Chapter 10 Enthalpy, Thermochemical Equations & Hess's Law
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20 Terms
What is pressure-volume work?
Work done when volume changes under constant pressure.
What is the equation for PV work?
w = -PΔV.
What happens to work when ΔV = 0?
No work is done.
What is enthalpy?
A thermodynamic quantity defined as H = U + PV.
What is ΔH?
The change in enthalpy of a system.
Under constant pressure, ΔH equals what?
Heat (q).
What is the enthalpy of reaction?
The difference between product and reactant enthalpies.
What does ΔH > 0 indicate?
An endothermic reaction.
What does ΔH < 0 indicate?
An exothermic reaction.
Is enthalpy an extensive or intensive property?
Extensive.
What does an extensive property depend on?
The amount of matter present.
Why must physical states be included in thermochemical equations?
They affect the enthalpy change.
What happens to ΔH when an equation is multiplied by a factor?
ΔH is multiplied by the same factor.
What happens to ΔH when an equation is reversed?
The sign changes.
What is Hess's Law?
The overall ΔH equals the sum of the ΔH values of individual steps.
Why does Hess's Law work?
Enthalpy is a state function.
What is the first step in solving Hess's Law problems?
Manipulate equations to obtain the desired reaction.
What do you do after manipulating Hess's Law equations?
Add the adjusted ΔH values.
What is the standard enthalpy of formation (ΔHf°)?
The heat change when 1 mole of a compound forms from its elements in their standard states.
What is the ΔHf° of an element in its standard state?
Zero.