Endothermic reactions Absorbing heat

An endothermic reaction absorbs heat energy. Energy must be added to the reactants for an endothermic reaction to occur. Since the energy is needed as a reactant, it is shown on the reactant side of the equation. 

Cold packs, used by doctors and athletes, lower the temperature of an injury. A cold pack contains two chemicals. The chemicals react with each other when they are mixed. The endothermic reaction absorbs heat from the injured part of the body. It helps to cool the body down. 

If the sum of the enthalpies of the products is greater than the sum of the enthalpies of the reactants, then A Hrxn is positive and the reaction is endothermic. Endothermic reactions absorb heat from their surroundings, simplest endothermic reactions break chemical bonds. 

As you learned in Chapter 1, chemical reactions can be exothermic or endothermic. Recall that an exothermic reaction releases heat, and an endothermic reaction absorbs heat. Figure 20.1 pictures an exothermic process as a reaction that s going downhill energetically and an endothermic process as a reaction that s going uphill. 

Heat Required = Q = specific heat capacity x mass x AT Specific heat capacity of H20(l) = 4.184 J/g "C Kinetic energy = mv /2 Exothermic reactions produce heat Endothermic reactions absorb heat... 

Reactions are either exothermic or endothermic. Exothermic reactions liberate heat endothermic reactions absorb heat. In an exothermic reaction, heat is a product and may be written on the right side of the equation for the reaction. In an endothermic reaction, heat can be regarded as a reactant and is written on the left side of the equation. Here are two examples ... 

Endothermic reactions absorb heat energy from the surroundings. The products of an endothermic reaction contain more potential energy or enthalpy than the reactants. For this type of reaction the enthalpy change is positive, because the chemical reactants gain heat from their surroundings. 

If an endothermic reaction absorbs heat, then why does it feel cold to the touch ... 

In a fuel cell electrochemical reactions take place leading to the direct conversion of chemical energy to electricity. The type of reaction that takes place plays a crucial role and, depending on its course, the fuel cell can achieve higher or lower efficiency. Exothermic reactions are characterized by the fact that the heat generated is put into the environment, whereas endothermic reactions absorb heat from the environment. Fuel cell efficiency is often based on only the chemical energy of the fuel delivered with such a definition, the heat supplied from the environment may result in the achievement of efficiency values above unity. Fig. 5.96 shows the theoretical efficiency of fuel cells from the supply of various compounds. 

The Powerformer reaction absorbs heat it is largely an endothermic reaction. This heat of reaction is in the order of 200-350 BTU/pound, depending on the type of feed. Since the individual Powerforming fixed bed reactors operate... 

Air Enthalpy change T he heat of reaction, or difference in strength between the bonds broken in a reaction and tire bonds formed. When All is negative, the reaction releases heat and is exothermic. When A IT is positive, the reaction absorbs heat and is endothermic. 

We can therefore report that AH = —208 kj because the enthalpy of the reaction mixture decreases by 208 kj in this reaction (Fig. 6.18). An endothermic process absorbs heat, and so when ammonium nitrate dissolves in water the enthalpy of the system increases (Fig. 6.19). Note that AH < 0 for exothermic reactions, whereas AH > 0 for endothermic reactions. 

FIGURE 6.19 If an endothermic reaction absorbs 100 k of heat at constant pressure, the height of the enthalpy "reservoir" rises by 100 k and AH = +100 kj. 

An endothermic process absorbs heat energy from its surroundings an exothermic process releases heat energy to its surroundings. If a reaction is endothermic in one direction, it is exothermic in the opposite direction. For example, the melting of 1 mole of ice water is an endothermic process requiring 6.02 kJ of heat ... 

You can include the enthalpy of reaction as a heat term in the chemical equation. Because heat is absorbed in an endothermic reaction, the heat term is included on the reactant side of the equation. 

Endothermic reactions absorb energy, or heat. When baking soda and vinegar react, the reaction absorbs energy, and you can measure that by taking the temperature before and after the reaction. Photosynthesis is another endothermic reaction plants absorb... 

These values of A Hr are standard state enthalpies of reaction (aU gases in ideal-gas states) evaluated at 1 atm and 298 K. 7VU values of A are in kilojoules per mole of the first species in the equation. When A Hr is negative, the reaction hberates heat, and we say it is exothermic, while, when A Hr is positive, the reaction absorbs heat, and we say it is endothermic. Tks Table 2-2 indicates, some reactions such as isomerizations do not absorb or liberate much heat, while dehydrogenation reactions are fairly endothermic and oxidation reactions are fairly exothermic. Note, for example, that combustion or total oxidation of ethane is highly exothermic, while partial oxidation of methane to synthesis gas (CO + H2) or ethylene (C2H4) are only slightly exothermic. 

If a reaction is exothermic, then the reaction generates heafi which tends to increase the reactor temperature, while if a reaction is endothermic, the reaction absorbs heat, which tends to cool the reactor. 

If the AH listed for a reaction is negative, then that reaction releases heat as it proceeds — the reaction is exothermic (exo- = out). If the A//listed for the reaction is positive, then that reaction absorbs heat as it proceeds — the reaction is endothermic (endo- = in). In other words, exothermic reactions release heat as a product, and endothermic reactions consume heat as a reactant. 

For a chemical reaction at constant volume, W = 0, so AE = Q. If the reaction absorbs heat from the surroundings, Q is positive, and the reaction is endothermic (AE is positive). 

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. 

We ve said on several occasions that chemical and physical processes occur spontaneously only if they go "downhill" energetically so that the final state is more stable and has less energy than the initial state. In other words, energy must be released for a process to occur spontaneously. At the same time, though, we ve said that some processes occur perfectly well even though they are endothermic and absorb heat. The reaction of barium hydroxide octahydrate with ammonium chloride shown in Figure 8.6, for example, absorbs 80.3 kj of heat (AH° = +80.3 kj) and leaves the surroundings so cold that moisture freezes around the outside of the container. 

When we sweat, water evaporates from the surface of the skin. Because it takes a lot of energy (heat) for water to change from the liquid phase to the gas phase, when sweat evaporates, it actually takes some energy (heat) away from the body. This is what scientists call an endothermic reaction. An endothermic reaction absorbs energy. 

Changing Temperature Exothermic reactions evolve energy from the system, and endothermic reactions absorb energy into the system from the surroundings. Therefore, you can think of heat as a product in exothermic reactions and as a reactant in endothermic reactions. Therefore, increasing temperature favors endothermic processes. 

In the case of endothermic reactions, the reverse is true, if the temperature is raised then we achieve greater product concentrations and K increases its value as T is raised. Here the reaction absorbs heat since AH > 0, so if T is increased the reaction moves in the direction in which it can absorb the additional heat supplied from the rise in temperature, i.e. in the forward direction. 

The net heat that is liberated in the overall combustion reaction is called heat of combustion, and is equal to the heat released by the exothermic reactions minus the heat absorbed by the endothermic reactions. The heat of combustion is partially stored in the combustion product as sensible energy, thus increasing their temperature, and partially lost to the environment. The maximum temperature that the combustion products can ever reach is given for the ideal conditions of no heat losses or adiabatic combustion. This is the called adiabatic temperature and it sets the theoretical maximum limit for the temperature of a flame (for methane/air mixtures this is 1950°C at atmospheric conditions). The tabulated values of adiabatic temperature for different gaseous fuels can be found in [1,2],... 

Let s take a moment to recap. Basically, we re saying that the internal energy of a collection of molecules or atoms is the sum of their (1) rotational, (2) translational, and (3) vibrational energy. The amount of internal energy that a system contains can be changed by heat. Heat can be added to or subtracted from a system in many ways, but the method of primary interest to us is by chemical reaction. When a chemical reaction gives off heat, it is termed exothermic. When a chemical reaction absorbs heat, it is termed endothermic. 

When heat is liberated, the heat content (enthalpy), //, of the molecules themselves must decrease the change in heat content. A//, is therefore given a negative sign. (In the case of an endothermic reaction, where heat is absorbed, the increase in heat content of the molecules is indicated by a positive AH.)... 

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