7 Thermochemistry Free Online FlashCards

What are the system and surroundings?

The system is the matter being studied; the surroundings are everything else.

How does heat differ from temperature?

Heat is energy transferred because of a temperature difference; temperature measures a thermal property of matter. Heat is energy in transit, not a substance stored in an object.

What do the signs of $q$ indicate?

By the chemistry sign convention, $q>0$ when heat enters the system and $q<0$ when heat leaves it.

What do the signs of $w$ indicate?

By the chemistry sign convention, $w>0$ when work is done on the system and $w<0$ when work is done by the system.

How are internal-energy change, heat, and work related?

The first law gives $\Delta U=q+w$: a system’s internal-energy change equals heat transferred to it plus work done on it.

How does volume change determine pressure–volume work?

At constant external pressure, $w=-P_{\text{ext}}\Delta V$. Expansion has $\Delta V>0$ and therefore $w<0$; compression has $\Delta V<0$ and $w>0$.

How do state functions differ from path-dependent quantities?

Internal energy is a state function, so its change depends only on initial and final states. Heat and work are path-dependent: their amounts depend on how the change occurs.

What is enthalpy, and when does it equal transferred heat?

Enthalpy is defined as $H=U+PV$. At constant pressure, when the only work is pressure–volume work, the heat transferred to the system is $q_p=\Delta H$.

What do the signs of reaction $\Delta H$ reveal?

An exothermic reaction has $\Delta H<0$ and releases heat; an endothermic reaction has $\Delta H>0$ and absorbs heat.

How does changing a thermochemical equation affect $\Delta H$?

Reversing a thermochemical equation reverses the sign of $\Delta H$. Multiplying every coefficient by a factor multiplies $\Delta H$ by the same factor.

How is reaction enthalpy calculated from formation enthalpies?

Calculate it as $\Delta H^\circ_{\text{rxn}}=\sum n\Delta H^\circ_f(\text{products})-\sum n\Delta H^\circ_f(\text{reactants})$, using balanced-equation coefficients for $n$.

What does Hess’s law allow you to calculate?

Hess’s law states that enthalpy changes for steps add to the overall enthalpy change because enthalpy is a state function.