y11 electrolysis test Flashcards
how to describe current in metal/graphite
the flow of electrons
how to describe current in liquid/dissolved ionic compound
the flow of ions
when are ionic compounds able to conduct and why
only when molten or dissolved in water, because the ions are able to move and carry charge
why can’t simple covalent molecules conduct
because they have no overall charge or free elctrons to carry charge
positive ion =
cation
negative ion =
anion
what is electrolysis
the decomposition of compounds using electricity
oxidation means…
the loss of electrons
reduction means…
the gain of electrons
Describe the electrolysis of molten lead bromide
-Pb 2+ ions are attracted to the cathode (negatively charged electrode) and gain electrons, becoming Pb atoms
- Br- ions are attracted to anode (positively charged electrode) and lose electrons, becoming Br2 molecules
- at cathode grey lead metal is deposited on surface of electrode
-at anode brown bromine gas is given off
Using example of electrolysis of PbBr2, write half equation at cathode, and whether it’s reduction or oxidation
Pb2+ + 2e- => Pb
Reduction
Using example of electrolysis of PbBr2, write half equation at anode, and whether it’s reduction or oxidation
2Br- => Br2 + 2e-
Oxidation
what is an ore
a rock that contains enough metal for profitable extraction of that metal
how can carbon be used to extract metals
Due to a displacement reaction, carbon can displace metals below it in the reactivity series, and seperate it from the ore.
method of extraction for elements above carbon in reactivity series
electrolysis
disadvantage of electrolysis
high energy cost for both heat and electricity
which metals don’t require extraction
metals at bottom of reactivity series: copper, silver, gold, platinum are found ‘native’ and don’t need to be extracted
ions present in water
small amount of H+ and OH- ions
what happens at the anode (which types of ions are oxidised)
If present, halide ions will be oxidised to form halogen molecules, but if not hydroxide ions are oxidised to form oxygen (gas)
what happens at cathode (which elements are reduced)
If the element present is lower than hydrogen in the reactivity, that metal will be formed. If higher, hydrogen gas will form. In other words, it’s whatever is lower in reactivity series.
what are the products from the electrolysis of sodium chloride solution
hydrogen, chlorine and sodium hydroxide
test for:
- H2 gas
- O2 gas
- Cl2 gas
H2 = extinguishes lit splint with a squeaky pop
O2 = relights a glowing splint
Cl2 = bleaches damp (preferably blue) litmus paper
test for
H+ ions
OH- ions
in a solution
Acids contain H+ ions, and bases contain OH- ions, so you can use a few drops of universal indicator. If H+ - would go red/orange, if OH- would go purple/blue
what is electroplating
when electrolysis is used to create a thin layer of a metal on a conductive object
explain the mass changes during the electrolysis of copper sulphate
-in this set up, instead of using a graphite electrode, your cathode is a thin sheet of pure copper, and your anode is impure copper
-the electrolyte (CuSO4) is added
-the copper atoms at the anode lose electrons (oxidised), becoming ions
-the anode becomes thinner due to atom loss, and the impurities fall to the bottom (as sludge)
-the copper ions are attracted to the cathode, and are reduced to form purified copper atoms
-the cathode gradually becomes thicker
-gain in mass at cathode is same as loss in mass at anode
what is the anode half equation for the oxidation of hydroxide ions
4OH- –> O2 + 4e- + 2H2O
what is the cathode half equation for the reduction of hydrogen ions
2H+ + 2e- –> H2
