9.2    Catalysis

What is a catalyst?

• A catalyst is a substance that increases the rate of reaction but is chemically unchanged at the end of the  reaction
• Catalysis is the increase in the rate of chemical reaction by a catalyst
• Characteristics of catalysts:
  1. Catalysts are needed in small amounts This is because a catalyst is not used up at the end of a reaction, the same catalyst can be used to catalyse a large number of reactions.
  2. Catalysts do not initiate the reaction, they accelerate it
  3. Catalysts do not alter the enthalpy change of a reaction
  4. Catalysts can be poisoned by impurities, thus losing its catalytic abilities
  5. Most catalysts are transition metals or compounds of them
  6. Catalysts are usually specific, a reaction can only be catalysed by a specific catalyst.
• Examples of catalysts used in industries:

The role of catalyst

• A catalyst works by providing an alternative route for the reaction to This alternative route has a lower activation energy. This increases the proportion of reacting particles with energy greater than the activation energy. As a result, the frequency of effective collision increases. More products are formed on per unit time and the rate of reaction is higher.
• A catalyst neither alters the energies of reacting particles nor lower the original activation It provides an alternative route with lower activation energy.

• An example is the decomposition of hydrogen peroxide into water and oxygen

2H2O2(aq) → 2H2O(l) + O2(g)

The reaction proceeds very slowly at room temperature and there are no observable changes. However, when a little manganese(VI) oxide, MnO2 is added, effervescence is seen immediately. This suggests that oxygen gas is released.

Autocatalysis

• For most reactions, the rate of reaction decreases with time because the concentration of reactants decreases
• However in some reactions, one of the products can act as a catalyst for that reaction. In these reactions, the rate is low at the beginning because there is no catalyst but increases as soon as the product(catalyst) is being formed. After that, the rate decreases again because the concentration of reactant  decreases

Enzymes as catalysts

• Enzymes are proteins which have catalytic function and can act as biological catalysts that catalyses biological reactions in living organisms
• Enzymes are highly speciftc, catalysing only one type of reaction.
• Enzymes are also very sensitive to changes in pH and temperature. Most enzymes can only function in a small range of temperature(usually 37 °C, the body temperature) and pH

• Enzymes are super-efficient catalysts, they are much more efficient compared to inorganic catalysts
• Enzymes function via the lock-and-key mechanism. According to this model, the substrate(reactant) molecule and the active site of the enzyme have complementary shapes so that the substrate fits in precisely
• The substrate binds to the active site of the enzyme just like a key binds to a lock. Bond-breaking and bond-forming processes then take place, transforming the substrate into products