The flow of energy

1. The flow of energy Heat and work

2. Objectives • When you complete this presentation, you will be able to • define energy, heat, and work • describe the flow of energy in exothermic and endothermic processes • use the proper units when calculating energy, heat, and work • perform specific heat calculations

3. Energy transformations • Energy is the capacity for doing workor supplying heat. • Energy is detected only by its effects: • the motionof a bounced basketball • The heatgenerated by a chemical reaction • Thermochemistry is the study of changesin chemical reactions and changes of state. • Chemical potentialenergy is the energy stored in chemical bonds.

4. Energy transformations • Chemical potential energy is seen in the combustion of methane in a laboratory burner. • Methane, CH4, is added to the burner with oxygen gas, O2. • A spark is added to the mixture and there is a flame. • C-H (in CH4), and O-O (in O2) bonds are broken. • C-O (in CO2) and O-H (in H2O) bonds are formed. • Energy is released.

5. Energy transformations • Looking at the reaction CH4 +2 O2 → CO2+ 2 H2O

6. Energy transformations • Looking at the reaction CH4 +2 O2 → CO2+ 2 H2O First, we break bonds.

7. Energy transformations • Looking at the reaction CH4 +2 O2 → CO2+ 2 H2O Then, we make bonds. + Energy

8. Energy transformations • Heat, symbolized by q, • is the transfer of energyfrom one object to another • due to the temperature difference between the two objects. • Heat always flows from a warmobject to a coolerobject. • If the two objects are in contact, and remain in contact, they will come to the same temperature.

9. Exothermic and endothermic Processes • A chemical reaction is part of a system. • Everything outside the system is called the surroundings. Surroundings System

10. Exothermic and endothermic Processes • Energy is not created or destroyed. • The total amount of energy in the system and the surroundings must remain the same. Surroundings System

11. Exothermic and endothermic Processes • If the chemical reaction in the system uses energy • then energy is transferred from the surroundingsinto the system. • This is an endothermicprocess • In an endothermic process, the system gains heat while the surroundings cool down. Surroundings System energy

12. Exothermic and endothermic Processes • If the chemical reaction in the system makes energy • then energy is transferred from the system out to the surroundings. • This is an exothermicprocess • In an exothermic process, the system cools down while the surroundings gain heat. Surroundings System energy

13. Units for measuring heat flow • We measure heat flow in two common units: • the calorie(cal) • the joule(J) • The calorie (with a lower case “c”) is ... • a non-SI unit. • the amount of heat to increase the temperature of water by 1°C. • equal to 0.001 Calories (food calories or Cal). • There are 1,000 cal in 1 Cal.

14. Units for measuring heat flow • We measure heat flow in two common units: • the calorie(cal) • the joule(J) • The joule is ... • the SI unit for heat (and energy and work). • the energy required to apply 1 newton of force (about 3.6 ounces) over a distance of 1 meter. • equal to 0.2390 cal. • There are 4.184 J in 1 cal.

15. Units for measuring heat flow • We measure heat flow in two common units: • the calorie(cal) • the joule(J) • We will be using the joule exclusively in this course. • You will not be required to convert from J ➔ cal or from cal ➔ J.

16. 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 ... • mass • 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.

17. Heat capacity and specific heat • 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. • Different objects have different specific heats.

18. Heat capacity and specific heat • Water has a very highspecific heat. • 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. • Farmers use this to protect crops in danger of freezing.

19. Heat capacity and specific heat • Water has a very highspecific heat. • 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. • This is why the filling in a hot apple pie is more likely to burn your tongue than is the crust.

20. Heat capacity and specific heat • 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 = • We can use this to calculate the specific heat of any material. q m∙∆T

21. Heat capacity and specific heat 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? m = 95.4 g ∆T = Tf – Ti = 48.0°C – 25.0°C = 23.0°C q = 849 J C = q m∙∆T 849 J = 0.387 J/g∙°C = (95.4 g)(23.0°C)

22. Heat capacity and specific heat 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. • 37.0 g of iron heated to 50.0°C using 415 J of heat. • 15.0 g of benzene heated to 52.0°C using 705 J of heat. • 78.0 g of calcium heated to 30.0°C using 252 J of heat. C = 0.449 J/g∙°C C = 1.74 J/g∙°C C = 0.646 J/g∙°C

23. summary • Energy is the capacity for doing work or supplying heat. • Thermochemistry is the study of energy changes in chemical reactions and changes of state. • Chemical potential energy is the energy stored in chemical bonds. • Heat, symbolized by q, is the transfer of energy from one object to another due to the temperature difference between the two objects.

24. summary • A chemical reaction is part of a system. • Everything outside the system is called the surroundings. • Energy is not created or destroyed, which means that the total amount of energy in the system and the surroundings must remain the same.

25. summary • If the chemical reaction in the system uses energy, then energy is transferred from the surroundings into the system (an endothermic reaction). • If the chemical reaction in the system makes energy, then energy is transferred from the system into the surroundings (an exothermic reaction).

26. summary • We measure heat flow in two common units: the calorie (cal), which we will not use and the joule (J), which we will use. • 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 mass and its chemical composition.

27. summary • 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. • 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.