Transition metals

• Transition metal - an element which has an incomplete d-sublevel in its atoms or one of its stable ions
• copper is a transition metal as one of its stable ions (Cu2+) has an incomplete d-sublevel
• Zinc and its compounds do not show transition metal properties as:

  • Zn does not have an incomplete d-sublevel
  • it has a full 3d10 sublevel
• properties of transition metals

  • form coloured ions - Cr3+ green, Cr2O72- orange
  • have ions with variable oxidation states - Fe2+, Fe3+
  • effective catalysts - "catalytic activity" Ni in hydrogenation, Fe in Haber process
  • form complex ions
• redox titrations

  1. write half equations
  2. combine to make redox equation
• we use the redox titration of potassium manganate (VII) and Fe (II) to estimate the amount of iron in a sample of blood, ore, iron tablet, an alloy, or fertiliser
• potassium manganate (VII) is dark purple hence no need for indicator in the titration
• potassium manganate is the oxidising agent

  iron is the reducing agent
• during the potassium manganate titration we observe purple solution (MnO4-) turning colourless in the conical flask as it reacts with Fe2+ to form Mn2+
• at the end point the Fe2+ is used up and the next drop of manganate has nothing to react with, so isn't reduced and colours the solution pale pink
• we use H2SO4 as other acids can get oxidised or reduced themselves. will affect titre as they could react with manganate or iron
• reduction of vanadium

  • vanadium has 4 different variable oxidation states (+5 to +2)
  • reduction occurs in acidic conditions
• the vanadate (V) ion is found in solid ammonium vanadate (NH4VO3)

  • VO3- is difficult to reduce due to negative charge
  • we add dilute HCl to ammonium vanadate to form a yellow solution of VO2+
  • this is not redox because the oxidation state of vanadium stays at +5
  • 2H+ + VO3- --> VO2+ + H2O
• we add excess of the reducing agent Zinc to VO2+ to begin reduction

  • zinc is a powerful reducing agent
  • acidic conditions with HCl
• vanadium is reduced by zinc

  • VO2 + (+5) yellow solution
  • VO 2+ (+4) blue solution
  • V3+ (+3) green solution
  • V2+ (+2) purple solution
• vanadium should be reduced in the absence of air to prevent oxidation
• to stop the reduction of vanadium at either +4 or +3, add a weaker reducing agent than zinc such as iron (II)
• there are two types of catalysis - heterogeneous and homogeneous
• heterogeneous catalyst

  • catalyst in different state to the reactants
  • e.g. solid catalyst with gaseous reactants
• homogeneous catalyst

  • catalyst in the same state as the reactants
  • e.g. aqueous catalyst with aqueous reactants
• the oxidation of iodide ions by peroxodisulfate (S2O8 2-) using Fe2+ or Fe3+ catalyst

  • without the catalyst the reaction is slow as two negative ions must collide successfully
  • this causes repulsion and a higher activation energy
• catalysis mechanism of Iodide with S2O8 2- and iron

  • Fe2+ acts as the catalyst and is oxidised to Fe3+ which acts as the catalyst intermediate
  • 2I- + 2Fe2+ --> 2Fe3+ + I2
  • 2Fe3+ + S2O8 2- --> 2SO4 2- + 2Fe2+
  • oppositely charged ions collide causing lower activation energy and more successful collisions per second
• Fe works as a catalyst

  • has ions with variable oxidation states that can act as catalysts
  • metals like Zn or Na do not work was they only have one ion
  • both Fe2+ and Fe3+ can be catalysed as the catalysis steps can be done in either order
• autocatalysis is a type of homogeneous catalysis where a product made in the reaction acts as a catalyst. an example of this is the reaction between manganate ions and ethanedioate ions
• potassium manganate is titrated against ethanedioate. the colour of manganate is slow to fade initially but after a short time, the colour fades instantly
• in autocatalysis, at the start of the reaction there is no catalyst present. this results in a slow reaction as:

  • high activation energy
  • caused by repulsion between negative ions trying to collide
• manganate and ethanedioate react to form Mn2+ which acts as the autocatalyst. this provides an alternative reaction pathway and the colour fades instantly
• catalysis mechanism of autocatalysis

  • Mn2+ reacts with MnO4- to form Mn3+ which acts as the catalyst intermediate
  • Mn3+ then reacts with ethanedioate to form carbon dioxide and reform the Mn2+ catalyst
• the catalyst mechanism allows for lower activation energy which increases the rate of reaction, however in autocatalysis the rate is slow at first before the autocatalyst is formed
• in an uncatalysed reaction collisions between gas particles are random and uncontrolled. this leads to less successful collisions per second
• 3 prinicples of heterogeneous catalysis

  1. one reactant adsorbs (attached) onto the active sites on the surface of the catalyst
  2. the other reactant collides successfully with the adsorbed reactant as there is a high concentration of the adsorbed reactant on the surface. this results with the reaction occuring on the surface of the catalyst
  3. the product is made on the surface must desorb to release the active site for further catalysis
• benefits of adsorption

  • when one reactant adsorbs, the collisions are more controlled as one reactant is stationary
  • some very good catalysts weaken the bonds in the adsorbed reactant to increase the rate further
• catalyst efficiency

  • transition metal catalysts are expensive therefore we must maximise the surface area exposed
  • the catalyst is powdered to increase the surface area
• powdered catalyst is coated onto an inert support medium to prevent the catalyst from being lost over time. the inert support medium must have

  • high surface area - honeycomb shape
  • unreactive
  • high melting point - ceramic
• catalyst poisoning

  • impurities can be present in reactants. this is often from when the reactant was manufactured.
  • the impurities will adsorb onto the active site, often irreversibly, preventing the reactants from adsorbing
  • results in slower reaction
  • can purify the reactants to prevent poisoning e.g. by fractional distillation
• catalyst poisoning

  • products block the active site by not desorbing efficiently
  • blocks the active site over time, resulting in a gradual decrease in rate of reaction
• in the Haber process, the iron catalyst is poisoned by the impurity CO

  • the impurity comes from H2 as it is made from CH4 and H2O
  • the CO would block the active site
• the contact process

  • SO2 + 1/2 O2 --> SO3 using V2O5 catalyst
  • SO2 is cheap and SO3 is used to make H2SO4 which is sold for profit
  • catalysis mechanism
  • V2O5 + SO2 --> SO3 + V2O4
  • V2O4 + 1/2 O2 --> V2O5
• complex ion - a central transition metal ion surrounded by coordinately-bonded ligands
• ligand - molecule/atom/ion that can donate at least one lone pair of electrons to the central metal ion