A student places equal masses of limestone rocks into two beakers. However, the student crushes the ones in one of the beakers to make a fine powder. Then, he adds hydrochloric acid to both of them. Which takes longer? Would one of them produce more CO2?

First of all let's think of which one would take longer. The one that would take longer would be the slower of the two reactions, and therefore would have a lower rate of reaction. So now we have to think: what affects rate of reaction? From your notes, you may recall that the main factors affecting rate of reaction are temperature, concentration, pressure, surface area and catalysts. Are any of those different between the two? We know temperature and pressure are the same, as they are both in the same room. We have not been told there is a catalyst, and the same hydrochloric acid is used, so therefore it must have the same concentration. Therefore the only thing that could be different between the two is the surface area.But... which one has the largest surface area, the rocks or the fine powder? Thinking of things this small and in 3D makes it hard to tell which one has the highest surface area, so let's try and use knowledge from our everyday lives to work it out. For example, imagine you are running a restaurant and you have some square tables. If you make a longer table (e.g: putting 3 tables together), you will be able to fit 8 chairs around the table. If you use these 3 tables and make smaller ones, you will be able to fit 4 chairs around each table, and therefore 12 chairs in total. And now you've worked it out! The more divided the tables are, the more chairs that can fit; the more divided the chemical is into pieces, the larger the surface area. So therefore, the powder has a larger surface area. Will this increase or decrease the rate of reaction? Well let's go back to our restaurant: if you have more chairs, you can have more people eating at the same time, and therefore you would make money faster. This is exactly what happens in chemistry; the HCl particles need to be in contact with the limestone, so therefore the more point of contacts there are (the more chairs), the faster the reaction happens. There you have your answer, the beaker with the rocks will take longer!Finally, be careful of not falling into the trap of thinking the powder would make more CO2 than the rocks; even if it is faster, you still have the same amount of limestone, so therefore the same amount of CO2 would be produced!