Activation energy and catalysis Flashcards
activation energy is
the minimum energy colliding particles must possess for a reaction to occur
the activation energy represents the energy that the colliding particles must obtain in order to reach the energy level of
the transition state
once the energy level of the transition state has been reached, the particles can
react to form the products and release energy as they do so
a homogeneous catalyst is
a catalyst in the same phase as the reactants
many aqueous reactions in solution are catalysed by
the hydrogen ion H+
an example of a reaction involving a homogeneous catalyst is
-the formation of oxygen from ozone
-the chlorine in CFCs is radicalised with ultraviolet radiation from the sun, and the Cl* radical acts as a catalyst for the following reaction
2O3(g) = 3O2(g)
the mechanism for the catalysed reaction of the formation of O2 from O3 using chlorine radicals is
Cl(g) + O3(g) = ClO(g) + O2(g)
ClO(g) + O3(g) = 2O2(g) + Cl(g)
the 2-step mechanism for the homogeneously catalysed reaction between peroxydisulfate ions (S2O8 2-) and iodide ions (I-) with Fe2+ as the catalyst is
Step 1:
S2O8 2- (aq) + 2Fe2+ (aq) = 2SO4 2- (aq) + 2Fe3+ (aq)
Step 2:
2Fe3+ (aq) + 2I- (aq) = 2Fe2+ (aq) + I2(aq)
a heterogeneous catalyst is
a catalyst which is in a different phase to that of the reactants
two examples of industrial processes where heterogeneous catalysts are used are
- Haber process
- Contact process
in the Contact process, the heterogeneous catalyst is
solid vanadium(V) oxide V2O5
in the haber process, the heterogeneous catalyst used is
solid iron
iron is able to act as a catalyst in the haber process because
- it can form a interstitial hydride with hydrogen molecules
- the hydrogen atoms in the hydride are held btween the metal ions in the lattice
- the hydrogen atoms then react with nitrogen molecules that are absorbed onto hte metal surface
the three stages in catalysis involving surface absorption are
- Absorption- the reactants are first absorbed onto the surface of the catalyst
- Reaction- the reactant molecules are held in positions that enable them to react together
- Desorption- the product molecules leave the surface
for catalysed reactions involving absorption, the rate of reaction is dependant on
how fast the reactant molecules are absorbed and how fast the products desorb, so increasing the pressure when the catalyst surface is covered with molecules will have no effect
