# hess law calculator

If you chose to work through chapter 5 in the book, you would be confident that you could do any chemical energetics calculation that you were given. Be careful to count up all the atoms you need to use, and make sure they are written as they occur in the elements in their standard state. Notice that you may have to multiply the figures you are using. Why have I drawn a box around the carbon dioxide and water at the bottom of the cycle? Note what happened to the enthalpies: multiplied or divided as well as the sign change when I reversed the third data equation. The big advantage of doing it this way is that you don't have to worry about the relative positions of everything on an enthalpy diagram. This is because I know only NH4Cl is on the reactant side in the target equation. However many stages the reaction is done in, ultimately the overall enthalpy change will be the same, because the positions of the reactants and products on an enthalpy diagram will always be the same. You will see that in the examples below. 1) The chemical reaction for the ΔH fo for Mg(NO3)2(s) is this: 2) We need to rearrange the three data equations to yield the above reaction when added together. 3) The first step in the solution to part (b) is to write all three data equations: 4) We have to modify the data equations so as to recover the target equation when we add them together: The first two data equations are left untouched. You could set out the above diagram as: Hess's Law says that the overall enthalpy change in these two routes will be the same. After canceling, we recover our target equation. This page explains Hess's Law, and uses it to do some simple enthalpy change calculations involving enthalpy changes of reaction, formation and combustion.

If you look at the change on an enthalpy diagram, that is actually fairly obvious. Then fit the other information you have onto the same diagram to make a Hess's Law cycle, writing the known enthalpy changes over the arrows for each of the other changes. In this case, there is no obvious way of getting the arrow from the benzene to point at both the carbon dioxide and the water. You can do calculations by setting them out as enthalpy diagrams as above, but there is a much simpler way of doing it which needs virtually no thought. Forgetting to do this is probably the most common mistake you are likely to make. Let's do that (the flip only, the division by 2 happens below) and keep everything else the same: 3) I know that I have two (and only two) missing products. Write a thermochemical equation which represents the standard enthalpy of formation of ethanol. This last cancel will reduce the O2 from 6⁄2 to 5⁄2, which is what we want. . The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. However many stages the reaction is done in, ultimately the overall enthalpy change will be the same, because the positions of the reactants and products on an enthalpy diagram will always be the same. Problem #17: (a) The standard enthalpy of formation of ethanol, C2H5OH(ℓ), is −278 kJ mol¯1. Hess’s law is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician. 1) The heat of formation for sucrose is this reaction: 2) Let us manipulate the three data equations as follows: 3) When the three data equations are added, the 12CO2 will cancel out as will the 11H2O and the 12O2. It is completely irrelevant whether a particular enthalpy change is positive or negative. Note that the standard state of carbon is graphite, not any of its other allotropes (such as diamond or buckminsterfullerene). You will notice that I haven't bothered to include the oxygen that the various things are burning in. 4) When three three equations are added, the carbon and the hydrogen go away. What two substances in the data equations will make two products and involve N, H, and Cl? It is useful to find out heats of extremely slow reaction.

Just write down all the enthalpy changes which make up the two routes, and equate them. Hess's Law is saying that if you convert reactants A into products B, the overall enthalpy change will be exactly the same whether you do it in one step or two steps or however many steps. Problem #23: Determine the enthalpy of reaction for: given the following enthalpies of combustion: 1) Write out the complete combustion equation for each enthalpy given: 2) We need to rearrange our three data equations to give the end result of: 3) Rewrite the equations as follows (our goal is to cancel anything not appearing in the final answer): 5) Add the three equations, cancelling the following three items: Problem #24: Calculate the enthalpy change (ΔH°) for the combustion of liquid propane In either case, the overall enthalpy change must be the same, because it is governed by the relative positions of the reactants and products on the enthalpy diagram. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. 3) To obtain the final answer, add up the three enthalpy values from the changed data equations. Add the three ΔH values for the final answer. 1) Manipulate the three data equations as follows: 3) Cancel what appears on both sides to obtain: Problem #19: Some heats of combustion are given below: Determine the heat of formation for sucrose. Remember that you have to go with the flow of the arrows.

If you have never come across this reaction before, it makes no difference. If you are interested, you could rework the calculation using a value of -393.5 for the carbon and -285.8 for the hydrogen.

Hess’s law is useful to calculate heats of many reactions which do not take place directly. The formation enthalpies are widely-available on the Internet as well as being available in your textbook. You need to take care in choosing your two routes. Hess's Law says that the overall enthalpy change in these two routes will be the same. That doesn't make it any harder! Choose your starting point as the corner that only has arrows leaving from it. Working out an enthalpy change of formation from enthalpy changes of combustion. I have labelled the vertical scale on this particular diagram as enthalpy rather than energy, because we are specifically thinking about enthalpy changes. Here it is: 5) When you add the three equations, the 2H2 , 1⁄2Cl2 and 1⁄2N2 all cancel, leaving only this: 6) Add the three enthalpies for the final answer of −82.805 kJ. I can only give a brief introduction here, because this is covered in careful, step-by-step detail in my chemistry calculations book. That is because carbon and hydrogen won't react to make benzene. The pattern will not always look like the one above. Hess's Law says that the enthalpy changes on the two routes are the same. We calculate it through multiple steps, where it is not possible to directly measure it; and this is where Hess's Law comes in. We have to make that go away while leaving one Mg on the left- hand side.

The amount of oxygen isn't critical because you just use an excess anyway, and including it really confuses the diagram. Given it and the following data: Determine the identity of the two missing products and calculate the ΔHrxn for this reaction: 2) I know that the second data equation will have to be flipped and divided by 2. The big advantage of doing it this way is that you don't have to worry about the relative positions of everything on an enthalpy diagram. 3) We manipulate the three equations as follows: Notice how all three equations stop being formation reactions. Multiplying eq 2 by 3 means we must also multiply eq 1 by 3. (In diagrams of this sort, we often miss off the standard symbol just to avoid clutter.). Problem #15: Calculate the enthalpy change for the reaction: 1) We need to rearrange the three data equations so that, when they are added together, the target equation emerges. You will need to use the BACK BUTTON on your browser to come back here afterwards. Look at the second and third data equations. Because I wanted to illustrate this problem! You mustn't, for example, write the hydrogens as 5H(g), because the standard state for hydrogen is H2. If you have read an earlier page in this section, you may remember that I mentioned that the standard enthalpy change of formation of benzene was impossible to measure directly. Hess's Law is the most important law in this part of chemistry and most calculations follow from it. Drawing the box isn't essential - I just find that it helps me to see what is going on more easily. You may add the three equations, if you wish, to satisfy yourself that they do, in fact, add up to the target equation of: The NIST Chemistry Webbook lists a 2008 determination for this value to be 49.

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