Energy Energy Ability to do Work l Potential

Energy

Energy: Ability to do Work l Potential Energy = Energy of position. – Also called STORED ENERGY. l Kinetic Energy = Energy of motion. l Radiant = Electromagnetic energy. – Ex: Sunlight

Kinetic Energy l K. E. = ½ X Mass X Velocity 2 = ½ m. V 2

Potential Energy l l Kleenex Box Spring Rubberband Popper l All these can have P. E. = energy of position = stored energy l Potential Energy can be converted to Kinetic Energy

Electromagnetic Radiation l l l l Sunlight – Visible radiation Ultraviolet radiation Infrared radiation Gamma rays X-rays Microwaves Radiowaves Applet spectrum

(The speed of light)

Magnets l The potential energy in the system of two magnets depends on their…

Types of Energy (Not a complete list!)

Units of Energy l SI system: unit of energy is the JOULE. (J) Ø l 1 Joule = amount of energy required to lift a golf ball 1 meter. Other Units: calorie, Calorie, BTU’s Ø Ø 1 calorie = 4. 18 Joules 1 Calorie = 1000 calories = 1 kilocalorie

Energy in Chemistry l Chemical – energy stored in bonds l Heat – – a form of ENERGY that flows from a warmer object to a cooler object. (Macroscopic) ENERGY associated with the motion of atoms & molecules in matter. (Microscopic)

Bonding PE Curve Energy stored in chemical bonds oo nuclei approach each other high PE Separated atoms / High PE o o—o Stable Bond min PE o

Stability and Energy l If PE is high, stability is low. – – l You standing on the edge of a cliff Two highly separated atoms If PE is low, stability is high. – – You standing at the bottom of the cliff Two atoms joined together in a bond

Heat Energy l Symbol for heat energy = Q or q. l Heat depends on the amount of substance present. l We measure changes in heat.

Temperature l A measure of the average kinetic energy of the particles of a substance. l Swimming Pool vs. Teacup l Temperature is NOT energy. – Temperature does not depend on amount of substance; energy does.

Changing KE with Temperature At a given temperature, molecules have a distribution of velocity. 273° Slower molecules 373° 473° Faster molecules At higher temperatures molecules are moving at higher velocities

Law of Conservation of Energy l Energy is neither created nor destroyed in an ordinary chemical or physical change. Energy before = Energy after Energy can be converted from one form to another. - potential to kinetic - radiant to electric - electric to heat - chemical to kinetic - chemical to electrical

Energy Changes All physical and chemical changes are accompanied by… So chemists are interested in energy changes. Thermochemistry!

Energy Transfer l Measure changes in heat. – That is, the amount of energy transferred from one substance to another. l You can measure the energy lost somewhere …or the energy gained somewhere else. l However, you cannot measure the absolute heat content of a system.

Energy of Universe is Conserved! Environment System Energy can move between the system and the environment

Perspective l When we talk about energy changes, we need a convention because direction is important. l Labels are from the system’s perspective!

Exothermic Change l System releases heat to the environment – What happens to the temperature of the environment? l EXO - energy leaves system (exits). l What happens to the energy level of the system? – What happens to temperature of system?

Exothermic Change Temperature of environment Environment Temperature of system System Energy

Exothermic Change l System has a net loss in energy! l Environment has a net gain in energy! Energy lost = Energy gained

Endothermic Change l System absorbs heat from environment – What happens to temperature of environment? l Endo - Energy enters system l What happens to the energy level of the system? – What happens to temperature of system?

Endothermic Change Temperature of environment Environment System Energy Temperature of system

Endothermic Change l System has a net gain in energy! l Environment has a net loss in energy! Energy lost = Energy gained

Heat Flow l Heat flows from hotter object to cooler object. l Cold pack on leg: Heat flows from the leg to the cold pack! – Leg cools down; cold pack warms up.

Heat Transferred l Quantity of Heat Transferred depends on: Temperature change – Mass of substance – Specific Heat of substance – Ø Specific to a particular substance

Specific Heat l The amount of heat required to raise the temperature of one gram of a substance by one degree C. l Symbol Cp l Specific heat is a physical constant. – Different for each pure substance.

Specific Heat Constants Material Cp Joules/gram deg C Silver 0. 230 Gold 0. 129 Graphite 0. 720 Diamond 0. 502 Copper 0. 385 Brass 0. 377 Mercury 0. 139 Water 4. 184 Ice 2. 01 Ethanol 2. 46

Calculating Heat Transferred Simple system: Pure substance in a single phase. To calculate heat gained or lost, use: Q = m. C T Q = amount of heat transferred m = mass of substance C = specific heat capacity of the substance. T = temperature change = Tfinal – Tinitial