thermochemistry some say the world will end in fire, some say in ice, from what i’ve tasted of...
TRANSCRIPT
Thermochemistry
Some say the world will end in fire,
Some say in ice,
From what I’ve tasted of desire
I hold with those who favor fire.Robert Frost
Fire and Ice
Principles of Heat Flow
• Thermochemistry is the study of heat flow
• System: the part of the universe on which we are focused
• Surroundings: exchange energy with system (in close contact with system)
State Properties
• Composition• Temperature• Pressure• (remind you of stp?)
• Heat flow (q)• q is + when heat flows
into the system from the surroundings (heating of a cup of coffee)
• q is - when heat flows out of the system into the surrounds (cooling of a cup of coffee)
if X is a state property, then ∆X = Xfinal-Xinitial
Heat Flow is NOT a state property
Endo- and Exothermic Processes
• An endothermic process (q>0) in which heat flows form the surroundings into the reaction system.
• And exothermic process(q<0) in which heat flows from the reaction system into the surroundings
system
surroundings
IN
OUT
endothermicexothermic
Magnitude of Heat Flow
q = C x ∆t
∆t = tfinal - tinitial
For a pure substance
q = m x c x ∆t
m is mass
c is specific heat
Specific heat is the amount of energy necessary to raise the temperature of 1 gram of a substance 1 degree C.
t is temperature
q is heat flow
C is heat capacity
q is measured in joules
Example
• How much heat is needed to raise the temperature of a 3000 gallon pool from 10 C to 20 C?
The specific heat of water is 4.18 J/goC
1. Q = m x c x ∆t
2. Plug and chug
Calorimetry
• Calorimeter: device that measures the heat flow in a reaction
• qreaction = -q calorimeter
• qreaction = -Ccal x ∆t
• “old” heat unit is the calorie
• 1 cal = 4.184 J (c)• 1 kcal = 4.184 kJ (C)
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Coffee Cup Calorimeter
• Heat from the reaction inside the innermost cup flows out to the water in the outer cup.
• Ccal = mwater x cwater
• qreaction =-mwater x cwater x ∆t
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Bomb Calorimetry
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Example
The reaction between H and Cl :
H2(g) + Cl2(g) --> 2HCl(g)
can be studied in a bomb calorimeter. It is found that when a 1.00 g sample of H2 reacts completely the temperature rises from 20.00 C to 29.82 C. Taking the heat capacity of the calorimeter to be 9.33 kj?oC, calculate the amount of heat evolved in the reaction.
• Which equation to use?
qreaction = -Ccal x ∆t
Plug and chug
Answer: -91.6 kJ
exothermic
Example
When 1.00 g of CaCl2 is added to 50.0 g of water in a coffee-cup calorimeter, it dissolves
CaCl2(s) --> Ca2+(aq) + 2Cl-(aq)
And the temperature rises from 25.00 C to 28.51 C. Assuming that all the heat given off by the reaction is transferred to the water, calculate q for the reaction system.
• Which equation?
•exothermic
Answer: -734 J
Plug and chug
qreaction = -mwater x cwater
Enthalpy (H)
• Enthalpy is a type of chemical energy (heat content)
• Under constant P, heat flow for a reaction system is equal to the difference in enthalpy between products and reactants
qreaction= ∆H=
Hproducts- Hreactants
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Enthalpy continued
• For an exothermic reaction
q = ∆H< 0
• For an endothermic reaction
q = ∆H> 0
The enthalpy of a substance, like its volume, is a state property
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Thermochemical Equations
• A chemical equation which shows the enthalpy relation between products and reactants
NH4NO3(s) --> NH4+
(aq)+ NO3-(aq)
∆H = +28.1 kJ
The sign of ∆H indicates endothermic (+) or exothermic (-)
H2(g)+ Cl2(g) --> 2HCl ∆H = -185kJ The value quoted for H applies when products and reactants are at the same temp (usually 25C)
The phases must be specified--significant differences at different phases
The coefficients represent numbers of moles
Rules of ThermochemistryThe magnitude of ∆H is
directly proportional to the amount of reactant or product
The ∆H for a reaction is equal in magnitude but opposite in sign to ∆H for the reverse reaction.
Heat of fusion : heat absorbed when a solid meltsHeat of vaporization: heat absorbed when a liquid vaporizes
-185 kJ 1 mol H2
1 mol Cl2 -185 kJ
-185 kJ
2 mol HCl
H2(g)+ Cl2(g) --> 2HCl ∆H = -185kJ
2HCl (g)--> H2 (g) + Cl2 (g) ∆H = 185 kJ
H2(g)+ Cl2(g) --> 2HCl(g)
∆H = -185 kJ
Example
Enthalpy of Formation
Example
Example