Energy­ ability to do work or produce heat
­2 forms: Kinetic and Potential
­KE = energy in motion
­PE = energy due to the position
­Depends on the objects composition (type of atoms, number and type of chemical bonds, and arrangement)
Law of Conservation of Energy ­ in a chemical reaction or physical process energy can be converted but never created or destroyed.
Heat (q) ­energy in the process of flowing from a warmer to a cooler object.
­calorie: a measure of the amount of energy required to raise the temperature of 1 gram of water, 1 degree Celsius.
NOTE: Food Calories are capitalized.
1 nutritional Calorie = 1000 calories
SI Unit for Energy is the Joule (J)
1 calorie = 4.184 Joules 1
Let's Practice using these conversion factors!
1. A fruit­and­oatmeal bar contains 142 nutritional Calories. Convert this energy to calories. 142,000 cal
2. An exothermic reaction releases 86.5kJ. How many kilocalories of energy are released?
20.7 kcal
3. What is the equivalent in joules of 126 Calories? 5.27x105J
4. Convert 109 kilocalories to kilojoules.
455 kilojoules
Specific Heat (c) ­ the amount of heat required to raise the temperature of one gram of that substance by one degree celcius.
­Every substance has its own
­c of H20 = 4.184 Joules/g oC
Calculating the Heat Absorbed
Concrete has c= 0.84J/g oC, which means 1 gram of concrete absorbs 0.84 Joules when the temperature is increased by 1oC
pg 520
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q = cm T
q= heat absorbed or released
c= specific heat
m= mass of sample in grams
T = Change in Temperature (oC)
Tfinal­Tinitial
Substances both absorb and release heat
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How do we get calorimeter information on packaged foods?
­The results of combustion reactions carried out.
calorimeter­ an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process.
Heat gained by one object is heat lost by another.
Enthalpy­ heat content of a system at constant pressure.
­can't measure actual enthalpy BUT we can calculate the change in enthalpy for a reaction. Called: Enthalpy (heat) of reaction or Hrxn
= Hfinal­Hinitial
Exothermic Reactions are ALWAYS negative
Ex: 4Fe(s) + 3O2(g) ­­> 2Fe2O3(s) = ­1625kJ Endothermic Reactions are ALWAYS positive
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Lots of processes absorb/release heat even with no chemical reaction involved.
Think about it: You step out of a hot shower, you shiver as water evaporates from your skin. WHY? Your skin provides the heat needed to vaporize the water.
Molar enthalpy (heat) of vaporization­ heat required to vaporize one mole of a liquid ( Hvap)
Think about it: You drop an ice cube into a glass of water. The water cools. WHY? The water is providing the heat to melt the ice.
Molar enthalpy (heat) of fusion ( Hfus) ­heat required to melt one mole of a solid substance.
Vaporizing a liquid and melting a solid are BOTH endothermic processes ­positive values
pg 530
Due to being opposite processes. Think phase change diagram.
Ex
ot
he
rm
ic
Hvap = ­ Hcondensation
Hfus = ­ Hsolid
ic
m
er
th
do
En
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Interesting FACT
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PRACTICE, PRACTICE, PRACTICE
pg 521 #4­6
pg 522 #7,8,10
pg 525 #12­15
pg 986 #3­6
pg 532 #23­25
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