Formula sheet: Thermodynamics
Thermodynamics12 min readLesson 1 of 7
1.1 The two laws in one page
The first law is conservation of energy applied to a system: the internal energy changes by the heat added plus the work done on the system. The second law decides direction: the entropy of the universe increases in every spontaneous process. Gibbs energy combines the two into one practical condition at constant temperature and pressure.
1.2 Thermochemistry
1.3 Entropy and Gibbs energy
1.4 Conventions and common traps
The chapter mixes two conventions only when students forget to write them down. In the chemistry convention work done on the system is positive and work done by the system is negative; in engineering texts the sign is often reversed. Heat absorbed is positive. Every standard enthalpy value in this book is for the standard state at one bar, and the standard enthalpy of formation of an element in its standard state is zero by definition. When a question gives heat capacities in J and enthalpies in kJ, convert to one unit before substituting, and when a question gives a temperature in degrees Celsius, convert it to kelvin for every thermodynamic relation.
1.5 The four sign cases
| delta H | delta S | Spontaneous when |
|---|---|---|
| negative | positive | at all temperatures |
| negative | negative | below T = delta H / delta S |
| positive | positive | above T = delta H / delta S |
| positive | negative | at no temperature |
1.6 Constants and units
| Quantity | Value or unit |
|---|---|
| Gas constant R | 8.314 J K^-1 mol^-1 |
| Faraday constant F | 96500 C mol^-1 |
| 1 calorie | 4.184 J |
| 1 litre bar | 100 J |
| Standard pressure | 1 bar |
| Standard temperature in data tables | 298 K |
Exam
Watch the sign of work. Work done by the system is negative in the chemistry convention, and work done on the system is positive. A gas expanding against an external pressure does negative work, so its internal energy falls if no heat is supplied.
- Write the sign convention for q and w before starting any first law question.
- Convert litres and bars to joules where work is needed.
- Check whether the process is reversible before using the logarithmic work expression.
- Use delta n_g from the balanced equation when converting delta U to delta H.
- Apply Hess's law by adding equations and adding their enthalpies.
- Decide spontaneity from the sign of delta G, not from delta H alone.