chap 3- elements of the sea Flashcards
define electrolysis
passing an electric current through an ionic substance that’s molten or in solution and it breaks it down into the elements it’s made of
why can an aqueous solution cause H+ and OH- ions to be formed at the electrodes but in molten solutions only the ions that make up the salt are formed
because in a molten salt only the elements are present but an aqueous solution has water and those ions can be formed
what is the rule for what is formed at the cathode in an aqueous solution
if the metal is less reactive than hydrogen (silver or copper) the metal will be formed
if the metal is more reactive (group 1,2 and aluminium) hydrogen gas will be formed
what is the rule for what is formed at the anode
if the solution doesn’t contain a halide, oxygen will be formed from hydroxide ions or if the solution is dilute and contains a halide, oxygen will be formed
otherwise the halogen will be formed
what is brine
a solution if water with a high salt concentration (mainly NaCl) but also some bromine and iodine salts
how is brine made
by dissolving rock salt in water
what are the 4 steps for the electrolysis of brine
1) at the cathode
2H+ + 2e-= H2 (two hydrogen ions become one hydrogen molecule)
2) at the anode
2Cl- = Cl2 + 2e- (two chloride ions lose their electrons and become one chlorine molecule)
3) the sodium ions stay in solution as they’re more reactive than hydrogen
4) sodium ions and hydroxide ions are left behind while hydrogen and chlorine are removed so NaOH is left in solution
how is bromine extracted from brine
brine contains bromide
chlorine is more reactive than bromine so when you bubble chlorine gas through brine the chlorine will displace the bromine
2Br- + Cl2 = Br + 2Cl-
the bromine that is produced is collected, condensed into liquid and purified
how is iodine extracted from brine
brine contains iodide ions
chlorine is more reactive than iodine so when you bubble chlorine through brine it will displace the iodine
2I- + Cl2 = I2 + 2Cl-
the iodine is collected, purified and condensed into a grey solid
what is the oxidation state or a simple monatomic ion
the same as it’s charge
what is a redox reaction
a reaction where reduction and oxidation happen simultaneously
what happens to the oxidation state when one electron is lost
it will increase by 1 and decrease by 1 for each electron gained
what are iodine thiosulfate titrations used for
finding the concentration of the oxidising agent so the more concentrated the oxidising agent is the more ions will be oxidised by a certain volume of it
what are 4 things that could cause you to get inaccurate results in a titration
1) contaminated apparatus- ensure burette is clean rinse jt with sodium thiosulfate before starting
2) read burette correctly (from bottom of meniscus)
3) repeat until you get 3 concordant results so you can take an average
4) wash the flask between repeat experiments or use a new clean one
what happens if you add the starch solution too soon during the titration
the iodine will stick to the starch and won’t react as expected with the thiosulfate so the result is unreliable
what is volatility
the measure of how easy it is to vaporise something (turn from liquid to gas)
why do the halogens become less volatile as you go down group 7
because if the increasing strength of the instantaneous dipole induces dipole bonds
how do you make a hydrogen halide
adding concentrated acid to a solid ionic halide
mainly phosphoric acid is used
why can’t you make hydrogen bromide or hydrogen iodide using sulphuric acid
because the bromide and iodide ions are strong enough reducing agents to reduce the sulfuric acid
why do the bonds between the hydrogen and the halide get stronger as you go down group 7
the halogen atoms get bigger going down the group meaning the bonding electrons are further from the nucleus and shielded by more shells
why is hydrogen fluoride the only hydrogen halide that is a weak acid
it has a stronger bond than the others so doesn’t fully dissociate in water. (splits apart to form two ions)
how is an ammonium halide formed
ammonia is a base and so can accept a proton to form a positive ammonium ion. the positive ammonium ion can then bond with a negative halide ion to produce an ammonium halide
which hydrogen halides react with sulfuric acid and why
hydrogen fluoride and hydrogen chloride don’t react with sulfuric acid as they’re not strong enough reducing agents to reduce the sulfur.
hydrogen bromide and iodide do react with sulfuric acid though
reaction of hydrogen bromide and sulfuric acid
HBr reduces H2SO4 to SO2
2HBr + H2SO4 = Br2 + SO2 + 2H2O
reaction of hydrogen iodide and sulfuric acid
8HI +H2SO4 = 4I2 + H2S + 4H2O
Outline the test for halides and the results
add dilute nitric acid to remove unwanted ions
add silver nitrate solution and a precipitate will form
Fluoride- no precipitate as AgF is soluble
Chloride- white
Bromide- cream
Iodide- yellow
after doing the test for halides you can add ammonia solution to tell them apart more clearly. what does each halide show with ammonia
AgCl- dissolves in ammonia solution to give colourless solution
AgBr- precipitate remains unchanged if dilute ammonia solution is added but will dissolve in concentrated ammonia solution to give a colourless solution
AgI- precipitate doesn’t dissolve at all
why is chlorine transported as a liquid
more can be stored in a fixed volume under pressure than as gas
why does the inside of a tank transporting chlorine need to be dry
chlorine reacts with water to produce corrosive acids
what is one method of checking stored chlorine for leaks
take a stick with cloth soaked in concentrated ammonia solution over the end. if the storage cylinder is leaking then a white cloud of ammonium chloride will be seen
how could a reaction with 100% yield have a low atom economy
if a lot of the atoms from the reactants end up as by products rather than the desired product.
what is dynamic equilibrium
when the forward reaction is going at the same rate as the backwards reaction so the amounts of reactants and products are constant
what is the expression for calculating kC
(D)d(E)e/ (A)a(B)b if aA + bB = dD +
lower case letters are the number of moles
what does it mean if the value of kC is larger than 1
equilibrium lies to the right (there are more products than reactants present at equilibrium) the more above 1 the value is the further to the right equilibrium is
what does it mean if the value of kC =1
the amount of reactants and products are the same
what does it mean if the value of kC is smaller than 1
equilibrium lies to the left (there are more reactants than products present at equilibrium)
how does changing pressure affect the position of equilibrium
increasing the pressure shifts the equilibrium to the side with fewer gas molecules in order to reduce the pressure
decreasing the pressure shifts to the side with more gas molecules in order to raise the pressure
how does changing the temperature affect the position of equilibrium
increasing the temp means the equilibrium will shift in the direction of the endothermic reaction in order to absorb the heat
decreasing the temp means it’ll shift in the exothermic direction to try and replace the heat
define le chatelier’s principle
if a system is at equilibrium and a change is made in any of the conditions then the system will oppose the change
what is the deacon process
hydrogen chloride mixed with oxygen and passed over a catalyst producing chlorine and steam
4HCl + O2 2Cl2 +2H2O
what are the ideal and realistic conditions of the deacon process
ideally equilibrium is to the right which requires high pressure, low temp and excess oxygen but in reality it is compromised as high pressure is expensive, low temp reduces rate of reaction and excess oxygen poses safety risks
what are some of the trends of properties going down the group
become darker in colour melting and boiling point increase become less volatile reactivity decreases change from gases to liquids to solids at room temp
what is the oxidation number of all the halides
-1
what is an oxyanion
a negative ion with oxygen in it. ends in -ate to show the oxygen is present
how do you use oxidation states to balance redox equations
identify oxidation states for each element change in the reaction
balance the equation so the number of electrons gained is equal to the number lost
name the reagent and conditions needed to turn chloroethene into chloroethane
hydrogen and nickel (hot)
or platinum and room temp and pressure
what are the reagents and conditions needed to convert ethanol into an aldehyde in the lab
potassium/sodium dichromate
sulfuric acid
distil do not heat
what is the state and colour of iodine at room temp
solid
grey/black
what is the state and colour of bromine at room temp
liquid
brown/red
why is chlorine a gas at room temp
because there are instantaneous dipole-induced dipole forces between molecules
why is there a high salt concentration in the dead sea
water has been constantly evaporated for many years allowing salt to accumulate in the dead sea as there’s no outflow
why does the melting/boiling point increase going down group 7
the dipole induced dipole forces increase due to there being more electrons
how does fluorine appear at room temp
pale yellow gas
how does chlorine appear at room temp
pale green gas
why are halogens more soluble in organic solvents than in water
because they are non polar molecules but water is polar
what happens when a halogen reacts with a halide ion
displacement reaction eg
Cl2 + KI -> 2KCl + I2
