Question

Novelty devices for predicting rain contain cobalt(II) chloride and are based on the following equilibrium: $$ \underset{\text { Purple }}{\mathrm{CoCl}_{2}(s)}+6 \mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons \underset{\text { Pink }}{\mathrm{CoCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}(s)} $$ What color will such an indicator be if rain is imminent?

Short answer

The indicator will be pink when rain is imminent, signaling an increase in humidity.

Step-by-step solution

Understand the effect of increased humidity on the equilibrium

When humidity increases, more water molecules are present in the air. According to Le Chatelier's principle, when a system at equilibrium is subject to a change, the system will shift the equilibrium to counteract the change. In this case, the equilibrium system will shift in a direction that consumes more of the added species (water molecules) to re-establish equilibrium.

Identify the direction of the equilibrium shift

As we have an increased amount of water molecules in a humid environment, according to Le Chatelier's principle, the equilibrium will shift towards the right to consume more water molecules. This means more cobalt(II) chloride hydrate (CoCl2·6H2O) will be formed.

Determine the color of the indicator

Since the equilibrium shifts to the right and more cobalt(II) chloride hydrate is formed, the color of the indicator will be closer to the color of the CoCl2·6H2O. This compound has a pink color when solid. Therefore, the rain indicator will be pink if rain is imminent. So, the indicator will be pink when rain is imminent, signaling an increase in humidity.

Key concepts

Chemical Equilibrium

Understanding chemical equilibrium is essential for grasping how reactions behave over time. Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products over time. Imagine a busy city intersection where cars are entering and leaving at the same rate; the number of cars within the intersection area stays consistent.

In the context of a chemical reaction, this does not mean the reactants and products are in equal concentrations, but that their rates of formation are balanced. The position of the equilibrium gives us information about the proportions of substances in the mixture at equilibrium.

Humidity and Chemical Equilibrium

Humidity plays a significant role in influencing chemical equilibria, especially in reactions involving hydrates and water vapor. Le Chatelier's principle helps us understand this influence by stating that when a system in equilibrium is disturbed, it will adjust in a way that counteracts the disturbance.

When humidity increases, the air contains more water vapor. A system containing a hydrate will respond by shifting the equilibrium to use up excess water vapor. This can be observed in everyday phenomena such as the pink coloration of a humidity indicator, where the formation of a cobalt(II) chloride hydrate intensifies with higher humidity levels, indicating a shift in the chemical equilibrium towards the hydrated form.

Cobalt(II) Chloride Hydrate

Cobalt(II) chloride hydrate, with its distinct color changes, serves as an excellent example of a chemical hydrate and its equilibrium with water vapor. This hydrate specifically transitions between a purple anhydrous form, CoCl2, and a pink hydrated form, CoCl2·6H2O, in response to atmospheric humidity.

The vivid color change associated with cobalt(II) chloride provides a visual indicator of the equilibrium shift. When the hydrate is exposed to dry conditions, the pink color fades to purple as water molecules are released into the environment. Conversely, in humid conditions, the compound takes on a pink hue due to the formation of CoCl2·6H2O, revealing how humidity can directly impact a chemical's properties.