6.2.1 Heats of Reaction: Enthalpy Flashcards Preview

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Flashcards in 6.2.1 Heats of Reaction: Enthalpy Deck (11)
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1
Q

Heats of Reaction: Enthalpy

A
  • 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.
2
Q

note

A
  • 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.
3
Q

Which of the following is an example of an exothermic reaction?

A

A candle burning

4
Q

Enthalpy is a state function, but heat measured at constant pressure is not a state function.

A

false

5
Q

Which of the following is the equation for the change in enthalpy?

A

ΔH = q_p

6
Q

Which of the following describes an endothermic reaction?

A

A reaction that moves heat from the surroundings into the system

7
Q

Which of the following are state functions?

A
  • Enthalpy

- Heat measured at a constant pressure.

8
Q

Change in enthalpy is equal to heat measured under what condition?

A

constant pressure

9
Q

True or false?

In an endothermic reaction, heat moves away from the system and into the surroundings.

A

false

10
Q

Which of the following values does not represent an exothermic reaction?

A

ΔH = +890.32

11
Q

Which of the following statements is not true about exothermic reactions?

A

The change in enthalpy for an exothermic reaction is greater than zero.

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