Here’s How:
To decide which reactant is in excess (more than required for the reaction) and which is the limiting reactant (not in excess) you need to know two things: the mole ratio of the reactants and the number of moles of each reactant.
Mole Ratio:
The mole ratio of the reactants can be taken from the balanced equation i.e.
It is important to note that the equation must be balanced (have the same number of atoms of each reactant on each side).
In the above equation, the mole ratio of Magnesium to Hydrochloric Acid is 1 : 2.
Number of Moles of Each Reactant:
If you are not given them, you can work out the number of moles of each reactant using these formulae:
where:
n = number of moles
C = concentration
V = volume
m = mass
gfm = gram formula mass
Deciding Which Is In Excess/Limiting:
It is then relatively simple to work out which reactant is in excess.
Pick one reactant and place that into the mole ratio as shown, then work out how many moles of the other reactant would be needed for this reaction. If the amount of moles needed is less than the amount of moles given of this reactant, then it is in excess. If the amount of moles needed is greater than the amount of moles given then it is limiting.
(for this example assume the number of moles worked out for Magnesium is 2 and the number of moles worked out for Hydrochloric acid was 5)
Now you know that Hydrochloric Acid is in excess and thus Magnesium is the limiting reactant.
Taking it further:
After finding out which reactant limits the reaction, you may be asked to decide how much of a product is produced. This is pretty simple. You only need to know the mole ratio of the limiting reactant to the product and then you can work as above.
(for this example assume we were asked to find how much hydrogen was produced)
The mole ratio here is 1 : 1
So, two moles of Magnesium would give you two moles of Hydrogen.
(Using given Gram Formula Mass)
1 mol of Mg = 24.3g = > 2 mol of Mg = 48.6g
1 mol of H2 = 2g = > 2 mol of H2 = 4g
Thus, 4g of Hydrogen was produced.
Also See:
My IUPAC Nomenclature for Organic Chemistry Masterpost for High School Chemists
My PowerPoint Presentations Masterpost
