### Video Transcript

Which of the following reasons could explain why one gram of gold nanoparticles are a better catalyst than one gram of solid gold metal? (A) The nanoparticles have a smaller mass. (B) The nanoparticles have a smaller surface area. (C) The nanoparticles have a larger volume. (D) The nanoparticles have a larger surface area. Or (E) the nanoparticles have a greater mass.

We are asked to say why one gram of gold nanoparticles are a better catalyst than one gram of solid gold metal. Before we discuss catalytic ability, let’s have a look at one gram of nanoparticles and one gram of solid metal. Metals can take on various shapes, even sheets and wires, because they are malleable and ductile. However, for simplicity, let’s imagine one gram of gold metal as a cube. Its mass is one gram, its volume is equal to its length times height times width, and its total surface area equal to the length times the height of one side multiplied by the six sides of the cube.

One gram of gold nanoparticles, however, would consist of many tiny particles. If we imagine for simplicity that they are all cubic in shape, we could still say that the total mass is one gram. The total volume would be equal to this value here, although the volume of each individual particle would be very small. But the total surface area of all the nanoparticles is not equal to the surface area of the solid gold metal. In fact, the surface area of one gram of gold nanoparticles is very large indeed, although we won’t do a calculation to prove this. Comparatively, this value over here, the surface area of one gram of solid gold metal, is relatively small.

So far, we’ve seen that one gram of gold nanoparticles and one gram of solid gold metal both have the same mass. And so we can eliminate mass as a factor in catalytic activity and ability. Though each nanoparticle has a smaller individual mass than one gram of solid gold metal, there is still the same total mass of gold metal present. We can therefore eliminate answer options (A) and (E), since the nanoparticles do not have a smaller or greater mass but the same total mass as one gram of solid gold metal.

We have also seen that the total volume of one gram of gold nanoparticles and one gram of solid gold metal are equal. Because this factor is the same, it cannot explain why nanoparticles are better catalysts than solid metal. And we can therefore eliminate answer option (C). The total volume of the nanoparticles is not larger than the volume of the solid gold metal, but the same.

We are left with one factor that is different between one gram of nanoparticles and one gram of solid metal, and that is surface area. Gold in the form of nanoparticles has a much larger surface area than solid gold of the same mass. For the same mass of a substance, the smaller the particles are, the larger the surface area. And so more surface atoms are exposed to reactant particles. And thus more reactions are catalyzed per second, making these small nanoparticles better catalysts.

Finally, which of the following reasons could explain why one gram of gold nanoparticles are a better catalyst than one gram of solid gold metal? The answer is (D). The nanoparticles have a larger surface area.