Question

Humidity indicators are available that use the equilibrium reaction between the cobalt(II) hexahydrate ion (pink) and chloride ions, which forms the cobalt(II) tetrachloride ion (blue) and water. The equilibrium reaction used by these indicators is $$ \mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons\left[\mathrm{CoCl}_{4}\right]^{2-}+6 \mathrm{H}_{2} \mathrm{O} $$ If you started with a pink indicator, what would you expect to happen if you (a) blew a hair dryer on it? (b) put it over a steam vent? (c) left it in Houston before a rainstorm? (d) left it in Phoenix in the middle of the summer?

Short answer

(a) The indicator would turn more pink. (b) The indicator would turn more blue. (c) The indicator would turn more blue. (d) The indicator would turn more pink.

Step-by-step solution

Background Understanding

The color change in the humidity indicator is based on the equilibrium of the reaction between cobalt(II) hexahydrate ion, which is pink, and chloride ions to form cobalt(II) tetrachloride ion, which is blue, and water. The direction of the equilibrium shifts according to Le Chatelier's Principle, which states that any change in concentration, temperature, or pressure results in a shift in the position of the equilibrium to counteract the change.

Applying Le Chatelier's Principle (a) - Hair Dryer

Using a hair dryer on the indicator would increase temperature and cause evaporation of water. According to Le Chatelier's Principle, the equilibrium would shift to produce more water to oppose the decrease in water concentration due to evaporation. Thus, the reaction would shift to the left, resulting in a more pink color, which indicates the presence of more cobalt(II) hexahydrate ion.

Applying Le Chatelier's Principle (b) - Steam Vent

Placing the indicator over a steam vent introduces more water vapor to the system. This would increase the concentration of water in the air around the indicator, shifting the equilibrium to the right to decrease water concentration through formation of more blue cobalt(II) tetrachloride ion.

Applying Le Chatelier's Principle (c) - Houston Before Rainstorm

In Houston before a rainstorm, the humidity is high which means an increase in water vapor in the air. This would cause the equilibrium to shift to the right, reducing water vapor concentration by forming more blue cobalt(II) tetrachloride ion.

Applying Le Chatelier's Principle (d) - Phoenix in Summer

Phoenix is known for its dry heat, especially in the middle of the summer. Low humidity corresponds to lower water vapor concentration in the air. The equilibrium would shift to the left to produce more water, leading to a predominance of the pink cobalt(II) hexahydrate ion, and thus causing the indicator to appear more pink.

Key concepts

Chemical Equilibrium

Understanding chemical equilibrium is crucial when studying reactants and products in a reversible chemical reaction. At equilibrium, the rate at which the reactants produce products is equal to the rate at which products revert to reactants. This state does not mean the reactants and products are equal in quantity, but that their concentrations have stabilized and do not change over time.
Le Chatelier's Principle provides insight into how a system at equilibrium reacts to external changes. It states that if a dynamic equilibrium is disturbed by altering the conditions, the position of equilibrium will move in order to counteract the change. For instance, changing the concentration, temperature, or pressure in a reversible reaction will cause the system to adjust itself to minimize the effect of the disturbance.
In practical terms, if more of a reactant is added to a system, the system will adjust to produce more products. Conversely, if a product is removed, the system tries to produce more of it. If the system is heated or cooled, the equilibrium will shift in the direction that absorbs or releases heat, respectively. This responsiveness makes chemical equilibrium a central concept in chemical reactions, notably when designing industrial reactions, predicting reaction outcomes, and understanding natural processes.

Humidity Indicators

Humidity indicators are practical tools for visually representing the moisture content in the air. These indicators typically contain a humidity-sensitive chemical compound that changes color depending on the level of humidity. A common example involves cobalt(II) complexes that shift from pink to blue as humidity increases, which follows Le Chatelier's Principle.
For the indicators using cobalt(II) complexes, humidity changes alter the position of equilibrium in a reversible reaction. When dry air is present, the equilibrium moves to favor the formation of a compound with less water, resulting in a color change that signals a drop in humidity. Conversely, in a more humid environment, the equilibrium favors a water-rich compound, leading to a different color that indicates increased moisture. These indicators are invaluable in various applications, including monitoring the moisture levels in packaging, ensuring quality control in manufacturing industries, and even helping art conservators protect valuable artworks from moisture damage.

Cobalt(II) Complexes

Cobalt(II) complexes, such as cobalt(II) hexahydrate and cobalt(II) tetrachloride, play a central role in the chemistry of humidity indicators. These complexes are coordination compounds, where cobalt ions are at the center surrounded by ligands – molecules that donate electron pairs to form a chemical bond. The color change associated with these complexes is not only aesthetically significant but also acts as a chemical signature that informs us about the surrounding environment's humidity level.
Cobalt(II) complexes' ability to change color with humidity is deeply rooted in the ligands' nature. In the case of the humidity indicator, the water molecules act as ligands, coordinating to the cobalt ion. Changes in humidity affect the number of water molecules attached to the cobalt ion, leading to this shift in equilibrium and subsequent color change. These complexes are not only practical for everyday use in humidity indicators but also serve as an educational example of Le Chatelier's Principle in action and offer insight into complex ion chemistry's dynamic nature.