thermochemistry: the heat energy of chemical reactions
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Thermochemistry: The heat energy of chemical reactions. • Enthalpy is the amount of ________ transferred during a reaction. The symbol for the change in enthalpy is ∆H. - PowerPoint PPT PresentationTRANSCRIPT
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Thermochemistry:The heat energy of chemical reactions
• Enthalpy is the amount of ________ transferred during a reaction. The symbol for the change in enthalpy is ∆H.
• An endothermic reaction is one that ___________ heat from the surroundings. (___ ∆ H) An endothermic reaction feels ______.
Example--an “instant” ice pack• An exothermic process is one that _____________ heat to the
surroundings. (___ ∆ H) An exothermic reaction feels _____.Example--burning paper
heat
gains+ cold
loses– hot
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Thermochemistry:How to measure heat (Energy) changes
• A ____________ is the amount of energy (heat) required to raise the temperature of one gram of water by one degree Celsius.
• The “calorie” written on food is actually not one calorie in chemistry. It is actually 1 __________ (or ____calories) and is written with a capital C (Calorie) to keep the two separate.
• A ____________ is the SI unit for measuring the amount of energy or heat transferred in chemistry.
• Write down this conversion factor:
calorie (cal)
kilocalorie
Joule (J)
1 cal = 4.184 J
1000
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McDonalds McChicken
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McDonalds McDouble
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Crunchy Cheetos
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So what do we do with all these Calories?
• Your body will use these Calories as energy to do everyday activities but what if you don’t use all the calories you consume?
• Your body will either use the energy or it will store it as fat!
• So people who eat more food than their body can use exercise as a way of releasing the extra energy. But how much exercise do you really need to doin order to burn off those extra Calories?
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Exercise!• Running and walking are not the same!! Even though
you can run OR walk a mile, there is a difference. According to David Swain, a Ph.D. in exercise physiology, “When you perform a continuous exercise, you burn five Calories for every liter of oxygen you consume and running in general consumes a lot more oxygen than walking.”
• Running burns approximately 100 Calories per mile.
• SO WHAT DOES THIS MEAN?
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I LOVE FOOD! But I hate running!
• To run off the food previously mentioned, this is approximately how long you would have to RUN!
• 1 McChicken: 1.38 MILES• 1 McDouble: 3.10 MILES• 1 bag of Cheetos: 3.30 MILES
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Thermochemical Reactions• A thermochemical reaction is written as follows:
2S + 3O2 2SO3 + 791.4 kJ
• This equation represents an ___________ reaction since the heat is a ________.
H2 + Br2 + 72.80 kJ 2HBr
• This equation represents an ___________ reaction since the heat is a __________.
exothermicproduct
endothermicreactant
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Thermochemistry Problems• Problem 1: How much heat will be released when 6.5 moles of
sulfur reacts with excess oxygen according to the following equation? Also, tell whether it will be exothermic or endothermic! 2S + 3O2 2SO3 ∆H = -791.4 kJ
6.5 mol S X _______________
2 mol S
-791.4 kJ= -2572 kJ
-∆H means heat is a product!
Exothermic ~
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Phase Changes & Energy
Endothermic: melting, evaporating/boiling & sublimation
Exothermic: freezing, condensation, & deposition
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Endothermic
Exothermic
“Reaction Profiles”
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Calculations in Thermodynamics• In order to calculate how much heat is transferred by a
thermochemical reaction the equation we use is q = mc∆T
• q = the ______ lost or gained in the process• m = the _____ of the substance• c = the ________ _____ ________
The specific heat of water is 4.186 Joules/gram °C • ∆T = Change in temperature
heatmass
specific heat capacity
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Calculations Practice
• Example 1: How many Joules would it take to raise the temperature of 250 g of ice from -20 °C to -5 °C? (The specific heat of ice is 2.108 Joule/gram °C)
q = mc∆Tq = 250g (2.108 J/g °C)( 15 °C)q = 7905 Joules= 7.9 kJ
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More Practice• Example 2: How many calories would it take to raise the
temperature of 100 grams of Ethyl Alcohol from 30 °C to 50 °C? (The specific heat of Ethyl alcohol is 0.58 cal/g °C)
q = mc∆Tq = 100 g (0.58 cal/g °C) (20 °C)q = 1160 cal