Video: Linking Solid Shape to Reaction Rate

In a reaction with hydrochloric acid, which of the following forms of calcium carbonate would react most rapidly, assuming the same total mass of calcium carbonate is used? [A] A single sphere [B] A cluster of fine needles [C] A single thin plate [D] A fine powder

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Video Transcript

In a reaction with hydrochloric acid, which for the following forms of calcium carbonate will react most rapidly, assuming the same total mass of calcium carbonate is used? A) A single sphere. B) A cluster of fine needles. C) A single thin plate. Or D) A fine powder.

In this question, we’re adding solid calcium carbonate to hydrochloric acid. And we want to know what form of solid calcium carbonate will cause the resulting reaction to occur most rapidly. Regardless of the form of calcium carbonate we choose to add, the same reaction will occur. Namely, hydrochloric acid and calcium carbonate will react to form calcium chloride, water, and carbon dioxide. This reaction is occurring because particles of hydrochloric acid are colliding with the particles of calcium carbonate. As long as these particles collide with sufficient energy, they’ll be able to react.

Since the particles of hydrochloric acid and calcium carbonate must physically collide in order for the reaction to occur, let’s take a very zoomed in look at the different forms of calcium carbonate. So we can determine which one might react most rapidly. If we had a single sphere of calcium carbonate, the particles of hydrochloric acid would be able to collide with the particles of calcium carbonate on the surface of the sphere. Because the particles of hydrochloric acid can collide with the particles of calcium carbonate on the surface of the sphere, that means that they can react there. However, the hydrochloric acid particles can’t reach the particles of calcium carbonate on the inside of the sphere until after the particles of calcium carbonate on the surface of the sphere are gone.

Because the hydrochloric acid can’t reach the calcium carbonate on the inside of the sphere immediately. This means that if we added calcium carbonate in a single sphere, the reaction would occur rather slowly. Something that we can see from the sphere is that the form of calcium carbonate that will react the most rapidly will be the one with the most particles on the surface. Because the more particles of calcium carbonate there are on the surface, the more particles are available to immediately react with the hydrochloric acid.

Another way of saying that we’re looking for the form of calcium carbonate with the most particles on the surface is that we want the form of calcium carbonate with the largest surface area, so the correct answer choice will be the form of calcium carbonate with the largest surface area. One way of thinking about which form of calcium carbonate will have the largest surface area is recognizing that since all of the forms will have the same total mass, we can think about creating each form from the single sphere. For example, we could flatten the sphere to create a single thin plate, which would increase the surface area. So a single sphere doesn’t have the largest surface area and is therefore not the correct answer.

We could then cut up the single thin plate into a cluster of fine needles, which would further increase the surface area. So we know a single thin plate is not the correct answer because it doesn’t have the largest surface area. Finally, we could further increase the surface area by grinding the cluster of fine needles into a fine powder, so the cluster of fine needles is also not the correct answer choice. Therefore, the form of calcium carbonate that has the largest surface area and will therefore react most rapidly with hydrochloric acid is a fine powder.

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