Transcript Heat Energy - MullisChemistry

Energy
Energy is the ability to do work or transfer
heat.
Units of Energy:
-Joule (J)
1J=1
kg-m2/s2
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
Energy
Chemical Energy: Energy produced or
absorbed by the formation or breakage
of chemical bonds.
Thermal Energy: Heat Energy
Law of Conservation of Energy
The energy can NOT be created or
destroyed.
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)
Chemical Energy Graphs
Exothermic Rxn
Δ H is negative
Heat is released
Endothermic Rxn
Δ H is positive
Heat is absorbed
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
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
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.
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).
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.
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.
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.
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.
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
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
Specific Heat Capacities (Cp) for Selected Materials at 20°C
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 (-10°C)
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
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)
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.
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.
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.
Phase Changes
Consider water to remember the words for
most phase changes:
Condense
Steam
GAS
Freeze
Water
LIQUID
Evaporate
Ice
SOLID
Melt
Note that all phases still water (H2O). These are PHYSICAL changes.
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)
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.