Stoichiometry: Understanding the Relationship between Substances in Chemical Reactions | General Chemistry for MCAT PDF Download

Introduction

In our everyday lives, counting and weighing objects is a simple task. However, when it comes to atoms and molecules, things get more complex. Enter stoichiometry, the branch of chemistry that unravels the relationship between the quantities of substances involved in a chemical reaction. In this article, we will dive deep into stoichiometry, exploring its definition, key concepts, and practical applications through real-world problem-solving. Get ready to unravel the mysteries of chemical reactions and discover the power of stoichiometry!

Understanding Stoichiometry

Stoichiometry is defined as the branch of chemistry that focuses on the relationship between the relative quantities of substances involved in a chemical reaction. To illustrate this concept, let's consider a general chemical reaction involving two reactants, A and B, which react to form two products, C and D, respectively:

A + B → C + D

Stoichiometry helps us answer various questions, such as:

• If 'x' grams of A are present, how many grams of C or D (or both) will be produced?
• If 'y' grams of B are present, how many grams of C or D (or both) will be produced?
• If 'x' grams of A and 'y' grams of B are present, which reactant is the limiting reactant and which one is in excess?
• If 'x' grams of A react with 'y' grams of B, how much of B will remain unreacted or unconsumed after the reaction is complete?

How many grams of A are needed to produce 'z' grams of C?

Understanding stoichiometry involves solving real problems related to chemical reactions, reactants, and products. However, two important factors need to be kept in mind before starting:

• Always ensure that the chemical reaction being dealt with is balanced.

Have a clear understanding of the concept of a "mole" and the relationship between "amount (grams)" and "moles."

The concept of a "mole" can be briefly explained as follows:

Consider a simple balanced chemical reaction where hydrogen gas reacts with oxygen gas to produce water:

2H₂ + O₂ → 2H₂O

In this reaction, the numbers in front of the chemical symbols (in red) represent the moles of molecules involved. These numbers are also known as coefficients. For example, two moles of hydrogen gas react with one mole of oxygen gas to produce two moles of water. A mole refers to an extremely large number, approximately 6.023 x 10^23.

The periodic table provides information about elements. For instance, let's take chlorine (Cl) as an example. The periodic table displays three important details:

• Atomic symbol of chlorine.
• Atomic number of chlorine, which corresponds to the number of protons or electrons in an atom of chlorine.
• Atomic mass of chlorine, representing the total mass of protons and neutrons in an atom. Atomic mass is often denoted as "A" or atomic mass number.

The rule of thumb is that the atomic mass of an element (in grams) is equal to the mass of 1 mole of atoms of that element. For chlorine (Cl), one mole of chlorine atoms (6.023 x 10^23 chlorine atoms) weighs 35.453 g.

The same principle applies to molecules as well. For example, let's calculate the molar mass of water (H₂O). The molar mass of H₂O can be obtained by adding up the atomic masses: 2 (atomic mass of hydrogen) + 1 (atomic mass of oxygen) = 2(1.008) + 1(15.999) = 18.015 g. Hence, one mole of water molecules weighs 18.015 g.

Having clarity about the conversion between moles and mass (in grams) is crucial as it forms the basis of all stoichiometric calculations.

To demonstrate, let's find out how many moles of glucose are present in 25 g of glucose. The first step is to calculate the molar mass of glucose (C₆H₁₂O₆): 6(atomic mass of carbon) + 12(atomic mass of hydrogen) + 6(atomic mass of oxygen) = 6(12.011) + 12(1.008) + 6(15.999) = 72.066 + 12.096 + 95.994 = 180.156 grams.

Thus, 1 mole of glucose weighs 180.156 g. Using this information, we can determine that 1 g of glucose is equal to (1 mole/180.156 g) x 1 g. Therefore, 25 g of glucose is equal to (1 mole/180.156 g) x 25 g = 0.139 moles.

Concept of ‘limiting reactant’ and ‘excess reactant’

• The concept of a "limiting reactant" and an "excess reactant" can be explained as follows: The limiting reactant is the substance involved in a chemical reaction that restricts the amount of product that can be formed. The reaction will cease once all of the limiting reactant has been consumed. On the other hand, the excess reactant is the substance that remains unconsumed when the reaction stops because the limiting reactant has been completely used up.

• To illustrate this concept, let's consider an example involving 4 kids and 9 pairs of flip-flops. Each kid will wear a pair of flip-flops, leaving behind 5 pairs. In this analogy, the excess flip-flops represent the excess reactant, while the kids represent the limiting reactant. Regardless of the number of flip-flops available, only 4 pairs can be worn since there are only 4 kids. Similarly, in a chemical reaction, when the limiting reactant is depleted, the reaction halts, and any remaining excess reactant remains in the reaction mixture.
• The crucial task is to determine which reactant serves as the limiting reactant in a given chemical reaction. The easiest method to identify the limiting reactant and the excess reactant is by calculating the number of moles of each reactant and dividing it by its molar coefficient, as specified in the balanced chemical equation. The substance with the lowest value obtained is the limiting reactant.
• Additionally, the concept of theoretical yield and actual yield can be illustrated using the example of popcorn. In an ideal scenario, 36 popcorns should pop from 36 seeds (theoretical yield). However, only 27 popcorns actually pop (actual yield). The percent yield of the popcorn-making process can be calculated as (actual yield divided by theoretical yield) multiplied by 100, resulting in 75%. This percentage represents the efficiency of the process, indicating that it was only 75% efficient.

• This analogy can be applied to any chemical reaction. The theoretical yield refers to the amount of product that can be theoretically calculated based on the given amount of the limiting reactant and the mole ratios of the reactants and products. On the other hand, the actual yield is the quantity of product that is actually obtained in a reaction, which is often less than the theoretical yield due to side reactions and the formation of undesired products. Chemists evaluate the reaction's efficiency by comparing the actual and theoretical yields, calculating the percent yield of the reaction using the formula: (actual yield divided by theoretical yield) multiplied by 100.

Conclusion

Stoichiometry is the key to unraveling the complexities of chemical reactions. By mastering this fundamental branch of chemistry, you gain the power to predict the quantities of reactants and products involved in a reaction. Through a thorough understanding of stoichiometry, balancing equations, grasping the mole concept, and solving stoichiometric problems, you can embark on a journey of discovery in the fascinating world of chemistry. So embrace stoichiometry, unlock the secrets of chemical reactions, and pave the way for a deeper understanding of the molecular world around us.