O
(H2S - eggy smell, test with lead ethanoate/ bromide paper)
what are halides
reducing agents. down a group they have stronger reducing agents
Testing for halides - AgNO3, method
- make a solution of the halide
- acidify with dilute nitric acid - to prevent the precipitation of other salts
- add a few drops of silver nitrate solution
- treat any precipitate with dilute ammonia solution
- if a precipitate still exists, add concentrated ammonia solution
Testing for halides - AgNO3
why can’t this test, test for fluoride ions
as silver fluoride dissolves in water
Testing for halides - AgNO3
colours of precipitate
- white precipitate- silver chloride AgCl
- cream precipitate - silver bromide, AgBr
- yellow precipitate - silver iodide, AgI
Testing for halides - AgNO3
add dilute ammonia
- soluble - silver chloride AgCl
- insoluble - silver bromide, AgBr
- insoluble - silver iodide, AgI
Testing for halides - AgNO3
add concentrated ammonia
- soluble - silver chloride AgCl
- soluble - silver bromide, AgBr
- insoluble - silver iodide, AgI
halide precipitate equations
Ag+(aq) + X-(aq) -> Ag+X-(s)
when they dissolve in ammonia a colourless diamine complex is formed [Ag(NH3)2]+(aq)
OH-
Test for hydroxide ions: AQ solution
- Test a 1cm depth of solution in a test tube with red litmus paper or universal indicator paper
- record your observations. dispose of the test tube contents
- sodium hydroxide will turn damp red litmus paper blue
OH-
Test for hydroxide ions: ammonia (OH- ions forms in contact with water)
- take 5 drops of 1.0 moldm-3 ammonia solution and place on a filter paper and place inside a petri dish with lid
- dampen a piece of red litmus paper with distilled water and place on the other side of the petri dish
- replace the lid and observe over a few minuets
- ammonia solution vapours will turn damp red litmus paper blue
OH-
Test for hydroxide ions: ammonia
equation
NH3 + H20 -> NH4+ + OH-
this is alkali in aq solution it makes an OH- ions
SO4-2
testing for sulfate ions
- acidify the sample dilute HCL and then add a few drops of aqueous barium chloride
- if a sulfate is present then a white precipitate of barium sulfate is formed:
Ba2+(aq) + SO42-(aq) -> BaSO4(s)
BaSO4 is an insoluble precipitate
CO3 -
method for carbonate ions
- react carbonate with acid first. then test for CO2 product
- test CO2 in limewater
- if Co2 is present CaCO3. precipitate formed
define the term RAM
the average mass of an atom ofan element
compared to 1/12th of the mass of at atom of carbon 12
why the melting point of magnesium is higher than the melting point of sodium
- mg has more electrons than Na
- stronger attraction due to the delocalised sea of electrons
why is chlorine added to water even though its toxic
give an equation chlorine and cold water
- disinfects water
- health benefits outweigh the risks
Cl2 + H20 <=> HCL + HCLO
equation for the extraction of titanium using magnesium and what’s the role of magnesium
2Mg + TiCl4 -> 2MgCl2 + Ti
reducing agent, via a displacement reaction
equation for Cr2O72-
Cr2O72- + 6e- + 14H+ -> 2Cr3+ + 7H2O
giant covalent structures
- graphite
- diamond
- silicon
- silicon dioxide
these all have high melting points due to the strong covalent bonds
household bleach
chlorine reacts with cold aq sodium hydroxide
2NaOH(aq) + Cl2 -> NaCl(aq) + NaOCl(aq) + H20
medical use for magnesium hydroxide
indigestion relief
observation and equation when magnesium reacts with steam
bright white light
Mg (s) + H2O -> MgO(s) + H2(g)
equation for the reaction of drinking water with chlorine
Cl2 + 2H2O -> 2HCL + HClO
kills bacteria
effect of student rinsing the burette with deionised water before filling with sodium hydroxide
- increase final titre volume
- because sodium hydroxide solution would be more dilute
which element period 3 has the highest melting point
Aluminium
silicon
Sodium
Sulfur
silicon
which ion has the largest radius
F-
Mg2+
Na+
O2-
O2-
what is barium sulphate used for
medical tracer that allow internal organs to be traced
fule gas removal
calcium oxide reacts with sulphur dioxide to remove it from factory pollutant
CaO + 2H2O + SO2 -> CaSO3 + 2H2O
what will chlorine Cl2 displace
Br-
I-
what will bromine Br displace
I-
what will iodine displace
nothing
chlorine reacting with cold water
Cl2 + H2O -> ClO- + Cl- + 2H+
what happens to chlorine infront of UV light
2Cl2 + 2H2O -> 4HCL + O2
flame test for group 2
Ca2+
brick red
flame test for group 2
Sr2+
red
flame test for group 2
Ba2+
pale green
what happens when sodium chlorate is added to colourless solution of potassium iodide
- goes brown
- due to iodine
- because I- is oxidised
explain why CBr4 is not a polar molecule
- its symmetrical
- so dipoles cancel out
