Question

When asked, "What is the limiting reactant?" you might be tempted to choose the reactant with the smallest mass. Why is this not a good strategy?

Short answer

The limiting reactant is determined by stoichiometry, not by the smallest mass.

Step-by-step solution

Understanding Limiting Reactant

A limiting reactant is the reactant that gets completely consumed in a chemical reaction, and it determines the amount of product formed. The limiting reactant is not necessarily the one with the smallest mass but depends on the stoichiometry of the reaction.

Considering Reaction Stoichiometry

Look at the balanced chemical equation for the reaction. Each reactant and product appears in some ratio dictated by their coefficients. This ratio tells us how many moles of each substance react with one another and how many moles of product they form.

Calculate Moles of Each Reactant

To find the limiting reactant, calculate the number of moles of each reactant using the formula: \( \text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} \). This calculation will give a better sense of how much of a reactant is present compared to the others, in terms of reaction participation.

Compare Mole Ratios

Use the mole amounts from the previous step and compare them to the stoichiometric ratios from the balanced chemical equation. This comparison will tell you the proportions in which the reactants are actually available and help identify which reactant will be consumed first.

Identify the Limiting Reactant

Determine which reactant runs out first when compared to its required stoichiometric ratio. This reactant is the limiting reactant, as it limits the extent and amount of product that can be formed in the reaction.

Key concepts

Reaction Stoichiometry

Reaction stoichiometry involves understanding and using the quantitative relationships between reactants and products in a chemical reaction. This is fundamental because it allows you to predict how much of each substance is involved in the reaction, how much product will be formed, and which reactant limits the reaction's extent. The stoichiometry of a reaction is derived from its balanced chemical equation. Each coefficient in the equation represents the number of moles of that substance involved in the reaction, defining the mole ratio that must be maintained.

For example, consider a simple reaction between hydrogen and oxygen to form water:

• The balanced equation:
  \[ 2 ext{H}_2 + ext{O}_2 ightarrow 2 ext{H}_2 ext{O} \]
• This indicates that 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.
• Understanding this ratio is essential to determine the limiting reactant, as it specifies how much of each reactant is needed for complete conversion.

Without this understanding, it would be difficult to determine how variations in reactant quantities can affect the amount of product produced.

Balanced Chemical Equation

A balanced chemical equation is an expression that illustrates the identities and quantities of reactants and products involved in a chemical reaction. Balancing is crucial, as it reflects the conservation of mass, stating that matter is neither created nor destroyed in a chemical reaction, only rearranged.

To balance a chemical equation, follow these steps:

• Write the unbalanced equation indicating the reactants and products.
• Adjust the coefficients in front of each formula to ensure there is an equal number of each type of atom on both sides of the equation. Begin with elements that appear in only one reactant and product.
• Recheck your work to confirm all atoms balance across the equation.
• A correctly balanced equation serves as a template for reaction stoichiometry, providing the mole ratios needed to calculate reactant and product quantities accurately.
• For instance, in the reaction \( 2 ext{H}_2 + ext{O}_2 ightarrow 2 ext{H}_2 ext{O} \), it shows that for every 2 moles of hydrogen, 1 mole of oxygen is consumed while forming 2 moles of water.

This balance enables us to apply stoichiometry accurately to determine limiting reactants and predict reaction yields.

Mole Calculation

Mole calculations are an essential part of stoichiometry, providing a means to convert quantities of substances between mass, moles, and number of entities (like atoms or molecules). The mole is the SI unit that represents a fixed number of particles, \(6.022 \times 10^{23}\), known as Avogadro's number. This makes it easier to relate macroscopic quantities we can measure with microscopic quantities we can't see.

To calculate moles:

• Use the formula \(\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}\).
• Your starting point is the mass of the substance you have or need to react.
• The molar mass is obtained by summing the atomic masses of the elements in its chemical formula.
• This calculation allows you to determine how much of a substance is present in terms that relate readily to stoichiometric coefficients.

For example, if you have 32 grams of oxygen gas \(\text{O}_2\) with a molar mass of 32 g/mol, you have 1 mole. Knowing the number of moles helps you determine which reactant is limiting when compared with the required stoichiometric ratios in a balanced chemical equation. Remember, the concept of moles helps bridge the gap between the measurable world of grams and the theoretical world of molecules.