Heats of Reaction: Enthalpy
- Enthalpy (H) is the internal energy (E) plus pressure multiplied by volume (PV). The change in enthalpy (ΔH) equals the heat (qP) for a system at constant pressure.
- ΔH is negative for exothermic reactions, and positive for endothermic reactions.
note
- Enthalpy (H) is the internal energy (E) plus pressure
multiplied by volume (PV). Since E, P, and V are
state functions (path-independent), H is also a state function. - The change in enthalpy (ΔH) for a reaction is the difference between the enthalpy of the products and the enthalpy of the reactants. H equals the change in internal energy (ΔE) plus the external pressure (Pex ) multiplied by the change in volume (ΔV) plus the volume (V) multiplied by the change in pressure ( P). At constant pressure, this equation reduces to ΔH = qP .
- This equality doesn’t seem to make sense; enthalpy is a state function, but heat is not a state function. Heat is analogous to the path from your house to your friend’s house. The paths can have different lengths, but the shortest path has a specific length—the shortest path is a state function. When constraints are place on a non-state function, it can become a state function. Thus, heat at constant pressure is a state function.
- In an exothermic reaction (such as the combustion of
ethanol), the products have a lower enthalpy than the
reactants. Therefore, H is negative for an exothermic
reaction. In an exothermic reaction, heat flows from the
system to the surroundings. - In an endothermic reaction (such as the reaction between barium hydroxide octahydrate and ammonium nitrate), the products have a higher enthalpy than the reactants. Therefore, H is positive for an endothermic reaction. In an endothermic reaction, heat flows into the system from the surroundings.
Which of the following is an example of an exothermic reaction?
A candle burning
Enthalpy is a state function, but heat measured at constant pressure is not a state function.
false
Which of the following is the equation for the change in enthalpy?
ΔH = q_p
Which of the following describes an endothermic reaction?
A reaction that moves heat from the surroundings into the system
Which of the following are state functions?
- Enthalpy
- Heat measured at a constant pressure.
Change in enthalpy is equal to heat measured under what condition?
constant pressure
True or false?
In an endothermic reaction, heat moves away from the system and into the surroundings.
false
Which of the following values does not represent an exothermic reaction?
ΔH = +890.32
Which of the following statements is not true about exothermic reactions?
The change in enthalpy for an exothermic reaction is greater than zero.