Enthalpy Change of a Chemical Reaction

PURPOSE OF EXPERIMENT Determine the enthalpy magnesium metal with hydrochloric acid. 

In this experiment you will measure the heat evolved in the reaction of magnesium with hydrochloric acid. 

You will determine a value for the heat capacity of your calorimeter by studying a system for which the heat of reaction is known the formation of water from its ions in dilute solution. 

The calorimeter in this experiment will consist of two nested styrofoam cups. Add to your calorimeter about 50. mL of approximately 2 M hydrochloric acid, HCl, solution, measured accurately in your graduated cylinder (Laboratory Methods B). Record the volume and actual molarity of the HCl solution in TABLE 21.lA. Calculate the volume of sodium hydroxide, NaOH, solution (approximately 2 M) you need to neutralize your HCl solution. 

Measure this volume plus an additional 0.5 mL in your graduated cylinder. Record the volume and the actual molarity of this NaOH solution in TABLE 21.lA. Put it into another beaker. Measure the temperatures of both the HCl and the NaOH solutions, and adjust these temperatures using cool tap water surrounding the containers so that they are the same and preferably a few degrees below room temperature. Be careful to rinse and wipe your thermometer clean before shifting from acid to base or vice versa. Record these temperatures in TABLE 21.lA. Add 2 drops of phenolphthalein to your HCl solution. 

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The enthalpy change of a chemical reaction is known as the enthalpy of reaction, AHrxn- The enthalpy of reaction is dependent on conditions such as temperature and pressure. Therefore, chemists often talk about the standard enthalpy of reaction, AH°rxn - the enthalpy change of a chemical reaction that occurs at SATP (25 C and 100 kPa). Often, Alf n is written simply as AW°, The symbol is called nought. It refers to a property of a substance at a standard state or under standard conditions. You may see the enthalpy of reaction referred to as the heat of reaction in other chemistry books. 

You can represent the enthalpy change that accompanies a phase change—from liquid to solid, for example—just like you represented the enthalpy change of a chemical reaction. You can include a heat term in the equation, or you can use a separate expression of enthalpy change. 

You can calculate the enthalpy change of a chemical reaction hy adding the heats of formation of the products and subtracting the heats of formation of the reactants. The following equation can he used to determine the enthalpy change of a chemical reaction. 

In this section, you learned how to calculate the enthalpy change of a chemical reaction using Hess s law of heat summation. Enthalpies of reaction can be calculated by combining chemical equations algebraically or by using enthalpies of formation. Hess s law allows chemists to determine enthalpies of reaction without having to take calorimetric measurements. In the next section, you will see how the use of energy affects your lifestyle and your environment. 

Suppose that you need to find the enthalpy change of a chemical reaction. Unfortunately, you are unahle to carry out the reaction in your school laboratory. Does this mean that you cannot find the enthalpy change of the reaction Explain. 

Hess s Law It states, "the enthalpy change of a chemical reaction remains the same whatever be the intermediate steps". 

This law can be used to calculate the enthalpy change of a chemical reaction which is not possible by direct experiment and also for those reactions which take place very slowly. 

The enthalpy change of a chemical reaction (AH) is the amount of heat that is released or taken up during the course of the reaction. If AH is negative, heat is evolved and the reaction is called exothermic if AH is positive, heat is taken up and the reaction is called endothermic. 

We use units of kJ/mol for the heat of formation of a substance. But in writing the enthalpy change of a chemical reaction, we will use kJ as our preferred unit, not kj/mol. The reaction in this example illustrates why we do this. The value we calculated, AH° = -2219.9 kJ, is for a reaction in which one mole of propane reacts with five moles of oxygen to form three moles of carbon dioxide and four moles of water. So if we were to say -2219.9 kJ/mol, we would need to specify carefully which substance that mol refers to. We choose to write the AH value in kJ, with the understanding that it refers to the reaction as written. This is also dimension-ally consistent with Equation 9.12, provided that we treat the stoichiometric coefficients as carrying units of moles. You may see other texts that refer to values as per mole of reaction. ... 

Standard Heat of Reaction. This is the standard enthalpy change accompanying a chemical reaction under the assumptions that the reactants and products exist in their standard states of aggregation at the same T and P, and stoichiometric amounts of reactants take part in the reaction to completion at constant P. With P = 1 atm and T = 25°C as the standard state, AH (T,P) can be written as... 

How do we determine the energy and enthalpy changes for a chemical reaction We could perform calorimetry experiments and analyze the results, but to do this for every chemical reaction would be an insurmountable task. Furthermore, it turns out to be unnecessary. Using the first law of thermodynamics and the idea of a state function, we can calculate enthalpy changes for almost any reaction using experimental values for one set of reactions, the formation reactions. 

We use a short version of the seven-step method. The problem asks for the entropy and enthalpy changes accompanying a chemical reaction, so we focus on the balanced chemical equation and the thermodynamic properties of the reactants and products. 

Hess law states that the enthalpy change accompanying a chemical reaction is independent of the pathway between the initial and final states. ... 

Now let us consider how the standard enthalpies of formation can be used to calculate the standard enthalpy change for a chemical reaction. First, we calculate the total standard enthalpy of formation of all the products. We then calculate the total standard enthalpy of formation of all the reactants. The difference between the two totals is the... 

Given a listing of free energies of formation, the free energy change of a chemical reaction may he calculated in the same manner as you evaluated enthalpies of reaction and entropies of reaction. For the reaction that was discussed earlier,... 

The enthalpy of formation, which we have discussed in the previous section, offers an easy way to overcome this difficulty. We now introduce Hess s Law. Recall that enthalpy is a state function, and hence the enthalpy change depends on only the initial and fmal states. Hess s law is basically the same as stated above, but expressed in a different way Hess s law states that the enthalpy change for a chemical reaction is the same whether it takes place in one or several stages. Consider the combustion of methane again. ... 

The enthalpy change for a chemical reaction in which all reactants and products are in their standard states and at a specified temperature is called the standard enthalpy (written AFf°) for that reaction. The standard enthalpy is the central tool in thermochemistry because it provides a systematic means for comparing the energy changes due to bond rearrangements in different reactions. Standard enthalpies can be calculated from tables of reference data. For this purpose, we need one additional concept. The standard enthalpy of formation AH° of a compound is defined to be the enthalpy change for the reaction that produces 1 mol of the compound from its elements in their stable states, all at 25°C and 1 atm pressure. For example, the standard enthalpy of formation of liquid water is the enthalpy change for the reaction... 

The heat of reaction, or more accurately, the enthalpy change during a chemical reaction, A, indicates the amount of energy being absorbed or released when a chemical transformation takes place at given operating atmosphere and temperature of 298 K. The standard molar heat of reaction of a chemical reaction (expressed in energy per mole extent) is denoted by It is calculated by... 

The following section gives an example of a laboratory determination of the enthalpy change for a chemical reaction in which the reactants are mixed in water. After the reaction has taken place the final mixture consists of products, unused reactants and water. The principles of such measurements are as follows. 

The standard free enthalpy change for a chemical reaction can be calculated from tabulated values of various thermodynamic functions (1- ) For aqueous chemical reactions at 25 C, the standard free enthalpy change is computed from... 

One can, in principle, calculate the heat energy (enthalpy) change in a chemical reaction if the balanced chemical equation is known and the bond energies of all the bonds broken and made are known. For our purposes, we have only included a very abbreviated table (Table 12.1) of the types of bonds that might be involved in the combustion of a hydrocarbon or related fuel. 

The heat change of a chemical reaction is temperature-dependent. According to Kirchhoffs law, the partial derivative of the enthalpy change during the reaction, with respect to temperature, is equal to the difference between the initial and final molar specific heat of the system ... 

Figure 9.12 I This conceptual diagram shows how to use tabulated enthalpies of formation to calculate the enthalpy change for a chemical reaction. We imagine that first the reactants are converted to elements in their standard states, and then those elements recombine to form the products. Because enthalpy is a state function, we do not need to know anything about the actual pathway that the reaction follows.

Enthalpy, you may recall, is a state function (Section 6.3). This means that the enthalpy change for a chemical reaction is independent of the path by which the products are obtained. In 1840, the Russian chemist Germain Henri Hess, a professor at the University of St. Petersburg, discovered this result by experiment. Hess s law of... 

The heat evolved or enthalpy change in a chemical reaction under constant pressure is usually denoted by A//rxn and called the enthalpy of reaction. The enthalpy of reaction is negative for exothermic reactions and positive for endothermic reactions. 

Enthalpy changes of only one kind of chemical reaction need to be tabulated (although it is not uncommon to see tables of enthalpy changes of other reactions, like combustion reactions). A formation reaction is any reaction making 1 mole of a product using, as reactants, the product s constituent elements in their standard states. We use the symbol AfH to stand for the enthalpy change of a formation reaction, called the enthalpy of formation or (more loosely) the heat of formation. As an example,... 

Chemical (stoichiometric) equations may be combined by addition or subtraction. The standard heat (enthalpy) of reaction (Aff ) associated with each equation may likewise be combined to give the standard heat of reaction associated with the resulting chemical equation. This is possible, once again, because enthalpy is a point function, and these changes are independent of path. In particular, formation equations and standard heats of formation may always be combined by addition and subtraction to produce any desired equation and its accompanying standard heat of reaction. This desired equation, however, carmot itself be a formation equation. Thus, the enthalpy change for a chemical reaction is the same whether it takes place in one or several steps. This is referred to as the law of constant enthalpy summation and is a direct consequence of the first law of thermodynamics. 

The enthalpy change for a chemical reaction depends upon the number of moles undergoing chemical change, and AH is expressed in terms of kj/mol. To compare reactions under the same conditions, the following conventions are used. Since we are only interested in the difference in enthalpy between the initial state, the reactants, and the final state, the products, we need a reference point. We call this reference point the standard state. 

We can obtain an approximate value for the standard enthalpy change for a chemical reaction, AH°, by considering the energy of the bonds that are cleaved or formed in the process. The exact bond energies are not known for complex molecules, but average bond energies can be estimated based on simple, strucurally analogous compounds. Consider the substitution reaction of 2.2-dimethylpropane with chlorine. 

Hess s law the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place. 

The heat content of a system at constant pressure is called the enthalpy H) of the system. The heat absorbed or released during a change in a system at constant pressure is the change in enthalpy (A//). The enthalpy change for a chemical reaction is called the enthalpy of reaction or heat of reaction and is defined by this equation. 

The lattice model that served as the basis for calculating ASj in the last section continues to characterize the Flory-Huggins theory in the development of an expression for AHj . Specifically, we are concerned with the change in enthalpy which occurs when one species is replaced by another in adjacent lattice sites. The situation can be represented in the notation of a chemical reaction ... 

The heal of reaction (see Section 4.4) is defined as tlie enthalpy change of a system undergoing chemical reaction. If the retictants and products are at tlie same temperature and in their standard states, tlie heat of reaction is temied tlie standard lieat of reaction. For engineering purposes, the standard state of a chemical may be taken as tlie pure chemical at I atm pressure. Heat of reaction data for many reactions is available in tlie literature. ... 

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