The common units of energy are the joule, J, and the calorie, cal. A calorimeter is used to measure the heat released or absorbed during a chemical or phys-
ical change. Know how a calorimeter works.
The specific heat capacity is the amount of heat needed to change the temperature of
1 gram of a substance by 1 K, while the molar heat capacity is the heat capacity per mole.
The heat lost by the system in calorimetry is equal to the heat gained by the surroundings. The specific heat (c) of a solid can be calculated by: −(csolid×msolid×ΔTsolid) =cwater×
mwater×ΔTwateror by g= cmΔT.
The First Law of Thermodynamics states that the total energy of the universe is con-
stant. (Energy is neither created nor destroyed.)
The Second Law of Thermodynamics states that all spontaneous processes move in a
way that increases the entropy (disorder) of the universe.
The enthalpy change, ΔH, is equal to the heat lost or gained by the system under con-
stant pressure conditions.
ΔHvalues are associated with a specific reaction. If that reaction is reversed, the sign of
ΔHchanges. If one has to use a multiplier on the reaction, it must also be applied to
the ΔHvalue.
The standard enthalpy of formation of a compound, ΔHf°, is the enthalpy change when
1 mol of the substance is formed from its elements and all substances are in their stan-
dard states.
The standard enthalpy of formation of an element in its standard state is zero. ΔH°rxn=ΣΔHf°products −ΣΔHf°reactants. Know how to apply this equation. ΔH°rxnis usually negative for a spontaneous reaction. ΔS°=ΣS°products −ΣS°reactants. Know how to apply this equation. ΔS°is usually positive for a spontaneous reaction. The Gibbs free energy is a thermodynamic quantity that relates the enthalpy and
entropy, and is the best indicator for whether or not a reaction is spontaneous.
If ΔG°>0 the reaction is not spontaneous; if ΔG°<0, the reaction is spontaneous; and
if ΔG°=0, the reaction is at equilibrium.
ΔG°=ΣΔGf°products −ΣΔGf°reactants. Know how to apply this equation. ΔG°=ΔH°rxn−TΔS°rxn. Know how to apply this equation. For a system not at equilibrium: ΔG=ΔG°+RTln Q=ΔG°+2.303 RTlog Q. Know
how to apply this equation.
For a system at equilibrium: ΔG°=−RTln K=−2.303 RTlog K. Know how to apply
this equation to calculate equilibrium constants.136 Step 4. Review the Knowledge You Need to Score High