Terms Used in Thermochemistry: MDCAT Chemistry notes
Terms Used in Thermochemistry MDCAT notes: system and surroundings, state and path functions, internal energy, enthalpy, standard conditions, heat units.
System, surroundings and boundary
- System: the part of the universe under study. For water in a cup on a table, the system is the water.
- Surroundings: everything else around the system (the cup, table, air).
- Boundary: the real or imaginary surface separating the system from its surroundings.
| Type of system | Exchanges energy? | Exchanges matter? | Example |
|---|---|---|---|
| Open | Yes | Yes | Water boiling in an open beaker |
| Closed | Yes | No | Water in a sealed flask |
| Isolated | No | No | Liquid in an ideal thermos flask |
State functions and path functions
A state function depends only on the initial and final states of the system, not on the path by which the change was made.
- State functions: pressure, volume, temperature, internal energy ($E$), enthalpy ($H$).
- Not state functions: heat ($q$) and work ($w$) depend on the path taken.
- Processes such as freezing, decomposition and sublimation are changes, not state functions.
Internal energy and enthalpy
- Internal energy ($E$ or $U$): the total energy of the particles in the system (kinetic plus potential). Only the change $\Delta E$ can be measured. At constant volume, $\Delta E = q_v$.
- Enthalpy ($H$): $H = E + PV$. The heat exchanged at constant pressure is the enthalpy change: $\Delta H = q_p$.
Why reactions involve energy changes
Energy is absorbed to break bonds and released when new bonds form. The balance of the two is the heat of reaction.
Standard conditions
Standard enthalpy changes, written $\Delta H^\circ$, are measured at 298 K (25 °C) and 1 atm pressure, with substances in their standard physical states and solutions at 1 M concentration. Example: $\Delta H_f^\circ$ of $\mathrm{NH_4Cl(s)}$ is about $-314\ \mathrm{kJ\,mol^{-1}}$.
Units of heat and heat capacity
- SI unit of energy: joule (J). 1 calorie = 4.184 J (often rounded to 4.18 J). 1 kcal = 4.184 kJ.
- Heat capacity: heat needed to raise the temperature of a body by 1 K. Unit: $\mathrm{J\,K^{-1}}$.
- Specific heat capacity: per gram, $\mathrm{J\,g^{-1}\,K^{-1}}$. For water it is $4.18\ \mathrm{J\,g^{-1}\,K^{-1}}$.
- Molar heat capacity: per mole, $\mathrm{J\,K^{-1}\,mol^{-1}}$.
Sign conventions
- Heat absorbed by the system: $q$ positive. Heat released: $q$ negative.
- Work done on the system: $w$ positive. Work done by the system: $w$ negative.
- Exothermic $\Delta H$ negative; endothermic $\Delta H$ positive.
Key formulas
- $H = E + PV$; $\Delta H = q_p$; $\Delta E = q_v$
- $q = mc\Delta T$ (specific heat); $q = C\Delta T$ (heat capacity)
Worked example
Heat needed to warm 50 g of water by 10 K: $q = 50\times4.18\times10 = 2090\ \mathrm{J} \approx 2.09\ \mathrm{kJ}$.
Common MDCAT traps
- Enthalpy is a state function; work and heat are not.
- 1 calorie is 4.18 J, not 4.18 kJ.
- Heat capacity is in $\mathrm{J\,K^{-1}}$; $\mathrm{kJ\,mol^{-1}}$ is the unit of an enthalpy change.
- Standard state is 298 K and 1 atm, not 300 K or 273 K.
- The system is what you study (the water), not the container.
Quick revision
- State functions: P, V, T, E, H.
- Isolated system exchanges neither energy nor matter.
- $\Delta H$ is heat at constant pressure.
- Standard conditions: 298 K, 1 atm.
- 1 cal = 4.184 J.