CHEM 1310 Lecture Notes - Exothermic Process, Intermix Media, Joule

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Question:
A drink has 180 calories. Given that an average 150 lb person uses 800 kJ/hour while walking, how
long must that person walk to not store any energy? Assume energy only goes into walking
Answer
Convert from calories to kJ
o 180 Cal x 
   
o 180000 cal x 
   105 J
o OR 7.53 x 105/ 1000 J = 
 x=7.53 x 102 kJ
Now find time
o  
     
Note: energy will flow from high (hot) to low (cold), until both objects are at the same
temperature.
o Tsys < Tsurr - energy flows until Tsys = Tsurr
Heat
Heat capacity - C
o Amount of heat needed to change the temperature by 1 degree in the heat system
Molar heat capacity - the heat capacity of one mol of substance3
Specific heat capacity
o The heat capacity of 1 g of substance
o For water = 1 cal g-1 or C-1 = 4.184 J
Heat capacity - proportionality constants between q (quantity of heat) and T (temp)
o Are compound specific ie: is a property of the substance (state function)
More ways to store heat; the higher the heat capacity will be for the compound
o Note: how 'hot' an object is is NOT related to its heat capacity
Related to its surroundings
Law of the Conservation of Energy
Energy is neither created or destroyed
Total energy between the system and the surroundings, or the universe, is constant
Qsystem + qsurroundings = 0
Thus -qsys = + qsurr OR +qsys = -qsurr
Heat of a Reaction
Amount of heat exchanged or transferred during a chemical reaction in a system, between the
system and its surroundings at constant temperature
Heat of combustion = combustion reaction
Measured with isolated systems and thermometers
o Gives T, thus it may be used to find the quantity of heat (q), by a technique called
calorimetry
o Q = quantity of heat transferred
o C = heat capactiy
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Document Summary

Convert from calories to kj: 180 cal x, 180000 cal x. 105 j: or 7. 53 x 105/ 1000 j = x=7. 53 x 102 kj. Note: energy will flow from high (hot) to low (cold), until both objects are at the same temperature: tsys < tsurr - energy flows until tsys = tsurr. Heat capacity - c: amount of heat needed to change the temperature by 1 degree in the heat system. Molar heat capacity - the heat capacity of one mol of substance3. Specific heat capacity: the heat capacity of 1 g of substance, for water = 1 cal g-1 or c-1 = 4. 184 j. Heat capacity - proportionality constants between q (quantity of heat) and t (temp: are compound specific ie: is a property of the substance (state function) More ways to store heat; the higher the heat capacity will be for the compound: note: how "hot" an object is is not related to its heat capacity.

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