Stoichiometry and Chemical Reactions Study Guide

DO NOW Exercises

Calculation of Moles: Use the formula n = m/M, where:
- n = moles (mol)
- m = mass (g)
- M = molar mass (g/mol)
Examples:
- 3.50 g of NaOH results in 0.0875 mol.
- 1 kg of magnesium sulfate results in 8.31 mol.

Daily Review Concepts

Enthalpy: Total energy present in a substance.
Enthalpy Change: Heat gain or loss at constant pressure.
Endothermic Reaction: Positive ΔH, where enthalpy increases during the reaction.
Exothermic Reaction: Negative ΔH, where enthalpy decreases during the reaction.

Application Exercises

Ethanol (C₂H₆O):
- Calculate the number of molecules (N) in a beaker containing 0.180 kg of ethanol.
  - N = 2.35 × 10²⁴ molecules.
- Determine the number of hydrogen atoms present:
  - N[H] = 1.41 × 10²⁵ hydrogen atoms.

Learning Objectives

Learn to use balanced chemical equations for calculating the mass of substances via stoichiometry.
- Identify limiting reactants in chemical reactions.
- Calculate the masses of products in reactions using stoichiometry.

Concept Development

Law of Conservation of Mass: Mass in an isolated system is neither created nor destroyed during chemical reactions or physical transformations.
Balanced Chemical Equations: Show the ratios of reactants and products, ensuring the number of each atom type equals on both sides of the equation.
- Example Equation: H₂SO₄(aq) + 2 LiOH(s) → Li₂SO₄(aq) + 2 H₂O(l)
Identifying Limiting Reactants: The limiting reactant is the one that gets fully consumed in a reaction.
- Example: For HCl + NaOH, if HCl is present in a smaller mole amount, it is the limiting reactant.

Stoichiometry Fundamentals

Stoichiometric Ratios: Essential for conversions in reactions.
- Example: CH₄ + 2 O₂ → CO₂ + 2 H₂O has a ratio of 1:2:1:2.
Steps to Identify the Limiting Reactant and Product Mass:
- Calculate the moles of reactants from given masses.
- Divide by stoichiometric coefficients.
- Identify the lowest mole value as the limiting reactant.
- Use mole values with stoichiometric ratios to calculate product masses.

Practice Problems

Determine the mass of MgCl₂ produced from 4.6 g of HCl and 6.2 g of magnesium.
Calculate the mass of salt from 12.3 g of HNO₃ reacting with 18.4 g of CaCO₃.

Closure: Connecting Concepts

Understanding stoichiometry allows calculations of product masses because it relies on the Law of Conservation of Mass, which links reactant and product moles. However, it's important to determine limiting reagents first since not all reactants are always fully consumed. This knowledge is invaluable for chemists aiming to wield precision in manufacturing and sustainability efforts.