Section 15-2

1. Section 15.2 Heat • Describehow a calorimeter is used to measure energy that is absorbed or released. • Explain the meaning of enthalpy and enthalpy change in chemical reactions and processes. Section 15-2

2. Calorimetry • A calorimeteris an insulated device used for measuring the amount of heat absorbed or released in a chemical reaction or physical process.

3. How dose it work? • 1- A sample(food for example) is positioned in a steel inner chamber called the bomb, which is filled with oxygen at high pressure. • 2- Surrounding the bomb is a measured mass of water stirred by a low-friction stirrer to ensure uniform temperature. • 3- The reaction is initiated by a spark, and the temperature is recorded until it reaches its maximum.

4. You have to know that: • A known mass of water is placed in an insulated chamber to absorb the energy released from the reacting system or to provide the energy absorbed by the system. • Combustion reactions carried out in calorimeters

5. Determining specific heat: Using foam-cup calorimeter which are open to the atmosphere, so reactions carried out in them occur at constant pressure. Q:Suppose you put 125 g of water into a foam-cup calorimeter and find that its initial temperature is 25.60°C.Then you heat a 50.0g sample of the unknown metal to 115.0°C and put the metal sample into the water

6. Heat flows from the hot metal to the cooler water, and the temperature of the water rises. The flow of heat stops only when the temperature of the metal and the water are equal.( specific heat of water=4.184j/(g.C), T final for water = 29.3C, T final for metal=29.3C) • Equation for Calculating Heat: • q = c × m × ∆T • q = the heat absorbed or released • C= specific heat • M= mass • ∆T= is the change in temperature in °C, or Tfinal -Tinitial.

7. q water = 4.184 × 125 × (29.30°C - 25.60°C) • q water = 4.184 × 125 × 3.70°C • q water = 1940 J • The heat gained by the water, 1940 J, equals the heat lost by the metal, • q metal = q water • q metal = −1940 J • c metal × m × ∆T = −1940 J

8. c metal = -1940 J m × ∆T • c metal = -1940 J (50.0 g)(-85.7°C) = 0.453 J/(g·°C)

9. Chemical Energy and the Universe • Thermochemistryis the study of heat changes that accompany chemical reactions and phase changes. • The system is the specific part of the universe that contains the reaction or process you wish to study. Section 15-2

10. Chemical Energy and the Universe (cont.) • The surroundingsare everything else other than the system in the universe. • The universeis defined as the system plus the surroundings. Section 15-2

11. Chemical Energy and the Universe (cont.) • Chemists are interested in changes in energy during reactions. • Enthalpyis the heat content of a system at constant pressure. • The enthalpy change for a reaction is the enthalpy of the products minus the enthalpy of the reactants. • Enthalpy (heat) of reactionis the change in enthalpy during a reaction symbolized as ΔHrxn. ΔHrxn = Hfinal – Hinitial ΔHrxn = Hproducts – Hreactants Section 15-2

12. Chemical Energy and the Universe (cont.) • Enthalpy changes for exothermic reactions are always negative. • Enthalpy changes for endothermic reactions are always positive. Section 15-2

13. Chemical Energy and the Universe (cont.) Section 15-2

14. Chemical Energy and the Universe (cont.) Section 15-2

15. Q1:Describe how you would calculate the amount of heat absorbed or released by a substance when its temperature changes? • The heat absorbed or released equals the specific heat of the substance times its mass times its change in temperature

16. Q2 Explain why ∆H for an exothermic reaction always has a negative value? • ∆ H rxn = H products - H reactants and H products < H reactants . • Q3 Explain why a measured volume of water is an essential part of a calorimeter? • You need the mass of the water in order to calculate the heat absorbed or released • (q = cm∆T).

17. Q4 Explain why you need to know the specific heat of a substance in order to calculate how much heat is gained or lost by the substance as a result of a temperature change? The specific heat of a substance tells you the number of joules that are lost or gained for every degree change in temperature and for every gram of the substance.

18. Q5 Describe what the system means in thermodynamics, and explain how the system is related to the surroundings and the universe. • The system contains the process being studied. The surroundings are every- thing except the system, and the universe is the system and its surroundings.

19. A B C D In thermochemistry, the specific part of the universe you are studying is called ____. A.system B.area C.enthalpy D.surroundings Section 15-2

20. A B C D What is the heat content of a system at constant pressure called? A.heat of reaction B.heat of the system C.enthalpy D.entropy Section 15-2

21. A B C D What is the universe when using a bomb-calorimeter to measure heat absorbed by a substance in a process? A.the substance in the calorimeter B.the calorimeter itself C.the water in the calorimeter D.the calorimeter and all its contents Chapter Assessment 2

22. A B C D In which example is the ΔH° positive? A.an ice cube melting B.condensation forming on cold glass of water C.molten lava cooling to form solid rock D.water vapor changing directly into frost on windows

23. A B C D The specific heat of ethanol is 2.4 J/g ● C. How many degrees Celsius can a 50.0g sample be raised with 2400 J of energy? A.10° B.20° C.30° D.40° STP 1

24. End of sec 15.2