[Music] hi and welcome back to three sighs lessons code you K by the end of this video you should be able to describe and explain the effect of the concentration of reactants or the pressure of reactant gases on the way to the chemical reaction now this can't seem like a tricky topic so you might want to watch this video a couple of times over the last couple of videos we've looked at the rate of chemical reactions we've seen that reactions are rapid initially over time reactions slow down and finally reactions stop now we can explain the shape of this curve using a key idea in chemistry and that's called collision Theory collision theory states 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 going back to our graph we can explain the shape using collision Theory reactions are rapid initially because we've got a large number of reactant molecules this means that we've got a large number of collisions per second over time we actually slow down because the number of reactant molecules is running out that means that we've got a smaller number of collisions per second finally the reaction stops because all of the reactant molecules have run out this means that the number of collisions per second is now zero in this video we're looking at the effective concentration of reactants or the pressure of a gas on the weight of a reaction I'm showing you here two containers each containers holding particles that can react with each other we're going to assume that these particles are dissolved in water although I'm not showing you the water molecules and you'll notice that both of the containers have the same volume remember that the rate of reaction depends on the number of successive collisions per second in the right-hand container we've got twice the number of particles in the same volume in other words the concentration of particles is twice that of the container on the left this means that in the right-hand container the number of collisions per second will be twice that of the left-hand container because of that the rate of reaction in the right-hand container will be twice that on the left because the rate increases if we increase the concentration we can say that the rate is proportional to the concentration I should point out that this also applies to reactions in a gas in this case the right-hand container would have a higher gas pressure but the principle is the same now if we plot the quantity of product for these reactions then we can see that the lines look different firstly the higher concentration is a faster reaction and we can tell that because the line steeper secondly with the higher concentration we get more product at the end but because we started with more reactant molecules at the start in the next video we've been looking at a required practical investigating the effect of concentration on the wait for reaction remember you'll find plenty of questions on the effective concentration 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 and explain the effect of the concentration of reactants or the pressure of reacting gases on the rate of the chemical reaction [Music]