Chem 162 - Ch 9 Thermochemistry (stoichiometry)

Introduction to Thermochemistry

• Focus on thermochemistry and its application in stoichiometry to calculate heats of reactions.

Combustion of Sulfur

• Reaction: Sulfur (S₈) + 8 O₂ → 8 SO₂.

• Enthalpy change: ΔH = -2.39 x 10³ kJ for 8 moles of SO₂.

• This heat can be used as a conversion factor for stoichiometric calculations.

Example Problem: Heat Evolved from Burning Sulfur

• Question: How much heat is evolved by burning 15.0 grams of sulfur?

• Steps:

  • Convert grams of sulfur to moles:

    • Molar mass of S₈ = 256.5 g/mol.

  • Use enthalpy change per mole to find heat:

  • Heat evolved (Q) = moles of S₈ x ΔH.

  • Final answer: Q = -140 kJ (3 significant figures).

Daily Energy Requirements

• A 20-year-old man (67 kg): 1.3 x 10⁴ kJ daily energy requirement.

• A 20-year-old woman (58 kg): 8.8 x 10³ kJ daily energy requirement.

Combustion of Glucose Reaction

• Reaction: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O.

• Enthalpy change: ΔH = -2.82 x 10³ kJ.

• This represents energy obtained from glucose combustion (cellular respiration).

Calculating Glucose Consumption

• Process: Convert energy need to moles of glucose, then to grams.

For the Man:

• Energy needed: 1.3 x 10⁴ kJ.

• Conversion to moles of glucose:

  • Moles of glucose = Energy needed / ΔH = 1.3 x 10⁴ kJ / -2.82 x 10³ kJ/mol.

• Convert moles to grams:

  • Molar mass of glucose = 180.2 g/mol.

  • Final calculation gives ∼830 grams of glucose required.

For the Woman:

• Energy needed: 8.8 x 10³ kJ.

• Conversion to moles of glucose:

  • Moles of glucose = 8.8 x 10³ kJ / -2.82 x 10³ kJ/mol.

• Convert moles to grams:

  • Final calculation gives ∼560 grams of glucose required.

Importance of Negative Sign in Enthalpy

• The negative sign indicates energy released during reactions.

• It does not affect the calculated mass of glucose, ensuring it remains positive.

• The focus on signs is essential for accuracy in understanding thermodynamic implications.

Summary

• Stoichiometry can be applied to calculate heat changes and required reactants in thermochemical reactions, utilizing enthalpy as a conversion factor.