5.1: Energy

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5.1 Energy

• In this chapter well talk about energy in the
  form of heat Defn of energy? Heat?
• Energy-capacity to do work or transfer heat. SI
  unit is J.
• Work is force X distance
• Fact all things tend toward a lower energy
  level

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A Heat

• Heat--energy that is transferred from one object
  to another due to a temperature difference (heat
  flows from hot to cold). Unit?
• Joule (J)

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B Chemical Reactions

• Chemical reactions involve breaking bonds--This
  requires energy.
• They also involve making bonds which releases
  energy

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B. Chemical Reactions

• So--almost all chemical reactions result in a
  change of energy this is usually heat.
• Thermochemistry--the study of heat changes in
  chemical reactions (one part of thermodynamics).

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C. Exothermic Reactions

• Reactions that release heat
• Therefore, heat is a product
• Example combustion
• CH4 2O2 ? CO2 2H2O 890 kJ
• Energy to break the bonds is less than the energy
  released forming new bonds. (match)

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D. Endothermic Reactions

• Reactions that absorb (require) heat.
• Heat is a reactant
• Example a cold pack picks up heat from your
  skin, causing a reaction to take place.

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D. Endothermic Reaction

• Example
• 2 H2O 286 kJ ? 2 H2 O2

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E. Kinetic Energy

• Definition?
• Energy of motion
• Hot coffee--its hot because the molecules are
  striking the walls of the container (lots of KE)
  Ek ½ mv2
• Energy is joules what is a J?

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E. Kinetic Energy

• 1 J 1 kgm2/s2
• Sometimes we use calories
• 1 cal 4.184 J
• 1 cal is the amount of heat required to raise 1 g
  of water 1 C
• A food calorie is 1 Cal 1 kcal

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F. Potential Energy

• Stored energy, or energy due to position or
  location
• PE mgh

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G. Temperature

• Its just a measure of the hotness of a substance
• Not the same as heat
• Units are degrees C or F
• Absolute scale Kelvin (K)

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• SystemWhat we are interested in
• Surroundingseverything else
• Law of conservation of energy applies if our
  system is large enough.
• State function-value does not depend on history,
  but only the present condition (or state)
• It means you can take final - initial

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• Change in energy is a state function.
• Work is not a state function.
• Enthalpy is a state function.