1. Chapter 16: Thermochemistry

2. Chemical reactions involve changes in energy – Breaking old bonds of reactants and forming new bonds in products. The study of the changes in energy in chemical reactions is called thermochemistry These energy changes are usually in the form of heat. INTRODUCTION TO THERMOCHEMISTRY

3. Heat and Its Units  Heat can be thought of as a flow of energy due to a difference in temperatures.  Heat always flows from a region of higher temperature to region of lower temperature. –This is the 2 nd law of Thermodynamics  The joule (J) is the SI unit of energy (heat).

5.  A joule is a very small unit of heat; therefore, kilojoules (kJ) are commonly used.  1 kJ = 1000 J  Another common unit is the calorie (cal)  1 cal = 4.184 J

6. Specific Heat All substances change temperature when they are heated, but how much they change varies significantly from one substance to another. The specific heat of a substance is the amount of energy required to raise the temperature of one gram by one Celsius degree (1°C). Each substance has its own unique specific heat that can be used to identify it.

7. Specific heat varies depending on the state of the material. SubstanceSpecific Heat J/(g°C) SubstanceSpecific Heat J/(g°C) Water (liquid)4.18Iron0.449 Ethanol2.42Copper0.385 Ice2.01Silver0.235 Aluminum0.903Lead0.129

8. The specific heat of a substance can be used to determine the amount of heat needed to cause a specific temperature change. The equation that relates these quantities is: q = m x C p x  T q is the heat lost or gained, m is the mass of the sample, c p is the specific heat (of the element or compound), and ∆ T is the difference between temperatures.

9. Sample Problems Problem #1: Gallium is a solid metal at room temperature but melts at 29.9°C. If you hold gallium in your hand, it melts from your body heat. How much heat must 2.50 g of gallium absorb from your hand to raise its temperature from 25.0°C to 29.9°C? The specific heat of gallium is 0.372 J/g°C.

10. Problem #2: A chemistry student finds a shiny rock that she suspects is gold. She determines the mass of the rock to be 14.32 g. She then finds that the temperature of the rock rises from 25°C to 52°C upon absorbing 174 J of heat. Determine the specific heat of the rock. If the specific heat of gold is 0.128 J/g°C, is the rock composed of gold?

11. One Minute Paper You have one minute to answer these two questions concerning today’s lesson. –What was the most important thing you learned? –What is still muddy?