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A catalyst is a special ingredient in chemistry that makes reactions happen faster or in a different way without being consumed itself. Imagine it as a helper that speeds up a task without changing itself. Catalysts achieve this by offering a new route for the reaction, known as a reaction mechanism, which requires less energy than normal. This is because they lower the activation energy needed for the reaction.
The catalyst can hold onto molecules, change their shape or orientation, and create temporary states that make it easier for reactions to occur. By facilitating these changes, catalysts help convert reactants into products more efficiently.
Importantly, though catalysts speed up reactions, they don't alter the final balance of the reaction. They simply help it reach that balance more quickly.

A reaction pathway is the series of steps that transform reactants into products. Each step along the path involves breaking and forming chemical bonds. The pathway chosen by a reaction can determine how quickly products are formed and even what products are formed.
In catalyzed reactions, the presence of a catalyst creates an alternate reaction pathway. This alternate path has a lower energy barrier, which allows the reaction to proceed faster than it would otherwise.

• The pathway can be visualized as a mountain trail: without a catalyst, you have to climb over the highest peak. With a catalyst, there's a tunnel through the mountain, making the journey much easier and faster.
• Different catalysts may open up different pathways, which is why switching the catalyst can lead to different products.

Within a reaction pathway, chemical intermediates are the temporary species formed in between the start and end points of a reaction. They're like intermediate checkpoints in a long journey. These intermediates can tell a lot about how a reaction is proceeding.
When a catalyst is involved, the formation and stability of intermediates can be dramatically different from the uncatalyzed reaction or one involving another catalyst. These differences can lead to pathways with unique intermediates that ultimately result in different products.
For example, in the hydrogenation of alkenes with a platinum catalyst, intermediates bind to the catalyst surface, easing the addition of hydrogen atoms. Conversely, an acid catalyst might create a carbocation intermediate, which can lead to entirely different final products such as in polymerization.