Download Andrew Rosen 5.1 – The Nature of Energy Energy – The capacity to

Andrew Rosen
5.1 – The Nature of Energy
Energy – The capacity to do work or transfer heat (work or heat) and is an extensive property
Work – The energy used to cause an object to move against a force (
)
Heat (q) – The energy used to cause the temperature of an object to increase
Kinetic Energy – Energy of motion (
)
Potential Energy – Stored energy by virtue of position (
)
Chemical Potential Energy – Energy an object possesses by virtue of chemical composition
Thermal Energy – Form of KE of individual molecules and atoms
Joule (J) – SI unit of energy
calorie (cal) – Older non-SI unit for energy
Food Calorie (Cal) – 1000 cal
System – Portion we consider for study (can be open, closed, or isolated)
Surroundings – The rest of the universe
Open System – Matter and energy can be exchanged with the surroundings
Closed System – Systems that exchange energy but not matter with the surroundings
Isolated System – Neither energy nor matter can be exchanged (no perfect isolated system)
5.2 – The First Law of Thermodynamics
First Law of Thermodynamics – Energy is conserved and energy is neither created nor destroyed but may be converted from
one form to another (Discovered by Joule and Clausius)
Internal Energy (E) – Sum of all kinetic and potential energies of the components of the system
For q: Positive (+) means system gains heat and negative (-) means system loses heat
For w: Positive (+) means work is done on the system and negative (-) means work is done by the system
Endothermic – Systems absorbs heat
Exothermic – System loses heat
State Function – A property of a system that is determined only by the current state of the system and not the path ( )
The sum of heat and work is a state function
)
Internal Energy is a state function (E)
Pressure and volume are state functions
Enthalpy (H) is a state function
Heat Tax – Potential energy cannot be fully converted into work since some is lost as heat
5.3 – Enthalpy
Pressure-Volume Work (P-V work) – Work involved in the expansion or compression of a gas
If a piston compresses a system, work is done on (+) the system
If a piston decompresses a system, work is done by (-) the system
(
is a state function even though
is not)
Andrew Rosen
5.4 – Enthalpies of Reaction
Enthalpy (heat) of Reaction
– Enthalpy change that accompanies a reaction
Decreases (negative/exothermic) in spontaneous processes
Extensive property since it relies on the amount of reactant consumed
Enthalpy change is the same magnitude but negated for reverse reactions
Enthalpy change depends on the states of the reactants and products
Combustion reactions are heat-releasing exothermic reactions
Dissolution of solids is endothermic
Thermochemical Equations – Balanced chemical equations that show the associated enthalpy change
Calorimetry – Measurement of heat flow with a calorimeter
Heat Capacity ( ) – Temperature change an object experiences when it absorbs a certain amount of heat
Molar Heat Capacity (
Specific Heat
) – Heat capacity of one mole of a substance
– Heat capacity of one gram of a substance
Temperature can be in Celsius or Kelvins
Constant Pressure Calorimetry:
Because the calorimeter is insulated, the sum of all heats is zero
Bomb Calorimetry (Constant Volume):
Used for combustion reactions
To calculate the heat of combustion, the total heat capacity of the calorimeter is needed (
)
This is because the sum of all q values must equal zero
Heat transfer for bomb Calorimetry refers to
and not
5.6 – Hess’s Law
If a reaction is carried out in steps, the enthalpy change for the reaction equals the sum of the enthalpy changes for the individual
steps
∑
∑
5.7 – Enthalpies of Formation
Enthalpy of Formation (
): Enthalpy change for the reaction in which a compound is made from its constituent elements in
their most stable elemental forms and the stoichiometric coefficient for the product is one
Standard Enthalpies of Formation (
): Measured under standard conditions of 25 celsius and 1.00 atm
Pure elements have zero enthalpies of formation
The physical states matter with thermochemical equations
Andrew Rosen
5.8 – Foods and Fuels
Fuel Value – Energy released when one gram of any substance is combusted
Carbohydrates break down rapidly for quick energy but are not stored in high quantities
Fats are used as the body’s energy reserves since they are insoluble in water (easier storage) and produce more energy per gram
than either proteins or carbohydrates
The majority of energy consumption in the U.S. is from fossil fuels rich in carbon
Petroleum and natural gas are most easily transported since they are liquids
Cal is the most abundant fossil fuel in the world but are not environmentally friendly
Fossil Fuels – Fuels formed over millions of years from the decomposition of plants and animals
Natural Gas – Gaseous hydrocarbons
Petroleum – Liquid composed of hundreds of compounds (mostly hydrocarbons)
Coal- Solid that contains hydrocarbons of high molecular weight
Sulfur dioxide is produced from the burning of coal, which is a major pollutant
Fossil fuels release carbon dioxide, which destroys the Ozone
Nuclear Energy – Energy released in either the splitting or fusion of atomic nuclei