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CHEM 1211 - Principles of Chemistry I
Chapter 6 - Thermochemistry
6.1 The Nature of Energy
! we will define energy as the capacity to do work or to produce heat ! one of the most important characteristics of energy is that it is conserved ! the law of conservation of energy states that energy can be converted from one form to another but can be neither created nor destroyed " that is energy of the universe is constant ! energy can be classified as potential or kinetic " potential energy is energy due to position or composition (stored energy) " kinetic energy of an object is energy due to the motion of the object and is proportional the mass and velocity of the object " see Figure 6. ! there is a difference between heat and temperature " temperature is a property that reflects the random motions of the particles in a particular substance " heat involves the transfer of energy between two objects due to a temperature difference ! work is defined as force acting over a distance ! important statement - energy change is independent of the pathway but work and heat are both dependent on the pathway (much work - little heat or vice versa; but total energy stays the same) ! now the concept of state function or state property which refers to a property of the system that depends only on its present state " energy is a state function; work and heat are not state functions
Chemical Energy
! consider the combustion of natural gas (methane) which is used to heat houses; to discuss this reaction, we divide the universe into two parts: the system and surroundings ! the system is the part of the universe on which attention is focused and the surroundings include everything else in the universe ! when a reaction evolves heat it is exothermic ! when a reaction absorbs heat it is endothermic ! in any exothermic reaction, some of the potential energy stored in the chemical bonds is being converted to thermal energy (random kinetic energy) via heat " see Figure 6. " see Figure 6. ! the study of energy and it interconversion is called thermodynamics ! the law of conservation of energy is often call the first law of thermodynamics (the energy of the universe is constant)
! the internal energy E of a system can be defined most precisely as the sum of the kinetic and potential energies of all the “particles” in the system ! D E = q + w where D E is the change in the system’s internal energy, q is heat and w is work " the sign (positive or negative) reflects the system’s point of view;
- positive in
- negative out ! a common type of work associated with chemical processes is work done by a gas (through expansion) or work done to a gas (through compression) " therefore have w =- P D V " in dealing with “ PV ” work the P always refers to the external pressure - the pressure that causes a compression or that resists an expansion
6.2 Enthalpy and Calorimetry
Enthalpy
! so far have discussed the internal energy of a system ! a less familiar property of a system is its enthalpy, H , which is defined as H = E + PV where E is the internal energy of the system, P is the pressure of the system, and V is the volume of the system " since internal energy, pressure, and volume are all state functions, enthalpy is also a state function ! at constant pressure (where only PV work is allowed), the change in enthalpy D H of the system is equal to the energy flow as heat " this means that for a reaction studied at constant pressure, the flow of heat is a measure of the change in enthalpy for the system; for this reason, the terms heat of reaction and change in enthalpy are used interchangeably, for reactions studied at constant pressure ! for a chemical reaction, the enthalpy change is given by the equation
D H = H products (^) - H reactants
" positive change - endothermic reaction " negative change - exothermic reactions
Calorimetry
! device used experimentally to determine the heat associated with a chemical reaction is called a calorimeter; have, calorimetry, the science of measuring heat based on observing the temperature change when a body absorbs or discharges energy as heat ! the heat capacity C of a substance, which is a measure of this property, is defined as C = heat absorbed increase in temperature ! in defining the heat capacity of a substance, the amount of substance must be specified
been carried out under standard conditions " the standard state for a substance is a precisely defined reference state ! for most thermodynamic properties, we can only measure changes in the property; for example no method for determining absolute values of enthalpy " consider
D H (^) f= 34 kJ/mol o
" note that both elements in their standard states and one mole of product is formed ! enthalpies of formation are always given per mole and 1 mole or product with the product in its standard state " see Table 6. " see Appendix 4 ! the importance of the tabulated D H (^) fvalues is that enthalpies for many reactions can be o calculated using these numbers ! the enthalpy change of a given reaction can be calculated by subtracting the enthalpies of formation of the reactants from the enthalpies of formation of the products
! elements are not included in the calculation because elements require no change in form
6.5 Present Sources of Energy
! fossil fuels " see Figure 6.
Petroleum and Natural Gas
! how petroleum and natural gas were produced is not completely understood ! petroleum is thick, dark liquid composed mostly of compounds called hydrocarbons ! natural gas, usually associated with petroleum deposits, consists mostly of methane, but also significant amounts of ethane, propane, and butane ! petroleum consists mostly of hydrocarbons having chains that contain from 5 to more than 25 carbons; must fractionate the petroleum ! the petroleum era began when the demand for lamp oil during the Industrial Revolution outstripped the traditional sources: animal fats and whale oil ! in response to the increased demand, Edwin Drake drilled the first oil well in 1859 at Titusville, Pennsylvania " the petroleum was refined to produce kerosene (C 10 -C ) which served as an excellent 18 lamp oil " gasoline (C -C ) was of limited use and was often discarded 5 10 ! the development of the electric light decreased the need for kerosene and the advent of the
automobile signaled the gasoline age ! catalytic cracking ! tetraethyl lead added to gasoline to prevent “knocking” ! catalytic converter
Coal
! coal was formed from the remains of plants that were buried and subjected to high pressure and heat over long periods of time ! coal is an important and plentiful fuel in the United States furnishing approximately 20% of the our energy ! burning coal produces pollution
Effects of Carbon Dioxide on Climate
! see Figure 6.
6.6 New Energy Sources
! as we search for the energy sources of the future, we need to consider economic, climatic, and supply factors ! there are several potential energy sources: the sun (solar), nuclear processes (fission and fusion), biomass (plants), and synthetic fuels
Coal Conversion
! coal gasification; see reactions in text ! half of South Africa gasoline made by gasification of coal
Hydrogen as a Fuel
! per gram, energy generated by combusting hydrogen is approximately 2.5 times that of natural gas " only product of combustion is water ! even though it appears that hydrogen is a very logical choice as a major fuel for the future, there are three main problems; the cost of production, storage, and transport
Other Energy Alternatives
! oil shale ! ethanol ! methanol ! seed oil
