Energy transfer as heat

This chapter introduces the first law of thermodynamics and its applications in three main parts. The first part introduces the basic concepts of thermodynamics and the experimental basis of the first law. The second part introduces enthalpy as a measure of the energy transferred as heat during physical changes at constant pressure. The third part shows how the concept of enthalpy is applied to a variety of chemical changes, an important aspect of bioenergetics, the use of energy in biological systems. 

Similarly, heat is not a state function. The energy transferred as heat during a change in the state of a system depends on how the change is brought about. For example, suppose we want to raise the temperature of 100 g of water from 25°C to 30°C. One way to raise the temperature would be to supply energy as heat by... 

The quantity of energy transferred as heat is measured in joules, J. However, a unit of energy still widely used in biochemistry and related fields is the calorie (cal). The original definition of 1 cal was that it is the energy needed to raise the temperature of 1 g of water by 1°C. The modern definition is... 

If the reaction is exothermic, the energy transferred as heat to the calorimeter results in a rise in temperature, AT. The temperature increase is proportional to the energy transferred, and the constant of proportionality is called the heat capacity of the calorimeter, Qal ... 

When an infinitesimal quantity of energy dq is supplied as heat, the temperature rises by an infinitesimal amount dT Recalling from Section 6.8 that the rise in temperature is proportional to the energy transferred as heat, we can rewrite Eq. 15 in Chapter 6 for the transfer of an infinitesimal amount of heat ... 

The number of unknown variables for a single unit is the sum of the unknown component amounts or flow rates for ail inlet and outlet streams, plus all unknown stream temperatures and pressures, plus the rates of energy transfer as heat and work. The equations available to determine these unknowns include material balances for each independent species, an energy balance, phase and chemical equilibrium relations, and additional specified relationships among the process variables. 

Have you ever wondered why a heavy iron pot gets hot fast but the water in the pot takes a long time to warm up If you transfer the same quantity of heat to similar masses of different substances, they do not show the same increase in temperature. This relationship between energy transferred as heat to a substance and the substance s temperature change is called the specific heat. 

Recall that the specific heat is the quantity of energy that must be transferred as heat to raise the temperature of 1 g of a substance by 1 K. The quantity of energy transferred as heat during a temperature change depends on the nature of the material that is changing temperature, the mass of the material, and the size of the temperature change. 

In the above equation, Cp is the specific heat at a given pressure, q is the energy transferred as heat, m is the mass of the substance, and AT represents the difference between the initial and final temperatures. 

Specific heat is the relationship between energy transferred as heat to a substance and a substance s temperature change. 

You know that heat and temperature are different because you know that when two samples at different temperatures are in contact, energy can be transferred as heat. Heat and temperature differ in other ways. Temperature is an intensive property, which means that the temperature of a sample does not depend on the amount of the sample. However, heat is an extensive property which means that the amount of energy transferred as heat by a sample depends on the amount of the sample. So, water in a glass and water in a pitcher can have the same temperature. But the water in the pitcher can transfer more energy as heat to another sample because the water in the pitcher has more particles than the water in the glass. 

Section 12.2 defines specific heat capacity as the amount of heat required to increase the temperature of 1 g of material by 1 K. That definition is somewhat imprecise, because, in fact, the amount of heat required depends on whether the process is conducted at constant volume or at constant pressure. This section describes precise methods for measuring the amonnt of energy transferred as heat during a process and for relating this amonnt to the thermodynamic properties of the system under investigation. 

The total change in a system s internal energy is the sum of the energy transferred as heat and/or work ... 

Energy Transfer as Heat Only For a system that does no work but transfers energy only as heat (q), we know that = 0. Therefore, from Equation 6.2, we have A = q + 0 = q. The two possibilities are ... 

Heating value refers to the total energy transferred as heat in an ideal combustion reaction at base temperature and pressure. For net heating value, the water formed in the combustion appears as vapor in the products for gross heating value, the water formed in the combustion appears as liquid in the products. 

It is well known from thermodynamic principles that energy transferred as work is more useful than energy transferred as heat. Work can be completely converted to heat, but only a fraction of heat can be converted to work. Furthermore, as the temperature of a system is decreased, heat transferred from the system becomes less useful and less of the heat can be converted to work. A state property that accounts for the differences between heat and work is entropy, S. When heat is transferred into a closed system at temperature T, the entropy of the system increases because entropy transfer accompanies heat transfer. By contrast, work transfer (shaft work) is not accompanied by entropy transfer. When heat is transferred at a rate Q from a surrounding heat reservoir at a constant temperature, Treservoir, into a system, the heat reservoir experiences a decrease in entropy given by... 

An infinitesimal quantity of energy transferred as heat at a surface element of the boundary is written dq, and a finite quantity is written q (Sec. 2.5). To obtain the total finite heat for a process from q = fdq (Eq. 2.5.3), we must integrate over the total boundary surface and the entire path of the process. 

Energy Transferred as Heat Only For a system that transfers energy only as heat... 

We calculate AH of a process by measuring the energy transferred as heat at constant pressure (qp).To do this, we determine AT and multiply it by the mass of the substance and by its specific heat capacity (c), which is the quantity of energy needed to raise the temperature of 1 g of the substance by 1 K. 

The ability to measure temperature is thus based on energy transfer. The amount of energy transferred as heat is usually measured in joules. 

TransfGr of Energy The direction of energy transfer as heat is determined by the temperature differences between the objects within a system. The energy is transferred as heat from the hotter brass bar to... 

The enthalpy of reaction is the quantity of energy transferred as heat during a chemical reaction. You can think of enthalpy of reaction as the difference between the stored energy of the reactants and the products. Enthalpy of reaction is sometimes called "heat of reaction. ... 

Calorimetry is the study of heat transfer during physical and chemical processes. A calorimeter is a device for measuring energy transferred as heat. Here we explore three common types of calorimeters used in investigations of nutrients, fuels, and biological processes. 

According to the discussion in Sections 1.5 and 1.6, and the relation AU = qv, the energy transferred as heat at constant volume is equal to the change in internal energy, A [/, not AH. To convert from A1/ to AH, we need to note that the molar enthalpy of a substance is related to its molar internal energy by H = U + pV (eqn 1.12a). For condensed phases, pUm is so small that it maybe ignored. For example, the molar volume of liquid water is 18 cm mol" , and at 1.0 bar... 

A typical human produces about 10 MJ of energy transferred as heat each day through metabolic activity, (a) If a human body were an isolated system of mass 65 kg with the heat capacity of water, what temperature rise would the body experience (b) Human bodies... 

That is, the change in entropy of a system is equal to the energy transferred as heat to it reversibly divided by the temperature at which the transfer takes place. This definition can be justified thermodynamically, but we shall confine ourselves to showing that it is plausible and then show how to use it to obtain numerical values for a range of processes. 

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