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Thermochemistry & Thermodynamics
Thermodynamics tells you which way a reaction goes and how much heat it exchanges. You will use the first law, calorimetry, Hess's law, entropy, and Gibbs free energy to predict spontaneity and calculate heat and energy changes.
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What this chapter covers
- Energy, Work & Enthalpy. The first law and the sign conventions behind q, w, ΔU and ΔH are tested directly and hidden inside almost every passage.
- Calorimetry & Heating Curves. Heat calculations with q = mcΔT and phase-change plateaus show up in both calculations and graph questions.
- Hess's Law & Enthalpy Calculations. Because enthalpy is a state function, you can add reactions, use formation data, or count bonds. All three methods are tested.
- Entropy & the Second Law. Entropy explains why some endothermic processes still happen. Predicting the sign of ΔS is a quick, high-yield skill.
- Gibbs Free Energy & Spontaneity. ΔG combines enthalpy and entropy into one spontaneity test and connects directly to the equilibrium constant.
Key points to know
- ΔU = q − w: w is work done BY the system
- At constant pressure ΔH = q; at constant volume ΔU = q
- q = mcΔT off the plateaus; q = mΔH on a plateau
- Temperature is constant during a phase change
- Hess's law: ΔH is a state function, so step enthalpies add
- Reverse a reaction: flip the sign. Multiply: scale ΔH
- Entropy: solid < liquid ≪ gas; more moles of gas means ΔS > 0
- Second law: ΔS_univ > 0 for spontaneous processes
Try a practice question
Which of the following is a state function?
- Heat absorbed
- Work done
- Enthalpy
- The pathway used
Show the answer
C. Enthalpy depends only on the state of the system. Heat and work depend on the route taken between the two states, so they are path functions.
Common questions
What does this chapter cover?
Thermochemistry & Thermodynamics covers Energy, Work & Enthalpy, Calorimetry & Heating Curves, Hess's Law & Enthalpy Calculations, Entropy & the Second Law, Gibbs Free Energy & Spontaneity.
What should I know first?
ΔU = q − w: w is work done BY the system At constant pressure ΔH = q; at constant volume ΔU = q q = mcΔT off the plateaus; q = mΔH on a plateau
24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.