Chapter 11 Thermochemistry

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Chapter 11
Thermochemistry
Objectives:
• 1 Explain the relationship between energy
and heat.
• 2. Distinguish between heat capacity and
specific heat.
Thermochemistry
deals with the heat changes that
occur during a chemical reaction
Energy
Ability to do work or give heat
• Heat (q)
1. energy that is transferred from one
object to another because of
a
temp.change
2. flows from a warmer object to a
cooler object
• System (the universe)
1. surroundings (everything in the
universe)
2. ex. Mixture of chemicals in a beaker
(system)
(surroundings)
• Law of conservation of energy
In a chemical or physical process energy
is neither created nor destroyed.
It can become work, stored energy or
heat.
• Heat flow
1. Endothermic
flows into the system
positive sign
can feel cool
2. Exothermic
heat flows out of the system
negative sign
can feel hot
• Examples of endo/exo thermic
1. burning a piece of paper _________
2. melting ice cubes ____________
3. air in heated tire expands ________
4. cooking an egg __________
• Thermochemical equations
endothermic
2NaHCO3 + 129 kJ  Na2CO3 + H2O + CO2
OR
2NaHCO3  Na2CO3 + H2O + CO2 ΔH = 129 kJ
exothermic
C3H8 + 5O2 3CO2 + 4H2O + 2043 kJ
OR
C3H8 + 5O2 3CO2 + 4H2O ΔH = -2043 kJ
calorie
quantity of heat needed to raise the
temperature of 1 g of pure water 1°C
1. Calorie ( C ) - refers to food
2. calorie ( c ) – refers not to food
3. 1 Calorie = 1 kcal = 1000 cal
Joule (J) – SI unit of heat and energy
1. 1 J = 0.2390 calories
2. 1 calorie = 4.184 J
Ex. Convert 142,000 calories
to J
 Specific Heat capacity or specific heat
(C)
1. Amount of heat required to
raise the temperature of 1 g
of a substance by 1oC
2. q = C(m)(ΔT)
q= heat
m= mass
C = specific heat
ΔT = final T – initial T
4.Calculate the total amount of heat the
solar pond will absorb and release on a
typical day, if the mass of the water is
22,500 kg, the mass of granite is 14,500
kg. The change in temperature for both
is 22.0oC. The specific heat of water is
4.184 J/goC and granite is
0.803 J/goC.
Objectives:
 3. Construct equations that show the heat
changes for chemical and physical
processes.
 4. Calculate heat changes in chemical and
physical processes
More themrochemistry
Calorimeter- equipment used to measure
the amount of heat released or
absorbed during a change
Heat of reaction- amount of heat released
or absorbed during a chemical
reaction
Enthalpy
1. The sum of all energy plus a term
that takes into account the
pressure
and volume of the
substance.
2. Symbol
H
3. units: kJ/mole
Enthalpy change (∆H)
1. ∆H = Hproducts - Hreactants
2. ∆H value
+
-
Process heat
endo
absorb
exo
released
3. ∆H graphs
C3H8 (g) + 5O2 (g)3CO2(g) + 4H2O(g) +2043 kJ/mole
What can you tell from the equation?
exothermic
∆H = -2043 kJ/mole
heatproduct < heatreactant
exothermic
C (s) + H2O(g) + 113 kJ/mole --> CO(g) + H2O(g)
What can you tell from the equation?
endothermic
∆H = +113 kJ/mole
Hproducts > Hreactants
• Heat of reaction calculations
1. depend on the quantity of reactants
and products
2. depend on the state of the
reactants
and products
3. Defined in terms of 1 mole of
product
Examples of enthalpy change
1. How much heat will be released if 6.0 g
of carbon reacts with excess oxygen
according to the equation below?
C(s) + O2 (g) --> CO2 (g) ∆H = -394 kJ/mole
Using limiting reagents
How much heat will be transferred when 0.50
mol of TiO2 reacts with 1.60 mol of HCl
according to the following equation?
TiO2(s) + 4HCl(g) -->TiCl4(l) + 2H2O(g)
∆H=-67kJ/mole
• Heat of combustion
1. The heat released by the complete
combustion of 1 mole of a
substance
2. Defined in terms of 1 mole of
reactant
3. Ex.
How much heat will be released if 1.0 g
of hydrogen peroxide (H2O2)
decomposes?
2H2O2 (l)  2H2O (g) + O2 ΔH= -190 kJ/mole
Calorimetry
1. The accurate and precise
measurement of heat change for
chemical and physical processes
2. qrxn = -qH2O The negative means the
opposite sign for the
system
4. Example: A 13.7 g sample of
Pb(NO3)2 reacts with 85.0 g of
water. The temperature
drops
from 23.4oC to
19.7oC. Find the
∆H.
Pb(NO3)2 --> Pb+2 + 2 NO3-
Objectives:
• 5. Apply Hess’s Law of heat summation to
find heat changes for chemcial and
physical processes
• 6. Calculate heat changes using standard
heats of formation.
Hess’s Law
1. The summation of theoretical
equations and the enthalpy
2. ∆Hnet = ∆H1 + ∆H2
3. Applying Hess’s Law
Example
S(s) + O2(g) --> SO2(g)
∆H = -297 kJ
2SO3(g) --> 2SO2(g) + O2(g) ∆H = 198 kJ
Answer: 2S(s) + 3O2(g) --> 2SO3(g) ∆H = ?
Example 2:
H2S(g) + 3/2 O2(g) -->H2O(l) + SO2 ∆H = -563 kJ
CS2(l) + 3O2(g) --> CO2(g) + 2SO2(g) ∆H = -1075 kJ
CS2(l) + 2H2O(l) --> CO2(g) + 2H2S(g) ∆H = ?
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