Excess and Limiting Reactants!!

Limiting Reactant - The reactant in a chemical reaction that limits the amount of product that can be formed.  The reaction will stop when all of the limiting reactant is consumed.

Excess Reactant - The reactant in a chemical reaction that remains when a reaction stops when the limiting reactant is completely consumed.  The excess reactant remains because there is nothing with which it can react.

 

No matter how many tires there are, if there are only 8 car bodies, then only 8 cars can be made.  Likewise with chemistry, if there is only a certain amount of one reactant available for a reaction, the reaction must stop when that reactant is consumed whether or not the other reactant has been used up.

Example Limiting Reactant Calculation:

A 2.00 g sample of ammonia is mixed with 4.00 g of oxygen.  Which is the limiting reactant and how much excess reactant remains after the reaction has stopped?

First, we need to create a balanced equation for the reaction:

4 NH_3(g) + 5 O_2(g)4 NO_(g) + 6 H₂O_(g)

Next we can use stoichiometry to calculate how much product is produced by each reactant.  NOTE:  It does not matter which product is chosen, but the same product must be used for both reactants so that the amounts can be compared.

The reactant that produces the lesser amount of product: in this case the oxygen. Next, to find the amount of excess reactant, we must calculate how much of the non-limiting reactant (oxygen) actually did react with the limiting reactant (ammonia).

We're not finished yet though.  1.70 g is the amount of ammonia that reacted, not what is left over.  To find the amount of excess reactant remaining, subtract the amount that reacted from the amount in the original sample.

STOICHIOMETRY

 

Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. You might be looking at the amounts of substances before the reaction. You might be looking at the amount of material that is produced by the reaction. Stoichiometry is all about amounts.

What do you measure? It could be anything. When you're doing problems in stoichiometry, you might look at...

• Mass of Reactants (chemicals before the reaction)
• Mass of Products (chemicals after the reaction)
• Chemical Equations
• Molecular Weights of Reactants and Products
• Formulas of Various Compounds

Now an example. Let's start with something simple like Sodium chloride (NaCl). You start with two ions and wind up with an ionic compound (NaCl). When you look at the equation, you see it takes one atom of sodium (Na) to combine with one atom of chlorine (Cl) to make the salt. When you use stoichiometry, you can determine amounts of substances needed to fulfill the requirements of the reaction. Stoichiometry will tell you that if you have ten million atoms of sodium (Na) and only one atom of chlorine (Cl) you can only make one molecule of sodium chloride (NaCl). Nothing you can do will change that.

Example:

A piece of metallic iron was dissolved in concentrated HCl. The reaction formed H₂ and FeCl₂.

Balanced equation is -->    Fe +  2HCl =  FeCl₂  +  H₂

_{How many grams of Hydrogen gas will be produced when 300 g of Iron metal is reacted with sufficient HCl?}

_{Solution -->    300 g  x  1 mole  x  1  mole of H2  x  2  g   =  10.8 g of H2} 

                                   _{55.8 g      1 mole of Fe      1 mole}