Stoichiometry Guide

Stoichiometry is the quantitative study of reactants and products in chemical reactions, based on balanced equations. It allows chemists to predict yields, determine reactant needs, and analyze reaction efficiency. For example, in \( \ce{2H2 + O2 -> 2H2O} \), stoichiometry tells us how much water forms from given hydrogen and oxygen amounts. This guide covers moles, ratios, limiting reactants, and applications.

The Mole Concept

A mole is a unit representing \( 6.022 \times 10^{23} \) particles (Avogadro’s number), linking microscopic atoms to macroscopic masses:

\[ 1 \, \text{mol} = 6.022 \times 10^{23} \, \text{molecules/atoms} \]

Molar mass converts moles to grams (e.g., \( \ce{H2} \): 2 g/mol; \( \ce{O2} \): 32 g/mol). Example: 4 g \( \ce{H2} = 2 \, \text{mol} \).

Mole Ratios

Coefficients in balanced equations give mole ratios. For \( \ce{N2 + 3H2 -> 2NH3} \):

  • 1 mol \( \ce{N2} \) reacts with 3 mol \( \ce{H2} \).
  • Produces 2 mol \( \ce{NH3} \).

Problem: How many moles of \( \ce{NH3} \) from 2 mol \( \ce{N2} \)?

\[ 2 \, \text{mol} \, \ce{N2} \times \frac{2 \, \text{mol} \, \ce{NH3}}{1 \, \text{mol} \, \ce{N2}} = 4 \, \text{mol} \, \ce{NH3} \]

Mass calculation: \( 4 \, \text{mol} \, \ce{NH3} \times 17 \, \text{g/mol} = 68 \, \text{g} \).

Limiting Reactants

The limiting reactant determines the reaction’s yield when reactants aren’t in exact stoichiometric ratios. Example: \( \ce{2H2 + O2 -> 2H2O} \), with 3 mol \( \ce{H2} \) and 1 mol \( \ce{O2} \):

  • \( \ce{H2} \): \( 3 \, \text{mol} / 2 = 1.5 \, \text{mol} \, \ce{H2O} \) possible.
  • \( \ce{O2} \): \( 1 \, \text{mol} \times 2 = 2 \, \text{mol} \, \ce{H2O} \) possible.

\( \ce{H2} \) limits at 1.5 mol \( \ce{H2O} \); excess \( \ce{O2} = 0.25 \, \text{mol} \).

Applications

Stoichiometry is vital in:

  • Industry: Calculating fertilizer yields (\( \ce{NH3} \)) in the Haber process.
  • Medicine: Drug synthesis stoichiometry ensures correct dosages.
  • Environment: Assessing \( \ce{CO2} \) from fuel combustion.

It’s the backbone of chemical engineering and research.