Chemical Activity in Thermodynamics

Instructor: Dave Hays
Why do some chemical systems not behave ideally? Learn about chemical activity in this lesson, how to calculate it, and how intermolecular attractive forces between particles can affect it.

Unideal Conditions

Rachel walks into her chemistry class and sees a chemical equation written on the board: A + B → C. Like most of us would, she assumes that in this reaction all the reactants and products are behaving ideally. Therefore, she assumes that there are no interactions between any of the say, 100 particles of reactants A and B that would decrease the anticipated 100 particles of the product C.

In reality, particles interact with each other based on variables like temperature, pressure, and concentration in such a way that they reflect a reduced concentration. So, for example, the interaction of the particles of A may result in them acting as if there were 90 particles, not 100. The particles of B likewise may act as if there were 85 particles.

What Is Chemical Activity?

Rachel's teacher, Mr. Milani, says to use the term chemical activity, given the symbol a, to explain how chemists account for deviations from ideal behavior that may cause a species to 'act as if' there were an amount present different from its actual concentration.

For example, the chemical activity, a, of a gas is the ratio of the actual pressure of the gas to its ideal pressure under the given conditions of temperature and pressure. This relationship can be expressed by the following equation where p is pressure:

a = pgas / pgas, ideal

When the pressure of the gas (the numerator) is the same as its pressure under ideal conditions (the denominator), the ratio of the two pressures is the same. Therefore, the chemical activity of that gas has a value of 1, which is also the maximum value for the chemical activity of any species.

What about solutions in which a solute is dissolved in a solvent? For example, Rachel dissolves some solid sodium chloride (called a solute) into water (called the solvent) to make a solution. She points out to Mr. Milani that unlike gases, liquids are non-compressible, so how can the above equation be used?

Mr. Milani shows her a different equation for calculating the chemical activity of the dissolved solute:

a = γ * m

• a is the chemical activity
• γ is called an activity coefficient and is used to catch all deviations from ideal behavior
• m is the concentration of the solute in units of molality

Mr. Milani explains this equation to Rachel, as it can be confusing. For solutions, as the solution becomes more dilute, the solute concentration decreases and a approaches zero. Therefore, the activity coefficient gamma approaches one. The solution becomes ideal as the amount of solute decreases.

Intermolecular Attractive Forces

Rachel asks, ''Why does this happen? What causes chemical activities to be less than one?''

The answer lies in what are called intermolecular attractive forces, or the attractive forces between particles (atoms, molecules, or ions).

Molecules have various shapes and resulting symmetries. That the symmetry may be impacted depending on the nature of atoms that form it. For example, a molecule having a triangular shape is symmetrical if the atoms at all three corners of the triangle are the same.

However, if two of the atoms remain the same and one is changed, the symmetry is lost.

Rachel comments that molecules seem to have geometries that appear from their shapes. Mr. Milani agrees, and explains that depending on the atoms in the molecule and their arrangement (their molecular geometry), a molecule may have dipoles, or positive and negative regions, within it.

This arises from a property of atoms called electronegativity, and those dipoles may result in the overall molecule having a somewhat positively charged region and a somewhat negatively charged region. When this occurs, the molecule is said to be polar. Therefore, the molecule acts kind of like a really small magnet.

When polar molecules are brought together, the molecules tend to become attracted to each other in a negative-positive orientation, just like magnets!

Increased Concentration Reduces Chemical Activity

Mr. Milani asks Rachel, ''How do you think this property is expressed when you have either large numbers of molecules or, if the molecules are a gas, the gas pressure increases?''

Rachel responds that when the concentration of a molecule increases, there's more chance of polar molecules becoming attracted to each other because there are more of them present.

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