To solve this type of problem, organize the given chemical reactions, where the overall effect gives the required reaction. There are a few rules you need to follow when handling a reaction. So why didn`t I use more accurate values at all? Because I wanted to illustrate this problem! The answers you get to questions like these are often a little out of the way. The reason for this is usually either rounding errors (as in this case) or the fact that the data may come from one or more other sources. Trying to get consistent data can be a nightmare. This shows the enthalpy changes for an exothermic reaction using two different ways of switching from reactants A to product B. In one case, you perform a direct conversion. In the other, you use a two-step process involving certain intermediaries. That is the answer we are looking for. The ΔH of this reaction is the sum of the three ΔH values: you can perform calculations by plotting them as enthalpy diagrams as above, but there is a much easier way to do this that requires virtually no consideration.
If you look at the change in an enthalpy graph, it`s actually pretty obvious. To put this definition in mathematical terms, here is the equation of Hess`s law: I can only give a brief introduction here, as this is covered carefully and step by step in my chemistry calculus book. Now that we understand the concept and equation of Hess`s law, let`s expand our knowledge with practical problems. These word problems may require some manipulation of reactions (i.e. change of direction of the equation, multiplication, division), but the general idea is the same for all problems of Hess`s law. Let`s review some examples below! Hess`s law, also known as Hess`s law of constant heat summation, states that the total enthalpy of a chemical reaction is the sum of the enthalpy changes for the reaction steps. Therefore, you can find an enthalpy change by breaking down a reaction into component steps that have known enthalpy values. This example problem shows strategies on how Hess`s law can be used to find the enthalpy change of a reaction using enthalpy data from similar reactions. In this case, we try to find the standard enthalpy change of benzene formation so that this equation works horizontally. Since reaction (i) is the only one with N2H4(l) that is a reactant in the overall equation, it is assumed to go in the right direction. Then reaction (ii) has the product 2NH3 (g) on the right, so that this equation also remains the same. In the above attempt to find the total equation, the hydrogen gas of equations (i) and (ii) cancels each other out, which means that the hydrogen gas of reaction (iii) is the only one that enters the overall equation that belongs to the left.
For this reason, we can reverse the equation of reactants and products to go in the opposite direction; However, since the reaction goes in the opposite direction, enthalpy also becomes the “opposite”. When you change the direction of a reaction, the reverse of enthalpy becomes the new enthalpy. We combine these three reactions, modified as I said: Hess`s law is now understood as an expression of the fact that the enthalpy of a chemical process is independent of the path from the initial state to the final state (i.e. the enthalpy is a state function). According to the first law of thermodynamics, the change in enthalpy in a system due to a constant pressure reaction is equal to the absorbed (or negative heat of the heat released), which can be determined by calorimetry for many reactions. Values are usually given for reactions with the same initial and final temperatures and pressures (although conditions may vary as reactions progress). Hess`s law can be used to determine the total energy required for a chemical reaction, which can be broken down into synthetic steps that are easier to characterize individually. This allows for the creation of standard formation enthalpies that can be used to predict enthalpy changes in complex syntheses.
And now the calculation. It is enough to write down all the changes of enthalpy that make up the two routes and assimilate them. The Russian chemist and physicist Germain Hess developed the concepts of thermochemistry and physical chemistry.