[Music] hi and welcome back to three sight lessons cold dot u K by the end of this video you should be able to describe the effective surface area of solid reactants on the rate of chemical reactions in the last video we looked at how to explain rates of reaction using collision theory remember that chemical reactions can only take place when the reacting particles collide with each other and the collisions must have sufficient energy the rate of a chemical reaction is determined by the frequency of successful collisions and the word frequency means the number of successful collisions per second so in this video we're looking at how the surface area of a solid reactant affects the rate of a reaction I'm showing you here particles of a solid reactant these are reacting with particles in solution and those are shown in red I should point out that this would also apply if the solid was reacting with the gas now at the key point that you need to understand is that the particles in solution can only react with particles on the surface of the solid in other words these particles as you can see a lot of the particles in the solid are not on the surface and I'm showing those here these particles cannot react with the particles in solution in this diagram I've taken the same massive solid but now it's in two blocks rather than one with two blocks we now have a greater surface area than with one block so now all of these particles can react so because we've got more collisions per second the rate increases when we increase the surface area of a solid reactant now one key idea linked to this is surface area to volume ratio we've already looked at that in two videos smaller sized blocks of solid reactant have a greater surface area to volume ratio than larger blocks again this means that they've got more particles on the surface so there are more collisions per second and this increases the rate of the reaction now one way of investigating the effective surface area on the rate of reaction is to use this set up we saw a similar set up in the required practical on rate of reaction marble chips contain the chemical calcium carbonate this react with hydrochloric acid to produce carbon dioxide gas and we can see that here we can measure the volume of carbon dioxide gas and then use this to determine the rate of reaction we can then change the surface area of the marble chips now measuring the volume of a gas can be difficult using a measuring cylinder like this that's because the bubbles can be quite rapid we can get more accurate results by using a gas syringe like this one another way of carrying out this practical is by measuring the mass of carbon dioxide gas that's lost and I'm showing you that setup here in this experiment we place our reaction on a balance as carbon dioxide is produced the mass decreases we can use this to calculate the rate of reaction now one important point is that the cotton-wool allows carbon dioxide gas to escape however it also prevents acid from splashing out of the flask if any acid did splash out they would cause the master' fall more than it should and that would produce an anomalous result remember you'll find plenty of questions on the effective surface area on the rate of reaction in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe the effective surface area of solid reactants on the rate of chemical reactions [Music]