developing metals
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what is meant by the term ‘end point’? (acid/base)
the point where the alkali is exactly neutralised and the indicator changes colour
describe how a redox titration using Manganate(VII) ions is performed
• measure out quantity of reducing agent e.g. Iron(II) ions • add dilute sulfuric acid (xs) • add aqueous MnO4, swirl • stop when the mixture is tainted with pink
describe how an electrochemical cell can be constructed
two different metals dipped in salt solution dipped in salt solutions of their own ions and connected by a wire
what process occurs at the anode?
oxidation
what process occurs at the cathode?
reduction
in which direction does the flow of electrons go?
from the most reactive metal to the least
how do you construct a salt bridge?
soak a piece of filter paper in salt solution and draping it between two beakers
what does a negative electrode potential represent?
oxidation
what electrode potential represents the forward reaction?
positive
what is the definition of standard electrode potential?
the voltage measured under standard conditions when the half cell is connected to standard hydrogen electrode
what is the equation for the reaction at the hydrogen electrode?
2H+ + 2e- -> H2
which way does a reaction with a more positive standard electrode potential go?
forwards
how do you calculate Ecell?
(more positive-more negative)
which way do electrons flow in an electrochemical cell?
from the most reactive metal to the least
when might a prediction using Ecell be wrong?
• if reaction has a high activation energy - rate is so slow reaction does not appear to happen
what is rust?
Fe2O3.xH2O
what is the equation for rusting?
2H2O + O2 + 2Fe -> 4OH- + 2Fe2+
how does Fe2+ and OH- combine to form rust?
• Fe2+ and OH- combine to form Fe(OH)2 • Fe(OH)2 is oxidised further to form Fe(OH)3 • Fe(OH)3 gradually turns into hydrated Iron(III) Oxide
why is rust less likely to occur in alkaline conditions?
when OH- ions are added, equilibrium of the reaction between O2 and H20 moves left, equilibrium of iron half reaction moves right to get rid of electrons
describe two ways to prevent rusting
• painting or coat with a polymer • oiling • coating with a more reactive metal (sacrificial metal)
what is a transition metal?
a d block element that can form at least one stable ion with an incomplete d sub shell
why do transition metals have a variable oxidation state?
the energy levels of the 4s and 3d subshells are very close together this means different numbers of electrons can be gained or lost using similar amounts of energy
what is a complex ion?
a central metal atom or ion surrounded by co-ordinately bonded ligands
what is a ligand?
an atom or molecule that donates a pair of electrons to a central transition metal ion to form a co-ordinate bond