An N-heterocyclic carbene was added between the development of the first and second generations of one of these compounds used (10[1])for olefin metathesis. Lead acetate is used to generate an example of these substances invented by Lindlar that is poisoned to reduce (10[1])its activity. (10[1])Based on their phase relative to the reactants, these compounds are classified as heterogeneous or homogeneous. (10[1])These substances, which provide pathways with lower activation energy, are regenerated (10[2])at the end of a reaction. For 10 points, enzymes are biological examples of what type of substance that can speed up a chemical reaction? ■END■