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 ? H is negative Heat is released
Endothermic Rxn ? 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 energy is 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 1C.
• 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 Earths 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 1C
• Quantitative application q mc?T

16
Specific heat

• To calculate the energy transferred to or by a
  substance, use this formula
• q mc?T
• q energy in J
• m mass in grams
• c specific heat for the substance (J/g-C)
• ?T change in temperature (tfinal tinitial) in
  C

17
Specific Heat Capacities (Cp) for Selected Materials at 20C Specific Heat Capacities (Cp) for Selected Materials at 20C
Material Cp (Joules/g-C)
Air 1.00
Water (l) 4.18
Carbon dioxide 0.839
Aluminum 0.902
Brass 0.380
Copper 0.386
Gold 0.126
Iron 0.448
Lead 0.128
Silver 0.233
Zinc 0.387
Granite 0.79
CaCO3 0.852
Stainless steel 0.51
Glass 0.84
Ice (-10C) 2.05
Ethyl alcohol 2.45
Asphalt 0.92
Sandy clay 1.38
Quartz sand 0.83
Concrete 0.88
Tipler, Paul A., Physics for Scientists and
Engineers, 4th Ed., W.H. Freeman, (1999).
engineeringtoolbox.com
18
Enthalpy ?H

• Enthalpy is the amount of heat content used or
  released in a system at constant pressure
• Mathematically
• Sum of ?H products sum of ?H reactants ?H for
  the reaction (?Hrxn)

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 ?H - 2814 kJ
• ?H 6(?HfCO2) 6(?HfH2O) (?HC6H12O6) -6(?HfO2)
  
• (values from table)
• ?H 6(-393) 6(-286) (-1260) -6(0) -2814 kJ
• Recall that the negative ?H means that 2803 kJ
  are released in the combustion of 1 mole of
  glucose.
• ?H is negative This is an exothermic reaction.

20
Heat and Matter

• Adding heat energy increases the motion of a
  substances 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 4C
  and 0C.

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 liquids 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.