Specific Heat General Organic and Biological Chemistry Copyright

Specific Heat Specific heat § is the amount of heat needed to raise the temperature of 1 g of a substance by 1 °C (or K) § is different for different substances § in the SI system has units of J/g C § in the metric system has units of cal/g C General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 2

Examples of Specific Heats General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education,

Learning Check A. For the same amount of heat added, a substance with a large specific heat 1) has a smaller increase in temperature 2) has a greater increase in temperature B. When ocean water cools, the surrounding air 1) cools 2) warms 3) stays the same C. Sand in the desert is hot in the day and cool at night. Sand must have a 1) high specific heat 2) low specific heat General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 4

Solution A. For the same amount of heat added, a substance with a large specific heat 2) has a smaller increase in temperature B. When ocean water cools, the surrounding air 2) warms C. Sand in the desert is hot in the day and cool at night. Sand must have a 2) low specific heat General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 5

Heat Equation Rearranging the specific heat expression gives the heat equation. energy = specific heat x mass x change in temperature heat = c x m x ΔT Units: § Energy Joules (J) or calories (cal) § Mass grams (g) or kilograms (kg) § Temperature change °C or °F or K § Specific heat cal/g • °C or J/g • °C General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 6

Sample Calculation for Heat A hot-water bottle contains 750 g of water at 65 °C. Water has a specific heat of 1. 00 cal/g • °C. If the water cools to body temperature (37 °C), how many calories of heat could be transferred to sore muscles? STEP 1 Write the version of the formula needed. energy = c x m x ΔT STEP 2 Plug in given values energy = 1. 00 cal x 750 g x (65 °C - 37 °C) General, Organic, and Biological Chemistry g • °C Copyright © 2010 Pearson Education, Inc.

Sample Calculation for Heat STEP 3 Solve energy = 1. 00 cal x 750 g x (65 °C - 37 °C) g • °C energy = 1. 00 cal x 750 g x (28 °C) g • °C energy = 21, 000 calories General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 8

Example of Calculating Specific Heat What is the specific heat of a metal if 24. 8 g absorbs 65. 7 cal of energy and the temperature rises from 20. 2 C to 24. 5 C? STEP 1 Write the version of the formula needed. energy = c x m x ΔT energy = c m x ΔT General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 9

Example of Calculating Specific Heat What is the specific heat of a metal if 24. 8 g absorbs 65. 7 cal of energy and the temperature rises from 20. 2 C to 24. 5 C? STEP 2 Plug in given values energy = c m x ΔT ____65. 7 cal_____ = c 24. 8 g x (24. 5°C to 20. 2°C) General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 10

STEP 3 Solve ____65. 7 cal_______ = c 24. 8 g x (24. 5°C to 20. 2°C) ___65. 7 cal____ = c 24. 8 g x (4. 3°C) __65. 7 cal__ = c 106. 64 g • °C __0. 62 cal__ = c g • °C General, Organic, and Biological Chemistry c = 0. 62 cal/g • °C Copyright © 2010 Pearson Education, Inc. 11

Learning Check How many kilojoules are needed to raise the temperature of 325 g of water from 15. 0 °C to 77. 0 °C? 1) 20. 2 k. J 2) 84. 3 k. J 3) 105 k. J 84. 3 k. J General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 12