CHM 151 – Lecture 9 Notes on Chapter 4: The Mole & Chemical Equations

Instructor: Lauren N. West-Livingston, MD, PhD, MSL
Course: CHM 151 – Physical Sciences
Chapter: 4 – Parts 5-6

Chapter 4: The Mole & Chemical Equations

Chapter 4 Part 5: Limiting Reagents

Limiting Reagent:
- The reactant that is completely consumed first in a chemical reaction, thus determining the maximum amount of product formed.
Excess Reagent:
- The reactant that is present in a quantity greater than necessary to completely react with the limiting reagent.

A practical analogy for understanding limiting and excess reagents is drawing a comparison to hotdogs and buns. In this analogy, the limiting reagent could be the hotdogs if there are fewer hotdogs than buns (the excess reagent). This emphasizes how one component can limit the end result of a mixture.

Pick a Reactant:
- Choose one reactant to work with, and calculate how much of the other reactant is required for the reaction to occur.
Comparison:
- Compare the calculated amount required with the actual amount provided to identify which is the limiting reagent and which is the excess reagent.
Determine Theoretical Yield:
- Utilize the identified limiting reagent to calculate the theoretical yield of the reaction, or the amount of product that can be formed under ideal conditions.

Problem Statement:
If 5.00 g of hydrogen is combined with 25.0 g of oxygen, how many grams of water can be produced?

Pick a Reactant:
We will begin with hydrogen and determine how much oxygen is needed to fully react with 5.00 g of hydrogen.

Compare Amounts:
Upon calculating the required amount of oxygen to react with 5.00 g of hydrogen, we can conclude whether oxygen is the limiting reagent or if hydrogen is.
Identified Limiting Reagent: Oxygen.

Determine Theoretical Yield:
Use the limiting reagent (oxygen) to find the theoretical yield of water produced in the reaction. This is done by using stoichiometric principles based on the balanced chemical equation of the reaction.

Chapter 4 Part 6: Combustion Reactions

A Combustion Reaction is defined as any reaction in which a substance reacts with oxygen, producing products that are typically carbon dioxide and water.

If the reactant contains:

Combustion analysis is utilized to determine the empirical formula of a compound through the following steps:
- Pure oxygen is pumped through a tube containing the sample.
- The sample is heated to its combustion temperature, which ensures complete combustion.
- The remaining mass, after combustion, can be used to derive the empirical formula of the compound.

Problem Statement:
A hydrocarbon (denoted as CₓHᵧ) underwent combustion, producing 0.379 g of CO₂ and 0.1035 g of H2O.

Calculate Moles:
- Determine the number of moles of carbon derived from carbon dioxide (CO₂) and the number of moles of hydrogen derived from water (H₂O).

Calculation of Moles:

The number of moles of carbon in CO₂ can be calculated using the molar mass of carbon (approximately 12.01 g/mol).
- Moles of carbon = rac{0.379 ext{ g CO₂}}{44.01 ext{ g/mol}} imes 1 ext{ mol C}
- Moles of hydrogen in H₂O can be calculated using the molar mass of water (approximately 18.02 g/mol).
- Moles of hydrogen = rac{0.1035 ext{ g H₂O}}{18.02 ext{ g/mol}} imes 2 ext{ mol H}

Calculate the ratio of moles of carbon to moles of hydrogen obtained from the above calculations. Use this ratio to express the empirical formula. The empirical formula is determined by simplifying the ratio of elements to the smallest whole numbers.