S1, Ep 14: Limiting reactants as related to... baking!

Caroline Christian, Ph.D.

Limiting reactants are one of the hardest topics for general chemistry students to understand. These problems are when you are given information about two or more reactants and they are asking you about a product of a reaction. A reactant is a chemical species that is on the left side of the arrow, and products are on the right.

Think about limiting reactants this way: if you want to bake a cake, and you need four eggs, two cups of flour and one cup of sugar (reactants) for each cake (product). But you only have five eggs, six cups of flour, and three cups of sugar, how many cakes can you make, and which reactant is limiting? Well since you only have five eggs, and you need four to make a cake, you can only make one cake, and eggs are limiting.

Now, in chemistry it is more difficult, but usually the problems are written pretty straightforward; you have so much of this and this, how much of the product can you create. To calculate a limiting reactant, convert to...moles of course! This is because along with the ratio of number of atoms in a compound being related by moles, moles also relate the number of molecules or formula units in a chemical equation represented by the coefficients of the reaction.

For example: 2 H2 + O2 --> 2 H2O, this equation means that there are two moles of hydrogen that react with one mole of oxygen to create two moles of water. If in a problem they give you information in grams of the reactants, you need to convert them to moles. If you don't remember how to do this read the blog post Always convert to moles! Also, if they want the answer in terms of grams of product, you need to convert to grams from moles. This is also covered in this previous blog post. Now, I've got to go, because I am really hungry - in fact, I may make myself..some cookies!

04/13/2018