Download Thermochemistry

Thermochemistry
Grade 12 Chemistry SCH4U0
Mr. Krstovic
What did we learn so far?
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Heat is energy transferred from one place to
another
Thermochemistry is the study of the
energy and heat associated with chemical
reactions and/or physical transformations.
Calorimetry is the science of measuring the
heat of chemical reactions or physical
changes.
What did we learn so far?
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A calorimeter is a device used for
calorimetry.
Q = mcT
Specific heat capacity (c) has units of
J/gC
Difference in heat capacity of oil and
water (recall the demo in class)
Remember these three
processes?
Exothermic and Endothermic
Processes
If heat is transferred from surrounding to system.
Process is endothermic. Surroundings feel COOLER!
If heat is transferred from system to surroundings.
Process is exothermic. Surroundings feel HOTER!
Enthalpy
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Chemists give a special symbol, ΔH(delta H) to the
heat change in a reaction
This heat change is called
ENTHAPLY
ΔH (enthalpy) = energy absorbed or released to
the surroundings when a system changes from
reactants to products.
ΔHsystem = - Qsurrounding
Potential Energy Diagrams
Molar Enthalpy
ΔHx: the enthalpy change for a
physical, chemical or nuclear change
involving 1 mol of a substance
[Units: kJ/mol]
Types of Molar Enthalpies:
ΔHvap
ΔHsol
ΔHrx
ΔHfr
ΔHcomb (See Table1 p.306)
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Types of Molar Enthalpies
Example: ΔHcomb
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)
ΔHcomb = - 802 kJ/mol
Enthalpies of Reaction
What is H for the combustion of 11.0 g of
CH4 in excess oxygen?
 802 kJ
mol CH4
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11.0 g CH4 
= -550 kJ
16.04 g CH4 1 mol CH4
Example Question
In a calorimetry experiment, 7.46 g
of KCl is dissolved in 100.0 mL of
water at an initial T of 24.1 C. The
final T of the solution is 20C. What
is the molar enthalpy of solution of
KCl?
Methods of Communicating
Enthalpy Changes
Method 1: Energy value as part of the
reaction
H2O (l) + 285.8 kJ  H2 + 1/2O2
Mg (s) + 1/2O2 (g)  MgO(s) + 601.6 kJ
Method 2
Thermochemical Equations with ΔH
values next to the reaction
H2O (l)  H2 + 1/2O2
ΔH = +285.8 kJ
Mg (s) + 1/2O2 (g)  MgO(s) ΔH = - 601.6 kJ
Method 3
Standard Molar Enthalpy of Reaction
measured at SATP conditions (100 kPa; 25 C)
ΔHorx - this symbol distinguishes
standard molar enthalpies from molar
enthalpies
ΔHocomb = -726 kJ/mol CH3OH
Method 4
Potential Energy Diagrams