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Topic 10/11
Chapter 5: Thermochemistry
5.1 Energy
Energy WorkHeat-
Kinetic Energy
Kinetic energy is…
: KE =
Forms of Kinetic energy
Chemical Kinetic energy
Potential Energy
Potential energy is
Forms of potential energy
Chemical Potential energy
Electrostatic energy
Eel =
Example: electrons
Units of Energy
Definitions: System and Surroundings
System –
Surroundings –
Energy Flow:
Heat flows from….
Conversion of Energy
Kinetic / Potential energy -
Examples
Energy can be transferred as Heat or Work
Example:
5.2 First Law of Thermodynamics
1st law -
Exchange of Heat between System and Surroundings
Endothermic Exothermic -
5.3 Enthalpy
Endothermic and Exothermic
A process is endothermic when …
A process is exothermic when ….
Two definitions of Enthalpy:
H heat content
depends on…
H change in heat
Energy of states:
Gas
Gas
Liquid
liquid
Solid
solid
Sample 5.3 Indicate the sign of the enthalpy change, H in these processes carried out under atmospheric pressure and
indicate whether each process is endothermic or exothermic:
a) An ice cube melts b) 1 g of butane (C4H10) is combusted in sufficient oxygen to give complete combustion to CO2 and H2O.
Practice: molten gold poured into a mold solidifies at atmospheric pressure. With the gold defined as the system is the
solidification an exothermic or endothermic process? Explain
5.4 Enthalpy of Reaction
Enthalpy of reaction –
H =
Enthalpy is an extensive property
Enthalpy of the reaction in reverse:
Enthalpy depends on the states of reactants and products
Sample 5.4 How much heat is released when 4.50 g of methane is burned at constant pressure?
(1 mole of CH4 releases 890 kJ when burned.)
Practice: hydrogen peroxide can decompose to water and oxygen by the reaction
2H2O2  2H2O + O2
∆H = -196 kJ
Calculate the quantity of heat released when 5.00 g of H2O2 decomposes at constant pressure.
Heat of Reaction Table I
Thermochemical equations (regents)
5.5 Calorimetry
Calorimetry Heat Capacity –
Specific Heat –
C=
OR
q=
Regents Ex1: 100 grams of water at 250C loses 4200 joules of energy. What is its final
temperature?
Sample 5.5 a) how much heat is needed to warm 250 g of water from 22oC to 98oC?
b) What is the molar heat capacity of water?
Practice
a) Large beds of rocks are used in some solar heated homes to store heat. Assume that the specific heat of rocks is 0.82
J/gK. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0oC.
b) What temperature change would these rocks undergo if they emitted 450 kJ of heat? \
Regents Ex 2: What is the specific heat capacity (Joules per gram per OC) of a substance which absorbs 20. joules of heat
to raise 5.0 grams of the substance by 2.0 oC?
Regents ex3 What will be the temperature change in 100. grams of water if 0.100 moles of KNO3 are dissolved in it?
Ex4: Calculate the heat of solution for KNO3 in kJ/mole, if 10.1 grams of KNO3 is dissolved in 100. g of water causing its
temperature to decrease by 8.3 0C?
Ex5: Heat of fusion of a solid (Hf)
How much heat is absorbed per gram, if 10 grams of a solid melts in 300 grams of water, causing the water temperature
to decrease by 500C?
Sample problem 5.6: A student mixes 50 mL of 1.0 M HCl with 50 mL of 1.0 M NaOH in a calorimeter, the temp changes
from 21.0oC to 27.5oC. Calculate the enthalpy change per mole of HCl
Practice: When 50.0 mL of 0.100 M AgNO3 and 50.0 mL of 0.100 M HCl are mixed in a constant pressure calorimeter, the
temperature of the mixture increases from 22.30 oC to 23.11 oC. The temperature increase is caused by the following
reaction:
AgNO3 + HCl  AgCl(s) + HNO3
Calculate ∆H for this reaction in kJ/mol AgNO3, assuming that the combined solution has a mass of 100.0 g and a specific
heat of 4.18 J/goC
In terms of energy flow, explain why the temperature of the water in the calorimeter used in the investigation increases.
Show a correct numerical setup for calculating the number of joules of heat gained by the water in the investigation.
In the investigation shown, the change in heat of the copper is greater than the change in heat of the water. What error
could account for this apparent violation of the law of conservation of energy? [do not use human error as part of the
answer]
Bomb calorimetry
Sample 5.7: Bomb Calorimetry
The combustion of methylhydrazine (CH6N2) a liquid rocket fuel, produces N2(g) , CO2(g) and H2O (l):
2CH6N2 (l) + 5O2(g)  2N2(g) + 2CO2(g) + 6H2O (l)
When 4.00 g of methylhydrazine is combusted in a bomb calorimeter, the temperature of the calorimeter increases
from 25.00 0C to 39.50 0C. In a separate experiment the heat capacity of the calorimeter was found to be 7.794 kJ/0C.
Calculate the heat of reaction for the combustion of one mole of CH6N2.
How is C for the bomb calorimeter calculated?
A 1.630 g sample of naphthalene C10H8, is completely burned in a bomb calorimeter assembly and a temperature
increase of 8.46oC. If the heat of combustion of naphthalene is -5156 kJ/mol C10H8 what is the heat capacity of the
bomb calorimeter?
Practice: a 0.5865 g sample of lactice acid (HC3H5O3) is burned in a calorimeter whose heat capacity is 4.18 kJ/oC. The
temperature increases from 23.10oC to 24.95oC. Calculate the heat of combustion of lactic acid
a) per gram and
b) per mole.
5.6 Hess’s Law
Defined:
Example:
5.7 Enthalpies of formation
H the old way: combine H values for breakdown of each reactant and
formation of each product.
Better way:
H = n H
p
H = H
f,products
products
−H
reactants
– n H
f
Calculation of dH by Hf’s
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
f ,reactants