Using Hess's Law to Calculate the Change in Enthalpy of a Reaction

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  • 0:00 Intro to Hess's Law
  • 1:44 Thermochemical Equations
  • 3:02 Manipulating…
  • 5:39 Calculating Enthalpy Change
  • 7:58 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

Want to make sure you don't blow yourself up during a chemical reaction? This lesson will help you avoid this by teaching you Hess's Law. This is one way to calculate the heat transferred, or enthalpy change, of a chemical reaction.

Introduction to Hess's Law

Enthalpy, or enthalpy change, is how much energy (in the form of heat) has been transferred out or taken in during a chemical reaction. So, why do we need to know this? Let us say that you're doing a chemical reaction and don't, in the process, want to blow yourself up. To make sure this doesn't happen, it's good to know how much energy is going to be given off or absorbed during a chemical reaction. After all, a hugely exothermic reaction could really ruin your day, not to mention your hair.

One way we can measure enthalpy change is to use Hess's Law. We will define exactly what Hess's Law is a little later. A really important point you should know is that Hess's Law works because enthalpy is a state function. This is awesome news! With a state function, all you are interested in is where you start and where you finish. How you got there is not important.

Enthalpy change can be determined directly using calorimetry, or it can be calculated through multiple steps. We calculate it through multiple steps, where it is not possible to directly measure it; and this is where Hess's Law comes in.

Hess's Law is a really easy way to measure the enthalpy change of a chemical reaction. The enthalpy change is the same whether the reaction takes place in one step or in a series of steps. We use Hess's Law by adding or subtracting chemical reactions with the same products or reactants as the one we are interested in.

Thermochemical Equations

Okay, this adding and subtracting chemical reactions to find enthalpy change may not make too much sense right now. So, to help us understand more why Hess's Law works, let us start by looking at something called thermochemical equations. These are a special type of chemical equation that shows the enthalpy change going from reactants to products. Here is an example of a thermochemical equation: N2(g) + 2O2(g) --> 2NO2(g), delta H = +68 kJ.

Here you can see a normal chemical equation that you are used to. Nitrogen adds together with oxygen to form nitrogen dioxide. Always check that it is balanced, and you can see this is.

You will see on the right side a bit of extra information that you probably haven't seen before. This is the thermochemical bit, and it is the delta H, the enthalpy change value for this reaction. In this case, delta H is equal to +68 kJ. The positive sign tells us heat is absorbed by the reaction. A negative sign tells us that heat is released by the reaction.

So, for this reaction we now have two pieces of information: the chemical reaction itself, correctly balanced, and the amount of enthalpy change for that reaction. They are directly related and can be used in Hess's Law calculations.

Manipulating Thermochemical Equations

Now, let us return to our simple equation from before and see what we can do with it. For this reaction to form nitrogen dioxide, the enthalpy change is +68 kJ. This is the value if we did this reaction all in one step. You will see I have added a little 1 next to the delta H to highlight this:

thermochemical equation

It is also possible to carry out this reaction in two separate steps:

two thermochemical equations for steps

  1. So, let us look at the first step. Here one mole of nitrogen plus one mole of oxygen comes together to form two moles of nitrogen monoxide. This step has a delta H value of +180 kJ.
  2. Now there is a second step, where the two moles of nitrogen monoxide react with another mole of oxygen to form two moles of nitrogen dioxide. This step has a delta H of -112 kJ.

Now, you may see where we are going to go with this. We can do something clever here and add together these two steps to get the overall reaction. So, let us do this. The first thing to notice is that two moles of nitrogen monoxide appear on both sides, so we can simply cross them out because they cancel. Now, do be careful when you do cancellations like this that both the species are in the same phase. Here they are both gases, so we can.

And now if we add up both equations, you can see we have got back to our original reaction. Adding up our delta Hs, we have (180 kJ) + (-112 kJ) to equal +68 kJ. This is the value of delta H1. We can say that delta H1 = delta H2 + delta H3. This is really useful, particularly if we didn't know the value for delta H1.

This was a fairly simple example because we didn't need to do too much to our two steps to get our original equation back. However, it is possible to manipulate equations much more than this. You will see this when we go through the worked example next. A really key point to remember is you must manipulate both parts of the thermochemical reaction. In other words, whatever you do to the reaction you must do to the delta H value.

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