Overview
This lecture covers the technique of calorimetry, including its applications, calculations, types of calorimeters, and real-world examples. Key concepts include heat transfer, specific heat, and interpreting calorimetry data.
Calorimetry Basics
- Calorimetry measures heat transfer in chemical or physical processes using a calorimeter.
- The system is the substance undergoing change; the surroundings absorb or provide heat.
- Exothermic reactions release heat to surroundings (temperature increases), endothermic absorb heat (temperature decreases).
- Accurate calorimetry requires minimizing heat exchange with the external environment.
Types of Calorimeters
- Simple calorimeters (e.g., polystyrene cup/coffee cup) are common in teaching labs, but less accurate.
- Commercial calorimeters offer better insulation, stirring, and precise sensors.
- Bomb calorimeters operate at constant volume for high-heat and combustion reactions.
Calculating Heat Transfer
- At thermal equilibrium in ideal calorimetry: (q_{substance,1} + q_{substance,2} = 0).
- (q = cm\Delta T), where (c) is specific heat, (m) is mass, (\Delta T) is temperature change.
- For reactions: (q_{reaction} + q_{solution} = 0); often (q_{reaction} = -q_{solution}).
Worked Examples & Sample Calculations
- Determining unknown initial temperatures of substances by equating heat lost and gained.
- Identifying unknown metals via specific heat calculation after calorimetry.
- Calculating heat of reaction in solution by approximating solution as water.
- Using bomb calorimeters, account for both water and the calorimeter’s heat capacity.
Everyday Applications
- Hand warmers: exothermic crystallization of sodium acetate or oxidation of iron generates heat.
- Instant ice packs: endothermic dissolution (e.g., NH₄NO₃ in water) absorbs heat.
- Calorimetry can measure metabolic energy expenditure in humans with whole-body/room calorimeters.
Nutritional Calorimetry
- Energy in food is given in Calories (1 Calorie = 1 kcal = 1000 cal = 4184 J).
- Proteins and carbohydrates: ~4 Cal/g; fats: ~9 Cal/g.
- Food labels use the Atwater system, based on average energetic values of macronutrients.
Key Terms & Definitions
- Calorimetry — Measurement of heat transfer in physical or chemical changes.
- Calorimeter — Device to measure the amount of heat exchanged.
- Specific Heat ((c)) — Heat needed to raise 1 g of substance by 1°C.
- System & Surroundings — System is what's studied; surroundings exchange heat with the system.
- Exothermic — Process that releases heat ((q) negative).
- Endothermic — Process that absorbs heat ((q) positive).
- Bomb Calorimeter — Calorimeter for constant volume, used in combustion reactions.
- Nutritional Calorie (Calorie/Cal) — 1 Cal = 1000 calories (energy to heat 1 kg water by 1°C).
Action Items / Next Steps
- Practice calorimetry calculations using provided examples.
- Review Table 9.1 for specific heats.
- Complete assigned homework problems on calorimetry.