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Thermodynamics
Principles of Chemical Reactivity
Basic PrinciplesThermodynamics: The science of heat
and workEnergy: the capacity to do work
-chemical, mechanical, thermal, electrical, radiant, sound, nuclear-affects matter by raising its temperature,
eventually causing a state change-All physical changes and chemical
changes involve energy
Potential Energy: energy that results from an object’s
position -gravitational, chemical, electrostatic
Kinetic Energy: energy of motion
Basic Principles
Law of Energy Conservation:Energy can neither be created nor
destroyed-a.k.a. The first law of thermodynamics
-The total energy of the universe is constant
Temperature vs. Heat:– Temperature is the measure of an object’s
heat energy– Heat ≠ temperature
The Measurement of Heat
Thermal Energydepends on temperature and the amount (mass or volume) of the object
-More thermal energy a substances has the greater the motion its atoms/molecules have-Total thermal energy of an object is the sum of the individual energies of all atoms/molecules/ions that make up that object
SI unit: Joule (J)1 calorie = 4.184 JEnglish unit = BTU
Converting Calories to Joules
Convert 60.1 cal to joules
System: object or collection of objects being studied– In lab, the system is the chemicals inside the
beaker
Surroundings: everything outside of the system that can exchange energy with the system– The surroundings are outside the beaker
Universe: system plus surroundings
Exothermic: heat transferred from the system to the surroundings
Endothermic: heat transferred from the surroundings to the system
Basic Principles
Specific Heat Capacity (C)
amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius
SI Units: Specific heat capacity =J/g. °CSpecific heat of water = 4.184 J/g. °C
Heat Transfer
Heat transfer equationused to calculate amounts of heat (q) in
a substance
T C mq J
g J/g·°C
°C
q1 + q2 + q3 … = 0 or qsystem + qsurroundings = 0
Heat Transfer
Calculate the amount of heat to raise the temperature of 400 g of water from 10.0 oC to 100 oC
Heat Transfer
Calculate the amount of heat energy (in joules) needed to raise the temperature of 12.50 g of water from 45.0°C to 79.0°C
Heat Transfer
Specific heat of gold is 0.13
Therefore the metal cannot be pure gold.
A 1.6 g sample of metal that appears to be gold requires 5.8 J to raise the temperature from 23°C to 41°C. Is the metal pure gold?
Jg.°C
Changes of State
occurs when enough energy is put into a substance to over come molecular interactions
Solid-liquid: molecules in a solid when heated move about vigorously enough to break solid-solid molecular interactions to become a liquid
Liquid-gas: molecules in a liquid when heated move about more vigorously enough to break liquid-liquid molecular interactions to become a gas
Note: This happens in reverse by removing heat energy
Energy and Changes of StateHeat of fusion:
heat needed to convert a substance from a solid to a liquid (at its melting/freezing point)
333 J/g for water
Heat of vaporization: heat needed to convert a substance from a liquid to a gas (at its boiling/condensation point)
2256 J/g for water
Example: Calculate the amount of heat involved to convert 100.0 g of ice at -50.0°C to steam at 200.0°C.
The First Law of Thermodynamics
This law can be stated as, “The combined amount of energy in the universe is constant”
Also called-The Law of Conservation of Energy:– Energy is neither created nor destroyed in
chemical reactions and physical changes.
Changes in Internal Energy (E)
E is negative when energy is released by a system
-Energy can be written as a product of the process
kJ 10 3.516- E
kJ 10 3.516 OH 6 CO 5 O 8 HC3
3)(22(g)2(g))(125
Changes in Internal Energy (E)
E is positive when energy is absorbed by a system undergoing a chemical or physical change
– Energy can be written as a reactant of the process
kJ 10 3.516 E
O 8 HC kJ 10 3.516 OH 6 CO 53
2(g))(1253
)(22(g)
Enthalpy Changes for Chemical Reactions
Exothermic reactions: release energy in the form of heat to the surroundings (H < 0)
-heat is transferred from a system to the surroundings
Endothermic reactions: gain energy in the form of heat from the surroundings (H > 0)
-heat is transferred from the surroundings to the system
For example, the combustion of propane:
Combustion of butane:
Enthalpy Changes for Chemical Reactions
Exothermic reactions generate specific amounts of heat– Because the potential energies of the products are
lower than the potential energies of the reactants
Endothermic reactions consume specific amounts of heat– Potential energies of the reactants are lower than the products
H for the reverse reactionis equal, but has the opposite sign to the forward reaction
Thermochemical Equations
balanced chemical reaction with the H value for the reaction
H < 0 designates an exothermic reaction: heat is a product, the container feels hot
H > 0 designates an endothermic reaction: heat is a reactant, the container feels cold
kJ 3523 - H OH 6 CO 5O 8 HC orxn)(22(g)2(g))12(5
CalorimetryAn experimental technique that measures the heat
transfer during a chemical or physical process
Constant pressure calorimetry:A styrofoam coffee-cup calorimeter is
used to measure the amount of heat produced (or absorbed) in a reaction
– This is one method to measure qP (called H) for reactions in solution
qreaction + qsolution = 0
Note: Assuming no heat transfer to the surroundings
CalorimetryIf an exothermic reaction is performed in a
calorimeter, the heat evolved by the reaction is determined from the temperature rise of the solution– This requires a two part calculation
When we add 25.00 mL of 0.500 M NaOH at 23.000oC to 25.00 mL of 0.600 M CH3COOH already in the calorimeter at the same temperature, the resulting temperature is observed to be 25.947oC. Determine heat of reaction and then calculate the change in enthalpy (as KJ/mol) for the production of NaCH3COO.
CH3COOH(aq) + NaOH(aq) NaCH3COO(aq) + H2O(l)