elements of the sea Flashcards
calculation for % atom economy
% atom economy = ( Mr desired products / Mr reactants) x 100
aqueous rules of electrolysis at the cathode
hydrogen gas formed unless less reactive metal (silver, gold, or copper) present
aqueous rules of electrolysis at the anode
oxygen gas formed unless halide ions present
molten rules of electrolysis at cathode
cations gain electrons to form atoms
molten rules of electrolysis at anode
anions lose electrons to form atoms
process of extraction of bromine from seawater
chlorine more reactive than bromine and iodine
Br and I displaced by Cl
When chlorine gas is bubbled through brine, the chlorine displaces the bromine as it is more reactive, forming bromine gas
process of extraction of chlorine from seawater
electrolysis of seawater which contains conc. NaCl
- formation of hydrogen at cathode (as Na more reactive than hydrogen)
- formation of chlorine gas at anode
electrolysis definition
the passing of an electrical current through an aqueous or molten substance, breaking it down
anode definition
the positive electrode in electrolysis
cathode definition
the negative electrode in electrolysis
electrolyte definition
the substance (molten or aqueous) being broken down in electrolysis
oxidation definition
loss of electrons
oxidising agent definition
substance that oxidises another substance, gaining electrons
reduction definition
gain of electrons
reducing agent definition
substance that reduces another substance, losing electrons
redox reaction definition
a reaction where oxidation and reduction occur
disproportionation definition
a reaction where one molecule is both oxidised and reduced
oxidation state rule halogens
-1
unless with more reactive halogen
or unless with O (except F)
oxidation state rule for group 1
+1
oxidation state rule for group 2
+2
oxidation state rule for oxygen
-2, unless with F or as a peroxide
oxidation state rule for hydrogen
+1
oxidation state rule for atoms in elements
0
relationship between oxidation state and oilrig
if oxidation state decreases, atom has been reduced
if oxidation state increases, atom has been oxidised
purpose of iodine thiosulfate titrations
to find the concentration of an oxidising agent, e.g. iodate
process of iodine thiosulfate titrations
a) oxidising iodine
1. measure 25cm3 oxidising agent using volumetric pipette
2. add to excess of acidic potassium iodide solution
b) finding number of moles of iodine produced
1. titrate above solution with soidum thiosulfate
2. when solution = pale yellow, add drops of starch solution, will turn it black
3. continue titration, complete when solution is colourless
appearance of fluorine
gas at room temp
pale yellow colour
appearance of chlorine
gas at room temp
greenish yellow
appearance of bromine
liquid at room temp
reddish brown
appearance of iodine
solid at room temp
grey-black colour
general properties of halogens
highly reactive
non-metals
diatomic
not very soluble (covalent + non-polar)
halogen trends
as you go down the group
- boiling and melting point increases
- volatility decreases (due to stronger id-id bonds)
- reactivity decreases
reactivity of halogens
- remove electrons from other elements to complete their outer shell (oxidising agents)
- decrease going down the group due to valence electrons further from core so attraction weaker + more electron shielding
aqueous chlorine colour
colourless
aqueous bromine colour
orange
aqueous iodine colour
brown
halide ions + silver ions
chlorine - white precipitate
bromine - cream precipitate
iodine - yellow precipitate
silver halides + ammonia
silver chloride - dissolves in dilute ammonia
silver bromide - dissolves in concentrated ammonia
silver iodide - insoluble in ammonia
making hydrogen halides with sulfuric acid
hydrogen chloride - made by adding conc. sulfuric acid to solid ionic chloride
bromide + iodide - can’t be made this way as Br and I are strong enough reducing agents to reduce sulfuric acid
with bromine -> makes Br2 gas + sulfate salt + water + SO2
with iodine -> makes I2 gas + H2S + water
hydrogen halides when heated
HF and HCl -> stable + won’t break
HBr -> will slightly dissociate
HI -> will fully dissociate
hydrogen halides with water
HF -> doesn’t fully dissociate in water, making it a weak acid
HCl, HBr, HI -> fully dissociates in water, strong acid
hydrogen halides with ammonia
forms ammonium halide
NH4X (aq)
hydrogen halides with sulfuric acid
HF and HCl -> do not react with sulfuric acid
HBr -> Br2 + SO2 + H2O
HI -> I2 + H2S + H2O
risks of chlorine
a toxic gas, corrosive, irritates the respiratory system, risk of fires (oxidising agent)
transportation of chlorine
stored in cylinders
transported by road or rail, in pressurised tank containers
transported as a liquid (more liquid can be stored in a fixed volume than gas)
venting mechanism to stop pressure getting too high
uses of chlorine
sterilising water
in bleach
removes stains
criteria for dynamic equilibrium
- concentrations of the reactants and products stay constant
- the forward and reverse reactions are both occurring
- the rate of the forward reaction equals the rate of the reverse reaction
equilibrium constant
Kc
described position of reaction at equilibrium
temperature dependent
if Kc is greater than 1
there is a greater concentration of products over reactants at equilibrium
if Kc is less than 1
there is a greater concentration of reactants over products at equilibrium
Kc expression
aA + bB <=> cC dD
Kc = ([C]^c [D]^d) / ([A]^a [B]^b
Kc = [products] / [reactants]
calculating Kc with correct units method
- write Kc expression
- sub in values + solve
- for units - sub in moldom-3 into Kc expression and solve by cancelling units
effect of changing pressure on equilibrium
Increasing pressure shifts the equilibrium to the side with the fewest moles of gas
Decreasing pressure shifts the equilibrium to the side with the most moles of gas
effect of changing temperature on equilibrium
Increasing the temperature shifts the equilibrium in the endothermic direction
Decreasing the temperature shifts the equilibrium in the exothermic direction
