Thermochemistry II

1. Thermochemistry II Heat capacity and Calorimetry

2. Calorie • A calorie is a unit of heat. • It is defined as the quantity of heat (q) needed to raise one gram of pure water one degree C.

3. calorie versus Calorie • Dietary units of energy are Calories. • 1000 calories = • 1 kilocalorie = • 1 Calorie

4. Joules • Joule is the SI unit for energy. • 1 joule= 0.2390 cal • 1 cal = 4.184 joule

5. Heat Capacity • The amount of heat needed to increase the temperature of an object exactly 1 degree C is the heat capacity of an object. • Different objects have different heat capacities. • Which heats faster, Cu or Al?

6. Heat Capacity

7. Specific heat capacity • Specific heat capacity or Specific Heat, is the amount of heat it takes to heat 1 gram of a substance 1 degree C.

10. Examples • If a chunk of silver has a heat capacity of 42.8 J/°C. If the silver has a mass of 181 grams, calculate the specific heat of silver. • Answer: • 0.236 J/(g °C)

11. Calorimetry

12. Enthalpy • ∆H is the heat under constant pressure. • What pressure is constant? • Atmospheric Pressure.

13. q= ∆H = m x C x ∆T • ∆H is the heat under constant pressure. • C is the heat capacity. • In calorimetry, C is the heat capacity of water, the item heated.(system or surrounding?) • ∆T is the change in temperature. • T final – T initial = ∆T

14. Bomb Calorimetry

15. ∆H versus q • In a bomb calorimeter, the pressure is not constant. • With combustion under a constant volume, pressure will increase. • Therefore we must use q and not ∆H.

16. Example: • Go to worksheet.