catalysts Flashcards
what is a catalyst
speed up the rate of reaction providing an alternative pathway with a lower activation energy without getting used up itself
why do transition metal form good catalyst
as they form variable oxidation states
what is a heterogenous catalyst
where the reactants and catalyst are in a different physical state
what is a homogenous catalyst
when the catalyst and the reactant are in the same physical state
what is autocatalyst
the catalyst is a product of the reaction which is the same state as the reactant product
what is the structure of a catalyst and why
it is a honeycomb structure which provides a large surface area and minimises cost of energy ,
how is it made
a thin layer of expensive metal is placed on the structure so molecule can recat at a given time
how does reaction occur in general
adsorption then reaction the desorption
where does the reaction take place
at the surface of the catalyst
what happens frst
reactants are adsorped to the binding sites on the surface of the catalyst and form weak chemical bonds with the active site
what happened next
bonds in reactants are weakened and react to form other bonds
what happened in the last stage
the products are desroped
what are the benefits of adsorption
weakens the bonds within the reactants so lower activation energy required to break bonds
increase surface concentration of reactant molecules so increase collision frequency
provides reactant with correct orientation for reaction to occur
what does the strength of adsorption determine
the effect of the catylitic ability
what if it is too weak
silver is too weak and the reactants are too mobile
what if adsorption is too strong
tungsten which means the reactant and products cannot leave active site once formed
how do catalyst get poisoned
the impurities are strongly adsoped on the surface of the catalyst reduces the surface area for the catalyst and slows down rate of reaction
why is it more expensive
decreases the amount of product formed in a certain time or with a certain amount of energy eventually need to be replaced
why was lead poisonous
it was included in petrol and used to bind to active site and decrease the surface area for reaction to occur
why is sulphur poisonous
it poisons the iron in the haber process and reduces the surface are afor reaction to take place n
how do catalyst work in homogenous system
as the two ions are negatively charged, so they repel each other so unlikely to collide the catalyst is a poisitve ion so can attract and activation energy lowered
give an example of a homogenous catalyst
s208 and iodine with iron
what happens to the rate of reaction in an autocatalyst recation
at the start of reaction the rate of reaction is slow as both reactants repel each other so activation energy is high, as the catalyst is formed so can attract the reactants and the activation enrgy is lowered and reaction increases
