In a reaction, one reactant can be finished earlier while other reactants are still available to react in the reaction medium. That finished reactant is defined as limiting reagent and reaction will be immediately stopped when this limiting reagent is finished. In this tutorial, we will discuss followings to get a good knowledge about limiting reagent in chemistry.
Reactants react with each other in a different ratios in different reactions and several examples for this case are explained below.
In the beginning of a reaction, there is a certain amounts (mol) of reactants and let's discuss with an example.
As an example, we can consider sodium hydroxide ad hydrochloric acid reaction. In the beginning of reaction, there are 1 mol of sodium hydroxide and 0.9 mol of hydrochloric acid. We have seen that sodium hydroxide and hydrochloric acid react with 1:1 ratio according to the stoichiometric ratio of balanced equation.
That means, for a complete reaction between sodium hydroxide and hydrochloric acid, same amount is required. But, in this example, there is only 0.9 moles of hydrochloric acid is available to react with 1 mol of sodium hydroxide. So, total amount of hydrochloric acid is not enough is react with total sodium hydroxide amount. Therefore, total hydrochloric acid amount is reacted and consumed and some sodium hydroxide remain in the final solution.
Calculation steps can be shown as below for better explanation.
To decide the limiting reagent, we should know the amount of reactants. Because, both are aqueous solutions, we can find the amount by given data (concentration and volume).
Because, now we know the initial amounts of reactants, we can write balanced equation to find the what is the limiting reagent.
For a complete reaction, similar amounts of KOH and HNO3 are required. Therefore, for 0.0025 mol of KOH, 0.0025 mol HNO3 is required. But, in this case, there is only 0.0024 mol of HNO3.
Therefore, total amount of HNO3 is not enough to react with 0.0025 mol of KOH. Therefore, HNO3 becomes the limiting reagent.
You were given 40 cm3 of 0.1 M ammonia solution and you have added 10 cm3 of 0.1 M HCl solution. Find which reagent will be the limiting reagent?
Ammonia and hydrochloric acid reacts with each other and form ammonium chloride as the product. Ammonium ion shows acidic characteristics. Therefore, if both ammonia and ammonium chloride exist in a considerable concentration after the reaction, final solution can be a buffer solution.
Then, we should calculate how much HCl and NH3 are consumed during the final reaction.
Both reactants react with each other in 1:1 ratio. Because, initial ammonia amount is higher than added amount of HCl, ammonia remains in the final solution with ammonium chloride. Therefore, HCl becomes the limiting reagent in this reaction.
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