Q. Use of Reactants in Excess?
Ans.
Consider a recipe for pancakes that calls for 2 cups of pancake batter mix and 1 cup of water. The result of this recipe is 14 pancakes.
Let's say you have only one cup of pancake mix. Can you still make the recipe? Of course! just make half the recipe. In this case the pancake mix is the limiting reagent.
What if you have 4 cups of water but only 2 cups of pancake mix? The two reactants aren't present in the correct ratio, so one of these is the limiting reagent. It seems that the limiting reagent is the pancake mix and that there are 2 cups of water in excess. A reactant in excess will always have an amount left over after the other reactant is used up.
Now in chemical terms, let's examine the idea of limiting reagent and excess.
2 Mg + O2 -> 2 MgO
Let's say in our case 1 mole of oxygen reacts with 3 moles of magnesium. You still obtain 2 moles of magnesium oxide. There is 1 mole of magnesium in excess, which tells you oxygen is the limiting reagent.