paper 1 Flashcards
(23 cards)
transitional metals- variable oxidation states
which compounds tend to be oxidising agents?
compounds with high oxidation states eg MnO4-
transitional metals- variable oxidation states
which compounds tend to be reducing agents?
compounds with low oxidation states eg V2+ and Fe2+
transitional metals- variable oxidation states
what are the four main oxidation states of vanadium and the colours?
VO2+ Oxidation state +5 ( a yellow solution)
VO2+ Oxidation state + 4 (a blue solution)
V3+ Oxidation state + 3 (a green solution)
V2+ Oxidation state + 2 (a violet solution)
transitional metals- variable oxidation states
how does V 5+ normally exist and in what compound?
what can you say about this compound
how would you liberate the vanadium ion to form a coloured solution
usually exists as a solid compound in the form of VO3- usually in NH4VO3 (ammonium vanadate (V) )
it is a reasonably strong oxidising agent
addition of acid to the solid will turn into yellow solution containing VO2 +
transitional metals- variable oxidation states
how do you reduce vanadium?
addition of zinc
(solution will reduce the vanadium down through each successive oxidation state, and the colour will successively change from yellow to blue to green to violet.)
transitional metals- variable oxidation states
what is a strong reducing agents that can reduce most transition metal ions from a higehr oxidation state to the lowest state eg Fe3+ to Fe2+
zinc with acid (ie H2SO4)
transitional metals- variable oxidation states
how does tollen’s reagent work [Ag(NH3)2]+?
what is the reduction and oxidation 1/2 equn
Red 1⁄2 eq: [Ag(NH3)2]+ + e- –> Ag +2NH3
Ox 1⁄2 eq: CH3CHO + H2O –> CH3CO2H + 2H+ + 2e-
transitional metals- variable oxidation states
redoc titration between Fe2+ and MnO4- (purple) (self indicating titration) equation
MnO4- + 8H+ + 5Fe2+ –> Mn2+ + 4H2O + 5Fe3+
transitional metals- variable oxidation states
2) which acids are not suitable to supply the 8H+ ions for manganate titrations?
1) which should you use
(some acids are not suitable as they set up alternative redox reactions)
1) USE ONLY DIULATE H2SO4 - but insufficient volumes of it will mean solution is not acidic enough and MnO2 will be produced instead of Mn2+ which is brown so will mask the colour change and lead to a greater volume of manganate being used (innacurate) MnO4-(aq) + 4H+ (aq) + 3e- –> MnO2 + 2H2O
2)
- using a weak acid = same effect, not enough H+ ions supplied, need 8
- cannot be HCL as Cl- would be oxidised to Cl2 by MnO4- as the Ecell of the MnO4-/Mn2+ > Ecell Cl2/Cl-.
MnO4- + 8H+ + 5e- –> Mn2+ + 4H2O Ecell= 1.51V
Cl2 + 2e- –> 2Cl- Ecell = 1.36V
This would lead to a greater volume of manganate being used and poisonous Cl2 being produced.
- cannot be nitric acid as it is an oxidising agent, it oxidises Fe2+ to Fe3+ as Ecell NO3-/HNO2 > Ecell Fe3+/Fe2+
would lead to smalled volume of managate being used
what is the euqation for the oxidation of hydrogen peroxide
(half-equn)
H2O2 —> 2H+ + O2 + 2e-
transitional metals- variable oxidation states
what is the generall half equation for the reduction of manganate ions
MnO4- + 8H+ + 5e- –> Mn2+ + 4H2O
transitional metals- variable oxidation states
oxidation euqation of ethandioate ions
C2O42- –> 2CO2 + 2e-
transitional metals- variable oxidation states
ratio MnO4- : C2O4 2-
2:5
transitional metals- variable oxidation states
ratio of MnO4- to FeC2O4
why
3: 5
because 1 MnO4- reacts with 5 Fe2+ and 2 MnO4- react with 5 C2O4 2-
transitional metals- catalysis
Steps in heterogeneous catalysis
- Reactants form bonds with atoms at active sites on the surface of the catalyst (adsorbed onto the
surface) - As a result bonds in the reactants are weakened
and break - New bonds form between the reactants held close
together on catalyst surface. - This in turn weakens bonds between product and
catalyst and product leaves (desorbs).
transitional metals- catalysis
1) what helps determine effectiveness of the catalytic activity
2) what happens if adsorption[tion is too strong
3) what happens if adsorbtion is too weak
4) which meatls are good catalysts (why)
1) The strength of adsorption helps to determine the
effectiveness of the catalytic activity.
2) Some metals e.g. W have too strong adsorption and
so the products cannot be released.
3) Some metals e.g. Ag have too weak adsorption, and
the reactants do not adsorb in high enough
concentration.
4) Ni and Pt have about the right strength and are most
useful as catalysts.
transition metals- catalysis
catalyst for haber process
Fe
N2 + 3H2 –> 2NH3
transition metals- catalysis
catalyst for manufacture of methanol
Cr2O3
CO + 2H2 –> CH3OH
why can leaded petrol not be used in cars fitted with calalytic converter
lead strongly adsorbs onto the surface of
the catalyst.
what are the two steps in the autocatalysis by Mn2+ in titrations of C2O4
Step 1
4Mn2+ + MnO4- + 8 H+–> 5Mn3+ + 4 H2O
Step 2
2Mn3+ + C2O42- –> 2Mn2+ + 2 CO2
catalysis
what is necessary for a substance to act as a homogenous catalyst?
its electrode potential must lie in-between the electrode potentials of the two reactants. It will first reduce the reactant with the more positive electrode potential and then in the second step oxidise the reactant with the more negative electrode potential.
what does a buffer do when you add acid to it?
Ch3COO- from salt reacts with added H+
what is H2NCH2CH2NH2 (bidentate ligand called?)
ethane-1,2-diamine
