Limiting Reactant Definition Example Essay

Observing a Limiting Reactant

662 WordsJan 17th, 20113 Pages

Observing a Limiting Reactant
An experiment was carried out to predict the limiting reactant in a chemical reaction between Magnesium and Hydrochloric acid, using the mole concept.
Limiting Reactant:
It is the reactant that will deplete or will be used up first during a chemical reaction. Limiting reactant also determine how long the reaction will last for.
Balanced Equation:
Mg + 2HCl = MgCl2 + H2
The balanced equation is needed to determine the mole ratio between the two reactants. From the equation we know that the equation mole ratio is 1:2. However, we need the actual mole ratio to find out the limiting reactant. The actual mole ratio is calculated below:
Moles of 5.0g of Mg = 5.0/24.305 = 0.21 moles
Moles of…show more content…

The second piece of Mg was added to the test tube to see if all the HCl had been used up, reacting with first piece of Mg. However, there was a reaction between HCl and the second piece of Mg, meaning that excess HCl was added to the first piece of Mg, from which some remained unused. While adding HCl one drop at a time we noticed that HCl had been the limiting reactant throughout the experiment till the last stage when we added just enough HCL to completely dissolve the Magnesium strip. At this stage Magnesium was the limiting reactant. This proves that the hypothesis was correct.
Volume needed to completely react with the first strip of Mg:
Moles of Mg = 0.07/24.305 = 2.89 x 10-3
From the equation, we know that 2 moles of HCl reacts with every mole of Mg.
Therefore, 2 x (2.89 x 10-3) = 5.78 x 10-3
The concentration of HCl = 6M.
(5.78 x 10-3)/6 x 1000 ml
= 0.96 ml of HCl is needed to completely react with the first strip of Mg.
The mass of Mg must be measured at every stage to determine the ratio. The volume of HCl added to Mg, needs to be noted down too.
Patience is the key in such type of experiment; it seems we

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Excess and Limiting Reagents

Chemical reaction equations give the ideal stoichiometric relationship among reactants and products.

However, the reactants for a reaction in an experiment are not necessarily a stoichiometric mixture. In a chemical reaction, reactants that are not use up when the reaction is finished are called excess reagents. The reagent that is completely used up or reacted is called the limiting reagent, because its quantity limit the amount of products formed.

Let us consider the reaction between sodium and chlorine. The reaction can be represented by the equation:

It represents a reaction of a metal and a diatomic gas chlorine. This balanced reaction equation indicates that two Na atoms would react with two Cl atoms or one Cl2 molecule. Thus, if you have 6 Na atoms, 3 Cl2 molecules will be required. If there is an excess number of Cl2 molecules, they will remain unreacted. We can also state that 6 moles of sodium will require 3 moles of Cl2 gas. If there are more than 3 moles of Cl2 gas, some will remain as an excess reagent, and the sodium is a limiting reagent. It limits the amount of the product that can be formed.

Chemical reactions with stoichiometric amounts of reactants has no limiting or excess reagents.

Example 1

Example 2

Skill Developing Problems


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