TEMPERATURE, HEAT, AND ENERGY

State function Temperature Heat System Surroundings Exothermic Endothermic 

Energy cannot be created or destroyed, but it can be converted from one form to another. 

I AIMS To understand energy and its effect on matter. To understand the meanings of temperature and heat. 

When we use a Bunsen burner on an ice cube, some of the energy of the reaction between natural gas (the fuel for the burner) and the oxygen in the air is transferred to the ice, causing it to melt. This flow of energy is called heat. Heat can be defined as a flow of energy due to a temperature difference. 

Equal masses of hot water and cold water separated by a thin metal wall in an insulated box. 

The HjO molecules in hot water have much greater random motions than the H2O molecules in cold water. 

When a process results in the evolution of heat, it is said to he exothermic (exo- is a prefix meaning out of ). For example, when a match is struck, energy is produced as heat it is an exothermic process. Processes that ahsorh energy are said to he endothermic. Melting ice to form liquid water is a common endothermic process. Energy from a source such as a Bunsen burner is required to melt the ice. Energy is put into the ice to melt it. 

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The general concepts of energy, temperature, and heat were introduced under the broad classification of potential and kinetic energies. Both these forms were subclassified into external forms determined by the position and motion of a mass of matter relative to the earth or other masses of matter and into internal forms determined by the inherent composition, structure, and state of matter itself, independent of its external position or motion as a whole. 

The Nature of Energy Temperature and Heat Exothermic and Endothermic Processes Thermodynamics Measuring Energy Changes... 

In the broadest sense, thermodynamics is concerned with mathematical relationships that describe equiUbrium conditions as well as transformations of energy from one form to another. Many chemical properties and parameters of engineering significance have origins in the mathematical expressions of the first and second laws and accompanying definitions. Particularly important are those fundamental equations which connect thermodynamic state functions to real-world, measurable properties such as pressure, volume, temperature, and heat capacity (1 3) (see also Thermodynamic properties). 

In the finite-difference appntach, the partial differential equation for the conduction of heat in solids is replaced by a set of algebraic equations of temperature differences between discrete points in the slab. Actually, the wall is divided into a number of individual layers, and for each, the energy conserva-tk>n equation is applied. This leads to a set of linear equations, which are explicitly or implicitly solved. This approach allows the calculation of the time evolution of temperatures in the wall, surface temperatures, and heat fluxes. The temporal and spatial resolution can be selected individually, although the computation time increa.ses linearly for high resolutions. The method easily can be expanded to the two- and three-dimensional cases by dividing the wall into individual elements rather than layers. 

The room model consists of nodes, which are interconnected by heat exchange paths (Fig. 11.36). The nodes represent either surface temperatures of the individual walls or the zone air temperature. For each node, an energy balance is formulated. From the resulting set of equations, the temperatures and heat fluxes can be determined. 

When steam at the saturation temperature contacts a surface at a lower temperature, and heat flows to the cooler surface, some of the steam condenses to supply the energy. With a sufficient supply of steam moving into the volume that had been occupied by the steam now condensed, the pressure and temperature of the steam will remain constant. Of course, if the condensate flows to a zone where it is no longer in contact with the steam it can cool below steam temperature while supplying heat to a cooler surface. 

This method was first applied by McCormick27 and by Bywater and Worsfold11 to the system a-methylstyrene/poly-a-methyl-styrene, and the free energy, entropy and heat of polymerization as well as the ceiling temperature were determined. Similar studies concerned with the system styrene/polystyrene are being carried out in our laboratories. 

It is thus seen that heat capacity at constant volume is the rate of change of internal energy with temperature, while heat capacity at constant pressure is the rate of change of enthalpy with temperature. Like internal energy, enthalpy and heat capacity are also extensive properties. The heat capacity values of substances are usually expressed per unit mass or mole. For instance, the specific heat which is the heat capacity per gram of the substance or the molar heat, which is the heat capacity per mole of the substance, are generally considered. The heat capacity of a substance increases with increase in temperature. This variation is usually represented by an empirical relationship such as... 

Temperature and heat are not the same. Temperature is a measure of the intensity of the energy in a system. Consider the following experiment Hold a lit candle under a pail of water with one-half inch of water in the bottom. Hold an identical candle, also lit, under a pail full of water for the same length of time. To which sample of water is more heat added Which sample of water gets hotter ... 

Figures 4.6—4.8 are the results for the stoichiometric propane-air flame. Figure 4.6 reports the variance of the major species, temperature, and heat release Figure 4.7 reports the major stable propane fragment distribution due to the proceeding reactions and Figure 4.8 shows the radical and formaldehyde distributions—all as a function of a spatial distance through the flame wave. As stated, the total wave thickness is chosen from the point at which one of the reactant mole fractions begins to decay to the point at which the heat release rate begins to taper off sharply. Since the point of initial reactant decay corresponds closely to the initial perceptive rise in temperature, the initial thermoneutral period is quite short. The heat release rate curve would ordinarily drop to zero sharply except that the recombination of the radicals in the burned gas zone contribute some energy. The choice of the position that separates the preheat zone and the reaction zone has been made to account for the slight exothermicity of the fuel attack reactions by radicals which have diffused into...

The heat capacity is the amount of energy required to increase the temperature of a unit mass of material. It is commonly measured using a differential scanning calorimeter (DSC). The heat capacity depends on the resin type, additives such as fillers and blowing agents, degree of crystallinity, and temperature. A temperature scan for the resin will reveal the Tg for amorphous resins and the peak melting temperature and heat of fusion for semicrystalline resins. The heat capacities for LDPE and PS resins are shown in Fig. 4.15. 

The obvious way to heat or cool a chemical reactor is by heat exchange through a wall, either by an external jacket or with a cooling or heating coil. As we discussed previously, the coolant energy balance must be solved along with the reactor energy balance to determine temperatures and heat loads. 

TEMPERATURE. The thermal state of a body, considered, with reference to its ability to communicate heat to other bodies (J. C. Maxwell). There is a distinction between temperature and heat, as is evidenced by Helmholtz s definition of heat, [energy that is transferred from one body to another by a thermal process), whereby a thermal process is meant radiation, conduction, and/or convection. 

Just as the macroscopic mechanical energy equation is used to determine the relations between the various forms of mechanical energy and the frictional energy losses, so the thermal energy equation, expressed in macroscopic form, is used to determine the relation between the temperature and heat transfer rates for a flow system. 

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