Heat and Energy

1
Heat and Energy

• Chp 10

2
Energy

• The ability to do work or produce heat
• 2 types
• Kinetic
• Due to motion
• Affected by mass and speed of object
• Potential
• Due to position or composition
• Can be converted to mechanical energy

3
Law of Conservation of Energy

• Energy can be converted to new forms, but it
  cannot be created or destroyed
• A change in the energy of an object is a result
  of work being done on it
• System part of universe we are focused on
• Surroundings rest of universe
• If one object loses energy, than another must
  gain it (if system gains, then surroundings lose)

4
Thermodynamics

• Study of energy
• Heat is a type of energy
• When objects experience friction, some of their
  mechanical energy is converted to heat energy (no
  longer useful to us)
• 2 reaction types
• Exothermic heat is released to surroundings
• Endothermic heat is taken in from the
  surroundings

5
Heat and Temperature

• NOT the same thing, but related
• Temperature the average kinetic energy of
  molecules in a substance
• Heat the total kinetic energy of molecules in a
  substance
• Can have a lot of heat with little temperature if
  a lot of molecules are involved

6
Laws of Thermodynamics

1. The energy of the universe is constant.
2. The universes entropy (disorder) is always
   increasing.
3. Heat always flows from hot to cold, never the
   other way.

7
Energy Equation

• ?E q w
• The signs for q and w are from the systems point
  of view ( indicates it was gained by the system,
  - indicates the system lost it)
• Ex. How much energy is there if a system does 10
  kJ of work and absorbs 15 kJ of heat?
• Since the system does the work it is -10, since
  the heat is absorbed its 15 so
• ?E 15 -10 5 kJ

8
Heat Equation

• Heat is affected by temperature, mass and an
  objects specific heat capacity (a measure of how
  quickly it changes temperature)
• High shc means it changes temp slowly (ex. Water)
• Low shc means it changes temp quickly (ex. Sand)
• Q sm?T
• Q is quantity of heat (in joules)
• S is specific heat capacity (in J/goC)
• M is mass (in grams)
• ?T is change in temperature (in celsius)

9
An Example

• How much heat is needed to raise 7.4 g of water
  from 29 to 46 oC?
• Q 4.184 (7.4)(46-29)
• Q 526 J
• This only works if no phase change occurs (cannot
  melt, boil, etc)

10
Enthalpy

• The flow of energy in a reaction at constant
  pressure (symbol is H)
• The same for a process whether it occurs in one
  step, or in a series of steps (Hesss Law)
• Measured using a calorimeter
• Burn the object and measure the change in
  temperature in a water bath that the object is
  submerged in

11
Using Hesss Law

• Rearrange equations to cancel out compounds not
  present in answer
• To cancel, the compounds must have the same
  coefficient and be on opposite sides of the arrow
  (one product and one reactant)
• Whatever is done to the equation, must also be
  done to its enthalpy
• If you multiply to get a coefficient, multiply
  the enthalpy
• If you flip the equation, you flip the sign (
  becomes -)

12
How we use energy

• When we use energy, we degrade its usefulness
  only
• Quantity stays the same, but quality decreases
• Fossil fuels
• Coal
• Oil
• Natural gas
• Solar energy
• Nuclear fusion