Review of Concepts for Enthalpy

[105764441]

Enthalpy: The study of heat released or absorbed in chemistry
The amount of heat released or absorbed at constant pressure is called enthalpy.
q=is heat
qp=heat is being given off under constant pressure conditions

If the reaction gives off a net release of heat, the reaction is considered to be exothermic. So, delta H is negative. The reaction releases heat to the environment and warms it.
Exothermic reactions release heat.

If the reaction requires heat to proceed, the reaction is considered to be endothermic. The reaction requires a heat input.

Enthalpy is a state property. The value of enthalpy is determined by its current state.
State properties can be added and subtracted.

Work (w) and heat (q) depend on path taken (they are not state properties) For example, if I walk straight up a mountain, but my friends walk around and around and around the mountain to get to the same elevation, we have both gotten to the same elevation. Thus, elevation is a state property. However we have doen different ammount of work, thus woprk is NOT a state property.

Phase Changes

Vaporization
means that a liquid is turning into a vapor.
Enthalpy Equation for a liquid turning into a vapor:
Delta H of the vaporization state - Delta H of the liquid state = delta H of the vaporization state, which is a positive value. This makes sense given that water must be heated to create steam. This is vaporization. If a pahse change needs an input of heat, then the reaction is endothermic, and the phase change has a delt H that is positive because the enthalpy of the products is far greater than the enthalpy of the reactants. You can see this on an enthalpy graph because the products will be much higher than the reactants on the graph, form which you may gather that the enthalpy of the products is greater.

Melting, otherwise known as fusion
Melting means that a solid undergoes a phase change and becomes a liquid. It is pretty intuitive to gether from this that some input of heat is required. An easy example of this to cememnt this idea would be ice melting into water. We know that the ice melts because it is heated up, therefore, the phase change requires an input of heat, and thus it is an endothermic reaction. If we take a look at the equation, we will notice that the delta H of the water minus the delta H of the ice will result in a positive delta H for the reaction, which again helps us conclude that the reaction is endothermic.

Sublimation: this is a term I was at first unfamiliar with. But all it means is a solid turning into a vapor. You may think to yourself, "when does this happen?" However, you have likely seen this happen before when dry ice is placed in a room temperature environment. The ice condenses as it interats with the air and lets of CO2 gas, or vapor. Why does this happen? Again, it has to do with the solid interacting with the air. The air is warmer than the ice, so, the ice is being heated, aka an input of energy, and a phase change of solid to vapor results. Thus, this reaction is ALSO endothermic. Delta H of sublimation= The delta H of the vapor minus the delta H of the solid, which is a positive value!

So, when is a phase change exothermic, aka it requires heat? Well, all of these reactions are actually exothermic in the opposite direction. So, a vapor becoming a liquid, or liquid becoming a solid, or a gas becoming a solid all release heat to the environment in order to change phases.

Think about is. Water turning to ice must be cooled, so the water must release heat to the environment. If your windows are fogges up in you car, you must cool the fog or gas to then create water which then drips off the windows, so this foggy gas will release heat to the environment as it becomes a liquid again.

[kjwong02]

Hi! I really appreciate the reviews of concepts for enthalpy you made. It seems you've puts lot of effort into it! I think you've made a typo in the second to last paragraph. I think you meant that an exothermic phase change releases heat as opposed to requires heat.