Module 5: Transition metals
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What’s the second property of transition metals?
Variable oxidation states
What do the variable oxidation states mean?
That they can undergo redox reactions and change from one state to another.
What’s a good reducing agent for transition metals?
Zinc metal as in acidic solutions it will reduce most transition metal ions from higher to lower states, oxidising itself to zinc (II) in the process. Zn (s) ——> Zn(II) + 2e-
How do you reduce vanadium (V) to vanadium (II) using zinc?
• dissolve ammonium metavanadate (NH4VO3) in sodium hydroxide solution. • Add acid to ensure the dioxovanadium (V) ion (VO2+) is the main ion present. • Transfer to a conical flask and add a small amount of zinc powder and HCl. Use a stopper at the neck of the flask so that H2 can escape but it minimised reoxidation from O2 in the air. • Swirl slowly • yellow - blue - green -violet are observed.
What’s responsible in the colour change for the rediction of vanadium?
VO2+ = yellow VO^2+ = blue V^3+ = green V^2+ = violet
How is vanadium reoxidised?
——> In air it turns green immediately (back to V(III) ——> If it’s in air for a long time it will turn blue as the air oxidises it back to V(IV) ——-> If you add nitric acid (powerful oxidising agent) to V(II) solution it will also oxidise back to vanadium(IV) which is blue.
What can also be added to reoxidise vanadium ions?
Potassium dichromate.
What can also be reduced by zinc metal and HCl?
Dichromate ions Cr2O7^2- : oxidation state: +6, orange solution. Cr^3+: oxidation state: +3, green solution. The green solution can be reoxidised to the orange dichromate with dilute acid (HNO3) and hydrogen peroxide (H2O2).
What will Cr3+ oxidise to when it’s in the presence of an alkali and H2O2?
Yellow chromate (VI).
What’s the maximum oxidation state of chromium?
+6
What are the 2 common substances containing chromium?
CrO4^2- Chromate (VI) Yellow solution stable only in alkaline conditions Cr2O7^2- Dichromate (VI) orange solution stable in acid only
Write the equation for moving between chromate (VI) and dichromate (VI)
2CrO4^2- + 2H+ <——-> Cr2O7^2- + H2O
What’s the maximum oxidation state of Manganese?
+7
What’s the common reaction involving manganese?
The use of manganese to oxidise other materials.
What are the 2 common ions for Manganese?
Mno4- Manganate (VII) purple [Mn(H2O)6]2+ /Mn2+ Manganese Pale pink/colourless
What are the types of redox reactions studied?
Determining the % of mass of iron in iron tablets or impure iron samples using the oxidising agent acidified potassium dichromate. Determining the concentration of H2O2 in hair bleach using the oxidising agent acidified potassium manganate. Titration of thiosulphate with iodine to subsequently work out the mass of copper.
What are the half equations for Mno4^- plus iron?
• 5e- + MnO4- + 8H+ ——-> Mn2+ + 4H2O • Fe2+ ——> Fe3+ + e- Combined: MnO4- + 8H+ + 5Fe2+ ——-> Mn 2+ + 4H2O + 5Fe^3+
What is a catalyst?
A substance that increases the rate of reaction but isn’t used up in the reaction.
How does a catalyst work?
It provides an alternative route with a lower activation energy.
How does a catalyst affect a reaction at equilibrium?
A catalyst has no effect on the position of equilibrium as it increases the rate of forward and backwards reaction equally. It only decreases the time taken to reach equilibrium.