Question

The enthalpy change, ?H, is the quantity of heat transferred into or out of a system as it undergoes a chemical or a physical change at constant pressure. In short: ?H is heat change at constant pressure. Note that there really isn't much new here. We've already discussed heat a lot during our thermodynamic discussions. Since heat is not a state function, it turns out that the amount of heat absorbed or released as a system undergoes a change is dependent on how that change occurs - the pathway of the change. This means that if a reaction occurs at constant pressure it might have a different value for q than if it occurred while pressure was changing. Since we often carry out reactions at constant pressure - picture an open beaker on a lab bench - we give q at constant pressure a special name, enthalpy. If you see the term enthalpy or its symbol, H, you know it means "heat at constant pressure". We can symbolize this as qp, where the subscript p indicates "constant pressure". ?H = qp The process of water condensing at 100 °C is Endothermic with ?H > 0 and w > 0 Endothermic with ?H > 0 and w < 0 Endothermic with ?H < 0 and w < 0 Exothermic with ?H < 0 and w < 0 Exothermic with ?H < 0 and w > 0 Exothermic with ?H > 0 and w > 0

          The enthalpy change, ?H, is the quantity of heat transferred into or out of a system as it undergoes a chemical
or a physical change at constant pressure.
In short: ?H is heat change at constant pressure.
Note that there really isn't much new here. We've already discussed heat a lot during our thermodynamic
discussions.
Since heat is not a state function, it turns out that the amount of heat absorbed or released as a system
undergoes a change is dependent on how that change occurs - the pathway of the change. This means that if a
reaction occurs at constant pressure it might have a different value for q than if it occurred while pressure was
changing. Since we often carry out reactions at constant pressure - picture an open beaker on a lab bench - we
give q at constant pressure a special name, enthalpy.
If you see the term enthalpy or its symbol, H, you know it means "heat at constant pressure". We can symbolize
this as qp, where the subscript p indicates "constant pressure".
?H = qp
The process of water condensing at 100 °C is
Endothermic with ?H > 0 and w > 0
Endothermic with ?H > 0 and w < 0
Endothermic with ?H < 0 and w < 0
Exothermic with ?H < 0 and w < 0
Exothermic with ?H < 0 and w > 0
Exothermic with ?H > 0 and w > 0

The enthalpy change, ?H, is the quantity of heat transferred into or out of a system as it undergoes a chemical
or a physical change at constant pressure.
In short: ?H is heat change at constant pressure.
Note that there really isn't much new here. We've already discussed heat a lot during our thermodynamic
discussions.
Since heat is not a state function, it turns out that the amount of heat absorbed or released as a system
undergoes a change is dependent on how that change occurs - the pathway of the change. This means that if a
reaction occurs at constant pressure it might have a different value for q than if it occurred while pressure was
changing. Since we often carry out reactions at constant pressure - picture an open beaker on a lab bench - we
give q at constant pressure a special name, enthalpy.
If you see the term enthalpy or its symbol, H, you know it means "heat at constant pressure". We can symbolize
this as qp, where the subscript p indicates "constant pressure".
?H = qp
The process of water condensing at 100 °C is
Endothermic with ?H > 0 and w > 0
Endothermic with ?H > 0 and w < 0
Endothermic with ?H < 0 and w < 0
Exothermic with ?H < 0 and w < 0
Exothermic with ?H < 0 and w > 0
Exothermic with ?H > 0 and w > 0

Added by Blanca L.

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