Overview
This lecture covers the key concepts, definitions, and equations of thermochemistry, including internal energy, heat, work, phase changes, thermochemical equations, heats of formation, and Hess's Law.
Internal Energy, Heat, and Work
- The change in internal energy (ΔE) of a system is ΔE = q + w.
- q (heat) is positive when absorbed (endothermic) and negative when released (exothermic).
- Work (w) is calculated as w = -PΔV; positive when work is done on the system, negative when done by the system.
- 1 calorie (cal) = 4.184 joules (J); 1 kilocalorie (kcal) = 1,000 cal.
- 1 L·atm = 101.3 J (useful for unit conversions with gases).
Calculating Heat and Energy Changes
- Use q = m·c·ΔT for temperature changes, where m = mass, c = specific heat, and ΔT = temperature change.
- For water, c = 4.184 J/g°C (or J/g·K).
- For phase changes, use q = m·ΔH (ΔH = heat of fusion/vaporization) or q = n·ΔH (n = moles).
- Example: To melt 54 g of ice (heat of fusion = 6 kJ/mol), convert grams to moles, then multiply by heat of fusion.
Thermochemical Equations and Calculations
- Combustion reactions: balance atoms first, then calculate energy changes using mole relationships.
- To find heat released/absorbed: grams → moles → kJ by using balanced equations and stoichiometry.
- Example: 64 g O₂ yields energy by converting grams to moles and scaling by the balanced equation.
Heats of Formation and Reaction Enthalpy
- Standard enthalpy of formation (ΔH_f°): heat change for forming 1 mole of a compound from its elements in standard states.
- Reaction enthalpy (ΔH_rxn) = sum of products' ΔH_f° − sum of reactants' ΔH_f°.
- O₂ and other pure elements have ΔH_f° = 0.
Hess's Law
- Hess’s Law: Overall ΔH for a reaction = sum of enthalpies for steps that combine to give the target reaction.
- Reverse reactions or multiply coefficients as needed, adjusting ΔH accordingly.
- Add or subtract ΔH values to get the target reaction's enthalpy.
Key Terms & Definitions
- Internal Energy (ΔE) — total energy change of a system (q + w).
- q (Heat) — energy transferred due to temperature difference.
- w (Work) — energy from volume change (w = -PΔV).
- Enthalpy of Formation (ΔH_f°) — heat change for forming 1 mole of a substance from its elements.
- Hess's Law — total enthalpy change is the sum of enthalpy changes for individual steps.
Action Items / Next Steps
- Practice stoichiometric and enthalpy calculations using provided equations and unit conversions.
- Review conversion factors for energy units and standard enthalpies of formation.
- Balance and analyze simple thermochemical equations as exercises.