Module 3 - [ch8] (reactivity trends) Flashcards
what are the group 2 elements also known as
alkaline earth metals
what is the most common reaction of group2 elements
redox
loses its two outermost electrons and be reduced
acts as a reducing agent
group2 metal + oxygen
metal oxide
group2 metal + water
alkaline hydroxide + hydrogen
M(OH)2 + H2
how does reactivity of the group2 metals change going down the group
becomes more vigorous down the group
reactivity increases down the group
metal + acid
salt + hydrogen
why does reactivity increase down group 2
the atoms of group2 lose 2 electrons to form 2+ ions
ionisation energy decreases down the group
due to increases atomic radius, electron shielding
less energy needed for reaction to occur down the group
group2 oxide + water
release hydroxide ions (OH~)
only slightly soluble in water
when solution becomes saturated
Ca+(aq) + OH~(aq) -> Ca(OH)2(s)
solid precipitate formed
solubility of hydroxides in water
increases down the group
down group contain more OH~ (aq) ions and are more alkaline, pH increases
what are group 2 compounds often used as
bases
oxides
hydroxides
carbonates
group2 compound uses
Ca(OH)2
calcium hydroxide
agriculture - added to fields as lime to increase pH of acidic soils
neutralises acidic soil forming neutral water
Mg(OH)2
magnesium hydroxide
antacids to treat acid indigestion
CaCO3
calcium carbonate
+ HCl (stomach acid)
equation for antacid
CaCO3(s) + HCl(aq) -> CaCl(aq) + H2O(l) + C2O(g)
OR
Mg(OH)2(s) + 2HCl(aq) -> MgCl2(aq) + 2H2O(l)
trend in boiling points of halogens
increases down group
more electrons
stronger london forces
more energy required to break intermolecular forces
how do halogens exist
as diatomic molecules
most common type of reaction for halogens and why
redox
each halogen atom has 7 electrons on outer shell
only needs to gain one for stable noble gas configuration
each halogen atom reduced to form 1- halide ion
acts as an oxidising agent
what experiment can be carried to show how the reactivity of halogens decrease down a group
displacement reaction
oxidation
if the halogen is more reactive it displaces the halide in solution
colour change
Cl2(aq) Br2(aq) I2(aq) Cl~ X none none Br~ orange X none I~ purple purple X
how can you tell apart Cl2, Br2 and I2
solution in water
Cl2 - pale green
Br2 - orange
I2 - brown
not very clear, hard to distinguish between orange and brown
+ organic non-polar solvent
cyclohexane
shake
non-polar halogens dissolve more readily in non-polar solvent
Cl2 - pale green
Br2 - orange
I2 - violet
fluorine
pale yellow gas
extremely reactive
astatine
extremely rare
radioactive and decays rapidly
never been seen
predicted to be the least reactive halogen
reactivity of halogens
reactivity decreases down group
atomic radius increases
shielding increases
nuclear attraction decreases
more difficult to attract electron to outer shell
what is disproportionation
redox reaction in which the same element is oxidised and reduced
what is chlorine used in
water purification
chloric acid produced (HClO) kills bacteria
chloric acid(I) acid also acts as weak bleach
what type of reaction is chlorine and water
give the equation
disproportionation
Cl2(aq) + H2O(l) -> HClO(aq) + HCl(aq)
household beach
what is the equation
NaClO
Cl2(aq) + 2NaOH(aq) -> NaClO(aq) + NaCl(aq) H2O(l)
disproportionation
reaction between chlorine and water is limited by low solubility of chlorine in water
if water has sodium hydroxide much more chlorine dissolves
benefits and risks of chlorine use
BENEFITS
kills bacteria so we can have safe to drink water
RISKS
chlorine is extremely toxic
respiratory irritant in small concentrations
large conc, fatal
may react with organic hydrocarbons in the water to form chlorinated hydrocarbons which are suspected of causing cancer
carbonate test
bubble through lime water
if it goes cloudy
CO2 present
what is limewater
saturated aqueous solution of calcium hydroxide Ca(OH)2
CO2 reacts with this to form fine white precipitate CaCO3 which turns limewater ‘cloudy’
sulfate test
Add dilute hydrochloric acid and Ba 2+ (aq) ions to solution (barium chloride, not when testing a mixture tho)
they will react with sulfate ion SO4 2- to form BaSO4
solid white precipitate
testing for halide ions
aqueous silver nitrate AgNO3 to an aqueous solution of a halide
Ag+ ions react with halide ions to form solid precipitates
Cl - white
Br - cream
I - yellow
what test often accompanies the halide test with silver nitrate and why
test of solubility of the precipitate with aqueous ammonia
precipitate colours can be hard to distinguish between
Cl - fully soluble
Br - partially soluble
I - insoluble
what is the sequence of tests
carbonate
sulfate
halides
why is the carbonate test done first
you’re looking for a gas
neither halide nor sulfate produce bubbles with dilute acid, only carbonates
so bubbles definitely = carbonate
how should you carry out a carbonate test on a mixture of ions
carry the carbonate test out first
if you see bubbles continue adding dilute nitric acid until the bubbling stops
so all carbonate ions have been removed
why should dilute nitric acid be used in the carbonate test when testing a mixture of ions
sulfuric acid has sulfate ions which will show up in sulfate test
Hydrochloric acid has chloride ions which will also show up in halide test
testing a mixture of ions
carbonate test
dilute nitric acid
sulfate test
excess Ba(NO3)2(aq)
filter
halide test
AgNO3(aq)
add NH3 to confirm precipitate
why should an excess of Ba(NO3)2 (aq) be used
so all sulfate ions present will precipitate out as Barium sulfate
how do you test for ammonium ion NH4+
add aqueous sodium hydroxide NaOH(aq)
ammonia gas produced (won’t see bubbles as it is soluble in water)
warm the mixture, ammonia gas released
test with moist pH indicator paper - blue paper (smell)
NH4+(aq) + OH~(aq) -> NH3(g) + H2O(l)
which group2 element doesn’t react with water
berrylium
magnesium + water
very slow reaction
equation for carbonate ion test
CO3 2- + H+ -> CO2 + H2O
equation for sulfate test
Ba2+ + SO4 2- -> BaSO4
equation for ammonium test
NH4+(aq) + OH~ -> NH3(aq) + H2O(aq)
