Download Calorimetry Notes

5/2 Opener
 What is the difference between a dissolution and
a chemical reaction?
Thermochemistry
Define Heat
Compare Endothermic and Exothermic
reactions
Know units
Energy Transformations
 Thermochemistry – the study of energy changes
that occur during chemical reactions and changes
in state
 Chemical potential energy – energy stored in the
chemical bonds of a substance
Heat
 Represented by q
 One effect of adding heat to an object in an
increase in its temperature
 Heat always flows from a warmer object to a
cooler object
Processes
 System – the part of the universe on which
you focus your attention
 Surroundings – everything else in the
universe
 Law of Conservation of Energy – in any
chemical and physical process, energy is
neither created or destroyed
Endothermic Process
 The system gains heat as the surroundings cool
down
Exothermic Process
 The system loses heat as the surroundings heat up
Units for Measuring Heat Flow
 Calorie (cal) – the quantity of heat needed to
raise the temperature of 1 g of pure water 1°C
 Joule (J) – the quantity of heat needed to raise
the temperature of 1 g of pure water 0.2390°C
 1J = 0.2390cal 4.184J = 1cal
Heat Capacity and Specific
Heat
 Heat capacity depends on the mass and chemical composition
of the object
 Specific heat is the amount of heat it takes to raise the
temperature of 1g by 1°C
C=
q ___ or = ________heat__________
m x ∆T
mass(g) x change in °C
Plicker Review
In what direction does heat flow?
A. Forward B. High to Low
C. Low to High
D. Up
The part of the universe on which you focus your attention is called:
A. Surroundings
B. Earth
C. System
D. Energy Region
In what process does the system gains heat as the surroundings cool
down?
A. Endothermic B. Caloric C. Exothermic
D. Potential
In what process does the system lose heat as the surroundings heat up?
A. Endothermic B. Caloric C. Exothermic
D. Potential
Ch 17.2 Measuring and Expressing
Enthalpy Changes
SWBAT:
Describe Calorimetry
Use Thermochemical Equations
Calorimetry
 The precise measurement of the heat flow into or out of a system
for chemical or physical processes.
 Enthalpy (H) of the system is the heat content of a system at constant
pressure
 A calorimeter is an insulated device used to measure the
absorption or release of heat in chemical or physical processes
 Constant-Pressure Calorimeters – foam cups which do not let much
heat in or out, open to the atmosphere.
 Measures enthalpy (H), q = ∆H
 qsys = ∆H = -qsurr = -m x C x ∆T
 Negative ∆H values are exothermic reactions, while positive ∆H values are
endothermic
 Constant-Volume Calorimeters, or bomb calorimeter
Complete Model 3
Enthalpy Calculation
When 25ml of water containing .025mol of HCL at 25*C is
added to 25.0 ml of water containing .025mol of NaOH at
25*C in a calorimeter a reaction occurs. Calculate the enthalpy
change in joules if the highest temperature observed is 32*C.
Thermochemical Equations
 In a chemical equation, the enthalpy change for the reaction
can be written as either a reactant or a product
 A thermochemical equation includes the enthalpy change
 The heat of reaction is the enthalpy change for the chemical
equation exactly as it is written
 The heat of combustion is the heat of reaction for the
complete burning of one mole of a substance.
Using Heat of Reaction to Calculate
Enthalpy Change
2NaHCO3(s) + 129KJ Na2CO3(s)+H2O(g)+ CO2(g)
Is this reaction exothermic or endothermic?
Calculate the amount of heat required to decompose 2.24 mol
NaHCO3.
Ch 17.4 Calculating Heats of
Reaction
Hess’s Law
 Hess’s law allows you to determine the heat of reaction
indirectly
 Hess’s law of heat summation states that if you add two or
more thermochemical equations to give a final equation, then
you can also add the heats to give a final heat.
 Useful in calculating the heat of formation for just one of
multiple products
Calculating Enthalpy Change
Find the enthalpy change for the conversion of diamond to
graphite.
C(s, graphite) + O2(g)CO2(g) ΔHf=-393.5KJ
C(s, diamond) + O2(g)CO2(g) ΔHf=-395.4KJ
CO2(g) C(s, graphite) + O2(g)
C(s, diamond) + O2(g) CO2(g)
ΔHf=393.5KJ
ΔHf=-395.4KJ
C(s, diamond) C(s, graphite) ΔHf= -1.9KJ
Standard Heats of Formation
 The standard heat of formation (∆Hf0) of a compound is the
change in enthalpy that accompanies the formation of one
mole of a compound from its elements with all substances in
their standard states at 25˚C
 For a reaction that occurs at standard conditions, you can
calculate the heat of reaction by using standard heats of
formation
 ∆H0 = ∆Hf0(products) - ∆Hf0(reactants)
Calculating Standard Heat of Reaction
What is the standard heat of the reaction for :
2CO(g) + O2(g)CO2(g)
CO(g) ΔHf= -110.5
O2(g) ΔHf= 0
CO2(g) ΔHf= -393.5