Animated Solution for Chemistry - Surface Chemistry: Which of the following is not an example of heterogeneous catalytic reaction?

Imagine you are trying to push a heavy boulder over a hill. It takes a massive amount of energy. Now, what if someone dug a tunnel through the hill for you? You'd get to the other side much faster and with far less effort. In the world of chemistry, a catalyst is that tunnel. It provides an alternative pathway with a lower activation energy, speeding up the reaction without being consumed itself.

When we talk about catalysis, we often divide it into two main categories: homogeneous and heterogeneous. The distinction is beautifully simple. If the catalyst and the reactants are in the same physical state (like all liquids or all gases), it's homogeneous. But if they are in different states—for example, gaseous reactants flowing over a solid catalyst—it's heterogeneous catalysis.

Let's put on our detective hats and examine the four processes given in our problem to see which one doesn't fit the heterogeneous profile.

First, let's look at Haber's process, the reaction that literally feeds the world by producing ammonia for fertilizers. In this process, nitrogen gas ($N_2$) and hydrogen gas ($H_2$) react to form ammonia gas ($N{H}_3$). But they don't do it alone; they need a surface to break their strong bonds. That surface is provided by a solid iron ($Fe$) catalyst. Since the reactants are gases and the catalyst is solid, this is a textbook example of heterogeneous catalysis.

Next up is the hydrogenation of vegetable oils. If you've ever seen solid margarine or vanaspati, you've witnessed the result of this reaction. We take liquid vegetable oil and bubble hydrogen gas ($H_2$) through it. To make the reaction happen at a reasonable rate, we use finely divided solid nickel ($Ni$) as a catalyst. Liquid oil, gaseous hydrogen, and a solid catalyst—different phases everywhere! This is definitely heterogeneous.

Now, let's examine Ostwald's process, which is crucial for manufacturing nitric acid. The very first step involves the oxidation of ammonia gas ($N{H}_3$) with oxygen gas ($O_2$). To facilitate this, the gases are passed over a solid platinum ($Pt$) gauze. Once again, we have gaseous reactants and a solid catalyst. The verdict? Heterogeneous catalysis.

Finally, we arrive at the combustion of coal. Picture a piece of coal burning in a furnace. The solid carbon ($C$) in the coal reacts with oxygen gas ($O_2$) from the air to produce carbon dioxide gas ($C{O}_2$).

But wait... where is the catalyst?

There isn't one! The combustion of coal is a highly spontaneous, exothermic reaction. Once you provide the initial spark (activation energy), the reaction sustains itself vigorously. It doesn't need a catalyst to proceed. Therefore, since there is no catalyst involved at all, it cannot be an example of a heterogeneous catalytic reaction.

By carefully analyzing the physical states of the reactants and catalysts in each process, we've successfully unmasked the imposter. Haber's process, hydrogenation of oils, and Ostwald's process all rely on solid catalysts to speed up reactions involving gases or liquids. The combustion of coal, however, is a spontaneous process that requires no catalyst whatsoever.

This makes Option (b) the correct answer. Always remember, before classifying a catalytic reaction, make sure a catalyst is actually present!