Question

Which (if any) of the following is true regarding the limiting reactant in a chemical reaction? a. The limiting reactant has the lowest coefficient in a balanced equation. b. The limiting reactant is the reactant for which you have the fewest number of moles. c. The limiting reactant has the lowest ratio of moles available/ coefficient in the balanced equation. d. The limiting reactant has the lowest ratio of coefficient in the balanced equation/moles available. Justify your choice. For those you did not choose, explain why they are incorrect.

Short answer

The correct answer is option c: The limiting reactant has the lowest ratio of moles available/coefficient in the balanced equation. This statement correctly identifies the limiting reactant, as the reactant with the lowest ratio will run out first and determine the maximum amount of product produced. The other options are incorrect for various reasons, such as disregarding stoichiometry or incorrectly calculating ratios.

Step-by-step solution

Option a Analysis

The limiting reactant has the lowest coefficient in a balanced equation. This statement is incorrect. The limiting reactant is the reactant that runs out first and thus determines the maximum amount of product produced. The coefficient in a balanced equation only shows the stoichiometric relationship between the reactants and products, and does not necessarily indicate which reactant will be the limiting one.

Option b Analysis

The limiting reactant is the reactant for which you have the fewest number of moles. This statement is only partially correct. While a reactant with fewer moles might be limiting, it is not necessarily true in all cases. To determine the limiting reactant, we also need to consider the stoichiometry of the reactants in the balanced equation.

Option c Analysis

The limiting reactant has the lowest ratio of moles available/coefficient in the balanced equation. This statement is correct. By dividing the moles of each reactant by the corresponding coefficient in the balanced equation, we obtain a ratio that indicates which reactant will run out first (the limiting reactant). When a ratio is the smallest among reactants, it means that there will be less of that reactant available per stoichiometric ratio, thus running out first and limiting the reaction progress.

Option d Analysis

The limiting reactant has the lowest ratio of coefficient in the balanced equation/moles available. This statement is incorrect. Dividing the coefficient by moles available won't necessarily provide any meaningful information about the limiting reactant. The correct way, as stated in option c, is by dividing the moles available by the coefficient. In conclusion, the correct answer is option c: The limiting reactant has the lowest ratio of moles available/coefficient in the balanced equation.

Key concepts

Stoichiometry

Stoichiometry is at the heart of chemical reactions. It is the field of chemistry that is concerned with the relative quantities of reactants and products in chemical reactions. In essence, stoichiometry is used to calculate the amounts of substances involved in a reaction based on the balanced chemical equation. It is founded on the law of conservation of mass, which states that in a chemical reaction, matter is neither created nor destroyed.

To use stoichiometry, one must first understand what a mole is. A mole is a unit that measures the amount of substance, defined as containing approximately \(6.022 \times 10^{23}\) entities (Avogadro's number). The mole allows chemists to count atoms, molecules, and ions by weighing them. Stoichiometry uses the mole concept to help scientists perform calculations about the masses of reactants consumed and the masses of products formed in a chemical reaction.

When calculating stoichiometric problems, chemists often use a mole ratio, which comes directly from the coefficients of a balanced chemical equation. The mole ratio is instrumental in figuring out how much of each reagent is needed or how much of a product will form under given conditions.

Chemical Reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. During the reaction, the bonds between atoms are broken in the reactants and new bonds are formed to create the products. This process involves energy changes, where energy is absorbed or released, which can be evident through heat, light, or sound.

Chemical reactions are represented by chemical equations. Reactants are listed on the left side, products on the right, and an arrow separates the two, indicating the direction of the reaction. There are different types of chemical reactions, like synthesis, decomposition, single replacement, double replacement, and combustion, each with unique characteristics.

To predict the outcome of a chemical reaction, knowledge of both stoichiometry and the limiting reactant is crucial. Identifying the limiting reactant allows chemists to determine the maximum amount of product that can be formed. As in the given exercise, understanding which reactant will run out first (the limiting reactant) is a key aspect of predicting the quantitative result of the reaction.

Balanced Chemical Equation

A balanced chemical equation is a written representation of a chemical reaction that adheres to the law of conservation of mass. Each side of the equation has the same total number of atoms for each element involved in the reaction. Balancing an equation ensures that the stoichiometry, or the quantitative relationship between reactants and products, is accurate.

To balance a chemical equation, one must adjust the coefficients, which are the numbers placed in front of the chemical formulas. These coefficients indicate the relative amounts of each substance involved in the reaction and are used to calculate the mole ratio. The incorrect assumption highlighted in the exercise is that the limiting reactant has the lowest coefficient in a balanced equation. However, coefficients alone do not reveal which reactant will run out first; they simply show the ratio needed to make the reaction occur.

The balanced equation is essential for calculating the correct ratios and determining the limiting reactant, as was correctly identified in the exercise. Without a balanced equation, stoichiometric calculations would be flawed, potentially leading to incomplete reactions or incorrect predictions of the quantities of products formed.