notes: 17.1 – thermochemistry – the flow of energy
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NOTES: 17.1 – Thermochemistry
– The Flow of Energy
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Thermochemistry
● THERMOCHEMISTRY: the study of energy (in the form of heat) changes that accompany physical & chemical changes
● Heat flows from high to low (hot to cool)
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Energy (E):
● measures the capacity to do work or supply heat
● energy is weightless, odorless, and tasteless
● can only be detected because of its effects
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Energy (E):
● energy stored within the structural units (i.e. molecules) of chemical substances is called: CHEMICAL POTENTIAL ENERGY (PE)
EXAMPLE: gasoline contains a large amount of chemical PE; when the gasoline burns in your car engine, the energy is released to do mechanical work
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Heat (q):
● measure of the total amount of energy (E) transferred from an object with high energy to an object with low energy
● cannot be detected by the senses or instruments…only changes caused by heat can be detected
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Magnitude of Heat Flow
● Units of heat energy:
1 kcal = 1,000 cal = 1 Cal (nutritional)
1 kJ = 1,000 J1 calorie = 4.184 J1 kcal = 4.184 kJ
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Heat (q):
● heat always flows from a warmer object to a cooler object
EXAMPLE: when you put an ice cube into a cup of hot water, heat will flow from the hot water to the ice cube… the ice melts and the water’s temp. decreases
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Temperature (T):
● Measure of average kinetic energy
● Indicates relative amounts of energy
● Indicates the hotness or coldness of an object
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Thermochemistry
● all chemical reactions and changes in physical state (melting, freezing, etc.) involve either the release or absorption of HEAT
● in thermochemistry we examine the flow of heat from a “system” (i.e. a chemical reaction) to its surroundings (or vice versa)
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How does energy change?
● Law of Conservation of Energy:
-In any chemical reaction or physical process, energy is neither created nor destroyed;
-all of the energy involved in a process can be accounted for as: work, stored energy, or heat.
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Thermochemistry● exothermic reactions: release energy in
the form of heat; show a negative value for quantity of heat (q < 0)
● endothermic reactions: absorb energy in the form of heat; show a positive value for quantity of heat (q > 0)
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EXOTHERMIC CHANGE:
● losing energy
● products have less energy than the reactants
● ΔH = (-)
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ENDOTHERMIC CHANGE:
● gaining energy
● products have more energy then reactants
● Δ H = (+)
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Endothermic or Exothermic?● H2O (l) H2O (s)
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Endothermic or Exothermic?● H2O (l) H2O (s)
Exothermic (liquid water freezing to solid ice)
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Endothermic or Exothermic?● Melting - solid to liquid● Boiling - liquid to gas● Condensing - gas to liquid● Freezing - liquid to solid● Sublimation - solid to gas
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Endothermic or Exothermic?● Melting - solid to liquid
endothermic● Boiling - liquid to gas
endothermic● Condensing - gas to liquid
exothermic● Freezing - liquid to solid exothermic● Sublimation - solid to gas endothermic
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In conclusion…● Solid Liquid Gas (endothermic)
● Gas Liquid Solid (exothermic)
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Cup of water at 30o C vs. tub full of water at 28o C:
● Which has a higher kinetic energy?
● If the cup of water was poured into the tub of water, would there be a transfer of heat at that moment? Would the temperature change?
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Heating and Cooling Curves:● When a plot of
temperature vs. time is plotted as a pure substance is heated or cooled, a plateau is observed at the substance’s melting and boiling points.
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Heating and Cooling Curves:● In other words, as
energy is added or removed from a substance, there is no change in temperature at the pure substance’s melting and boiling points until it has completely melted or boiled.
● mmkay