15. Transition metals ( CATALYST)
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Define what is meant by the term catalyst
A substance that increases the rate of reaction by providing an alternative pathway with a lower activation, without itself being used up in the reaction
Give an advantage of heterogenous catalysts
They can be filtered/ removed easily at the end of the reaction
What characteristics of transition metals make them good catalysts
They can form more than one stable oxidation states so they can accepts and loose electrons easily. Therefore, they can easily be oxidized and reduced again or vice versa
Define what is meant by a heterogenous catalyst, and give 4 examples
A substance which speeds up the rate of a reaction without itself being used up that is in a different physical state as the reactants The reaction occurs on the surface of the catalyst. Eg: Iron in the Haber process, Nickel in the hydrogenation of alkenes, platinum/ rhodium/ palladium in catalytic convertors and V2O5 in the contact process
Give the overall equation for the contact process
SO2 (g) + ½ O2 (g)→ SO3 (g)
Give the equations and change in oxidation numbers to show how V2O5, a heterogenous catalyst, is used in the contact process
SO2 + V2O5 → SO3 + V2O4 –> vanadium is reduced from +5 to +4 V2O4 + ½ O2 → V2O5 –> Vanadium is oxidized back from +4 to +5
How do homogenous catalysts catalyze a reaction
Through an intermediate
Given the overall equation, S2O82-(aq) + 2I-(aq) ➝ 2SO42-(aq) + I2(aq), give the equations that show Fe2+ acting as a catalyst within this reaction
2Fe2+(aq) + S2O82-(aq) ➝ 2SO42-(aq) + 2Fe3+(aq) –> The Fe2+ reduces Sulphur, so Fe must be oxidized 2Fe3+(aq) + 2I-(aq) ➝ I2(aq) + 2Fe2+(aq) –> The Fe3+ oxidizes the Iodide ions, so itself must be reduced back to 2+
What happens if Fe3+ is used as the catalyst instead
Earlier equation becomes flipped
What type of reaction is the following: S2O82-(aq) + 2I-(aq) ➝ 2SO42-(aq) + I2
Redox
Describe what is meant by the term autocatalysis
Autocatalysis occurs when a reaction product acts as a catalyst for the reaction. This means no catalysts are added, but one of the products made during the reaction become the catalyst
Describe how autocatalysis effects the rate of a reaction
Initially there is no catalyst and the rate of reaction is quite slow. Once the product that acts as a catalyst is formed, rate of reaction increases, before slowing down and eventually stopping when the reactants become used up
Why might a reaction be slow and require a large amount of energy to cause (large activation energy)
Reactants may be 2 negative ions that are repelling each other
Give an example of an ion involved in autocatalysis
Mn2+