Heat in Chemical Reactions and Processes

Heat changes that occur during chemical and physical processes can be measured using an insulated device called a calorimeter. In one type of calorimeter, the temperature change of a known mass of water is used to determine the amount of energy released or absorbed by a system undergoing a chemical or physical change. 

The following example problem shows you how to determine the specific heat of an unknown substance by using calorimetry data. 

A calorimeter contains 195 g of water at 20.4°C. A 37.8-g sample of an unknown metal is heated to 133°C and placed into the water in the calorimeter. Heat flows from the metal to the water until both reach a final temperature of 24.6°C. What is the specific heat of the metal  

Note that three significant figures are retained for precision in the calculation. At the end of the problem, you should round off the final answer to two significant figures. 

160 Chemistry Matter and Change Solving Problems A Chemistry Handbook 

You have learned that some chemical reactions and processes must absorb energy in order to occur. These are called endothermic reactions. Others release energy and are called exothermic reactions. Whenever you cook food using methane or propane gas in your kitchen range, you utilize the heat released in the combustion of these fuels. But how do you measure the amount of heat released or absorbed when chemical reactions such as these occur  

calculate the heat gained by the water. For this you need the specific heat of water, 4.184 J/(g-°C). 

The heat gained by the water, 1900 J, equals the heat lost by the metal, metai so you can write this equation. 

In the measured mass of water has an initial temperature of 25.6°C. A piece of hot metal is added. The metal transfers heat to the water until metal and water attain the same temperature. That final temperature is shown in as 29.3°C. 

solve the equation for the specific heat of the metal, hy dividing 

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Several reasons have been proposed to account for the effect of microwave heating on chemical reactions and catalytic systems. The results summarized in 1 to 7, above, show that under specific conditions microwave irradiation favorably affects reaction rates of both the liquid- and gas-phase processes. This phenomenon has been explained in terms of microwave effects, i. e. effects which cannot be achieved by conventional heating. These include superheating, selective heating, and formation of hot spots (and possibly nonthermal effects). 

THERMOCHEMISTRY. That aspect of chemistry which deals with die heat changes which accompany chemical reactions and processes, the heal produced by them, and die influence of temperature and odier thermal quantities upon them. Tt is closely related to chemical thermodynamics. The heat of formation of a compound is the heat absorbed when it is formed from its elements in their standard states. An exothermic reaction evolves heat and endothermic reaction requires heat for initiation. 

The affinity of irreversible processes is a thermodynamic function of state related to the creation of entropy and uncompensated heat during the processes. The second law of thermodynamics indicates that all irreversible processes advance in the direction of creating entropy and decreasing affinity. This chapter examines the property affinity in chemical reactions and the relation between the affinity and various other thermodynamic quantities. 

Development of optimization methods for MEIS with variable flows of a substance participating in chemical reactions and transfer processes of heat, mass, and electric charges. 

The subject of thermochemistry deals with the heat changes resulting from chemical processes its laws are direct consequences of the first law of thermodynamics. As most reactions are carried out under constant pressure, our treatment will be restricted to a discussion of enthalpy changes. A corresponding set of equations could easily be obtained for the internal energy. In this section we discuss heat changes in chemical reactions and the thermochemistry of solutions. 

One of the aims of the calorimetry is the development of experimental methods for the direct determination of the quantities of heat produced in chemical reactions and in associated physical processes [18,19]. 

One of the main advantages of reaction calorimetry on the larger scale is the possibility of inserting into the reactor special analytical probes for on-line measurements. Some preliminary results obtained by coupling an ultrasonic sensor with calorimetry are presented in Fig. 5.17. The sensor is directly inserted into the reactor, its contribution in terms of heat accumulation having been previously determined so that the calorimetric signal is only related to the chemical reaction and process. At the moment, only the sound wave measurement is compared to the... 

We wOl also consider how to measure heat changes in chemical processes (calorimetry), how to establish standard values for enthalpy changes in chemical reactions, and how to use them to calculate AH values for reactions we can t actually study experimentally. 

Mass transfer processes are complicated, usually involving turbulent flow, heat transfer, multiple phases, chemical reactions, unsteady operation, as well as the influences from internal construction of the equipment and many other factors. To study such complicated system, we propose a novel scientific computing framework in which all the relevant equations on mass transfer, fluid-dynamics, heat transfer, chemical reactions, and all other influencing factors are involved and solved numerically. This is the main task and research methodology of computational mass transfer (CMT). 

Polymeric materials also experience deterioration by means of environmental interactions. However, an nndesirable interaction is specified as degradation rather than corrosion because the processes are basically dissimilar. Whereas most metallic corrosion reactions are electrochemical, polymeric degradation is physiochemical that is, it involves physical as well as chemical phenomena. Furthermore, a wide variety of reactions and adverse consequences are possible for polymer degradation. Polymers may deteriorate by sweUing and dissolution. Covalent bond rupture as a result of heat energy, chemical reactions, and radiation is also possible, typically with an attendant reduction in mechanical integrity. Because of the chemical complexity of polymers, their degradation mechanisms are not well understood. 

An industrial chemical reacdor is a complex device in which heat transfer, mass transfer, diffusion, and friction may occur along with chemical reaction, and it must be safe and controllable. In large vessels, questions of mixing of reactants, flow distribution, residence time distribution, and efficient utilization of the surface of porous catalysts also arise. A particular process can be dominated by one of these factors or by several of them for example, a reactor may on occasion be predominantly a heat exchanger or a mass-transfer device. A successful commercial unit is an economic balance of all these factors. 

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

The chemical engineer is concerned with the industrial application of processes. This involves the chemical and microbiological conversion of material with the transport of mass, heat and momentum. These processes are scale-dependent (i.e., they may behave differently in small and large-scale systems) and include heterogeneous chemical reactions and most unit operations. Tlie heterogeneous chemical reactions (liquid-liquid, liquid-gas, liquid-solid, gas-solid, solid-solid) generate or consume a considerable amount of heat. However, the course of... 

Most of the remainder of this chapter is devoted to a discussion of the magnitude of the heat flow in chemical reactions or phase changes. However, we will focus on a simpler process in which the only effect of the heat flow is to change the temperature of a system. In general, the relationship between the magnitude of the heat flow, q, and the temperature change, At, is given by the equation... 

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