Stoichiometry​ Limiting reagents​ Everything you always wanted to know

You already know... In the book you read about putting together tool kits. As you are putting them together, sometimes you run out of items while you have others left over. The item you run out of first is the "limiting factor" or "limiting agent."​

Another example Suppose you have a bike shop. You have 55 frames, 40 handlebars, 40 wheels, 35 seats, and 50 pedals. ​ How many bikes can you make?​

The Answer Yes that's right! 20!! The limiting factor is the wheels. You only have enough wheels for 20 bikes. You will have 25 frames left, along with 20 handlebars, 15 seats, and 10 pedals. No wheels left.​

The Connection Chemical reactions are the same. Often you will run out of one reactant while you may have extra of another.​

Limiting Reagent Problems (A reagent is another name for a reactant)​ There are several ways to work out a limiting reagent problem, but the simplest is just to do the problem twice. The correct answer is the lesser amount of product.​

Example Given 500. grams of iron reacts with 500. grams of oxygen, how much product will you get?​ --Note: if you are given the amounts of both reactants, it is a limiting reagent problem.​

Write the equation Fe + O2 → Fe2O3 Then balance it 4Fe + 3O2 → 2Fe2O3 500.g 500.g ?g

Now make boxes!! Remember to set up the units first!!!!! 500.gFe mol Fe mol Fe2O3 g Fe2O3 g Fe mol Fe mol Fe2O3 Remember to set up the units first!!!!! Now finish it. I’ll wait ….. BoxesareourfriendsBoxesareourfriendsBoxe sareourfriendsBoxesareourfriends

I got 715 grams You too?

Now do it again starting with... The oxygen 500.gO2 mol O2 mol Fe2O3 g Fe2O3 g O2 mol O2 mol Fe2O3 Finish it up. I’ll wait… BoxesareourfriendsBoxesareourfriendsBoxesareourf riendsBoxesareourfriendsBoxesareourfriendsBoxesa reourfriendsBoxesareourfriendsBoxesareourfriends

I got 1660 g (remember sig figs) So… What’s the answer? It’s the smaller one: 715 g Fe2O3

Don’t read this Another way to do this: Change both given reactants to moles. Divide the first by the second. Call this “A” Divide the coefficient of the first by the coefficient of the second. Call this “B”

Don’t read this If A>B, the second reactant is the limiting reactant. If A<B, the first reactant is the limiting reactant. Now you just have to do one row of boxes, starting with the limiting reactant.

Percent Yield When you do a stoichiometric calculation, the amount of product calculated is just theoretical. The actual yield of product is always less, for various reasons.

Percent Yield The formula is: experimental yield x100 theoretical yield Usually you would have to calculate the theoretical yield (stoichiometry). The experimental yield would have to be given in the problem.

Now do Exercise 25.5