Achieve Week #5 and #6 Question #10

[Diego Salgues 2K]

The small bags of silica gel you often see in a new shoe box are placed there to control humidity. Despite its name, silica gel is a solid. It is a chemically inert, highly porous, amorphous form of SiO2. Water vapor readily adsorbs onto the surface of silica gel, so it acts as a desiccant.

Despite not knowing mechanistic details of the adsorption of water onto silica gel, from the information provided, you should be able to make an educated guess about the thermodynamic characteristics of the process. Predict the signs of Delta G, Delta H, and Delta S.

The answers are Delta G < 0, Delta H < 0, and Delta S < 0.

I understand Delta G is less than zero because the reaction is spontaneous. It's spontaneous because for the device to work well on its own, well, it shouldn't need the input of energy. It works well on it's own.

I understand Delta S is less than zero because the silica gel pack brings things together, and makes them less chaotic therefore decreasing S and having Delta S be a negative change.

However, I don't get why Delta H is less than zero, at least conceptually. I understand to have Delta G be < 0 and Delta S < 0, Delta H < 0 would need to be the case to make the Delta G < 0 statement true. However, disregarding that line of reasoning, why is Delta H less than zero?

[Gabriel McDonald 1J]

∆H < 0 because water absorbs onto the surface of silica gel. Water vapor from the air will condense and get absorbed by the gel, and since the process of condensation (vapor to liquid) is exothermic, ∆H < 0.

[Tomas Burgess 3F]

Hi! Delta H is negative because the process described involves the condensation of water vapor onto the surface of the silica gel when it is absorbed. This process has a negative delta H value because condensation is an exothermic process, because the water molecules lose energy as they are converted from the gas form to the liquid form.

[patienceokani]

Delta H is negative because water vapor to liquid is condensation and condensation is a exothermic because it is a phase change and has to release some of it's energy.

[305597516]

Delta H is less than 0 because this implies that an exothermic reaction has occurred at some point during the adsorption of fluid into the silica gel packets. If we look at this from the perspective of phase changes, it appears that vaporized water (from the humidity in the shoe box) is absorbed as water into the gel, causing the shoe box to dry up (the silica gel is referred to as a "dessicant" which is basically something that dries something else). Therefore, the process of condensation is occurring as vaporized water becomes water. Condensation is therefore an exothermic process, which is why delta H is negative. Hope this helped!

[Vanessa Wiratmo 3k]

Hello,

When looking at Delta H, is is important to know what it means when delta H is positive or negative. When delta H is positive, that means that the reaction is endothermic and heat is consumed. When delta H is negative, that means the reaction is exothermic is released.

In the question, water vapor is being absorbed. This process is also known as condensation, which is an exothermic process. Delta H is less than 0 when the reaction is exothermic.

[905715668]

the sign of delta H (for this specific problem) depends on the phase changes
when heat is added to a system then delta H is positive (liquid to gas or solid to liquid)