Honors Chemistry Presentation 09-08 The Flow of Energy Page 1 of 5 Objectives: When you complete this presentation, you will be able to o define energy, heat, and work o describe the flow of energy in exothermic and endothermic processes o use the proper units when calculating energy, heat, and work o perform specific heat calculations Energy Transformations Energy is the capacity for doing work or supplying heat. o Energy is detected only by its effects: the motion of a bounced basketball The heat generated by a chemical reaction Thermochemistry is the study of changes in chemical reactions and changes of state. Chemical potential energy is the energy stored in chemical bonds. Chemical potential energy is seen in the combustion of methane in a laboratory burner. o Methane, CH4, is added to the burner with oxygen gas, O2. o A spark is added to the mixture and there is a flame. o C-H (in CH4), and O-O (in O2) bonds are broken. o C-O (in CO2) and O-H (in H2O) bonds are formed. o Energy is released. Looking at the reaction CH4 +2 O2 → CO2 +2 H2 Honors Chemistry Presentation 09-08 The Flow of Energy Page 2 of 5 Heat, symbolized by q, o is the transfer of energy from one object to another o due to the temperature difference between the two objects. Heat always flows from a warm object to a cooler object. If the two objects are in contact, and remain in contact, they will come to the same temperature. Endothermic and Exothermic Processe A chemical reaction is part of a system. Everything outside the system is called the surroundings. Energy is not created or destroyed. The total amount of energy in the system and the surroundings must remain the same. If the chemical reaction in the system uses energy o then energy is transferred from the surroundings into the system. This is an endothermic process In an endothermic process, the system gains heat while the surroundings cool down. If the chemical reaction in the system makes energy then energy is transferred from the system out to the surroundings. This is an exothermic process In an exothermic process, the system cools down while the surroundings gain heat. Honors Chemistry Presentation 09-08 The Flow of Energy Page 3 of 5 Units for Measuring Heat Flow We measure heat flow in two common units: o the calorie (cal) o the joule (J) The calorie (with a lower case “c”) is ... o a non-SI unit. o the amount of heat to increase the temperature of water by 1°C. o equal to 0.001 Calories (food calories or Cal). o There are 1,000 cal in 1 Cal. The joule is ... o the SI unit for heat (and energy and work). o the energy required to apply 1 newton of force (about 3.6 ounces) over a distance of 1 meter. o equal to 0.2390 cal. There are 4.184 J in 1 cal. We will be using the joule, J, exclusively in this course. You will not be required to convert from J ➔cal or from cal ➔J. Heat Capacity and Specific Heat The amount of heat needed to increase the temperature of an object by exactly 1°C is called the heat capacity of that object. The heat capacity of an object depends on both its ... o mass o chemical composition It takes more heat to increase the temperature of 1 kg of water than it does to increase the temperature of 1 g of water. It takes more heat to increase the temperature of 1 kg of water than it does to increase the temperature of 1 kg of iron. The amount of heat needed to increase the temperature of an object with a mass of exactly 1 g by exactly 1°C is called the specific heat capacity or the specific heat, C, of that object. Honors Chemistry Presentation 09-08 The Flow of Energy Page 4 of 5 Different objects have different specific heats. Substance C(J/g∙°C) C(cal/g∙°C) Water 4.18 1.00 Ice 2.1 0.50 Steam 1.7 0.4 Aluminum, Al 0.90 0.21 Iron, Fe 0.46 0.11 Silver, Ag 0.24 0.57 Water has a very high specific heat. o C = 4.18 J/g•°C. This means that you must add a lot of heat to water to increase its temperature. It also means that we get a lot of heat out of water when we decrease its temperature. o Farmers use this to protect crops in danger of freezing. o This is why the filling in a hot apple pie is more likely to burn your tongue than is the crust. To calculate the specific heat, C, of a material you divide the amount of heat input, q, by the mass, m, times the temperature change, ∆T. C= q m ∙ ∆T We can use this to calculate the specific heat of any material. Example 1: The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat of copper? Honors Chemistry Presentation 09-08 The Flow of Energy Page 5 of 5 Sample Problems: Sample Problems: Find the specific heat of each of the following metals that are heated from an initial temperature of 25.0°C to the indicated final temperature using the indicated amount of heat. 1. 37.0 g of iron heated to 50.0°C using 415 J of heat. 2. 15.0 g of benzene heated to 52.0°C using 705 J of heat. 3. 78.0 g of calcium heated to 30.0°C using 252 J of heat.