Heats of Reaction: Enthalpy (6.2.1)

Question 1

• Enthalpy (H) is the internal energy (E) plus pressure multiplied by volume (PV).
The change in enthalpy (∆H) equals the heat (qP) for a system at constant
pressure.

Answer 1

• Enthalpy (H) is the internal energy (E) plus pressure multiplied by volume (PV).
The change in enthalpy (∆H) equals the heat (qP) for a system at constant
pressure.

Question 2

• ∆H is negative for exothermic reactions, and positive for endothermic reactions.

Answer 2

• ∆H is negative for exothermic reactions, and positive for endothermic reactions.

Question 3

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 (∆H) 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 (∆E) plus the external
pressure (Pex) multiplied by the change in volume
(∆V) plus the volume (V) multiplied by the change in
pressure (∆P). At constant pressure, this equation
reduces to ∆H = 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.

Answer 3

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 (∆H) 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 (∆E) plus the external
pressure (Pex) multiplied by the change in volume
(∆V) plus the volume (V) multiplied by the change in
pressure (∆P). At constant pressure, this equation
reduces to ∆H = 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.