Grade 8

Grade 8Chemical Reactions and Stoichiometry


Limiting reactant in chemical equations


In chemistry, reactions are often depicted with balanced chemical equations. In these reactions, the substances we start with are called reactants, and the substances that are formed are called products. When we have a chemical reaction, the amount of reactants will determine how many products can be formed. Understanding which reactant will limit the amount of product generated helps us predict the course of a chemical reaction.

What is the limiting reactant?

The limiting reactant in a chemical reaction is the substance that is used up first. When this reactant is completely used up, the reaction stops, even if other reactants are still present. This concept is important because it determines how much product can be formed in a reaction.

Let's consider a simple analogy: Imagine you are baking cookies. If you have 10 cups of flour, 5 cups of sugar, and 2 eggs, but the recipe calls for 1 egg, 2 cups of flour, and 1 cup of sugar for each batch of cookies, the number of eggs will limit how many batches you can bake. You will run out of eggs first, even if you have plenty of flour and sugar. Similarly, in a chemical reaction, the limiting reactant is the substance that restricts the formation of more products.

Chemical equations and stoichiometry

Before diving into limiting reactants, it's important to understand how chemical equations work. A balanced chemical equation shows the relationship between reactants and products in terms of their molecules or moles. Balancing a chemical equation requires that there be the same number of atoms of each element on both sides of the equation.

For example, consider the combustion of methane:

CH4   2O2 → CO2   2H2O

In this equation, 1 molecule of methane (CH4) reacts with 2 molecules of oxygen (O2) to form 1 molecule of carbon dioxide (CO2) and 2 molecules of water (H2O).

Identifying the limiting reactant

To identify the limiting reactant in a chemical reaction we follow these steps:

  1. Balance the chemical equation.
  2. Convert all given reactant quantities (usually in grams or liters) to moles.
  3. Use stoichiometry to determine the mole ratio in a balanced equation.
  4. Calculate the amount of product formed from each reactant.
  5. The reactant that produces the least amount of product is the limiting reactant.

A visual example: the reaction of hydrogen and oxygen

Let us illustrate this concept with a simple example of the reaction between hydrogen and oxygen to produce water:

2H2   O2 → 2H2O

Imagine you have 4 molecules of hydrogen (H2) and 3 molecules of oxygen (O2).

According to the balanced chemical equation, 2 molecules of H2 react with 1 molecule of O2 to form 2 molecules of H2O

With 4 H2 (which means we can make 4/2 = 2 batches of water) and 3 O2 (which means we can make 3 batches of water), hydrogen is the limiting reactant because it can make fewer batches. Thus, when hydrogen is completely used up only 2 molecules of H2O are formed.

Numerical example: iron and sulfur reaction

Consider the reaction between iron and sulfur to form iron(II) sulfide:

8Fe   S8 → 8FeS

Imagine you have 32 grams of iron and 16 grams of sulfur. First, convert the mass of each reactant to moles:

  • Atomic mass of Fe (iron) = 55.85 g/mol
  • Atomic mass of S (sulphur, S8 molecules) = 256.48 g/mol (32.06 g/mol for one atom)

Convert grams to moles:

Moles of Fe = 32 g / 55.85 g/mol ≈ 0.573 moles
Moles of S8 = 16 g / 256.48 g/mol ≈ 0.0624 moles

This balanced chemical equation implies:

  • 8 moles of Fe react with 1 mole of S8 to form 8 moles of FeS

Determine how many moles of FeS can be produced from each reactant:

0.573 moles of Fe will make 0.573 moles of FeS
0.0624 moles of S8 will make 8 * 0.0624 = 0.499 moles of FeS

Sulfur is the limiting reactant because it produces less (0.499) moles of FeS.

Conclusion

Identifying the limiting reactant helps to accurately predict the amount of product formed in a chemical reaction. In each case, the reactant that forms the smallest amount of product determines the extent of the reaction. Mastering this concept in chemistry is fundamental because it can affect how we perform experiments and industrial chemical processes, saving time and resources.

Understanding limiting reactants involves combining knowledge about chemical equations, stoichiometry, and logical problem-solving. With practice, this concept becomes intuitive and a powerful tool for anyone studying chemistry.


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