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Chapter 6
Thermochemistry
6.3
Title
 A "real-world" closed system and a chemist's conception of it
Caption
 (a) Preparation of sun tea utilizes a clear, sealed jar. (b) The sketch suggests that the system is "closed:" changes occur in the system as heat is absorbed from the surroundings, but no matter enters or leaves.
Keywords
 thermochemistry, closed system, model, heat
6.4a.e
Title
 Some components of internal energy
Caption
 Some key components of the intenal energy of molecules are illustrated: motion, stretching of bonds, bending of bonds, attractions and repulsions within and atom and intermolecular attractions/repulsions.
Keywords
 internal energy, kinetic energy, attraction, repulsion, gas
6.5
Title
 Heat transfer between a system & surroundings
Caption
 In their collisions with the hypothetical thin wall, the more energetic gaseous helium atoms of the system transfer some kinetic energy to the less energetic nitrogen molecules in the cooler surroundings. The average translational kinetic energy of the helium atoms decreases and that of the nitrogen molecules increases until the average translational kinetic energies of the two gases become equal. At this point of thermal equilibrium, the temperatures of the system and surroundings have become equal and heat no longer flows.
Keywords
 gas, kinetic energy, temperature, model
6.8
Title
 Pressure-volume work
Caption
 The gas expands when one of the weights is removed. The remaining weight is lifted through the distance h. The gas volume increases by dV, and the gas does work.
Keywords
 work, pressure, volume, piston, gas
6.11abc
Title
 Conceptualizing exothermic reaction
Caption
 In an exothermic reaction, chemical energy in a system is converted to thermal energy. (a) The thermal energy is retained in the isolated system. A maximum temperature increase occurs. (b) The thermal energy is released as heat to the surroundings as rapidly as it is formed. There is no temperature change in the system. (c) The system is not completely isolated from its surroundings, nor is the flow of heat instantaneous. There is both an initial increase in temperature in the system and a flow of heat to the surroundings.
Keywords
 calorimeter, exothermic, surroundings, system
6.12abc
Title
 Conceptualizing endothermic reaction
Caption
 In an endothermic reaction, thermal energy in a system is converted to chemical energy. (a) The loss of thermal energy is uncompensated in the isolated system. A maximum temperature decrease occurs. (b) The loss of thermal energy in the system is immediately compensated for by the absorption of heat from the surroundings. There is no temperature change in the system. (c) This situation is similar to that described in Figure 6.11(c), except that it involves a temperature decrease.
Keywords
 endothermic, calorimeter, surroundings, system
6.13
Title
 Comparing the heats of reaction at constant volume & pressure for exothermic reaction
Caption
 (a) The stops above and below the piston prevent the system volume from changing. The heat evolved, qV, is equal to DU. (b) The system is allowed to expand against the constant pressure of the weight. A small amount of work, w, and a quantity of heat, qP, are evolved. The magnitude of qP is slightly less than that of qV.
Keywords
 enthalpy, heat, thermochemistry, pressure, closed system
6.14
Title
 Visualizing a reaction carried out under the pressure of the atmosphere
Caption
 In the reaction: Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)the evolved H2(g) pushes air from the flask and surroundings. The expanding gas does work, represented by the blue arrows.
Keywords
 enthalpy, model, work, heat, thermochemistry, volume, chemical reaction
6.15
Title
 Enthalpy diagrams
Caption
 No numbers are shown on the enthalpy axis because absolute values of enthalpy cannot be measured. For an exothermic reaction (red), the products have a lower enthalpy than the reactants and DH is negative (DHÊ<Ê0). For an endothermic reaction (blue), the products have a higher enthalpy than the reactants and DH is positive (DHÊ>Ê0).
Keywords
 enthalpy, exothermic, endothermic, reaction, graph
6.16
Title
 Reversing a chemical reaction
Caption
 The forward reaction (shown in blue) is the dissociation at 25 ¡C of 1.00 mol HgO(s) into its elements and is accompanied by an increase in enthalpy of 90.83 kJ; DHÊ=Ê+90.83 kJ. In the reverse reaction (red), 1.00 mol HgO(s) is formed from its elements. This reaction is accompanied by a decrease in enthalpy of 90.83 kJ; DHÊ=Ê-90.83 kJ. When a reaction is reversed, the magnitude of DH remains the same, but its sign changes.
Keywords
 enthalpy, state function, thermochemistry
6.17
Title
 A simple calorimeter
Caption
 A simple calorimeter can be made using 2 polystyrene cups. The inner cup is closed off with a cork stopper through which a thermometer and stirrer are immersed into the calorimeter. The outer cup provides additional thermal insulation from the surroundings.
Keywords
 calorimeter, system, surroundings, isolated, thermochemistry
6.18
Title
 A bomb calorimeter
Caption
 The sample to be burned is placed in a small cup in the steel bomb, which is then filled with oxygen. The sample is ignited by an electric current. The heat of the reaction is determined from the temperature rise in the water that surrounds the bomb. The steel bomb confines the reactants and products to a constant volume.
Keywords
 calorimeter, system, surroundings, isolated, thermochemistry
6.19
Title
 Determining an unknown enthalpy change through an enthalpy diagram
Caption
 This diagram brings out the relationship of the unknown DH(a) to the known values of DH(b) and DH(c).
Keywords
 enthalpy, state function, reaction, graph, Hess' law
6.20
Title
 Example 6.17 illustrated
Caption
 Even without a calculator, it is possible to determine which substance should yield the most heat upon combustion.
Keywords
 combustion, enthalpy, example, enthalpy of formation
6.21
Title
 Petroleum distillation & gasoline production
Caption
 Crude oil is vaporized, and the fractional distillation column separates the components of the vapor according to their boiling temperatures. The lower-boiling components come off at the top of the column, and higher-boiling components come off lower in the column. A residue that does not boil collects at the bottom. The residue may be paraffin or asphalt, depending on the source of the crude oil.
Keywords
 petroleum, distillation, fractional distillation, fuel
6.22
Title
 Glyceryl trilaurate
Caption
 Glyceryl trilaurate is a typical fat. It is shown in the Lewis structure and as a space-filling model. Glycerol provides the three-carbon backbone (at the top), and lauric acid molecules contribute the three tails.
Keywords
 combustion, fat, model, space filling, calorie, energy, thermochemistry
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