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Limiting Reactant is a chemical reactant which limits the quantity of formed product. It gives the least yield of the obtained product that is calculated from the reactants available. This smallest yield of product is called theoretical yield.
When a chemical reaction is finished, the limiting reagent (or limiting reactant or limiting agent) is a reactant that is totally consumed. Because the reaction cannot proceed without it, the quantity generated is controlled by this reagent. Excess reagents or excess reactants occur when one or more additional reagents are present in greater quantities than those needed to react with the limiting reagent (sometimes abbreviated as "XS").
Read More: Concepts in Chemistry
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Key Takeaways: Theoretical yield, actual yield, reactants, limiting factor, stoichiometry, molar ratio, reduction, oxidation.
Procedure to Determine Limiting Reactant
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A chemical reaction will remain operational as long as the reactants are present. When one of the reactants is used, the chemical process comes to a halt. Furthermore, we refer to the first reactant as the limiting reactant formula because it restricts the number of products that can be formed. A variety of approaches exist for detecting the limiting reactant in a chemical process. It's not enough to look at the quantities of each reactant and choose the one that's the smallest.
Determining Limiting Reactant
However, stoichiometry plays a crucial role in deciding which reactant genuinely limits this reaction.
- First and foremost, if the chemical equation is not balanced, balance it.
- Then figure out how many moles of each reactant there are in the reaction.
- Calculate the total amount of products that can be obtained from the reaction.
- The limiting reactant is determined by which reactant produces the least amount of the product.
Read Also: Benzene
Method 1: Comparison of Reactant Amounts
When there's only two reactants, this approach is most useful. The balanced chemical equation used to calculate the proportion of the other reactant (B) required to react with A. B is in large amounts and A is the limiting reagent if the amount of B present exceeds the amount necessary. B is the limiting reagent if the amount of B available is less than what is necessary.
See the combustion of benzene,
2 C6H6(1)+15 O2(g)→12 CO2 (g)+ 6H2O (1)
This means that to react with 2 moles of benzene, 15 moles of molecular oxygen are required.
Cross-multiplication can be used to calculate the consumption of oxygen for other amounts of benzene. For example,
1.5 mol C6H6 × \( \frac{15 mol O2}{2 mol C6H4}\)= 11.25 mol O2
If 18 mol O2 is present, there will remain an excess of (18 - 11.25) = 6.75 mol unreacted oxygen after all of the benzene has been consumed. The limiting reagent is then benzene.
By comparing the mole ratio of O2 and C6H6 needed by the balanced equation with the mole ratio already present, this result can be verified:
Required amount: \(\frac{mol O2}{ mol C6H4}\)= \(\frac{15 mol O_2}{2 mol C_6H_6}\)=7.5 mol O2
Actual amount: mol \( \frac{mol O2}{ mol C6H4} = \frac{18 mol O_2}{1.5 mol C6H4}\)= 12 mol O2
Because the actual ratio is greater than the needed value, O2 is the surplus reagent, indicating as benzene is the limiting reagent.
Read More: Oxidation and Reduction
Method 2: Comparison of the amount of product that can be made from each reactant
The chemical equation is employed in this method to determine the quantity of one product that can be made from each reactant in the amount present. The limiting reactant is the one that can only produce the tiniest amount of the desired product. This approach is more simply extended to any number of reactants than the first. For example,
In the following thermite reaction, 20.0 g iron (III) oxide (Fe2O3) is reacted with 8.00 g aluminium (Al):
Fe2O3 (s) + 2Al (s) → 2 Fe (l) + Al2O3 (s)
Because the reactant amounts are given in grams, they must first be converted to moles before being compared to the chemical equation to see how many moles of Fe can be created from each reactant.
(i) Fe moles that can be created from the reactant Fe2O3
Mol Fe2O3 = grams Fe2O3/g mol- Fe2O3
= \( \frac{20.0g}{159.7g/mol}\)
= 0.125 mol
(ii) Fe moles that can be made from the reactant Al
mol AI = \(\frac{grams AI}{\frac{g}{mol} AI}\)
= \(\frac{800g}{\frac{26.98 g}{mol}} = 0.297 mol\)
mol Fe = 0.297 mol AI x \(\frac{2molFe}{2molAI} = 0.297 mol Fe\)
Limiting Reagent
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Limiting Factor
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A limiting factor is a system variable that causes a visible variation or another system measure. The limiting factor is shaped like a pyramid of organisms, with producers, consumers, and so on at the top. A factor that does not limit over one set of starting conditions may yet limit over another set of starting conditions, including the factor's own.
Limiting Factor
Only by contrasting a limiting factor with one or more non-limiting elements can a factor be identified as limiting. When the phrases are employed, disciplines differ in whether they accept the simultaneous occurrence of more than one limiting factor (which is then referred to as "co-limiting"), but they always demand the presence of at least one non-limiting element. When more than one element is present, there are various conceivable possibilities of limitation. The first scenario, known as single limitation, arises when the System is limited by only one factor, the one with the highest demand.
Read More: Value of Boltzmann Constant
Things to Remember
- When a chemical reaction is finished, the limiting reagent (or limiting reactant or limiting agent) is a reactant that is totally consumed.
- We refer to the first reactant as the limiting reactant formula because it restricts the number of products that can be formed. A variety of approaches exist for detecting the limiting reactant in a chemical process.
- When there's only two reactants the balanced chemical equation is used to calculate the proportion of the other reactant (B) required to react with A.
- The chemical equation is employed in this method to determine the quantity of one product that can be made from each reactant in the amount present. The limiting reactant is the one that can only produce the tiniest amount of the desired product.
Also Read:
Sample Questions
Ques. If 5.43 moles of Na react with 4.25 moles of O2 in the following equation, identify and recognise the limiting reactant:
4Na + O2 → 2Na2O (2 marks)
Ans: Comparing how much of the output each reactant will produce is one of the best and easiest techniques to discover a limiting reactant. The stoichiometry will be used to create a ratio between the reactants and products in the chemical equation as follows:
5.43 moles Na x 2 moles Na2O/4 moles Na = 2.72 moles Na2O
(5.43 moles of Na will produce 2.72 moles Na2O)
4.25 moles O2 x 2 moles Na2O/1 mole O2 = 8.50 moles Na2O
(4.25 moles of O2 will produce 8.50 moles Na2O)
In the process described above, the Na will create less Na2O than the O2. As a result, Na is the limiting reactant or reagent in this situation. Although the initial amount of Na is more than the initial amount of O2, the stoichiometry indicates that the Na will run out first.
Ques. In a 3L container filled with chlorine gas, 2.3 g of sodium metal is transported. If the masses of Na = 23 and Cl = 35.5, find the limiting reagent and the amount of surplus reagent present. (2 marks)
Ans: Balanced equation is given by,
2 Na +Cl2 →2 NaCl
1 x 0.1/2 = 0.05 moles of chlorine are required.
At STP, 1 mole of chlorine gas is 22.4 L.
As a result, 3 L = 3/22.4 = 0.1339 moles are estimated.
As a result, the limiting reactant is sodium metal.
The surplus Cl2 quantity is 0.1339 – 0.05 moles.
= 0.0839 moles occupy 0.0839 x 22.4 / 1
Approximately 1.88 L
Ques. Provided the reaction,
2 LiOH (aq) + H2SO4 (aq) = Li2SO4 (aq) + 2 H2O (l) (2 marks)
Ans: The molecular weight of H2SO4 is 98. This results in 1.02 moles. Because just one mole is required per reaction, the reaction can be repeated 1.02 times. The molecular weight of LiOH is 24. This results in 2.71 moles. Because each reaction requires two moles, the reaction can be repeated 1.35 times. H2SO4 is the limiting reactant based on this.
Ques. What is thermite? (2 marks)
Ans: Metal powder and metal oxide are combined in the pyrotechnic compound thermite. Thermite exhibits the exothermic reduction-oxidation (redox) reaction when heated or chemically activated. The majority of kinds are not explosive, although they can produce brief bursts of heat and high temperatures in confined spaces. It works in the same way as other fuel-oxidizer mixes like black powder.
Thermite
Ques. What is the mass of water created and the limiting reagent when 80g of H2 reacts with 80g of O2 to produce water? (3 marks)
Ans: The mass of O2 is 80 grams, and the mass of H2 is 80 grams.
O2 has a molecular weight of 32 grams.
H2 has a molecular weight of 2gm.
H2 moles = 80/2=40
O2 moles = 80/32=2.5
Because there are fewer moles available for reaction, oxygen is the limiting reagent in this case. 2.5 moles O2 requires 5 moles of H2.
Water formation reaction
1/2 O2(80gm) + H2(10gm) = H2O (90gm)
Water is formed by combining 5 molecules of H2 and 2.5 molecules of O2. Since 5 molecules of H2 weigh 10 grams and 2.5 molecules of O2 weigh 80 grams, the total weight of the H2O molecule is 90 grams.
Ques. Assume 2.38g of barium acetate was dissolved in 150ml of a 0.40 M ammonium sulphate aqueous solution. Assume when the barium acetate is dissolved in the solution, the volume of the solution does not change. What is the barium cation's final molarity in the solution? (3 marks)
Ans: BaSO4 + 2NH4 = Ba (CH3COO)2 + (NH4)2SO4 = Ba (CH3COO)2 + (NH4)2SO4 = Ba (CH3COO)2 + (NH4)2SO4 (CH3COO)
n=m/M
n (Ba (CH3COO)2) available=2.38/245.3 = 0.0097
n=C*V
n((NH4)2SO4) = 0.150*0.40 = 0.060
Because there is too much (NH4)2SO4, all of Ba (CH3COO)2 reacts to generate BaSO4 in the form Ba2+.
n(Ba2+) =n(BaSO4) =n (Ba (CH3COO)2)=0.0097
C(Ba2+) =0.0097/0.150= 0.06468= 0.065M because C=n/v.
Ques. What is yield? (3 marks)
Ans: Yield, often known as reaction yield, is a percentage representation of the mols of a product produced in comparison to the number of reagents consumed in a chemical reaction. One of the most significant characteristics to consider in organic and inorganic chemical reactions is yield. "Yield," "conversion," and "selectivity" are terms in use in chemical process engineering to represent ratios of how much of a reactant was used (conversion), how much preferable product was produced (yield), and how much unwanted product was formed (selectivity), expressed as X, Y, and S, respectively.
Ques. In a closed vessel, 1 g of magnesium is burned with 0.56 g of oxygen. Which reagent, and how much of it, is left over? (5 marks)
Ans: 2Mg + O2 ------> 2MgO
To make 2 moles of MgO, 2 moles of Mg must be combined with 1 mole of O2.
Calculate the exact number of moles.
mass/molar mass Equals moles
Mg= 1/24 = 0.041667
O2= 0.56/32 = 0.0175
MgO=
Mg: O2 mole ratio is 2:1.
identify the Mg mass that reacts:
This indicates that 2 moles of Mg react with 1 mole of O2.
If 1 + 2 then
Then 0.0175 = 0.0175 x 2 is the answer.
Magnesium =0.035 moles
Magnesium mass that reacts:
moles × molar mass = mass
= 0.035 x 24
0.84 gramme
That means that just 0.84 grammes of the 1 gramme of magnesium employed in the reaction is utilised.
As a result, magnesium is in excess by 0.16.
Ques. Consider this well-balanced reaction:
2CaO = 2Ca(s) + O2(g)
Consider the preceding reaction. What will be the limiting reactant if 60g of Ca is introduced in a reaction chamber with 32g of O2? (3 marks)
Ans: The molar ratio of the reactants is the most significant factor to consider when solving Limiting Reactant issues. The balanced formula informs us that the product must include 1 mole of O2 for every 2 moles of Ca. The numbers supplied in the equation are the real amounts we're given, and they may be matched to the molar ratio to figure out which reactant is the limiting reactant. Ca is the limiting reactant because there is only one mole of O2 in any condition where there are less than two moles of Ca. If there are two moles of Ca, any condition where there is fewer than one mole of O2 is a case where O2 is the limiting reactant.
Ques. What is the significance of limiting reagent? (3 marks)
Ans: The limiting reagent is the reactant that is totally consumed in a reaction, determining when the reaction comes to a halt. The exact amount of reactant required to deal with another element can be estimated using reaction stoichiometry. If the reactants are not blended in the proper stoichiometric proportions (as specified by the balanced chemical equation), one reactant will be consumed completely while the other will be left over. The limiting reagent is the one that has been completely consumed; it prevents the reaction from continuing because there is no longer any reactant to react with. It's like the weakest link in a chain in chemistry, limiting the amount of reaction that can happen.
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