Enthalpy changes matter

Batch calorimeters are instmments where there is no flow of matter in or out of the calorimeter during the time the energy change is being measured. Batch calorimeters differ in the way the reactants are mixed and in the method used to detennine the enthalpy change. Enthalpy changes can be measured by the various methods... 

The decomposition of N2 O4 requires a bond to break. This is the reason why the decomposition has a positive A 77 °. At the same time, the number of molecules doubles during decomposition, which is the reason AS° has a positive value. The positive enthalpy change means that energy Is removed from the surroundings and constrained, whereas the positive entropy change means that matter is dispersed. At temperatures below 315 K, the enthalpy term dominates and decomposition is not spontaneous, but at temperatures above 315 K, the entropy term dominates and decomposition is spontaneous. 

The AH is dependent upon the state of matter. The enthalpy change would be different for the formation of liquid water instead of gaseous water. 

Hess s law Hess s law states that if a reaction occurs in a series of steps, then the enthalpy change for the overall reaction is simply the sum of the enthalpy changes of the individual steps, heterogeneous catalyst A heterogeneous catalyst is a catalyst that is in a different phase or state of matter from the reactants. 

Carbon dioxide can be formed by the reaction of oxygen with carbon to form carbon monoxide, followed by the reaction of carbon monoxide with oxygen. Carbon dioxide can also be formed directly from carbon and oxygen. No matter which pathway is used, the enthalpy change of the reaction is the same. 

Because enthalpy is a state function, AH is the same no matter what path is taken between reactants and products. Thus, the sum of the enthalpy changes for the individual steps in a reaction is equal to the overall enthalpy change for the entire reaction, a relationship known as Hess s law. Using this law, it is possible to calculate overall enthalpy changes for individual steps that can t be measured directly. Hess s law also makes it possible to calculate the enthalpy change of any reaction if the standard heats of formation (AH°f) are known for the reactants and products. The standard heat of formation is the enthalpy change for the hypothetical formation of 1 mol of a substance in its standard state from the most stable forms of the constituent elements in their standard states (1 atm pressure and a specified temperature, usually 25°C). 

In this example the standard heat of formation of H20 is available for its hypothetical standard state as a gas at 25°C. One might expect the value of the heat of formation of water to be listed for its actual state as a liquid at 1 bar or l(atm) and 25°C. As a matter of fact, values for both states are given because they are both frequently used. This is true for many compounds that normally exist as liquids at 25°C and the standard-state pressure. Cases do arise, however, in which a value is given only for the standard state as a liquid or as an ideal gas when what is needed is the other value. Suppose that this, were the case for the preceding example and that only the standard heat of formation of liquid H20 is known. We must now include an equation for the physical change that transforms water from its standard state as a liquid into its standard state as a gas. The enthalpy change for this physical process is the difference between the heats of formation of water in its two standard states ... 

For the cycle, the total enthalpy change is zero. At the compressor, outside energy Wm is needed, and at the evaporator, heat transfer qm from the matter to be cooled is used to evaporate the refrigerant R-134a. 

It should be clear that AH refers to the (differential) enthalpy change accompanying unit advancement of the reaction Epure species A, are maintained at unit pressure (usually, 1 atm). How this reaction could be accomplished in principle is the subject matter of Exercise 3.7.3. Equation (3.10.5) is the fundamental relation of interest this expression is known as the van t Hoff equation (1879). ... 

Enthalpy change, A//, is equal to the heat involved in a process when the process is done under a constant pressure and involves no work except perhaps expansion (or contraction) against the atmosphere. When these conditions are not met. A// is a more fundamental quantity than heat. For example. A// is a state function, which means that the change in its value is independent of the path in going from the initial state to the final state. Another example of a state function is change in volume. For example, if a gas starts out occupying 2.5 L and finally occupies 4.5 L, AF = 2.0 L no matter if the gas is first expanded to 8.6 L, then contracted to 3.7 L, then expanded to 4.5 L, or if some other path were followed. (Heat, in contrast, does depend on the path, except when q = A//.) The A// value is important in both theoretical and practical terms in chemistry. 

To simplify matters, the value cited for Ajffvap of water in Example 4,33 was at 25 C and 1 atm. To calculate this value, if the final state for water is specified as H20(g) at 25 C and 1 atm, the following enthalpy changes should be taken into account you start with H20(l) at 25 C and 1 atm ... 

A thermochemical equation is a balanced equation that includes the heat of reaction (A//rxn). Keep in mind that the A//,xn value shown refers to the amounts (moles) of substances and their states of matter in that specific equation. The enthalpy change of any process has two aspects ... 

It s important to realize that when titanium(IV) chloride and water react, the reactants don t actually decompose to their elements, which then recombine to form the products. But that is the great usefulness of Hess s law and the state-function concept. Because A//°x is the difference between two state functions, minus A//°eaciams> it doesu t matter how the change actually occurs. We simply add the individual enthalpy changes to find A//°x ... 

We can now see why exothermic and endothermic spontaneous reactions occur. No matter what its enthalpy change, a reaction occurs because the total entropy of the reacting system and its surroundings increases. The two possibilities are... 

It does not matter that we cannot make this reaction occur cleanly and therefore cannot directly measure its enthalpy change. As seen above, indirect methods can be used. In such ways it is found that for this reaction is -17.9 kcal. Since we have arbitrarily taken the enthalpies of the reactants to be zero, it follows that on this basis the enthalpy of CH4 (g) is —17,9 kcal mol" h This is known as the enthalpy of formation, AHf, of methane. It.is important to use the term enthalpy of formation only for the formation of the compound from elements in their standard states. 

In Chapter 4 we dealt with enthalpy changes in chemical reactions and found that it was very useful to determine enthalpies of formation of compounds the standard enthalpy of formation, AH, is the enthalpy increase for the process in which a compound is formed, at one atmosphere pressure, from the elements in their standard states. If we know the standard enthalpies of formation of all compounds in a chemical reaction it is a simple matter see equation 4.31) to calculate the enthalpy change in the reaction. 

The way in which equilibrium constants vary with temperature is a matter of considerable importance in thermodynamics. It leads us to a very convenient - v GG-experimental procedure for measuring enthalpy changes in chemical reactions. -... 

As we showed in Section 7.19, the enthalpy change in any chemical reaction does not depend on the numerical values of the enthalpies of the elements that compose the compound. Because this is so we may assign any arbitrary, convenient values to the molar enthalpies of the elements in their stable states of aggregation at a selected temperature and pressure. Clearly, if we chose the required values randomly from the numbers in a telephone directory this could introduce a good deal of unnecessary numerical clutter into our work. Since the numbers do not matter, they can all be the same if they can all be the same, they all might as well be zero and eliminate the clutter entirely. 

For the cycle, the total enthalpy change is zero. At the compressor, outside energy IVin is needed, and at the evaporator, heat transfer from the matter to be cooled is used to evaporate the refrigerant R-134a. Enthalpy values of the cooling water from the steam table (Table Dl) are Hicw = 83.86 kJ/kg at Ti = 293.15 K //2 ew = 104.89 kJ/kg at T2 = 298.15 K... 

Now we know how to determine the standard enthalpy change for reactions at high temperatures, which, as a matter of fact, you may not wish to do very often, unless you get involved in heat flow problems. However, we will see later on that the method is very similar for other important properties, so it is the first of a set of procedures which are very useM. 

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