2.5. Henry's Law Constant

So we are looking at Aqueous Solubility and vapor pressure, two things which are equilibrium of A between its pure form and water and pure form and air, yeah. If I dump a large amount of chemical into water all of it dissolves, when it comes in contact with air how much of this will go into air? Now we don’t have a pure substance, we have a solution in contact with air.

In this section, we focus on two important properties of chemicals: Aqueous Solubility and Vapor Pressure. Aqueous solubility refers to how much of a substance can dissolve in water, while vapor pressure is about how easily a substance evaporates into the air. When we introduce a chemical into water, it dissolves until it reaches a point where no more can dissolve – this is known as its aqueous solubility. Once in contact with air, some of that dissolved chemical will also begin to move into the air. This next focus is not on pure chemicals anymore, but rather on the behavior of chemicals that are in solution when they are exposed to air.

Think of it like trying to mix sugar in water. When you keep adding sugar to water, it continues to dissolve until no more can dissolve – that’s the maximum amount, or solubility. If you leave the sugar-water mixture open to the air, some of the sugar molecules will escape into the air, demonstrating vapor pressure.

This is the equilibrium between the concentration of A in air versus the concentration of A in water, air and water are in contact with each other and they go to equilibrium.

The focus shifts to what happens when the solution is both in contact with air and water. The ratio of concentrations of a chemical in air and water, when they reach equilibrium, is fundamentally important. At equilibrium, the amount of chemical particles in the air versus the water will stabilize based on their interactions and properties. This ratio can tell us how likely the chemical is to be found in either medium.

Imagine a sponge soaked in water; if you hang it up in the air, eventually, some of that water will evaporate into the air until the sponge stabilizes at a certain level of humidity. The balance between the water in the sponge and the water in the air is like how A's concentration equilibrates between the two phases.

So, Henry’s Law constant is defined as the ratio of the concentration in air versus the concentration in water. In contact with each other this is the equilibrium ratio in which the concentration in the vapour phase and the liquid phase will distribute itself.

Henry's Law Constant is a key concept that quantifies how a chemical distributes itself between two phases, air and water. It’s defined as the ratio of the concentration of the chemical in air divided by the concentration in water at equilibrium. A high Henry's Law constant indicates that more of the chemical is likely to be found in the air rather than in water, meaning it volatilizes easily. Conversely, a low constant means the chemical prefers to stay dissolved in water.

Consider a bottle of soda. The carbon dioxide in the liquid is kept under pressure, resulting in a greater concentration of CO2 in the liquid than in the air above it. When you open the bottle, the CO2 escapes – illustrating a case similar to Henry's Law where solubility in water equates to its tendency to rise into the gaseous phase.

For example, if the Henry’s Law is very small. Henry’s Law is 0.00001 so you don’t like to put 0.00001 so you want to reverse it and say Henry’s Law is Henry’s constant is 1000.

In practice, when discussing Henry’s Law constant values, sometimes it’s more convenient to express it in inverse terms, especially when the values are very small. For example, if the constant value is 0.00001 for a substance, it can be expressed as 1000 for convenience to indicate its preference for being in water compared to air. This can sometimes cause confusion but is used commonly in literature.

Think of a flea market where some items sold are very cheap and others are very expensive. Instead of saying the price of a cheap item is $0.01, a seller might say it is$100 per 1,000 items to emphasize value in a different way. This makes it more relatable and easier to discuss within the context of values that are commonly understood.

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