Energy Overview: Sources, Definitions, Laws, and Reactions

1. Energy

2. Overview: • Energy on our planet • Definitions of Energy • System and Surroundings • Exothermic and Endothermic reactions • 1st law of Thermodynamics • 2nd law of Thermodynamics

3. Energy: Source Most of all of the energy on planet Earth comes to us from the sun.

4. Energy: Source Most of all of the energy on planet Earth comes to us from the sun.

5. Energy: Definition Energy is defined as the ability to do work or transfer heat The SI unit of energyis the joule (J). (1 J = 0.2388 calorie) The SI unit of poweris the watt (W). (1 W = 1 J/s) [energy/time] kg m2 1 J = 1  s2

6. WORKED EXAMPLE 15.1Power and Energy Conversion How much electrical energy, in joules, is consumed by a 75-W bulb burning for 1.0 hr? Solution One watt is 1 J/s, so we know that 75W = 75J/s. The bulb burns for 1 hour so

7. Energy Energy can be considered to exist in two forms: • Potential energy: Energy of position or stored energy. • 2. Kinetic energy: Energy of motion.

8. 1 KE =  mv2 2 Energy Potential energy (PE) is energy an object possesses by virtue of its positionor chemical composition. Kinetic energy (KE) is energy an object possesses by virtue of its motion. Total Energy = PE+ KE

9. Energy • Energy can be converted from one type to another. • For example, the cyclist above has potential energy as she sits on top of the hill • As she coasts down the hill, her potential energy is converted to kinetic energy. • At the bottom, all the potential energy she had at the top of the hill is now kinetic energy.

10. Energy

11. System and Surroundings • The system includes the molecules we want to study (here, the hydrogenand oxygen molecules in thechamber). • The surroundingsare everything else (here, the cylinder and piston). • The universe is therefore equal to system + surroundings Two common ways energy is transferred between the system and the surroundings: Workand Heat

12. Energy and the Life Support System The biosphere is the thin film of air, water, and soil where life exists. Only a small amount of the energy the biosphere receives is used to support life. 30% of solar radiation is reflected back to space. 23% of solar radiation powers the water cycle. <0.02% is used by green plants to power photosynthesis. Photosynthesisproduces oxygen and stores energy for all animals on the planet. ENERGY (J) + 6CO2 + 6H2O C6H12O6+ 6O2

13. Energy and Chemical Reactions Endothermicreactions absorb heat energy from the surroundings. ENERGY (J) + REACTANTS PRODUCTS ENERGY (J) + 6CO2 + 6H2O C6H12O6+ 6O2

14. Energy and Chemical Reactions Exothermic reactionsrelease heat energy to the surroundings. REACTANTS PRODUCTS + ENERGY (J) C6H12O6+ 6O26CO2 + 6H2O + ENERGY (J)

15. Energy and Chemical Reactions Thermochemicalequations show both the mass and ENERGY relations: CH4(g) + 2 O2(g)CO2(g) + 2 H2O(l)+ 890.4 kJ 890.4 kJ are released for the: • Full consumptionof: • 1 mole or CH4(g) • 2 mole of O2(g) • Full productionof: • 1 mole of CO2(g) • 2 mole of H2O(l) The full consumption of 4 mole of O2(g) releases? 890.4 kJ 4 mol O2 X 2 mol O2 = 1780.8 kJ

16. WORKED EXAMPLE 15.2Energies of Chemical Reactions Burning 1.00 mol of propane releases 2201 kJ of energy. How much energy in kilojoules is released when 15.0 mol of propane is burned? Solution We start with 15.0 mol C3H8and use the balanced equation to form a conversion factor, just as we did with chemical conversions in Chapter 5.

17. Energy and Chemical Reactions

18. Energy and Chemical Reactions Factors that affect the rateof a chemical reaction include: • Temperature: Increasing temperature increases reaction rates. • Concentration reactants: Reaction ratesare dependent on reactant concentration. As concentration increases, rate increases. • Presence of catalysts: Catalysts increase the rate of reactions by lowering activation energy. Zn(s)+ 2HCl(aq)  ZnCl2(aq)+ 4 H2(g)

19. The First Law of Thermodynamics Energycan neither be created nor destroyed. This is also known as the law of conservation of energy. In other words, the total energy of the universe is a constant (DEuniv = 0); if the system loses energy, it must be gained by the surroundings, and vice versa. DEuniv = 0 = DEsys + DEsurr DEsys = -DEsurr

20. Energy spontaneously transfer from hotter objects toward colder objects. Energy flows downhill The energy available to do work is continually decreasing The Second Law of Thermodynamics

21. The Second Law of Thermodynamics Entropyis a measure of dispersal of energy (disorder). • The second law states that the disorder (or entropy) of a system tends to increase • Low entropy (S) = low disorder • High entropy (S) = greater disorder

22. Summary Energy is defined as the ability to do work or transfer heat Total Energy = Potential E+ Kinetic E 3. The energy of the universeis equal to system + surroundings 4. Exothermicreactionsrelease heat energy Endothermic reactionsconsumes energy 5. 1st law: the total energy of the universe is a constant 6. 2nd law: The energy available to do work is continually decreasing 7. Entropyis a measure of dispersal of energy (disorder).