6.1 – The Enthalpy of Mixing¶
6.1.0 – Learning Objectives¶
By the end of this section you should be able to:
- Understand what a what the enthalpy of mixing is.
- Explain when to account for the enthalpy of mixing.
6.1.1 – Introduction¶
This notebook covers the enthalpy that is generated through the mixing of two different compounds. In the majority of cases, the enthalpy is negligible, however; there are certain cases which cause a large shift in enthalpy.
6.1.2 – Example¶
Remember how your chemistry teacher told you to never add water to acid? Watch the video below. Jump to 9 minutes in the video
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from IPython.display import YouTubeVideo
YouTubeVideo("_f0vK8Wffgg",560,315,rel=0)
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The cups (reaction vessels) get extremely hot and in some of Cody’s iterations, the boiling water splashes out of the cup and melts the plate it is sitting on. This is due to the bonds of molecules breaking and forming, releasing energy and causing a temperature shift. This is known as the enthalpy of mixing (or enthalpy of dilution).
Food for thought: Cody notes that both cups melted because of the enthalpy of dilution so “it didn’t really matter whether he put acid into water or water into acid. So why is it recommended to avoid putting water in acid? Use your thermo knowledge to help you.
The expanded form of this enthalpy (for an acid) is
Where
\(H_{acid_{aq}}\) is the enthalpy of the acid solution at a specified \(T\) and \(P\)
\(H_{acid_{l}}+H_2O\) is the enthalpy of the acid and water (solute and solvent) at the same temperature and pressure.
As there is a release of heat, Q must be negative. This means that the enthalpy of the acid solution is less than the disassociated acid and water.
6.1.3 – When to consider enthalpy of mixing/solution¶
Most of the time, the enthalpy of mixing is negligible so it is assumed to be ideal mixing \(\Delta H_{mix} = 0\)
This section aims to look at cases where enthalpy of mixing is non-ideal such as the disassociation of strong acids and bases, and certain phase change mixtures. Like other enthalpy values, the enthalpy of mixing may be found in various databases such as Perry’s handbook.
Important: The \(\Delta H\) values in Perry’s handbook for the enthalpy of solution are negative.
Food for thought solution: If you watch the video you can see that the left cup is reacting more vigorously than the right cup. This is because the right cup has more water and thus, a higher heat capacity. This difference is due to the fact that water has a higher heat capacity than the acid. In the right cup, there is a large amount of water in the cup throughout the reaction so the overall temperature of the system increases at a low rate. On the other hand, the left cup has a large amount of acid in the cup throughout the reaction so the overall temperature of the system increases at a higher rate.
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