[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to explain what's meant by a limiting reactant you should then be able to explain the effect of a limiting reactant on the amount of products in a reaction and this topic is higher only we're going to stop by taking a look at a relatively simple chemical equation that we've seen before this is a balanced chemical equation magnesium plus chlorine produces magnesium chloride now remember that we do not write the number one in front of any chemical so this equation tells us that one mole of magnesium reacts with one mole of chlorine to form one mole of magnesium chloride remember that in chemical reactions the starting materials are called the reactants so in this reaction the reactants are the magnesium and the chlorine what we make is called the product so in this reaction the product is the magnesium chloride now if we used exactly one mole of magnesium and exactly one mole of chlorine then at the end of the reaction we'd make exactly one mole of magnesium chloride and we'd have no reactants left over however in most chemical reactions it's just not practical or convenient to measure your reactants as carefully as this so we normally use more of one reactant than the other for example in the reaction here we might use not5 mol of magnesium but one mole of chlorine we do this to make make sure that all of the Magnesium fully reacts we call the reactant that's fully used up the limiting reactant and in this case that's the Magnesium the reactant that's not used up is called the excess reactant and that's the chlorine so let's see what would happen in this reaction as the reaction proceeds the Magnesium would all get used up we would use up .5 moles of the chlorine and we'd make 0.5 mol of magnesium chloride now in the exam you could asked to predict the outcome of a reaction taking into account which reactant is in excess and which is limiting let's look at the typical question how many moles of zinc iodide would be produced if we used 0.5 moles of zinc and one mole of iodine calculate the mass of product looking at the chemical equation we can see that one mole of zinc reacts with one mole of iodine to make one mole of zinc iodide we've been given .5 moles of zinc which makes that the limit reactant that means that we must make .5 mol of zinc iodide in this reaction the amount of iodine has no effect on the amount of product because the iodine's in excess there'll be some iodine left over at the end of the reaction the question also asks us to calculate the mass of product to do this we use the equation Mass equals number of moles multiplied by relative formula mass the relative formula mass of zinc iodide is 319 multiplying .5 by 319 gives us a final mass of 159.5mm 1 Mo of hydrochloric acid to make one Mo of sodium chloride because we're only using .25 mol of hydrochloric acid this makes the hydrochloric acid the limiting reactant this means that we can make .25 moles of the sodium chloride to work out the mass we multiply the number of moles by the realtive formula mass so that's .25 multiplied by 58.5 giving us a final mass of 14.6 G to one decimal place okay here's one more question for you how many moles of copper will be produced if we use 0.5 moles of copper sulfate and 1 mole of magnesium calculate the mass of copper produced pause the video again and try this yourself the equation tells us that one mole of copper sulfate reacts with one mole of magnesium to make one mole of copper we have .5 moles of copper sulfate which means that we can make .5 moles of copper to find the mass we multiply the number of moles of copper by the relative atomic mass so that's 0.5 multiplied by 63.5 which gives us a final mass of 31.75 G remember that you'll find plenty more questions on calculations involving limiting reactants in my revision workbook which you can get by clicking on the link above okay so hopefully now you should be able to explain what's meant by a limiting reactant you should then be able to explain the effect of a limiting reactant on the amount of products in a reaction [Music]