Energy

1. Energy Energy is the ability to do work or transfer heat. Units of Energy: -Joule (J) 1 J = 1 kg-m2/s2

2. Energy • Kinetic Energy: Energy of an object in motion • Potential Energy: Energy of an object with respect to the position of the object in relation to another object

3. Energy • Chemical Energy: Energy produced or absorbed by the formation or breakage of chemical bonds. • Thermal Energy: Heat Energy

4. Law of Conservation of Energy • The energy can NOT be created or destroyed.

5. Some Types of Energy • Kinetic • Thermal (the internal energy of an object due to the kinetic energy of its atoms and/or molecules) • Mechanical (the energy associated with motion and position of an object) • Potential • Chemical (energy due to chemical reaction) • Gravitational (object can “fall”) • Elastic (energy stored as a result of deformation of an elastic object)

6. Chemical Energy Graphs Exothermic Rxn Δ H is negative Heat is released Endothermic Rxn Δ H is positive Heat is absorbed

7. Conservation of Energy • Energy cannot be created or destroyed. • When we see energy change, it is not lost, just transferred, perhaps converted to another type of energy

8. Heat Energy • Temperature = Measurement of the average kinetic energy of molecules in a substance. • Heat = Energy that is transferred from one substance to another. • Internal energy = Total amount of energy a substance contains. (Most I.E. is kinetic.) • More molecular movement = more kinetic energy = higher temperature

9. Temperature measurement • Temperatures are measured using the Celsius and Kelvin scales. • Kelvin is based on the amount of energy in a substance. • At 0 Kelvin, there is no movement, no kinetic energy. This temperature is called absolute zero. • Recall: K = C + 273.

10. Transfer of Heat • Can occur by 1. Conduction 2. Convection 3. Radiation • Is measured as a temperature change in a substance. • Heat is measured in Joules (like work).

11. Conduction is heat energy transferred when one substance comes in contact with another. • Metal spoon in boiling liquid • Depends on collision between molecules of a substance.

12. Radiation is the transfer of energy by infrared waves. • Sun warming your skin: The molecules in your skin vibrate faster when struck by IR waves. • For radiation, matter is not needed to transfer heat energy. • Radiant energyis any energy transferred by radiation.

13. Convection is heat energy transferred by movement of a fluid. • Ex: Warm air from a wood-burning stove • A convection current is the pattern of heat energy movement. • Warm fluid expands and is less dense than surrounding fluid: Warm rises and cool sinks. • Convection currents continue to form as long as there is a heat source.

14. Calories • Another way to measure heat • 1 calorie = 4.186 J • 1 calorie = the amount of heat needed to raise 1 g of water 1°C. • Food Calories are actually kilocalories. • When calorie is spelled with capital C, it is a food calorie. • A resting 150-lb person gives off heat at a rate of ~1200 Calories in 24 hours.

15. Thermal Properties of Materials Local surface temperatures on the Earth’s surface depend on: • Reflectivity • Is the proportion of radiation that is reflected • Reflected energy does not raise temperature • Specific heat capacity • Quantity of thermal energy needed to raise the temp of 1 g of a material by 1°C • Quantitative application: q = mcΔT

16. Specific heat • To calculate the energy transferred to or by a substance, use this formula: q = mcΔT • q = energy in J • m= mass in grams • c = specific heat for the substance (J/g-°C) • ΔT = change in temperature (tfinal – tinitial) in °C

17. Tipler, Paul A., Physics for Scientists and Engineers, 4th Ed., W.H. Freeman, (1999). & engineeringtoolbox.com

18. Enthalpy: ΔH • Enthalpy is the amount of heat content used or released in a system at constant pressure • Mathematically: Sum of ΔH products – sum of ΔH reactants = ΔH for the reaction (ΔHrxn)

19. Enthalpy Example • Chemical reactions: When bonds are broken, the energy in those bonds is available to be used in the products or is released as heat. • Example = combustion reaction, such as for glucose: C6H12O6 + 6O26 CO2 + 6H2O ΔH = - 2814 kJ ΔH = 6(ΔHfCO2) + 6(ΔHfH2O) –(ΔHC6H12O6) -6(ΔHfO2) (values from table) ΔH = 6(-393) + 6(-286) –(-1260) -6(0) = -2814 kJ • Recall that the negative ΔH means that 2803 kJ are released in the combustion of 1 mole of glucose. • ΔH is negative: This is an exothermic reaction.

20. Heat and Matter • Adding heat energy increases the motion of a substance’s molecules. • Phase changes occur when energy changes. • When ice melts, the temperature changes only when ice has melted.

21. Thermal Expansion • Thermal expansion occurs when heat causes the molecules of a substance to spread out. • Occurs in solids, liquids and gases. • Examples include: • Roads and bridges in the hot sun • Boiling liquids • Air in a tire (After driving, the friction increases the heat and tire expands.) • Exception: Water expands as it cools between 4°C and 0°C.

22. Phase Changes • Consider water to remember the words for most phase changes: Steam Condense GAS Freeze Water Evaporate LIQUID Melt Ice SOLID Note that all phases still water (H2O). These are PHYSICAL changes.

23. Phase Changes, Cont. • Another word for changing to the gaseous state is vaporization. • Vaporization includes evaporation (molecules leave the liquid’s surface) AND sublimation (solid to gas, such as dry ice, CO2)

24. Physical and Chemical Changes • 1st test: Is something new made? • Does the new substance have a different chemical formula than the original substance? • If yes, then it is a chemical change. • Examples of new substances: • Gas (bubbles) • Energy (heat/light) • Precipitate (solid – may be a different color) • Remember- If the substance only changes phase, it is a physical change.