Unit 2: Section 2 - Group 2 and group 7 elements CDS * Flashcards
Group 2 - the alkaline earth metals uses of the group 2 elements Group 7 - the halogens halide ions tests for ions
how many outer electrons do group 2 elements have?
they all have 2 electrons in their outer shell (s2)
they form 2+ ions
what happens to atomic radius down group 2?
atomic radius increases
more electron shells as you go down group
what happens to 1st ionisation energy down group 2?
more electron shells more shielding greater atomic radius weaker forces of attraction between outer electron and positive nucleus ionisation energy decreases
what happens to reactivity down group 2?
1st ionisation energy decreases
easier to lose electrons to react
reactivity increases
what is the general trend in melting points down group 2?
metallic structure
ions get bigger, same number of delocalised electrons
greater distance between delocalised electrons and positive nuclei, so weaker forces of attraction
less energy required to break bonds
melting point decreases
what is a metallic structure?
positive ions in crystal structure surrounded by delocalised electrons
how does the trend in melting points down group 2 differ?
magnesium has a different crystal structure (arrangement of the metallic ions changes) so it has the lowest melting point
what happens to oxidation state of group 2 elements when they react?
they are oxidised from an oxidation state of 0 to +2
M —-> M2+ + 2e-
0 +2
what happens when group 2 metals react with water?
a metal hydroxide and hydrogen is produced
they react more readily down the group as ionisation energies decrease
how does each group 2 element react with water?
Be - doesn’t react
Mg - very slowly
Ca - steadily, fizzes and forms white solid
Sr - fairly quickly, fizzes and forms colourless solution
Ba - rapidly, fizzes and forms colourless solution
how soluble are group 2 compounds containing singly charged negative ions (hydroxides)?
e.g. OH-
solubility increases down the group
how soluble are group 2 compounds containing doubly charged negative ions (sulfates)?
e.g. SO4 2-
solubility decreases down the group
what happens to the solubility of group 2 hydroxides down the group?
it increases Mg(OH)2 - insoluble Ca(OH)2 - slightly soluble Sr(OH)2 - soluble Ba(OH)2 - soluble
what happens to the solubility of group 2 sulfates down the group?
it decreases MgSO4 - soluble CaSO4 - slightly soluble SrSO4 - insoluble BaSO4 - insoluble
what is the test for sulfates?
add dilute HCl
if acidified barium chloride (BaCl2) is added to a solution containing sulfate ions then a white precipitate of barium sulfate is formed
Ba 2+(aq) + SO4 2-(aq) —> BaSO4(s)
what are group 2 elements known as?
alkaline earth metals, they are often used for neutralising acids
what common group 2 compounds are used for neutralising acids?
calcium hydroxide (Ca(OH)2) - aka slaked lime, used in agriculture to neutralise acidic soils magnesium hydroxide (Mg(OH)2) - used in some indigestion tablets as an antacid H+(aq) + OH-(aq) --> H2O(l)
what is an antacid?
a substance that neutralises excess stomach acid
what is barium sulfate used for?
barium sulfate is opaque to X-rays.
its used in barium meals to help diagnose problems with the oesophagus, stomach or intestines
barium sulfate coats the tissues, making them show up in x-rays, showing the structure of the organs
its insoluble so doesn’t damage tissue
how is magnesium used to extract titanium?
titanium(IV) oxide (TiO2) is converted to TiCl4, by heating it with carbon in a stream of chlorine gas
it’s purified by fractional distillation
reduced by magnesium in a furnace at almost 100*C
TiCl4(g) + 2Mg(l) –> Ti(s) +2MgCl2(l)
what is wet scrubbing?
the process by which acidic sulfur dioxide can be removed from flue gases by reacting with an alkali.
how can calcium be used to remove sulfur dioxide?
powdered calcium oxide and calcium carbonate can be used in wet scrubbing.
a slurry is made by mixing CaO or CaCO3 with water. its then sprayed onto the flue gases, producing calcium sulphite
what is the reaction between calcium oxide or carbonate with sulfur dioxide in flue gases?
CaO(s) + 2H2O(l) + SO2(g) —-> CaSO3(s) + 2H2O(l)
CaCO3(s) + 2H2O(l) + SO2(g) —-> CaSO3(s) + 2H2O(l) + CO2(g)
what colour is gaseous fluorine?
pale yellow
what colour is gaseous chlorine?
green
what colour is liquid bromine?
orange/red-brown
what colour is solid iodine?
grey
what is fluorine’s physical state?
gas
what is chlorine’s physical state?
gas
what is bromine’s physical state?
liquid
what is iodine’s physical state?
solid
what happens to the boiling point of elements down group 7?
they increase
the size and relative mass of the molecules increases.
so more electrons so van der Waals forces increase
what happens to the electronegativity of elements down group 7?
it decreases
halogens are highly electronegative
larger atoms have more shielding and have a larger atomic radius so have weaker forces of attraction between the nucleus and the shared electron pair
what is electronegativity?
the tendency of an atom to attract a bonding pair of electrons
what happens to reactivity down group 7?
it decreases
atoms are larger
outer shell further from the nucleus so weaker FOA
what happens to oxidising ability down group 7?
it decreases
atoms are larger
more shielding
harder to gain electrons
what is the rule for how halogens displace?
a halogen will displace a halide from solution if the halide is below it in the periodic table
how can displacement reactions be used to identify which halogen is present in a solution?
if a more reactive halogen displaces another, the halogen that is displaced is then present, and whatever colour it is will be the colour observed
e.g. colourless KBR reacts with Cl2 to form KCl and Br which will form an orange solution
what happens when you mix chlorine with cold, dilute, aqueous sodium hydroxide?
you get sodium chlorate(I) solution (NaClO) which is bleach
2NaOH(aq) + Cl2(g) –> NaClO(aq) +NaCl(aq) +H2O(l)
oxidaton state of chlorine is both higher and lower - disproportionation
what is bleach used for?
water treatment
to bleach paper and textiles
cleaning, killing bacteria
what happens when chlorine is mixed with water?
it undergoes disproportionation, and the product consists of a mixture of chloride and chlorate(I) ions
chlorine can decompose water in sunlight to form chloride ions and oxygen
Cl2 + H2O HCl +HClO
how does chlorine kill bacteria in water?
chlorate(I) ions kill bacteria. so adding chlorine to water can make it safe to drink or swim in, but chlorine is toxic
how is chlorine an important part of water treatment?
it kills disease-causing microorganisms
some chlorine persists in water and prevents reinfection
prevents growth of algae, eliminating bad tastes and smells
removes discolouration caused by organic compounds
what risks are involved in using chlorine to treat water?
chlorine gas is very harmful
water reacts with organic compounds in water to form chlorinated hydrocarbons
how is chlorine gas harmful?
irritates respiratory system if breathed in
liquid chlorine on skin or eyes causes severe chemical burns
how are chlorinated hydrocarbons harmful?
many are carcinogenic, but the increased risk of cancer is small compared to risks from untreated water
what happens to the reducing power of halides down the group?
increases
ions get bigger, electrons further away from positive nucleus
extra inner electron shells, so more shielding
weaker FOA, easier to lose electrons to act as a reducing agent
what happens when NaF or NaCl reacts with H2SO4?
NaHSO/ Na2SO4 formed
HF or HCl is formed
misty fumes are observed
not a redox reaction
what happens when NaBr reacts with H2SO4?
HBr formed misty fumes observed HBr is a strong reducing agent so reacts with H2SO4 in a redox reaction SO2, choking fumes produced orange Br2 fumes produced
what happens when NaI reacts with H2SO4?
misty HI fumes produced HI reduces H2SO4 choking SO2 and purple I2 fumes produced HI reduces SO2 H2S formed - toxic, smells of rotten eggs solid iodine formed
what is the test for halides?
dissolve in water
add dilute nitric acid to remove ions that might interfere with test
add few drops of silver nitrate solution (AgNO3) silver halide precipitate formed
add ammonia solution (NH3), each silver halide has different solubilities in ammonia
what coloured precipitates do each halide produce in test for halides with AgNO3?
fluoride - no precipitate
chloride - white precipitate
bromide - cream precipitate
iodide - yellow precipitate
what happens to different silver halides in ammonia?
chloride - white precipitate dissolves in dilute NH3
bromide - cream precipitate dissolves in concentrated NH3
iodide - yellow precipitate is insoluble in NH3
how to use flame tests to identify group 2 ions?
dip a nichrome wire loop in concentrated hydrochloric acid
dip wire loop into unknown compound
hold loop in the clear blue part of a Bunsen burner flame
observe colour change
what colour flame do different group 2 ions burn?
calcium - brick red
strontium - red
barium - pale green
how can you use NaOH to identify group 2 ions?
dissolve in water
add NaOH dropwise to a test tube containing the metal ion solution
observe the precipitate that forms
keep adding NaOH until it is excess, record any changes you see
what precipitates form from mixing halide ions with NaOH?
magnesium (Mg(OH)2) - slight white precipitate, white precipitate with excess
calcium (Ca(OH)2) - slight white precipitate, slight white precipitate
strontium (Sr(OH)2) - no change(colourless solution), slight white precipitate
barium (Ba(OH)2) - no change, no change
how can you test for ammonia gas using litmus paper?
ammonia gas is alkaline, so it can be tested for using a damp piece of red litmus paper, it will turn blue if ammonia is present
how can you test for ammonium ions using NaOH?
if hydroxide ions in NaOH are added to a solution containing ammonium ions, ammonia gas and water is produced
NH4+ + OH- –> NH3 +H2O
ammonia can then be tested for
how can you test for sulphates using HCl and barium chloride?
add dilute HCl
add barium chloride solution
if a white precipitate of barium sulfate forms, sulfate was present
how can you test for hydroxides using indicator?
hydroxide ions make solutions alkaline, so a pH indicator should reflect this if hydroxide ions are present
how to test for halides using silver nitrate?
dissolve in water add dilute nitric acid then silver nitrate solution fluorine - colourless solution chlorine forms white precipitate bromide gives cream silver bromide iodide gives yellow silver iodide
how to test for carbonates using HCl?
add dilute HCl
if solution contains carbonate ions it will fizz as carbon dioxide is released
CO3 2- + 2H+ –> CO2 + H2O
then carbon dioxide can be tested for
how to test for carbon dioxide?
using limewater
CO2 turns limewater cloudy
bubble the gas through a test tube of limewater, and observe is limewater goes cloudy
what happens to atomic radius down group 7?
increases
more electron shells
so bigger atoms
what happens to 1st ionisation energy down group 7?
decrease
atoms get bigger
more shielding
weaker attraction from nucleus to electron in outer shell
what colour is chlorine water?
colourless
what colour is bromine water?
orange
what colour is iodine solution?
brown
what are halogens known as?
salt makers e.g. NaCl
How to test for group 2 ions?
using H2SO4
using NaOH
how to test for group 2 ions using H2SO4?
dissolve ions in water
add H2SO4
group 2 sulfates have different solubilities so observe solutions for changes
what precipitates do group 2 sulfate ions form?
MgSO4 - very soluble - colourless solution
CaSO4 - soluble - colourless solution
SrSO4 - slightly soluble - white precipitate
BaSO4 - insoluble - white precipitate
what colour are iodine fumes?
purple
